blog

by Alma González (University of Guanajuato, Mexico) and Martine Lokken (Autonomous University of Barcelona, Spain)

24 April 2025

DESI’s educational program for high school students, DESI High, is now back in session!

Designed so students can work with DESI data directly from the telescope, the program also introduces fundamental concepts in cosmology and helps develop computer programming skills. The program was a huge hit with students in online events held during the pandemic and for a couple of years thereafter. However, it has been a while since we offered an event, especially in person.

But now DESI High is coming back in full swing. We are re-factoring and improving our Jupyter notebooks, testing new platforms to host them, and more importantly, we are adding examples that illustrate how to access the now publicly-available DESI data and new notebooks.  Beyond that, we are aiming to offer new tutorial sessions on how to use such materials, both in-person and online. In February, we hosted the first two events of 2025, in Cozumel, Mexico and Catalonia, Spain.

Our first event, held on February 8th, was co-organized with the Cozumel Planetarium, CHA’AN KA’AN, who invited us to commemorate the International Day of Women and Girls in Science (February 11th). Of the approximately 20 participants who joined us on the day of the event, many were female students from the Quintana Roo area, which is located in the southern part of Mexico. Over a two-hour hybrid session, the participants interacted with DESI members, including  Andrea Múñoz from the National Autonomous University of Mexico (UNAM, Phd Student), Diego González from University of Guanajuato (UG, Msc student), Maria Pineda (UNAM, undergraduate student), John Suarez from Monterrey Institute of Technology in Guadalajara (researcher), Fanny Rodríguez (UNAM, MsC student), a non-DESI undergraduate student Daisy Torres (UG) and Alma Gonzalez (UG, researcher). Mariana Vargas (UNAM, researcher), also a DESI member, participated as well in preparations for the event.

The session began with an introduction to DESI science as well as time for the students to get to know the instructors’ motivations to get into science and what their daily life is like working in DESI. Then, the instructors and students delved into the DESI High notebooks in Spanish! For this session, we partnered with Camber Cloud, who kindly provided test accounts for our students so that they could experience the entire process of making a copy of the DESI High repository, running the notebooks, and saving their progress. While we also offer other platforms to just plug-and-play, this gave students to the opportunity to see the entire process from start to finish, which all worth it. We are expecting to have a follow up session with this team.

At the second event, on Feb 22, approximately thirty high school students from across the province of Catalonia, Spain, joined us at the Autonomous University of Barcelona campus to learn how DESI is measuring our universe’s expansion. For the cosmology edition of the “Bojos per la Física” (“crazy for physics”) annual program, three cosmologists from the Institut de Física d’Altes Energies (IFAE) led the students through the introductory DESI High tutorial notebook. This notebook uses Python code to demonstrate how measuring the spectra of distant galaxies translates into understanding the universe’s contents. The students also got an overview of how the DESI telescope works, and examined a prototype of the Guide-Focus-Acquisition (GFA) cameras that IFAE engineers developed to help guide the DESI robotic fiber array.

The day’s activities were run by DESI members Laura Casas (Phd student) and Martine Lokken (Postdoc), and Dane Cross (PhD student in the DES collaboration), who also shared what their day-to-day research life is like. Many of the students are interested in pursuing physics in the future, some in astrophysics and cosmology, and this program helped them understand the current research landscape as well as a typical day in the life of a cosmologist.

In the next few months, the DESI High team will be finalizing updates to the repository. Besides updating the current Jupyter notebooks to work with data from the DESI’s first data release (public as of 19 March), we will also introduce some new notebooks. One will feature a shorter but more straightforward introduction to Dark Energy and DESI. Another one will illustrate how we measure distances in cosmology. And a third will introduce Baryon Acoustic Oscillations! With these visually interesting, step-by-step Jupyter notebooks, we look forward to many future events that use the DESI High tutorials to spread the excitement of cosmology and DESI discoveries!

— edited by Joan Najita

Data and Analysis also in Demand

by Joan Najita (NOIRLab)

14 April 2025

DESI’s 19 March 2025 press release, which reported the likelihood that dark energy is evolving, sparked excitement and engagement across the globe, from both the general public and the scientific community. Announced by Berkeley Lab and 23 collaborating DESI institutions that published partner releases, the news led to significant media coverage.

As of 27 March 2025:

• More than 1,500 articles reporting the news had appeared worldwide in 67 countries.
• Dozens of the world’s most respected science news and media outlets covered the story.
• Articles appeared in 35 languages, with one quarter of the articles published in a language other than English.

For Berkeley Lab Science Writer Lauren Biron, who organized the press release effort for the collaboration, “It was a real privilege to be trusted with DESI’s secrets ahead of time, and to play a small part in telling DESI’s story. The collaboration put such effort and care into the results and the announcement, so it was gratifying to see DESI’s news spread far and wide. This is certainly one of the most far-reaching results I’ve ever been involved with as a science writer.”

Biron attributes part of the media success to the beautiful visualizations of the DESI data created by NOIRLab, Fiske Planetarium, and DESI’s Claire Lamman: “Those maps and flythroughs of the galaxies conveyed at a glance the impressive scope of the experiment, and evoked a sense of wonder and awe at our universe. They made it easy for folks to get excited about the science.”

In creating his visualizations, color selection was important to NOIRLab’s Ron Proctor, who found the white-to-blue color scheme best at highlighting the intricate structure in the DESI dataset. The color scheme also evokes a connection to human experience. To DESI Director Michael Levi, the color scheme suggested “the white surf of the nearby galaxies and the deep sea of those faraway.” Surprised and delighted to see his work featured in the New York Times and on the cover of Le Monde, Proctor says, “It feels good to know I’ve contributed to something that is so meaningful and effective.”

The media coverage surpassed even that of last year’s April 2024 press release. While the earlier release, which reported the first hints of evolving dark energy based on the first year of DESI data, had been widely covered in the media, this year’s release, which reported results from the first 3 years of DESI data, generated more news articles (1500+ vs. 1200+) in more languages. It also resulted in much higher engagement (the number of likes, shares, and comments on those articles received online): a whopping 191,000 this year vs. 32,000 last year.

Beyond the media coverage, the response from the scientific community has also been strong, with the DESI papers reporting the science results and the Year-1 data release attracting a lot of attention.

As of 9 April 2025,

• 575 TB of data have been downloaded from the NERSC, the Berkeley Lab computing facility (with 197 TB downloaded directly from data.desi.lbl.gov, and 378 TB using globus to access the NERSC data transfer nodes).
• 15,000 new users have visited data.desi.lbl.gov.
• The 3 science papers reporting the main results have been downloaded 21,500 times from data.desi.lbl.gov/doc/papers, and potentially many more from the arXiv preprint server.
• Value Added Catalogs provided by the science collaboration in association with the data release have been downloaded 12,000 times.

With the potential to spark completely new ideas and applications than those they were originally designed to address, who knows what new discoveries will emerge from these DESI data?!

Note: Sincere thanks to Stephen Bailey (Berkeley Lab) for compiling and sharing the data usage statistics.

by Gillian Beltz-Mohrmann (Argonne National Laboratory)

In April 2024 DESI released the BAO results from its first year of data, reporting a surprising result: a slight preference for a model with a time-varying dark energy equation of state. This is in stark contrast to the standard LCDM cosmological model, which includes a constant dark energy density (i.e., not varying over time). The new preference for a time-evolving dark energy, a model known as w0waCDM, was strengthened further when the DESI data were combined with other datasets (Cosmic Microwave Background measurements and Type Ia Supernovae distances). Because the preference for w0waCDM based on the Year 1 data was moderate, DESI would have to collect more data to get a clearer picture of whether this was an anomalous result or whether we had discovered something new about the cosmos.

The December 2024 meeting of the DESI Collaboration came with an exciting prospect: the unblinding of the BAO results from the first 3 years of DESI data! This historic event would give us the first indication of whether the w0waCDM cosmology would still be preferred with the inclusion of more data.

The “live unblinding,” at which the collaboration would witness the results for the first time, was scheduled for Thursday evening. But before this could occur, a final consensus had to be reached about whether to indeed proceed with the unblinding. A Tuesday evening session was scheduled to discuss last minute analysis choices and decide whether all of the necessary checks had been passed in order to proceed with the unblinding. The stakes of the session were high: once the data was unblinded, it could not be blinded again. After a long discussion, which lasted about an hour longer than it was scheduled to, the decision was made: the unblinded analysis would proceed as scheduled!

Tuesday’s session was led by Sesh Nadathur, an STFC Ernest Rutherford Fellow at the Institute of Cosmology and Gravitation at the University of Portsmouth. Recalling the experience, Sesh said, “Tuesday was a long and exhausting day – the whole collaboration felt they had a stake in the results, and we must have had 100 people in the room or joining the call. That was a great feeling, but I also felt a lot of responsibility to ensure we took everyone with us and everyone bought into our decision to move ahead with the unblinding. At the same time, we knew the decision had to be agreed upon in that session otherwise the whole timeline would have been thrown off! By the end, my overwhelming emotion was just relief that we’d gotten there.”

The BAO analysis team spent the next two nights getting very little sleep in order to ensure the timely delivery of the results. Cristhian Garcia-Quintero, a NASA Einstein Fellow at Harvard University and a member of the analysis team, recalled, “Once we got the green light on Tuesday, I was very nervous because we needed to have the unblinded results ready to present by Thursday. There was a lot to do in a short amount of time, and everyone was waiting on the results.” Sesh added, “It was only possible to complete the analysis because of the incredible work that Cristhian and Uendert Andrade (a postdoctoral fellow at the University of Michigan) did on those two days, with massive help from many others in the months leading up to the meeting to set everything up to process all the results so quickly.”

By Thursday morning, Sesh and Cristhian were able to examine the final results for the first time. Over the next few hours, Sesh worked hard to put his presentation together in time for the evening unblinding session, a process he describes as “Actually the best experience of my career so far! It was that extremely rare combination of learning the answer to a very important scientific question – and recognizing immediately that it was very important, rather than slowly realizing this over time, which is the more usual case – and for a few hours being one of only two people in the whole world who knows it.”

Sesh and Cristhian spent all of Thursday deftly deflecting questions about the status of the unblinded results. Cristhian found that “Lots of people were joking with me throughout the day, asking for a sneak preview of the results.” Sesh had a similar experience: “All day people asked me subtle questions about the results and tried to read my expression to guess which way things had gone. I had to try to keep a poker face!”

By the end of the day, the two had artfully managed to keep the results concealed from everyone. Sesh even went so far as to refuse to upload his slides to the meeting website until after his presentation. He explained, “I really wanted to make the result reveal in the plenary talk as theatrical and memorable an experience for everyone as possible. This was not just about keeping the results secret until then, but also about trying to present as comprehensive an overview of the cosmology results as possible, and being prepared to answer all questions on the spot. I didn’t want people to come away with questions, and only slowly discover things in the weeks afterwards, because this could dilute the experience.”

As Sesh prepared to deliver his talk, the energy in the room was palpable. Cristhian recalled, “Once we were in the unblinding session, I could see the anticipation in everyone’s faces and feel it in the room. The biggest difference compared to the Year 1 unblinding was that in Year 1 the dark energy results came as a surprise, but in Year 3 there was a lot of expectation from the collaboration.” As an attendee at the meeting, I can honestly say that I have never been so excited about a conference presentation. It was a privilege to bear witness to the unveiling of results that had the potential to reveal something new and strange about the Universe.

Building the drama of the moment, Sesh made us wait for the results while he explained the science behind the BAO measurement (and then added a couple of extra slides just for fun). Finally, we arrived at the moment we had all been waiting for: the big reveal of the constraints on the dark energy equation of state. Was the preference for evolving dark energy from the Year 1 analysis a fluke? Or is DESI was onto something big?

At last, the results were revealed: the preference for a time-evolving dark energy was confirmed! With the increase in data from Year 1 to Year 3, the constraints on the cosmological parameters governing the dark energy equation of state were tightened, strengthening the results and making history in the process. The findings provide further indication that a cosmological constant is not the origin of cosmic acceleration, but rather that dark energy is a kind of dynamically evolving fluid that pervades all of space. With this result, a whole new era of cosmology begins.

Reflecting on the time since the unblinding, Sesh said, “The past three months getting the results ready for publication have been the hardest I’ve worked — a stretch of about 120% effort! I was anticipating this of course. With a subject is as sensitive and important as this, we’ve really had to make sure every argument is absolutely watertight and backed up by multiple calculations and tests. It’s a very important and impactful result, but one that is naturally going to face a lot of skepticism. Working to present the arguments in the most logical and convincing form, and crafting a paper that people will hopefully also enjoy reading — that’s a challenge I have actually enjoyed thinking about. I hope we have achieved it!”

— Edited by Joan Najita

To learn more:

Read about how blinding works and what it’s like to work this way in an interview with Sam Brieden, Uendert Andrade, and Juan Mena-Fernández.

Read about the Year 3 BAO results in the LBNL press release.

On March 19, DESI released a set of papers providing the measurements and interpretation of Baryon Acoustic Oscillations (BAO) based on Data Release 2 (DR2) – the first three years of survey data. This page contains a guide to the publications and main results. The papers are available here and on arXiv.

In addition to these new results, Data Release 1 (DR1) is now publicly available, with its corresponding paper summarized at the end of this guide. For summaries of the papers based on DR1 results, see our 2024 paper guides from April 4 and November 19.

Helpful links

• A press release containing a high-level overview of our main results: https://newscenter.lbl.gov/2025/03/19/new-desi-results-strengthen-hints-that-dark-energy-may-evolve/
• A press release covering Data Release 1: https://newscenter.lbl.gov/2025/03/19/desi-opens-access-to-the-largest-3d-map-of-the-universe-yet/
• DESI’s YouTube channel, including a playlist of videos covering the DR2 BAO results: https://www.youtube.com/@DESISurvey
• A short video on our results for the general public: https://youtu.be/Td0jakzT-Lk
• A list of key publications from DESI: https://data.desi.lbl.gov/doc/papers/
• For more background on DESI’s science, see our public webpages.
• DESI’s public data releases, now including DR1: https://data.desi.lbl.gov/doc/releases/

The DR2 results fall into seven main analysis categories as shown in the first figure below. The two on the left-hand side highlighted in green focus on galaxy and quasar clustering, as well as clustering of the Lyman-alpha forest, and include the BAO results released on March 19. Also highlighted in green is cosmological inference, which is based on measurements from both the Lyman-alpha forest and galaxy and quasar clustering. The bottom figure lists the papers released on March 19, featuring two key papers along with five supporting papers.

March 19 Paper Summaries

BAO Measurements from the Lyman-alpha Forest

Baryon Acoustic Oscillations (BAO) are a powerful standard ruler in cosmology, originating from ripples in the density of matter from the early universe. This ruler is used to constrain the universe’s expansion history by measuring the clustering of various tracers of the matter-density field. The most distant tracer used by DESI is the Lyman-alpha (Lyα) forest, a collection of absorption lines seen in the spectra of high-redshift quasars that map the distribution of neutral matter in the intergalactic medium. DESI’s DR2 results provide the most precise measurement of the BAO scale above redshift two.

DESI DR2 Results I: Baryon Acoustic Oscillations from the Lyman Alpha Forest

arXiv: 2503.14739

Summary: We have obtained the most precise measurement of the baryon acoustic oscillation scale above redshift two ever recorded, with a statistical precision of only 0.65 percent. The extremely high quality of this measurement when the universe was only about 3 billion years old is an important part of DESI’s new results on the history of cosmic expansion.

Validation of the DESI DR2 Lyα BAO analysis using synthetic datasets

Corresponding author: Laura Casas

arXiv: 2503.14741

Summary: The second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI), containing data from the first three years of observations, doubles the number of Lyman-α (Lyα) forest spectra in DR1 and it provides the largest dataset of its kind. To ensure a robust validation of the BAO analysis using Lyα forests, we have made significant updates to both the mocks and the analysis framework used in the validation, which we present in this paper. The figure presents the BAO measurement results from two sets of mocks: a stack of 100 Saclay mocks (previously used in DR1) and 300 CoLoRe-QL mocks (a new set of Gaussian Lyα mocks that incorporate a quasi-linear input power spectrum to model the non-linear broadening of the BAO peak). Our goal was to recover the true BAO parameters within one-third of the statistical uncertainty from DESI DR2 (represented by the black dotted line in the figure). The results indicate we are very close to meeting this criterion, and we discuss the small bias observed in Section 5 of the paper.

Construction of the Damped Lyα Absorber Catalog for DESI DR2 Lyα BAO

Corresponding author: Allyson Brodzeller

arXiv: 2503.14740

Summary: Damped Lyman-alpha absorption (DLA) systems are neutral hydrogen reservoirs with column densities N(HI)>2×10²⁰ cm^-2 observable in the Lyman-alpha forest of some quasars. The absorption profile of DLAs consist of broad damping wings that extend for thousands of km/s, compromising a significant fraction of the Lyman-alpha forest when present. It is therefore crucial to identify, catalog, and mask DLAs to mitigate their impact on Lyman-alpha forest clustering. This paper presents the DLA catalog strategy for the DR2 Lyα BAO measurement. The catalog is constructed using three DLA classification algorithms: the Gaussian process model from Ho et al. (2021), the CNN from Wang et al. (2022), and a spectral template DLA classifier which this work introduces. Our final DLA catalog for Lyman-alpha BAO is estimated to be 76% pure and 71% complete.

BAO Measurements from Galaxies and Quasars & Cosmological Constraints

The rest of DESI’s BAO results come from using galaxies and quasars as tracers of large-scale structure. Cosmological constraints are then derived from the combination of these measurements with those from the Lyman-alpha forest (described above). This dataset – more than twice the size of DR1 – is the largest ever used to measure BAO. These new results provide the best measurements of the BAO scale to date, enabling precise constraints on dark energy.

DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints

arXiv: 2503.14738

Summary: This paper presents the measurement of baryon acoustic oscillations in six different galaxy and quasar samples from the first three years of DESI observations, and explores the implications of these results for cosmology when combined with the CMB, supernovae and weak lensing. The evidence for a time-evolving dark energy equation of state has increased since the Year 1 results, which is very exciting! We’ve also performed a lot of extra tests this time which make us confident that the result isn’t driven by some unknown effect in any of the data.

Validation of the DESI DR2 Measurements of Baryon Acoustic Oscillations from Galaxies and Quasars

Corresponding author: Uendert Andrade

arXiv: 2503.14742

Summary: The DESI DR2 BAO analysis significantly improves constraints on cosmic expansion by leveraging a larger dataset of galaxies and quasars compared to DR1. Our results confirm the robustness of BAO as a standard ruler and achieve a factor of ~2 improvement in precision, reducing statistical uncertainties to ~0.24%. A key plot (below) showcases the stability of BAO constraints across different data vectors and modeling choices, ensuring the reliability of our findings for cosmological inference.

Extended Dark Energy analysis using DESI DR2 BAO measurements

Corresponding author: Kushal Lodha

arXiv: 2503.14743

Summary: In this paper, we perform an extensive analysis of dark energy using the latest DESI data, combined with CMB and SN Ia observations. Using a variety of parametric and non-parametric methods, our results indicate that extending the standard ΛCDM model with a two-parameter w(z) sufficiently captures trends in the current data. The evidence for dynamic dark energy, especially at low redshift (z<0.3), is robust across various methods.

The comparison of Gaussian Processes reconstruction of the dark energy equation of state w(z) with the w₀w_a parameterization using DESI, CMB, and Union3 data is shown in the figure below. See Figure 10 in the paper for more details.

Constraints on Neutrino Physics from DESI DR2 BAO and DR1 Full Shape

Corresponding author: Willem Elbers

arXiv: 2503.14744

Summary: We have pushed measurements of the Universe’s most elusive particles—neutrinos—to new limits by analyzing the positions of millions of galaxies. The results indicate that the combined mass of all three neutrino types is less than 0.0642 electron volts—a value that creates tension with the lower limit of 0.059 eV established by laboratory experiments. Statistical methods surprisingly indicated physically impossible negative masses, pushing the tension to a significance of 3σ. When allowing for evolving dark energy, the tension disappeared with a revised upper limit of 0.163 eV—potentially signaling new physics beyond the standard cosmological model.

Data Release 1 (DR1)

In addition to the papers described above, DESI’s first year data release (DR1) is now publicly available. The corresponding paper below provides an in-depth overview of this release.

Data Release 1 of the Dark Energy Spectroscopic Instrument

arXiv: 2503.14745

Summary: This paper describes DESI public Data Release 1, covering the first year of DESI main survey observations and a consistent reprocessing of Survey Validation data previously released in the Early Data Release. DR1 includes high-quality redshifts for 18.7M objects, of which 13.1M are spectroscopically classified as galaxies, 1.6M are quasars, and 4M are stars.

Joan Najita (NOIRLab)

10 March 2025

“Perhaps only in a world of the blind will things be what they truly are.” — José Saramago, Blindness

We’re all human. Science is a human endeavor, carried out by humans with all their imagination, creativity…and biases. Sometimes we explore the Universe without expectations, but often we have an idea of what we may find (a hypothesis, a hunch). These ideas motivate and guide us in our exploration. But they can also introduce confirmation bias, “the tendency to process information by looking for, or interpreting information that is consistent with…existing beliefs.” (Britannica). In its quest to make precision cosmological measurements, DESI uses a process called “blinding” in its analysis methodology in order to avoid the inaccuracies that can arise from confirmation bias.

We sat down with DESI scientists Sam Brieden (U. Edinburgh), Uendert Andrade (U. Michigan), and Juan Mena-Fernández (LPSC, Grenoble) to learn about how blinding works, the rationale behind it, and what it’s like to work this way.  

Q: What is the general purpose of blinding in research?

Sam: Research is never carried out in a vacuum, it is always bound to existing knowledge and expectations. Every researcher or group of researchers is subject to external influences that may affect the way an analysis is performed. For example, a new analysis result that is in agreement with current consensus and does not contradict any prior results, is more likely to be accepted by the community. On the other hand, if a new result is unexpected, it is more likely to raise eyebrows. Due to this “pressure”, researchers unconsciously tend to examine unexpected results more closely and, say, include more cross-checks and caveats, or a less optimistic error estimation. In other words, there is a trend in research that expected results tend to be treated with less scrutiny than required, while unexpected results may be tweaked until they agree better with expectations. In “The Neglect of Experiment,” Allan Franklin describes the process this way:

“Although each experiment was honestly made, they were, except for the first, conducted in light of previous results. In any experiment, the sources of error, particularly systematic error, may be hidden and subtle. This is particularly true of…technically difficult experiments. The question of when to stop the search for sources of error is then very important. One psychologically plausible end point is when the result ‘seems’ right.”

Uendert: Working with blinded data can feel both liberating and challenging. On the one hand, it frees researchers from preconceived notions about the results, encouraging a more objective and unbiased analysis. On the other hand, it requires a strong trust in the analysis pipeline and the blinding process itself, for which we have to creatively design tests and then carry them out. The anticipation of discovering the true cosmological parameters after unblinding adds an exciting layer of mystery to the research.

Q: What is it like to experience the “unblinding”? Is it exciting? What emotions do you have?

Juan: I have experienced several unblindings, and these are always extremely exciting. The unblinding is typically done in a video call meeting in which everyone in the collaboration can connect, and so there are a lot of people watching. This usually makes me feel nervous, especially if I’m deeply involved in the analysis (partly because the analysis could have taken months or even a year to carry out). There are lots of things to prepare for an unblinding, such as slides to present the project and scripts that will create the unblinded plots and output the results. The most exciting part is that moment in which you press the button to run your script and obtain the final unblinded measurement!

Uendert: For me, as you might imagine, the moment of unblinding is filled with a mix of excitement, anticipation, and a bit of nervousness. It’s the culmination of years of hard work, and there’s a tangible sense of revealing the Universe’s secrets. The experience is akin to opening a highly anticipated gift; you’re eager to see what’s inside but also hoping it meets your expectations. Regardless of the outcome, it’s a pivotal moment that deepens our understanding of the cosmos.

Sam: I was really excited in anticipation of the internal full-shape unblinding event on 12 June 2024 — which revealed the results that were eventually published on 19 November 2024 — since I developed both the Blinding and the full-shape (i.e., ShapeFit) methodologies during my PhD. Coincidentally, my wife and I were expecting a child, and we went to hospital on the same day, so I missed the DESI unblinding in favor of another (and for me personally even more exciting) unblinding event!

Q: What aspect of the DESI results are you looking forward to next?

Juan: I’m always looking forward to seeing the constraints in cosmological parameters inferred from the data, in DESI or in any other experiment.

Uendert: I’m particularly looking forward to seeing how the DESI results will refine our understanding of the dark energy equation of state and its potential evolution over cosmic time. The scale and precision of DESI’s dataset offer an unprecedented opportunity to probe the dynamics of the Universe’s expansion and the nature of dark energy. This could lead to groundbreaking insights into one of the most profound mysteries in cosmology.

Cosmological measurements often make use of fun and interesting ideas that depart from our everyday experience. Here cosmologist Jamie McCullough explains and helps us visualize several of the concepts involved in plans to measure the distribution of dark matter in the universe and probe the growth of cosmic structure over time.

by Jamie McCullough

26 February 2025

When DESI measures the spectrum of light from a galaxy – i.e., the intensity of light as a function of wavelength or energy – we learn a lot about the physics of what is happening both inside the galaxy and in the space between the galaxy and us.

A particularly important property we can measure is a galaxy’s redshift. As galaxies recede from us in an expanding universe, their light is pulled to less energetic wavelengths, or redshifted, by the same doppler effect that makes train whistles pitch low as they travel away from us. In an expanding universe, more distant objects are redshifted more than nearby objects. As a result, if we know how the universe is expanding, we can learn the distance to the galaxy from a measurement of its redshift. With the ability to measure the spectra of as many as 5000 galaxies at a time, DESI is uniquely suited to measuring the distances to galaxies across the night sky and thereby mapping the visible universe. You can see redshift in action for a galaxy in the figure below. Here the vertical dashed line marks a bright emission line of oxygen in star-forming galaxies. The number in the upper left corner is the redshift z = v/c where v is the recession velocity and c is the speed of light.

However, the visible universe accounts for only a small fraction of the matter we know exists. The vast majority doesn’t interact with light in any known way – it’s dark matter. The spatial distribution of this dark matter is of great interest, because it drives the movement of galaxies in the universe. Mapping the dark matter — which is thought to be arranged in long filaments, i.e., in a dark cosmic web — requires more information than DESI alone can provide. If we combine the galaxy distance measurements from DESI with the measurement of galaxy shapes from imaging surveys (like, for example, the Dark Energy Survey (DES), the Kilo-Degree Survey (KiDS), and the Hyper Suprime-Cam (HSC) survey), we can map the dark structure using a method called weak gravitational lensing. This method relies on an effect from general relativity that massive objects warp the geometry of space and time. As a result, an image of a distant galaxy will be distorted as light from the galaxy passes massive objects on its way to us.

To understand how weak lensing works, we can first look at the case of a more straightforward example. In what’s called “strong lensing,” we can readily see this warping in action, as we find images of background galaxies stretched and distorted tangentially around a very massive foreground object like a galaxy cluster. This stretching is very similar to the optical effect you might see looking through the thick base of a wine glass. With a thicker piece of glass we see more refraction, just as we see more distortion behind more massive galaxy clusters. You can see this effect in the simulation below and in this other simulation, in which a moving massive galaxy passes first in front of a grid of shapes, and then in front of the Hubble Deep Field. We see that these strong lenses can stretch a background galaxy’s light into continuous rings called “Einstein rings”. They can even produce more than one image of the same background galaxy! What happens all depends on how the background and foreground objects line up.

However, the typical (weak lensing) distortion that a galaxy’s light experiences on its way to us is not as dramatic as in the above examples. Warping from large-scale structure changes galaxy shapes much less, on the order of a mere percent. As we can see in the prior simulations, the amount of distortion depends very strongly on the distance to the background object and to the lensing structure, so understanding those distances is crucial. We expect that if we measure enough of these galaxy shapes and their correlations with one another, we can trace those dark cosmic filaments dominated by dark matter. You can see a toy model of weak lensing below, where a hypothetical background grid of perfect circles becomes displaced, magnified, and sheared about the hidden structure shaded in blue.

If we measure the distances and observed shapes of these galaxies and correlate them with one another, we can devise a relationship for how alike any two shapes are as a function of their separation (𝜉±). With these weak gravitational lensing measurements and different cosmological models, we can find the one that best explains our observations and produces constraints on how clumpy our universe is (as measured by a quantity called S8) and how much matter there is in the universe (as measured by a quantity called 𝛺m).

With the incredibly precise distances we get from DESI and the increasingly precise measurements we are now getting for galaxy shapes, we can perform these lensing measurements better than ever before – making it a very promising way to probe the growth of cosmic structure over time and learn about all the contents of our universe — both the luminous and the dark!

— Edited by Joan Najita

20 February 2025

BaoBan, DESI’s ambassador for Education and Public Outreach, recently dropped in on this year’s Tohono O’odham Nation Rodeo Parade, greeting parade spectators from high atop the Kitt Peak National Observatory (KPNO) parade float. The DESI project is being carried out at KPNO, which is located atop I’oligam Du’ag, in the homeland of the Tohono O’odham Nation. A coyote from the wilds of Arizona, BaoBan also took a star turn at last year’s Rodeo Parade.

At this year’s parade, held on 1 February 2025 in Sells, Arizona, BaoBan appeared on the KPNO parade float alongside images of Tohono O’odham employees and those who have supported the Observatory over the past 60 years. Decorated with colorful images of stars and planets, the float joined in on the parade theme “Celebrating Our O’odham Superheroes” with BaoBan sporting his own superhero cape. NOIRLab volunteers also walked alongside the float clad in vibrant capes and masks.

Now in its 86th year, the Tohono O’odham Nation Rodeo and Fair is an important tradition that celebrates Tohono O’odham culture and history. In addition to the all-Indian rodeo, the event featured live music, fairground rides, exhibitions and food booths. At the NOIRLab information booth, volunteers, including DESI astronomers, invited fair attendees to view the Sun through telescopes and to visit Kitt Peak to learn more about projects such as DESI. A special thanks to BaoBan and all volunteers for their help making this event a “super” success!

Joan Najita (NOIRLab)

8 February 2025

One of the remarkable things we’ve learned about galaxies over the past few decades is that they often come with a special surprise in the middle — a huge black hole. Weighing in at more than a million times the mass of the Sun, these “supermassive black holes” are found at the centers of almost all galaxies similar to or more massive than the Milky Way. Why do galaxies have these black holes? And how did they get there? Do much lower mass galaxies also have central black holes? These questions have motivated the search for black holes in low-mass “dwarf galaxies,” systems that may provide vital clues regarding the origin of black holes in the Universe.

A recent paper, led by Ragadeepika Pucha (U. Utah), takes a big step in this direction. Analyzing DESI spectra of dwarf galaxies, the researchers identified candidate black holes that are actively accreting matter from their surroundings. The feeding process produces a characteristic spectrum of bright atomic emission lines that the researchers used to identify these systems as Active Galactic Nuclei (AGN). The study detected AGN signatures in over 2000 dwarf galaxies, more than tripling the number of known dwarf galaxies with candidate black holes. The detections, which extend to lower galaxy masses and higher redshifts (i.e., further back in time) than previously probed, suggest that black holes may commonly exist even in these very low-mass galaxies.

We sat down with Raga to learn more about this remarkable result.

Questions for Raga:

Q: Why was it important to increase the census of dwarf AGN? Was this difficult before DESI?

A: Dwarf galaxies are the most abundant galaxies in the Universe, and understanding their growth is crucial for piecing together the puzzle of galaxy formation and evolution. Because their lower mass (and therefore lower gravity) makes it more challenging for them to hang on to their gas, the energy output from the AGN, which can eject gas from a galaxy, can potentially have a major impact on a dwarf galaxy’s ability to continue to form stars. What happens exactly remains an open question. To explore this complex interplay between dwarf galaxy evolution and black hole evolution, we first need to establish a robust statistical sample of dwarf AGN candidates.

Historically, this has been a real challenge. Because low-mass galaxies are faint, it has been difficult to measure the spectra of a large sample and identify dwarf AGN candidates. But DESI is changing the landscape with its ability to measure spectra of many objects simultaneously. The early DESI data included spectroscopy of nearly 115,000 dwarf galaxies, from which we uncovered the largest sample of dwarf AGN candidates to date. Part of the success is the result of the smaller fiber size of DESI, which makes it easier to  focus on the light from the central region of the galaxy, where the AGN is, and ignore the starlight from the rest galaxy. As a result, we were able to identify fainter dwarf AGN candidates than in previous studies.

Q: It looks like you find that dwarf galaxies also commonly host black holes. Do these results tell us anything about the origin of supermassive black holes in general?

A: A major question about supermassive black holes is how they formed. What mass did the black hole start out with? What was its “seed” mass? Because lower mass galaxies are likely to harbor lower mass black holes, they may provide a bridge between stellar mass black holes (< 100 solar masses) — which are familiar to us from X-ray binaries and LIGO gravitational wave sources — and supermassive black holes at the centers of large galaxies. These “intermediate mass black holes”, while elusive to date, are theorized to be the “seeds” of supermassive black holes and the relics of the first black holes formed in the Universe.

With DESI, we’ve found the lowest mass black holes in galaxies to date. Since black holes can only grow over time and cannot disintegrate into smaller ones, these findings suggest that the black holes we are observing may be analogs of the primordial black holes that formed in the early Universe, i.e., the “seed black holes” of supermassive black holes. A very small number of our sources may even be primordial black holes, having persisted through the ages with little to no evolution.

Q: What do you find most interesting about the results?

A: We find that nearly 2% of dwarf galaxies host active black holes, a significant increase compared to the ~0.5% reported in earlier studies. This is an exciting result, as it suggests that we have been missing a substantial number of undiscovered black holes. It opens the possibility that even more black holes are concealed within these low-mass galaxies.

The ability to detect an active black hole depends on several factors, including the black hole’s mass, the availability of gas in its vicinity, the accretion rate, and the sensitivity of the instruments used to detect the emission from the resulting AGN. As a result, for a given galaxy mass, the black holes we observe will tend to be either the most massive or those with the highest accretion rates, depending on the specific telescope and instrument used.

Our findings show that the fraction of galaxies hosting actively accreting black holes increases with galaxy mass, reaching nearly 100% for the most massive galaxies. This suggests that when a massive galaxy has an active black hole, it is readily detected by DESI. In contrast, in lower-mass galaxies, the emission from ongoing star formation can mask or dilute the AGN signal, making it harder to detect their faint AGN. This does not imply that low-mass, star-forming galaxies do not host black holes, but rather that we are currently identifying all the actively accreting black holes that are detectable with our instruments.

Q: What drew your interest to this topic?

A: When I began my PhD, my primary goal was to delve into the field of galaxy formation and evolution. I was particularly drawn to dwarf galaxies, as they are the most common kind of galaxy in the universe, yet they remain poorly understood. What are their histories? Do they follow the same evolutionary path as more massive galaxies? Or does the energy released by their active black holes (if they have one) play a significant role in shaping their growth?

By sheer coincidence, my advisors, Stephanie Juneau and Arjun Dey, encouraged me to join the DESI collaboration, which turned out to be the perfect opportunity to dive deeper into this research. They were incredibly supportive of the idea to use DESI data to search for dwarf AGN candidates as a first step in understanding the evolution of dwarf galaxies in the universe.

An unexpected bonus was realizing that this project also ties into one of the most fundamental and exciting questions in present day astronomy: the formation of supermassive black hole seeds. The chance to simultaneously explore the evolution of dwarf galaxies and the origins of supermassive black holes has been deeply motivating. The interconnection between these two lines of inquiry, and the potential to advance our understanding on both fronts, has been a truly rewarding aspect of my research journey.

Q: What’s next for you?

A: With the largest sample of dwarf AGN and IMBH candidates now at our disposal, we are poised to tackle some of the most pressing questions in the study of supermassive black hole seed formation and the co-evolution of dwarf galaxies and their central black holes. My upcoming projects will focus on examining the relative effects of AGN versus star-formation feedback in dwarf galaxies, exploring the energetics related to these feedback mechanisms, and characterizing the population of dwarf AGN candidates identified through multi-wavelength and multi-diagnostic approaches.

My DESI collaborators and I also plan to investigate the demographics of these black holes, employing modeling techniques to study whether all galaxies have black holes (or what fraction do), as well as the black hole mass function in the universe. We will expand these analyses to include DESI Year 3 data, which will further enhance the scale and scope of our research.

Joan Najita (NOIRLab)

One of the current tantalizing mysteries of cosmology is the “Sigma-8 tension,” or the persistent disagreement between the predicted and observed amounts of “clumpiness” of matter in the Universe. Briefly, the small density fluctuations in the early Universe (as recorded in the cosmic microwave background or CMB) can be used to predict the expected matter density fluctuations at later times, up to the present day. While the observed clumpiness of the matter density distribution at later times agrees well with expectations from our current cosmological model, observations consistently find less clumpiness than predicted by the CMB. The difference may indicate exciting new physics, or more prosaically, systematic effects.

A new paper led by Tanveer Karim explores the Sigma-8 tension using data from the DESI Legacy Imaging Survey. Similar to previous studies, the new paper also finds significantly less clumpiness in the galaxy distribution than predicted by the CMB (i.e., the red and blue shapes are below the green shape in the lower left panel of the figure). However (and interestingly), the results also depend on how the effect of intervening dust in our own galaxy is taken into account. In other words, our view of the Universe from within a galaxy means that dust in the Milky Way can block light from fainter distant galaxies, altering the apparent clumpiness of the galaxy distribution. Much like explorers of old, astronomers need to “brush away” this surface dust to reveal the cosmological relics of interest beneath. Here the authors make this correction using a new map of Milky Way dust, derived from DESI data itself. The correction reduces the tension (blue shape), but the clumpiness of the galaxy distribution still differs from the CMB prediction (by 3-sigma).  We sat down with Tanveer to learn more about the results.

Questions for Tanveer:

Q: How do you interpret these results? Is your measurement of Sigma-8 significantly different from the CMB value? And should we be concerned? (Or maybe excited about the prospect of new physics?)

A: As a bit of background, our results add to the “sigma-8 tension” story in two ways. Firstly, we find that the tension is already present quite a long time ago, at a redshift of z ~ 1.1, when the Universe was not yet affected by dark energy (in LCDM cosmology). Secondly, our study examines the clustering of lower-mass blue galaxies rather than massive red ones. That is, we study emission-line galaxies (ELGs), which are similar to the mass of the Milky Way. Previous studies have used luminous red galaxies (LRG) (Sailer 2407.04607, Kim 2407.04606, White 2111.09898, Kitanidis 2010.04698) or unWISE galaxies (Farren 2309.05659, Krolewski 2105.0342) that are typically 100-1000 times more massive than ELGs.

So, what do our results mean? The more exciting interpretation is that we are perhaps seeing hints of something new (not the traditional constant dark energy) happening at z ~ 1.1. But if we consider the LRG and unWISE galaxies as well, then our result is the outlier. What could explain this? The key could be that the previous studies studied massive red galaxies while we are probing the clustering of lower-mass blue galaxies, like our Milky Way. Why does this matter? While the CMB traces the clustering of dark matter, galaxy clustering studies are observing normal matter. To compare these, we need to understand how galaxies form in dark matter clumps and how well different galaxy populations trace the dark matter. So naturally, galaxy formation and other processes can come into play. Our understanding of Milky-Way-mass galaxies is limited at such high redshift, so perhaps our results not only point to the impact of systematics but also signatures of unknown ELG galaxy physics!

Q: How do your results relate to other studies of clumpiness? The Sigma-8 tension is also reported by studies that use completely different measures of the clumpiness of the Universe (e.g., weak lensing, galaxy cluster counting). Does the Milky Way dust distribution also affect these studies? Or are these other studies affected by different systematic effects?

A: That’s an interesting question that has not been explored at length yet, although there may be a possible connection. Papers such as (https://arxiv.org/pdf/1808.03294) and (https://arxiv.org/abs/2306.03926) have shown that certain extinction maps actually retain imprints of the large-scale structure of galaxies, because they use far-infrared light to map dust. While most of the far-infrared light is produced by dust in the Milky Way, distant star-forming galaxies also contribute. If their emission is incorrectly attributed to Milky Way dust, the process of correcting for Milky Way dust will incorrectly imprint a signature of distant star-forming galaxies on images of the sky, which may affect these other measures to some extent.

In any case, and as far as I am aware, our paper is the first to show exactly how much these effects change the effect of Milky Way dust changes our cosmological interpretations. As for the other galaxy clustering measurements, I think one could argue that since the earlier studies were using more massive galaxies, they were less prone to extinction systematics. But a reanalysis of such works will be important to definitively rule out the role of Milk Way dust. After all, Milky Way dust has impacted cosmological results in the past, such as the false detection of the primordial B mode in the CMB by the BICEP telescope!

Q: Does dynamic dark energy play a role here? As you say in your paper, the negative pressure of dark energy inhibits the growth of large-scale structures over time, countering the effect of gravity. Earlier in 2024 DESI reported that dark energy may be dynamic and weakening. Does this effect matter in your study? If it does, do you take this development into account?

A: I am puzzled by the recent DESI Key Paper results—in a very positive way—as I am sure many of the DESI collaborators are! While I do not have a clear answer on how dynamic dark energy relates to a detection of Sigma-8 disagreement at z ~ 1.1, it is a line of questioning we should consider seriously in interpreting the upcoming Y3 dataset (next major dataset for DESI). The Y3 dataset will be much cleaner than the Y1 data in our current paper, and I expect it will show a more robust detection of the ELG-CMB lensing cross-correlation. We should consider the impact of dynamic dark energy in this context, where the simplest extension of our current study would be to measure not only Sigma-8 and Omega_matter but also the parameters describing the dark energy equations of state (w0, and wa).

Speaking of these earlier reports, it’s interesting that an echo of our current results is also found there. While the November 2024 Full-Shape and Redshift Space Distortion (RSD) results measure Sigma-8 with many tracers and find it to be consistent with the CMB, if you look at Figure 1 of the full-shape paper (https://arxiv.org/pdf/2411.12022), you will see that the ELGs on their own are also consistent with a lower Sigma-8! The two studies are done in different ways — the full shape result derives from the 3D clustering of ELGs, while our analysis is carried out in 2D using ELGs selected in a different way — it is interesting that our two independent methods yield similarly low Sigma-8s. This naturally links back to the first question and leads me to wonder, if not a signature of dark energy, could we be on the verge of understanding how these early star-forming galaxies were interacting with their dark matter haloes?

Q: How did you decide to work on this project? Were you surprised by the results?

A: After I finished my initial work on the ELG target selection as a first and second-year graduate student, I was interested in exploring how to use the DESI ELG sample to study cosmology. Extensive discussions with my thesis advisor, Daniel Eisenstein, and collaborators of this project, Sukhdeep Singh and Mehdi Rezaie, helped me understand that the high-redshift star-forming galaxies could be the key to unlocking the early large-scale structures. I never thought that I would have to learn so much about “local” structures, such as the Milky Way dust mapmaking and the Sagittarius Stream, to learn about the very distant cosmos, so it was both shocking and exciting to see how the science of the distant Universe and that of our local neighborhood is becoming more and more intertwined.

Q: What’s next for you?

A: As an Arts & Sciences Postdoctoral Fellow at the University of Toronto, I am currently finishing up a similar analysis using the same ELGs, but this time cross-correlating with the cosmic infrared background, to quantify the star-formation rate of these galaxies and their galaxy-dark matter halo connection. I am excited to see what more we can learn about the ELGs and whether a better understanding of their physics can help us better interpret the CMB lensing cross-correlation results. My ELG work also made me fall in love with these early star-forming galaxies, and so I am currently co-leading the Lyman-Break Galaxies (LBGs) Topical Team in the Dark Energy Science Collaboration (DESC). The hope is that with the upcoming Rubin Observatory, we will explore all the up to z ~ 5.5 using LBGs. The coming years of wide-field high-redshift surveys will be the era of ELGs and LBGs, and I am thrilled to see what these galaxies will teach us about the infancy of our Universe.

Joan Najita (NOIRLab)

DESI reaches 50M milestone

On 18 December 2024, DESI reached a new milestone, having measured the spectra of 50 million astronomical sources (36.3 million galaxies and quasars, and 13.7 million stars) over 819 nights of observations. The milestone is remarkable for both its speed and scope. As described by DESI team member Arjun Dey (NOIRLab), “When we originally proposed the DESI project, we forecast that we would measure spectra of about 38 million sources (30 million galaxies and quasars and 8 million stars) over the 5-year survey. We have now already exceeded that mark in just 68% of our official survey time.”  While the 50M milestone has been reached quicker than expected, many of the spectra were obtained in “bright time”, when the moon is up. The main “dark time” portion of the DESI survey is still underway. Currently ahead of schedule, it is expected to complete toward the end of 2025.

On November 19, DESI released another set of papers based on year one (Y1) data. The first set of papers, summarized here, was released on April 4 and focused on a particular feature seen in the clustering of tracers called the Baryon Acoustic Oscillation (BAO) standard ruler. These new results provide an extended, “full-shape” analysis of the Y1 data by looking at how galaxies and quasars cluster on different scales, both in the plane of the sky and along the line of sight in redshift space. This page contains a guide to the publications and main results. The papers are available here and on arXiv.

Helpful links

• A press release containing a high-level overview of our main results: https://newscenter.lbl.gov/2024/11/19/new-desi-results-weigh-in-on-gravity/
• A short video on our results: https://www.youtube.com/watch?v=cVkUumMP6CI
• A list of current papers: https://data.desi.lbl.gov/doc/papers/ 
• For more background on DESI’s science, see our public webpages.
• DESI’s Y1 data is not yet public, but you can find our early data release and any updates on this site: https://data.desi.lbl.gov/doc/releases/

The Y1 results fall into seven main categories. The three highlighted in blue, based on BAO measurements of galaxies and quasars (DESI 2024 III), BAO with the Lyman-alpha forest (DESI 2024 IV), and cosmological inference from these BAO measurements (DESI 2024 VI), were released on April 4. The three highlighted in pink are released on November 19; the construction of galaxy and quasar catalogs for cosmological analyses (DESI 2024 II), full-shape clustering measurements from galaxies and quasars (DESI 2024 V), and cosmological constraints from this full-shape modeling (DESI 2024 VII). This figure shows the publication organization, where each category corresponds to a key collaboration paper with several supporting papers. Summaries for the key papers highlighted in pink, as well as supporting papers, are listed below.

November 19 Paper Summaries

Construction of Galaxy and Quasar Catalogs

Creating galaxy and quasar catalogs is essential for DESI’s cosmological analyses. These papers describe how these catalogs were constructed from the forthcoming DESI Data Release 1 (DR1), and include studies of the effects of systematics and incompleteness on the clustering measurements.

DESI 2024 II: Sample Definitions, Characteristics, and Two-Point Clustering Statistics

arXiv: 2411.12020

Summary: This paper presents the details of the DESI ‘large-scale structure (LSS) catalogs’ constructed using data from DESI DR1. It further presents the details of how 2-point clustering measurements and their covariance are calculated using the LSS catalogs.

Characterization of DESI fiber assignment incompleteness effect on 2-point clustering and mitigation methods for DR1 analysis

Corresponding author: Davide Bianchi

arXiv: 2411.12025

Summary: The fiber assignment incompleteness, arising from the limited mobility of the robotic fiber positioner in the DESI focal plane, leads to a scale-dependent suppression of the observed galaxy clustering amplitude. If left uncorrected, this effect can significantly affect the inference of cosmological parameters. In this work, we summarize the methods used to simulate fiber assignment on both mock galaxy catalogs and real data, and we discuss the mitigation strategies we implemented to address this issue. We conclude that we can robustly correct for the fiber incompleteness in DESI DR1, as demonstrated in Figures 11 and 12.

Mitigating Imaging Systematics for DESI 2024 Emission Line Galaxies and Beyond

Corresponding author: Alberto Rosado-Marín

arXiv: 2411.12024

Summary: This paper details the angular systematic treatment used for the DESI DR1 emission-line galaxies (ELGs). Separately, we introduce a new methodology for systematic treatment by combining forward-modeling and regression. Furthermore, we present the impact of imaging systematics on the 2-point clustering measurements of BGS, ELGs, LRGs, and QSOs. We also assess the impact of imaging systematics on the BAO measurement.

Full-Shape Analysis from Galaxies and Quasars

The DESI key papers released in April focused on extracting information from the BAO scale. This new set of papers extends this analysis to extract cosmological information from the full shape of the 2-point clustering statistics, allowing for tighter constraints on cosmological parameters. This is the largest dataset ever used to perform a full-shape analysis, with over 4.7 million galaxy and quasar redshifts spanning 0.1 < z < 2.1. The combined precision on the amplitude of the redshift space distortion (RSD) signal, which probes the growth of structure, is 4.7%. Remarkably, this level of precision from just one year of DESI data is comparable to that of 20 years of data from the Sloan Digital Sky Survey (SDSS).

DESI 2024 V: Full-Shape Galaxy Clustering from Galaxies and Quasars

arXiv: 2411.12021

Summary: For the first time, we have performed a “full-shape” analysis of the galaxy 2-point statistics that extracts information beyond the cosmic ruler, the Baryonic Acoustic Oscillations (BAO), and probes the formation of large-scale structures under gravity. We use a sample of galaxies and quasars collected during the first year of DESI. Our galaxy full-shape analysis is in agreement with BAO for the background evolution and confirms the validity of general relativity as our theory of gravity at cosmological scales.

Exploring HOD-dependent systematics for the DESI 2024 Full-Shape galaxy clustering analysis

Corresponding author: Nathan Findlay

arXiv: 2411.12023

Summary: We explore how changing the model of how galaxies form affects the cosmology we infer. We find this to have a very minor effect (Figure 6) and include it as an extra uncertainty in our analysis following a new, more generally applicable approach.

Cosmological Inference

This set of papers provides a cosmological interpretation of the full-shape (FS) analysis described above, combined with the previous BAO analysis. These FS+BAO results constrain the density of matter, Ω_m, the amplitude of mass fluctuations, σ₈, and the Hubble constant, H₀, for a flat ΛCDM cosmological model. Additionally, combinations of DESI (FS+BAO) with CMB and Type 1a supernovae continue to favor an evolving dark energy component. The DESI (FS+BAO) data is also used to test for deviations from general relativity, and results show agreement with it.

DESI 2024 VII: Cosmological Constraints from the Full-Shape Modeling of Clustering Measurements

arXiv: 2411.12022

Summary: Analysis of the first year of DESI data, in combination with other probes, shows preference for a cosmological model where the dark energy density evolves in time. This corresponds to the preference of the parameter w₀ in the accompanying plot being different from -1, and w_a different from 0. This result persists when we go beyond our earlier analysis of baryon acoustic oscillations signature in the clustering of galaxies, quasars, and features in quasar spectra, and extend it to utilize the full clustering signal of these cosmological tracers. When combined with Planck18 data, our data also tightly constrains the sum of the neutrino masses, which has to be below 0.071 eV.

Modified Gravity Constraints from the Full Shape Modeling of Clustering Measurements from DESI 2024

Corresponding author: Mustapha Ishak

arXiv: 2411.12026

Summary: We have analyzed the data from the first year of DESI allowing for different deviations from general relativity. This analysis combines DESI with CMB data and DES Y3 and is performed at different cosmic times and scales using various methods. We conclude that current data is consistent with general relativity (e.g. μ₀ and Σ₀ are consistent with 0 in the plot below) and that it still favors an evolving dark energy (w_a not 0 and w₀ not -1 in the plot below).

BAO Measurements from Galaxies and Quasars

In addition to the papers above, an additional supporting paper for the key paper DESI 2024 III: Baryon Acoustic Oscillations from Galaxies and Quasars, released in April, is now available.

Analytical and EZmock covariance validation for the DESI 2024 results

Corresponding author: Daniel Felipe Forero Sánchez

arXiv: 2411.12027

Summary: In cosmology, estimating uncertainties in large-scale structure analyses is crucial. Two methods are used: analytical covariance, which is fast and flexible but limited in handling complex systematics, and sample covariance, which is more accurate but resource-intensive. For the completed DESI 2024 analyses, analytical covariance is chosen for Baryon Acoustic Oscillations (BAO), while corrected sample covariance is used for Full-Shape analysis due to better accuracy in Fourier space.

Q: You mentioned that stuff on small scales usually forgets about the cosmic web. Why doesn’t that happen here?

A: While galaxy orientations tend to “forget” their history in a very general sense, here we show that some memory is in fact preserved! This is likely a result of how the galaxies formed. My mental picture is matter (gas, dust, galaxies) being channeled along cosmic filaments and their angular momentum in that direction is preserved in the motions (and therefore positions) of the galaxies relative to each other and to the densest nearby regions, the large clusters / nodes where filaments meet.

Q: Can we make a map of dark matter using your technique?

A: Yes! Or, more specifically, a map of the tidal forces created by dark matter. This is similar to how weak lensing makes a “map” of dark matter between us and distant galaxies.

Q: What drew your attention to this topic? And why multiplets in particular?

A: Initially I studied the orientations of galaxies as a source of error in cosmological surveys. (For example, we expect the intrinsic alignments of galaxies to bias measurements of redshift-space distortions for DESI.) But eventually I became interested in their underlying cause. It’s  fascinating that things on relatively “small” scales (galaxies) can be connected to the largest structures in the universe!

Q: How does DESI help with this problem?

A: Because DESI measures the distances to galaxies, it helps us better determine which galaxies are close to each other along the line of sight. That helps us better identify multiplets and gives us better 3D information about their positions. These improvements help us find subtle correlations between galaxies and the cosmic web.

Q: What was your reaction to the results? Were you surprised?

A: Although I expected we would find some correlation, I was surprised at how clear the signal was! And I was also pleasantly surprised to find that we could detect a correlation in the most “difficult” sample: the faint, blue, distant galaxies. This is exciting because we do not see any evidence of alignment of these galaxies as individuals.

Q: What’s next on your horizon?

A: Getting my PhD! I’m excited to graduate and start postdoctoral research, where I hope to collaborate with theorists and experts in galaxy dynamics in order to better understand the modeling and applications of multiplet alignment.

More information about Claire’s recent work is available in this doodle summary of the actual paper and in this Halloween lunch talk (Claire’s presentation begins at 35:15).

by Andrea Muñoz Gutiérrez

On 20 June 2024, the DESI planetarium show “5000 Eyes” premiered in Mexico City at the Luis Enrique Erro Planetarium. Previously shown in traveling planetariums across the country, it is now, for the first time, part of the regular programming at the oldest planetarium in the nation. Authorities from several DESI institutions attended the premiere, along with numerous DESI scientists, both senior and early-career.

Dr. Axel de la Macorra and I spoke at the event on behalf of the DESI survey. Also present on stage to introduce the film were Dr. Abdel Pérez Lorenzana, Academic Secretary of Cinvestav (a DESI institution), and Dr. Tonatiuh Matos (DESI scientist), along with Dr. Ana Lilia Coria Páez and Dr. Omar Matamoros representing the Luis Enrique Erro Planetarium. Approximately 260 people enjoyed this inaugural screening of “5000 Eyes.”

Our goal for the event was to convey the wonder of the DESI project to both the authorities and the general public, and we had a significant impact. With the event covered by science communicators and the media, we conducted interviews and spoke with representatives from magazines, social media platforms, and newspapers, and even appeared on TV!

Attendees were wowed by the scale of the DESI project and very happy and thrilled by its results.

During the event, I was incredibly happy to share with colleagues, authorities, and the general public what we do in the collaboration and how we do it. When people approached us with questions, you could see the amazement and joy in their eyes, which is one of the greatest rewards a science communicator can experience.

Now, “5000 Eyes” will be part of the regular film schedule at this planetarium and will be shown at least three times a week. But there’s much more to come! Two planetariums in Guadalajara are set to host premieres in the next few months, and many other planetariums across the country are already interested in screening “5000 Eyes” to share with the public how DESI scientists are creating the largest 3D map in human history.

Over the past two weeks, we released another batch of supporting papers of fundamental importance for the ongoing DESI DR1 release. In particular, we present six new supporting papers within the category 

• DESI 2024 II: Sample definitions, characteristics, and two-point clustering statistics.

These papers are:

• Paper 1: Construction of Large-scale Structure Catalogs for Data from the Dark Energy Spectroscopic Instrument, corresponding author: Ashley Ross
• Paper 2: Forward modeling fluctuations in the DESI LRG target sample using image simulations, corresponding author: Hui Kong
• Paper 3: Impact and mitigation of spectroscopic systematics on DESI 2024 clustering measurements, corresponding author: Alex Krolewski
• Paper 4: ELG Spectroscopic Systematics Analysis of the DESI Data Release 1, corresponding author: Jiaxi Yu
• Paper 5: Mitigation of DESI fiber assignment incompleteness effect on two-point clustering with small angular scale truncated estimators, corresponding author: Mathilde Pinon
• Paper 6: Blinding scheme for the scale-dependence bias signature of local primordial non-Gaussianity for DESI 2024, corresponding author: Edmond Chaussidon

But also the following previous papers belong to this category:

• Paper 7: Production of Alternate Realizations of DESI Fiber Assignment for Unbiased Clustering Measurement in Data and Simulations, corresponding author: James Lasker
• Paper 8: Validating the Galaxy and Quasar Catalog-Level Blinding Scheme for the DESI 2024 analysis, corresponding author: Uendert Andrade

Finally, we also report the release of a supporting paper for the category DESI 2024 III: BAO measurements with galaxies and quasars:

• Paper 9: Fiducial Cosmology systematics for DESI 2024 BAO Analysis, corresponding author: Alejandro Pérez Fernández

For an overview of all the main categories in which the DESI 2024 publications are organised, see our April 4 Paper Guide. Paper 8 is already introduced in our April 11 Guide, and Papers 1-7 are described below, after some general remarks. 

The DESI 2024 II category encompasses all the steps starting from the observed galaxy and quasar redshifts and angular positions, correcting systematics related to target selection,  observing conditions and the geometry of the instrument (to name a few), towards defining the final “large-scale structure” (LSS) catalog, resembling the “true” (unbiased) observed three-dimensional density fluctuations of cosmological origin. Hence, from this final LSS or “clustering” catalog unbiased estimates of the two-point statistics can be obtained. These are subsequently used for the galaxy and quasar BAO (DESI 2024 III) and Full Shape (DESI 2024 V) analyses.

 As such, the systematic correction procedures motivated and validated in this set of publications represent a major stepping stone for us being able to interpret our 3D map in terms of cosmology. This is especially true for the Full Shape analysis yet to come, which is more sensitive to choices made in systematic corrections, whereas the BAO analysis would not be affected even if no systematic correction is applied.

Before delving into the technical details of the papers, let’s try to understand the systematics to correct for when converting DESI’s observations of the Bright Galaxy Survey (BGS), Luminous Red Galaxy (LRG), Emission Line Galaxy (ELG), and quasar (QSO) Samples into pure LSS clustering catalogs. Broadly, these systematics can be split into imaging, spectroscopic, and incompleteness systematics. 

Imaging Systematics

Before DESI started operations, we had to decide which targets to observe spectra from. These targets were selected from the imaging surveys (DES, DECaLS, MzLS and BASS) that provided images of billions of galaxies across the sky. For a general overview of the target selection process, see this blog article by Edmond Chaussidon and this overview of the target selection papers by Anand Raichoor and Christophe Yeche.

However, once the targets are selected, we have to make sure we understand how the observing conditions of each imaging survey and their differences in instruments and sky regions affects the density of target objects we are taking spectra from. As the light from distant galaxies travels through space, particularly through our own Milky Way galaxy and then Earth’s atmosphere, it gets messed up by various factors. For example, it might get slightly redirected or blocked by cosmic dust or changes because of air turbulence. We refer to these issues as “imaging systematics.” These are basically anything that can distort the galaxy images from what they truly look like.

To figure out how much these factors are affecting our galaxy images, we use something called survey property maps. These maps record the amount of cosmic dust, the number of stars in the field of view, and even issues introduced by the cameras and instruments themselves like noise and blurriness (‘seeing’). By understanding the correlations or “trends” between these maps and the galaxy images, we try to correct for these distortions and get a clearer, more accurate view of the galaxies in their surveys. 

For our LRG and ELG Samples we use the “Obiwan” software to inject galaxies to real images to reproduce the imaging systematics trends seen in real images. For LRG, this validation is shown in Paper 2. By comparing systematics trends between DESI LRGs and Obiwan LRGs, we find good agreement in depth. The trend with depth also depends on the intrinsic brightness of LRGs, and this dependency is also reproduced in Obiwan LRGs. Additionally, this study finds that the LRG systematics trend in the dust extinction map is mainly contributed by the large-scale structure systematics in the extinction map. This trend should not be completely corrected because it contains leaks from  large-scale-structure signals.

Spectroscopic systematics

While the previous systematics are related to the imaging surveys, as a next step we need to consider the systematics arising due to the observing conditions during the actual DESI operations measuring galaxy and quasar spectra. To each observed target spectrum, we fit a template spectrum to measure the corresponding redshift using the “Redrock” software (add link). However, only if the fit passes a certain signal-to-noise and goodness of fit threshold, the redshift measurement is considered “successful”, otherwise it is considered as “failed”. The ratio of successful redshift measurements with respect to all attempts is called “redshift success rate”. The success rates are very high (~99%) for BGS and LRG, but significantly less for the more distant (and hence fainter) ELG (72.4%) and QSO (66.2%) Samples. For ELG, the “failed” attempts are mostly related to the [OII] spectral line being either too faint or out of the DESI wavelength range. For QSO, there may be a variety of reasons such as confusion of the quasar targets with stars or other types of galaxies.

Of potential concern is the fact that the success rate specified above is not uniform and depends not only on the galaxy type but also on redshift, observing conditions, etc. We addressed this in Paper 3: 

As mentioned before, DESI uses galaxies as tracers of the underlying matter field, so we need to make sure that fluctuations in the observed galaxy density come from matter field fluctuations and not systematics. This paper assesses the impact of systematics coming from inhomogeneities in DESI observing conditions–for instance, some DESI tiles were observed longer than others (or some individual fibers are more sensitive than others) which potentially leads to fluctuations in galaxy density. We characterize the spectroscopic observing conditions with the “effective spectroscopic observing time,” which is exactly as it sounds: the length of time that the instrument would have to observe under ideal conditions to get the observed signal-to-noise of a DESI target. We show that these fluctuations are small, can be largely corrected by a simple weighting procedure, and have negligible impact on large-scale clustering. The plot shows how success rate depends on effective spectroscopic observing time, and how this trend is largely removed when the weights are applied.

For the ELG Sample, there is an additional complication arising from the fact that sometimes the [OII] emission line can be confused with other lines, for example from the atmosphere, but at the same time may appear as a reasonably good fit (see figure below). We refer to these line confusions as “catastrophic” redshift failures. Paper 4 investigates this issue, finding that the “catastrophic redshift failure rate” of the DESI ELG Sample is 0.26%, which can be accounted for with an appropriate weighting scheme validated in that paper. To mitigate the sky confusion effect, one could also remove the contaminated redshift range 1.32<z<1.33.  In any case, the catastrophic redshift failures lead to negligible systematic biases in the full-shape analysis of  at most 0.2 times the statistical uncertainty.

Incompleteness systematics 

All systematic effects having to do with the fact that DESI does not observe ALL existing galaxies but only a subset defined by the targets, the successful redshifts, and the geometry of the spectroscopic instrument may be described by this umbrella term. 

Let’s start with the instrument: In each exposure, spectra are obtained through 5000 fibers mounted at robotic positioners located within the focal plane. The fibers are automatically positioned towards the targets, but they can only be positioned within a certain patrol radius meaning, amongst other things, that two fibers can not be positioned infinitely close together, we refer to this fact as “fiber collision”. Hence, we are missing out on very close pairs of galaxies on the sky, which means that we are underestimating the “true” clustering of galaxies on small scales.

To capture this effect of “missing galaxies”, we rely on mock galaxy catalogs, obtained from large N-body Abacus simulations to represent a “full” galaxy sample. Next, we simulate how DESI would observe galaxies within this mock catalog given the instrument geometry to obtain a “mock observed” galaxy Sample. Comparing it to the “full” Sample, we can infer what is the effect of fiber collision and reproduce it on the real data using a probabilistic weighting scheme. This is exactly what has been done in Paper 7.

A more conservative approach relies on cutting out all the information arising from pairs of galaxies with angular separation smaller than θ ~ 0.05 degrees, corresponding to the angle covered by two neighboring fibers separated by 10.4 mm (see scheme below). We refer to this as the “θ-cut” method.

Paper 5 shows that by removing all the small pairs with angular separation θ < 0.05 degrees and imposing this condition in our models, we are able to completely remove the systematic uncertainty associated to fiber collisions, as demonstrated in the figure below.

Finally, the results of all the weighting schemes presented until now come together in the LSS catalogue production pipeline presented in Paper 1. However, these different weights are not the end of the story, there are plenty of things to take into account. For instance, the fact that our observed galaxies are “incomplete”, means that in order to measure the actual clustering of galaxies, we cannot compare our map simply to a uniform distribution of galaxies. We need to compare to a random distribution of galaxies (without clustering) that matches exactly the sky area and mean density evolution with redshift of the DESI observations. This is tricky, given that some sky areas have yet been observed only once, while in other regions the observed tiles are overlapping to increase completeness. 

To visualize this problem see here a figure from Paper 1. The left column shows actual observed galaxies and the right column shows the galaxies we would have observed if galaxies were distributed randomly, i.e., if there was no BAO scale or gravitational clustering at all! Our actual clustering measurements are always defined as the excess clustering of the data with respect to that random Sample. Also, it displays how the tiling pattern evolved between the first year of operations (upper row) up to a few weeks before today, i.e. almost three years of operations. As you can see, the three-year sample is much more regular, so we are already looking forward to perform cosmological analyses on that more complete Sample!

Modeling systematics

Decoupled from the systematics related to the instrument and observing condition, we also need to make sure that the theoretical systematic error budget arising from our cosmological models is under control. For the full-shape analysis, this has been studied in detail in the papers presented in our April 11 Guide, and for the BAO analysis from galaxies and quasars we tested the impact of the reconstruction scheme, the galaxy-halo connection and the BAO theory (for details, see our April 4 Guide.

Today, we release an additional paper (Paper 9) presenting an in-depth study of the impact of the so-called “fiducial cosmology assumption” BAO analyses are subject to. In a nutshell, to perform the BAO analysis with galaxies and quasars, all redshift measurements must be transformed to Cartesian coordinates under the assumption of a fiducial cosmological model, such that we can measure the full three-dimensional clustering statistics. At the same time, a template for the BAO peak at a given fiducial value of the sound horizon must be used to infer the excess probability of clustering in units of the sound horizon. In theory, that quantity is independent of the fiducial cosmology, however, it is essential to explicitly test whether this is indeed the case in practice. In Paper 9, the authors test the robustness of the BAO measurements against the choice of fiducial cosmology for both cases (coordinates and templates) individually and deliver an estimate of the associated contribution to the systematic error budget in the context of the DESI DR1 BAO analysis. They conclude a contribution of 0.1% in the dilation parameters.

Blinding

To shield ourselves from confirmation bias we decouple the analysis of systematic uncertainties presented before from the cosmological analysis by blinding the data at the catalog level until the cosmological analysis settings are determined. The Blinding scheme for BAO and RSD analyses has been presented in Paper 8 (see April 11 Guide). While that scheme works by displacing galaxies in their position along the line of sight, we implemented an independent catalog-level weighting scheme changing the galaxy density on large scales mimicking the presence of primordial non-Gaussianity (PNG).   

Paper 6 presents the blinding scheme applied in DESI DR1 to mask the large scale dependent bias signal in the power spectrum that is generated by the presence of PNG. This is particularly relevant since the large scale modes of the power spectrum are heavily contaminated by the dependence on the imaging properties of the target density. With this blinding, we can therefore perform a confirmation bias free analysis and be able to provide a robust measurement compare to Planck18. Although DR1 data will not have the statistical power to reach similar constraints than Planck, one can expect competitive constraints with the final data set of DESI.

Conclusion

The set of papers published today mark an important milestone towards the cosmological analysis of our DESI 2024 DR1 Sample. The work that has been put into validating the LSS pipeline is invaluable for the full shape analysis yet to come and we are looking forward to unblinding our full-shape and RSD measurements very soon!

Joan Najita (NOIRLab)

30 May 2024

This past weekend, Kitt Peak National Observatory (KPNO) hosted an Open Night for members of the Tohono O’odham Nation. KPNO, where the DESI project is underway, is located atop I’oligam Du’ag (Manzanita Shrub Mountain) in the Tohono O’odham homeland. KPNO Open Nights celebrate the relationship between the Nation and the Kitt Peak astronomy community and express the community’s appreciation for the privilege of carrying out research at a site that is of deep historical and cultural significance to the Nation.

For the 25 May 2024 event, KPNO opened its doors, welcoming 600 Tohono O’odham community members to the observatory. Tribal members of all ages joined in a wide variety of activities, including solar and night-time telescope viewing, Waila music, hands-on activities, and observatory tours.

Visitors to the 4m Mayall Telescope were greeted by volunteers — including DESI collaboration members Dick Joyce, Luke Tyas, Chris Brownewell, Bob Stupak, Yuanyuan Zhang, and Joan Najita — who described the DESI project, its amazing technology and goals, and how the observations are carried out. Elsewhere on the mountain, DESI collaboration member and Mid-Scale Observatories Director Lori Allen greeted visitors as they arrived and helped them view highlights of the night sky through a small telescope.

BaoBan, DESI’s ambassador for Education and Public Outreach, made several guest appearances. A coyote from the wilds of Arizona, BaoBan appeared on DESI-themed souvenir postcards created for the event and in the inaugural KPNO Newsletter for the Tohono O’odham Nation, both of which were distributed to visitors. BaoBan has previously appeared at the Tohono O’odham Rodeo, in comic strips, and other DESI-related public engagement activities.

The 25 May Open Night was long anticipated, with the recent coronavirus pandemic and Contreras fire of 2022 having disrupted the usual 2-3 year cadence of these events. With the mountain now reopened and normal observatory operations resumed, the Kitt Peak astronomy community was eager to restart the series. More than 70 volunteers from NOIRLab and Kitt Peak facilities worked together with local Tucson community organizations in hosting the event.

For Bob Stupak, NOIRLab Electronics Technician and DESI collaboration member, welcoming visitors at the Mayall was “a great time,” an opportunity to share the excitement of DESI with the community, resulting in smiles all around. He noted, “I was working at the top of the visitor’s elevator and everyone leaving the building seemed to have been really impressed.”

Further details about the event are available in a NOIRLab press release.

Joan Najita (NOIRLab) and Luke Tyas (LBL)

Science communicators across the internet are discussing DESI’s Year 1 results and their implications for the fate of the Universe. Here are some of the highlights.

World Science Festival: Is Dark Energy Decaying? 

Brian Greene sits down with Michael Levi (LBL) to discuss DESI’s revolutionary observations that may upend our understanding of the cosmos. In an hour-long conversation that takes us from the discovery of dark energy to the birth of the DESI project and on to its Year 1 Key Project results, Greene and Levi highlight the cutting-edge nature of the story. That is, we’re finding an intriguing hint that dark energy is weakening (i.e., becoming “less pushy” over time), and but we’re not completely sure. Commenting on the uncertainty, and how we’ll know more soon, “This is why I love doing physics,” says Levi, “It’s detective novel that you get to read and discover, and it was written by the Universe!” As for the bigger picture meaning of the results, should they hold up, the weakening accelerated expansion of the universe seems reminiscent of the end of the inflationary expansion phase that marked the birth of the Universe. Levi finds it “appealing that we may be living in an epoch that ties back to the beginning of time.”

PBS Nova: New Map of the Universe Hints that Dark Energy May Be Evolving

This 3-minute video, which includes commentary from David Kaiser (MIT) and Priya Natarajan (Yale), describes how the DESI results could “fundamentally change what we know about the Universe and how it may come to an end.” Kaiser says the new DESI map of the Universe is more than a simple accounting where stuff is at any given moment. It also captures the Universe’s dynamics, creating something more like a movie that shows how fast space was stretching at different times over the Universe’s history. Natarajan emphasizes the possible cosmological implications of the results, noting that “If dark energy was a constant, the fate of the Universe would be grim.” Instead The DESI results appear to “open the door to the possibility of changing dark energy models”, which for Natarajan, “means that we have more exciting (possible) fates that await us.”

NPR: This week in science: Pompeiian frescoes, dark energy and the largest marine reptile  — NPR’s Mary Louise Kelly talks with Emily Kwong and Rachel Carson of Short Wave about how dark energy may be changing, with commentary from Priya Natarajan (Yale).

Dr Becky: Does the expansion rate of the Universe CHANGE over time?! DESI 1 year results — In this 12-minute video, astrophysicist Dr Becky Smethurst (Oxford) explains Baryon Acoustic Oscillations in words normal people can understand and walks the listener through the main plots and results from the Year 1 Key Project papers and press release.

Anton Petrov: Strange Expansion of the Universe Results from the Most Accurate Map — With a phenomenal 268,000 views as of this writing, this 12-minute video from scientist and educator Anton Petrov describes the use of Baryonic Acoustic Oscillations as a cosmological standard ruler and the implications of the DESI results for our understanding of what the universe “was doing billions of years ago, what it’s going to be doing in the future, and how all of this ends.”

AM24:First Year Results from the Dark Energy Spectroscopic Instrument (DESI) — Three talks by DESI scientists at the April 2024 meeting of the American Physical Society unveil DESI’s cutting-edge results. First, Hee-Jong Seo (Ohio University) presents the BAO results from galaxies and quasars at z < 2. Next, Julien Guy (Berkeley Lab) presents BAO results from the Lyman-alpha forest at z > 2. Finally, Mustapha Ishak (UT Dallas) presents the cosmology implications of the measurements.

Joan Najita (NOIRLab)

The original discovery, in 1998, that the expansion of the Universe is accelerating rather than slowing with time, could be understood as evidence that the vacuum of space possesses a tiny amount of energy of its own, i.e., a “dark energy.” That idea found an echo in an earlier proposal from Einstein of a “cosmological constant,” a concept that has been at the heart of our standard model of the Universe for the past two decades.

The model has been popular. In finding explanations for how the world and the Universe works, physicists sometimes use aesthetics as a guide and are drawn to explanations that have a simplicity or elegance, often of a mathematical kind. The standard model fits the bill. Explaining the attractiveness of the standard model and the role of the cosmological constant, Licia Verde told Quanta magazine: “It’s simple. It’s one number. It has some story you can attach to it. That’s why it’s believed to be constant.”

The new DESI results shake that foundation.

If dark energy isn’t a strict constant, who knows how it evolves. That uncertainty, and the potential unshackling from a cosmological constant, opens a richer set of potential futures for us. If dark energy continues to significantly weaken with time, it may become not only “less pushy,” but perhaps even “sucky,” causing the Universe to contract rather than expand. On the other hand, it may also grow stronger with time or possibly just fade away.

Commenting on these options, Durham University Professor Carlos Frenk told the Guardian that if DESI’s hints are right, our earlier understanding “goes out the window and essentially we have to start from scratch, and that means revising our understanding of basic physics, our understanding of the big bang itself, and our understanding of the long-range forecast for the Universe.”

The news may be soothing to some of us, at a human level.

In covering the DESI results, NPR characterized our current model of the Universe as painting a bleak picture of the future, i.e., that if dark energy is constant, it “will continue to push everything in the Universe apart. So much so that one day other galaxies won’t be visible from Earth. Even the stars in our own galaxy will die out, leaving behind a cold dark nothing…” Pretty depressing. But Priya Natarajan told NPR that the possibility of a changing dark energy opens the way to a happier ending: “Our fate may not be as lonely and desolate and grim as we imagine.”

Our understanding of exactly what the future holds depends on what we learn about whether and how dark energy is evolving. We should know more soon. The reported results are based on just the first year of data from the DESI survey, which is designed to extend over 5 years. So stay tuned!

Read other news reports on the recent DESI results here.

In this blogpost we introduce another batch of supporting papers released yesterday on April 11, just one week after releasing our cosmological results using BAO from galaxies and quasars and the Lyman-alpha forest. 

Yesterday’s papers do not exhibit new results, but represent major stepping stones towards the cosmology results from the RSD (aka Full Shape) analysis we plan to release soon. They fall into the two categories: 

• DESI 2024 II: Sample definitions, characteristics, and two-point clustering statistics.
• DESI 2024 V: Analysis of the full-shape of two-point clustering statistics from galaxies and quasars

DESI 2024 II: Sample definitions, characteristics, and two-point clustering statistics.

These papers describe the methods by which we ensure that all results properly take into account systematic effects, including: incomplete galaxy sampling, human biases, and imaging systematics. For a general overview of how DESI selects its targets, see this blog post, and for more information about survey validation see this blog post.

Most papers attributed to this category are yet to come out. But one of them, the paper presenting the DESI Blinding strategy, was already released yesterday, given its synergy with the BAO papers released a week ago, and with the Full Shape papers also released yesterday. It represents a major stepping stone validating the DESI 2024 Blinding strategy for the BAO and RSD (Full Shape) analysis.

Validating the Galaxy and Quasar catalog-level Blinding Scheme for the DESI 2024 analysis

Corresponding Author: Uendert Andrade

Arxiv: https://arxiv.org/abs/2404.07282 

Summary:

Short: This paper introduces the blinding strategy ensuring a data analysis without confirmation bias, validating it using mock catalogs and blinded data. 

Long: This paper introduces the galaxy and quasar BAO and RSD blinding scheme, where galaxy redshifts are displaced in two ways, such that overall they mimic i) a dark energy expansion history different than in the fiducial model with cosmological constant and 2) a different growth of structure history corresponding to a different law of gravity. Additionally, galaxy weights are applied to mimic the effect of primordial non-Gaussianity. BAO fits and full-shape fits (ShapeFit) are applied to one realization of Abacus mocks that was blinded according to 16 different varying dark energy and primordial non-Gaussianity scenarios. Additionally, the blinding scheme was applied to the blinded data and validated on that “double-blinded” catalog using BAO fits. 

This figure shows for one particular case the fitted isotropic and anisotropic BAO dilation parameters scaled to the expectation obtained from 8 different blindingalues of (w0, wa) and either positive (fnl=20) or negative (fnl=-20) primordial non-Gaussianity. Deviations from 1 are observed only for very extreme pairs of blinding values.

DESI 2024 V: Analysis of the full-shape of two-point clustering statistics from galaxies and quasars

This set of papers document various clustering statistics, modeling, and systematic analysis of DESI’s Year one galaxy and quasar samples. While there is yet more to come out, yesterday’s set of papers focus on the comparison between different Perturbation Theory models (based on Effective Field Theory (EFT)) and codes to the AbacusSummit LRG, ELG and QSO mocks. Overall, they find very good agreement among the different pipelines, corresponding to the Lagrangian and Eulerian Perturbation Theory (LPT and EPT) implementations within Velocileptors, as well as the EFT implementations of PyBird and FOLPSv, where the latter also features an improved model of the impact of massive neutrinos on structure formation.

Furthermore, the papers find excellent agreement between two very different approaches that are used nowadays to infer cosmological information from the full-shape of 2-point clustering statistics: 

1. Template fits: Here, templates of the two-point statistics at a fixed fiducial cosmology are used to extract physical information from the data, the so-called ‘compressed parameters’ such as the isotropic and anisotropic dilation scales, the growth rate, and the scale-dependence, or shape. The latter is a rather new observable proposed in the ‘ShapeFit’ method, which represents the state-of-the-art method when it comes to template fits. Cosmological parameters are obtained by fitting cosmological models to these compressed parameters measured in each redshift bin. This is very similar to the philosophy behind the BAO analysis, where the (compressed) BAO scaling parameters are measured first in each redshift bin and cosmological parameters are obtained in a second step. 
2. Full modeling fits: Here, the step of measuring compressed parameters is avoided. Instead, the 2-point statistics of all redshift bins are directly fitted according to the cosmological model. This is similar to the philosophy behind the analysis of cosmic microwave background (CMB) or weak lensing, where the 2-point statistics are also fitted directly, without an additional compression step in between.

Both these approaches have advantages and disadvantages. Template fits are designed to extract only the most robust information and allows for a modular interpretation. For example, they allow us to decouple the information on expansion history, growth history, and shape in an effective way. On the other hand, the extra compression step within the template fit method can erase some of the cosmological information within 2-point statistics. Direct fits allow us to squeeze all of the cosmological information out of the data. At the same time, their results are, by nature, model-dependent, and they do not provide the same means of performing diagnostic tests such as template fits.

For the DESI 2024 Full Shape analysis, we therefore plan to explore both approaches, and yesterday’s papers lay out the pathway of how to carry out both types of analysis in a robust way.

A comparison of effective field theory models of redshift space galaxy power spectra for DESI 2024 and future surveys

Corresponding Authors: Mark Maus, Yan Lai, Hernan E. Noriega and Sadi Ramirez-Solano

arXiv: https://arxiv.org/abs/2404.07272 

Summary:

Short: This paper models the redshift-space galaxy power spectrum into the quasi-linear regime with several different EFT models, compares the different models to each other, and tests each using the AbacusSummit simulations.

Long: This paper demonstrates the level of consistency between the different effective field theory models used for fitting galaxy power spectra in redshift space. We show, by fitting to Abacus cubic mocks, that velocileptors (Lagrangian and Eulerian PT versions), PyBird, and FOLPSv give consistent constraints in LCDM and ShapeFit parameters with differences in means of <0.1sigma. We also fit to noiseless theoretical data vectors created by each model while varying scale cuts, and show that for kmax=0.18 h/Mpc the systematic errors are far below the statistical errors for all parameters at precisions corresponding to 8 (Gpc/h)3 volumes.

An analysis of parameter compression and full-modeling techniques with Velocileptors for DESI 2024 and beyond

Corresponding author: Mark Maus 

arXiv: https://arxiv.org/abs/2404.07312 

Summary:

Short: This paper includes validation testing of various features of the analysis using the Velocileptors pipeline in combination with AbacusSummit mocks. Studies the dependence of the results on parameter compression, scale cuts, joint fitting, beyond-Lambda CDM modeling, inclusion of external data, and more.

Long: We present systematics tests and comparisons of three different modeling methods (Full-modeling, ShapeFit, and standard template) within velocileptors for modeling the galaxy power spectrum in redshift space using a Lagrangian effective field theory framework. We fit Abacus N-body simulations created to mimic the LRG, ELG, and QSO tracers that DESI targets, and show that ShapeFit and Full-modeling have consistent constraints and similar constraining power on LCDM models. We demonstrate the behavior of the three modeling methods for a variety of fitting settings with/without including BAO information in order to describe optimal fitting settings for velocileptors for DESI Y1 analyses and beyond.

We demonstrate constraints on the LRG mock data for the three modeling methods in the right panel of Fig. 3, also shown here:

Comparing Compressed and Full-modeling Analyses with FOLPS: Implications for DESI 2024 and beyond

Corresponding author: Hernan E. Noriega

arXiv: https://arxiv.org/abs/2404.07269 

Summary: 

Short: This paper explores potential sources of systematic error in the full-shape analysis and compression techniques, using the AbacusSummit mocks.

Long: This work validates the robustness of the theoretical modelling of FOLPS, which properly takes into account the presence of massive neutrinos. The study finds that potential modelling errors are fully sub-dominant for DESI’s statistical precision. The research also compares Full-Modeling and ShapeFit fitting approaches, demonstrating their agreement. Overall, this work paves the way for a robust analysis of the DESI power spectrum.

A comparison between Shapefit compression and Full-Modelling method with PyBird for DESI 2024 and beyond

Corresponding author: Yan Lai

arXiv: https://arxiv.org/abs/2404.07283 

Summary: 

Short: This paper shows that the Shapefit compression matches cosmological constraints using traditional full-shape analysis for ΛCDM, wCDM, and oCDM models.

Long: In this paper, we compare the constraints of cosmological parameters from Shapefit and Full-Modelling with PyBird. We do this with the DESI cubic box mocks for Luminous Red galaxies (LRG), Emission Line Galaxies (ELG), and Quasi Steller Object (QSO) for the LCDM, wCDM, and oCDM models. We found for all three cosmological models tested, the constraints from Shapefit and Full-Shape are consistent with each other. Furthermore, the constraints from both methodologies agree with the underlying cosmology.

Full Modeling and Parameter Compression Methods in configuration space for DESI 2024 and beyond

Corresponding author: Sadi Ramirez-Solano

arXiv: https://arxiv.org/abs/2404.07268
Summary: 

Short: This paper includes similar studies as the other projects in this group, but for configuration space

Long: This work conducts a thorough comparison of various methodologies for modeling the full shape of the two-point statistics in configuration space. We investigate the performance of both direct fits (Full-Modeling) and the parameter compression approaches (ShapeFit and Standard) with CLPT-EFT. Our pipeline recovers unbiased cosmological parameter values for a 1-year DESI volume. We also present the comparisons of the configuration space version of different EFT models.

On April 4, DESI released a set of papers marking our first release of year one (Y1) results. This page contains summaries of our main results and a guide to the publications. The papers will be available on arXiv at 5pm PST on April 4, and until then are available here.

Helpful links

• A press release containing a high-level overview of our main results: https://newscenter.lbl.gov/2024/04/04/desi-first-results-make-most-precise-measurement-of-expanding-universe/ 
• A brief announcement on our webpage: https://www.desi.lbl.gov/2024/04/04/first-cosmology-results-from-desi-most-precise-measurement-of-the-expanding-universe/ 
• A list of current papers: https://data.desi.lbl.gov/doc/papers/ 
• For more background on DESI’s science, see our public webpages.
• DESI’s Y1 data is not yet public, but you can find our early data release and any updates on this site: https://data.desi.lbl.gov/doc/releases/

The Y1 results fall into seven main categories, three of these (highlighted in blue) are released on April 4:, BAO measurements with galaxies and quasars (DESI 2024 III), BAO with the Lyman-alpha forest  (DESI 2024 IV), and cosmological inference from BAOs (DESI 2024 VI). This figure displays the publication organization with the results released on April 4 highlighted in blue, and summaries of each can be found below.

April 4 Paper Summaries

BAO Measurements from Galaxies and Quasars

Baryon Acoustic Oscillations (BAO) are a powerful tool to measure cosmic expansion through the “standard rulers” created by expanding overdensities from the early universe. Using galaxies as tracers of these overdensities, this set of papers describe DESI’s galaxy BAO measurements. This is the largest dataset ever used to measure BAO, by both number of galaxies and volume. They are the most precise measurements of their kind, at 0.52%.

DESI 2024 III: Baryon Acoustic Oscillations from Galaxies and Quasars

Arxiv: 2404.03000

Summary: This is an overview of the main DESI Year-1 BAO results from galaxies and quasars.

Baryon Acoustic Oscillation Theory and Modelling Systematics for the DESI 2024 results (submitted in Feb 2024)

Corresponding author: Shi-Fan Chen

Arxiv:  2402.14070 

Summary: Baryon acoustic oscillations are one of the best standard rulers there are. In this paper the authors work out just how robust they are, and how to best fit and extract the signal from DESI data.

Optimal reconstruction of baryon acoustic oscillations for DESI 2024

Corresponding authors: Enrique Paillas, Zhejie Ding, Xinyi Chen

Arxiv: 2404.03005

Summary: This paper investigates different reconstruction settings to optimize BAO detection. Reconstruction is a sophisticated technique enabling a more precise (lower statistical error) and more accurate (lower systematic error) BAO measurement. When considering the galaxy distribution, unfortunately the pristine BAO signal is slightly erased and contaminated by the galaxies’ velocity originating from their mutual gravitational interaction. Luckily, from the observed galaxy density we can calculate the gravitational potential at each galaxy and hence estimate their velocities. Using that estimate, we can displace each galaxy to its initial position and hence “reconstruct” the initial galaxy field exhibiting the original, pristine BAO signal. This work shows on Abacus mock catalogs that after applying reconstruction (post-recon) the BAO peak in the two-point correlation function is enhanced (see figure) and slightly shifted (not visible by eye in this figure) towards the location predicted by the cosmological model. 

For non-cosmologists: the two-point correlation function is basically a histogram of the number of galaxy pairs that are separated by a distance “s”. It hence describes the excess probability of finding two galaxies separated by “s”.

Semi-analytical covariance matrices for two-point correlation function for DESI 2024 data

Corresponding author: Michael Rashkovetskyi

Arxiv: 2404.03007

Summary: This work improves and validates an efficient method for generating covariance matrices for clustering analyses using the correlation functions. In particular, the authors report a close agreement in projected errorbars for BAO scale parameters between the mock-based (more standard) and semi-analytical (faster) covariance matrices, as shown in the right figure.

HOD-Dependent Systematics for Luminous Red Galaxies in the DESI 2024 BAO Analysis

Corresponding author: Juan Mena-Fernández

Arxiv: 2404.03008

Summary: This paper investigates how the halo occupation distribution (HOD) modeling might affect the measurement of the BAO distance scale in the DESI Y1 analysis. This is done for LRGs, using several sets of Abacus simulations (that rely on dark matter only) with different HOD models (that populate dark matter halos with galaxies) in order to estimate the amplitude of the so-called HOD systematics. This work finds that the BAO measurements are robust enough against these kinds of systematics for the DESI 2024 analysis, and provides the estimated error budget.

HOD-Dependent Systematics for Emission Line Galaxies in the DESI 2024 BAO analysis 

Corresponding author: Cristhian Garcia-Qunitero

Arxiv: 2404.03009

Summary: This paper consists of a series of tests to assess the robustness of the BAO analysis against HOD-dependence in the ELG tracer, and compare the differences found with the forecasted error with one year of DESI data.

We found that our BAO fits are robust enough against this systematics for DESI 2024 results and provide the error budget for this particular systematics.

BAO with the Lyman-alpha forest

The Lyman-alpha forest refers to absorption lines in the light of distant quasars, which reveal the distribution of gas along the line of sight. This allows DESI to extend our BAO analysis beyond the galaxy tracers and measure the most ancient BAO signatures, up to when the Universe was just ⅕ its current age. DESI’s Y1 results provide measurements of the isotropic BAO scale at 1.1% precision, the most precise measurement in the redshift ranges of 2 < z < 4.

DESI 2024 IV: Baryon Acoustic Oscillations from the Lyman Alpha Forest

Arxiv: 2404.03001

Summary: This is the main paper for BAO with the Lyman Alpha Forest, presenting the Lyman Alpha forest BAO measurement from over 420,000 spectra and their correlation with >700,000 quasars at an effective redshift of z=2.33.

The Lyman-α forest catalog from the Dark Energy Spectroscopic Instrument Early Data Release (published in December 2023)

Corresponding author: César Ramírez-Pérez

Article: https://academic.oup.com/mnras/article/528/4/6666/7462317?login=false 

Summary: This publication presents and validates the Lyman-alpha forest fluctuations in the first DESI data release. To accomplish this measurement, the continua of DESI quasars were fitted not only in the Lyman-alpha forest region but also in featureless regions to the right of the Lyman-alpha emission line, which were used for calibration.

The image displays the observed flux (black) of a high-SNR DESI quasar. The different coloured lines show the expected flux for each of the regions (quasar continuum corrected by the mean absorption), for the Lyman-alpha forest region and the SIV, CIV and CIII calibration regions.

3D Correlations in the Lyman-α Forest from Early DESI Data (published in November 2023)

Corresponding author: Calum Gordon

Article: https://iopscience.iop.org/article/10.1088/1475-7516/2023/11/045

Summary: This is the first analysis of 3D Lyman-a correlations in early DESI data. The BAO peak is strongly detected, and the results are in good agreement with previous analyses of Lyman-a forest correlations from eBOSS. The figure here shows the measured auto-correlation in bins of mu = r_parallel / r, including eBOSS and DESI early data, and the best-fitting model.

Synthetic spectra for Lyman-α forest analysis in the Dark Energy Spectroscopic Instrument (submitted in January 2024)

Corresponding author: Hiram K. Herrera-Alcantar

Arxiv: 2401.00303

Summary: This paper presents the methodology followed to produce synthetic DESI Lyman-𝛼 datasets. We include all the steps from the raw transmitted flux generation to the addition of a continuum and instrumental noise to spectra. We perform a qualitative comparison of the results of DESI EDR+M2 simulated and observed data. And present a Forecast of the full DESI survey constraining power (show in figure).

Impact of Systematic Redshift Errors on the Cross-correlation of the Lyman-α Forest with Quasars at Small Scales Using DESI Early Data (submitted in February 2024)

Corresponding author: Abby Bault

arxiv: 2402.18009

Summary: 

This publication presents a measurement of the systematic redshift errors from the DESI EDR+M2 quasar sample. We find evidence for a redshift-dependent bias causing redshifts to be underestimated with increasing redshift. This bias stems from the templates used for redshift estimation in the EDR+M2 sample. After deriving new templates for the DESI Year 1 quasar sample we repeat our analysis and no longer find evidence of a bias.

The image shows the measured redshift errors for the EDR+M2 and Year 1 samples when the catalog is cut into four redshift bins. For the EDR+M2 data (light blue triangles) there is a clear trend where the measured errors increase with increasing redshift. For the Year 1 data (dark blue circles) this bias is no longer present. The measured error for the full Year 1 catalog, shown in the light red region, also shows that the bias has been mitigated.

Characterization of contaminants in the Lyman-alpha forest auto-correlation with DESI

Corresponding authors: Julien Guy, Satya Gontcho A Gontcho

Arxiv: 2404.03003

Summary: This paper studies the signal introduced by the instrumental processing of the data that contaminates the signal detected from the Lyman Alpha Forest (neutral hydrogen clouds). In addition, it also studies the signal introduced by clouds of other chemical species (not neutral hydrogen) that contaminates the Lyman Alpha Forest signal. The conclusion of this paper is a thorough characterization of the instrumental and astrophysical contaminants of the Lyman Alpha Forest signal.

Validation of the DESI 2024 Lyα forest BAO analysis using synthetic dataset

Corresponding author: Andrei Cuceu

Arxiv: 2404.03004

Summary: This paper documents the creation of mocks for the Lyman-alpha BAO validation, validation of the pipeline using those mocks.

Broad Absorption Line Quasars in the Dark Energy Spectroscopic Instrument Early Data Release (submitted in September 2023)

Corresponding authors: Simon Filbert, Paul Martini

arxiv: 2309.03434

Summary: Broad absorption line (BAL) quasars can have significant absorption troughs near or coincident with the locations of some of the most prominent emission lines in quasar spectra. This paper presents a study of the impact of these features on how accurately we can measure the recession velocity (or redshift) of BAL quasars and presents strategies to mitigate their impact. The figure shows the differences in the redshift before and after mitigating the impact of the BAL features for various types of BALs.

Cosmological Inference 

This final paper interprets the analysis documented above. DESI’s Year one BAO measurements constrain the density of matter in the universe, Ωm,  and the rate of expansion of the universe, H, relative to the sound horizon, r_d. These measurements reveal the growth rate of the universe over time, and, consequently, the impact of dark energy.

DESI 2024 VI: Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations

Arxiv: 2404.03002

Summary: This paper analyzes the BAO measurements from all tracers, which are complementary to each other. The results are consistent with SDSS and CMB measurements. DESI’s measurements are compatible with the standard cosmological model, LCDM, but slightly prefer a model of dark energy which evolves with time.

Joan Najita (NOIRLab)

12 February 2024 was a spectacular night for DESI: it broke its own record and acquired nearly 200,000 redshifts in a single night. The figure is remarkable, especially in the context of history.

Forty years ago, the first and second CfA redshift surveys acquired spectra of 2200 and 15,000 galaxies respectively, giving us the first glimpses of the large scale structure of galaxies. Rather than being distributed randomly, galaxies were found to cluster in a froth-like structure, arranged on bubble-like surfaces surrounding large volumes mostly devoid of galaxies. In these pioneering surveys, redshifts were painstakingly acquired one at a time. As a result, the first CfA redshift survey took 5 years to acquire its 2200 redshifts (1977-1982). In comparison, on 12 February DESI acquired 100 times that number in a single night.

Rather than taking spectra of galaxies one at a time, DESI can acquire 5000 spectra at once through its 5000 robotically positioned fibers. Tiny robots center each fiber on an object (galaxy, quasar, or star). After an exposure is done, the fibers are quickly repositioned, within 1-2 minutes, and DESI is ready to acquire spectra of another 5000 objects. On 12 February, the seeing at the Mayall Telescope was very good (0.65 arcseconds) and DESI was able to go through the setup process more than 40 times. (The gory details: DESI observed 41 dark tiles, 4 bright tiles, and 2 backup tiles that night.)