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by Nico Schuster (CPPM)

7 July 2026

When we look at the positions of the millions of galaxies DESI has measured, one thing jumps out immediately: galaxies are not spread out evenly. Instead, they clump and string together into a cosmic web: bright filaments of galaxies threaded around huge, almost empty voids, which are some of the largest structures in the universe. However, this web was formed by two invisible ingredients, whose nature we are trying to uncover. One ingredient, dark matter, pulls everything together, while the other, dark energy, pushes everything apart. Together, these two components make up about 95% of the universe, while everything we can actually see, every star and galaxy, is part of the remaining 5%. Although we cannot see these parts of the dark universe directly, we can see the structures they leave behind, how they evolve, and the imprints of this tug of war between gravity and expansion.

Build Your Own Universe

The Cosmic Web Explorer is a way to play with that tug of war yourself. A small cosmic structure simulation that runs in your browser—no install required—it follows tens of thousands of particles from the early universe down to today. Think of these particles as stand-ins for galaxies rather than galaxies themselves. They mark where matter piles up, which is where DESI’s galaxies live, but their number stays fixed from a redshift of 10 onward. You choose how much matter and dark energy your universe gets, hit play, and the web grows in front of you. Try it here. A quick note on what you are seeing: the simulation divides out the overall expansion of space (it uses comoving coordinates), so the box never changes size. What grows is the clustering and the web itself, but not the box.

Behind the scenes, the code came together quite fast, with heavy help from AI coding assistants, used as a way to test how far they could be pushed on specific coding tasks. The answer was surprisingly far, provided scientists stayed in charge of the physics, decided what to include, and where the simple approximations needed refinement, while AI handled the implementation.

DESI and the Cosmic Web Explorer

DESI, of course, does not get its cosmic web this easily. It spends years pointing at the night sky, measuring precise spectra of millions of galaxies, turning those into distances, and building the map of the web.

Put the simulation side by side with galaxies from the DESI Year 5 data (as in the image above) and the resemblance is obvious: very similar filaments and gaps. There is, however, a subtlety in those observed positions: they come from redshift, so each galaxy’s own velocity shifts where it appears along the line of sight compared to its true position. This redshift-space distortion is another thing DESI measures, and the Explorer has a redshift-space mode where you can switch it on and off. Underneath the web itself, two effects are competing. Gravity pulls matter together and grows the web. Dark energy works the other way, stretching space and slowing that growth down. The tool lets you control both. Turn up the matter density and structure clumps more strongly. Turn up dark energy, or change its equation of state (the parameters w₀ and w_a, which describe whether dark energy stays constant or evolves over cosmic time), and you can watch structure formation lose steam quickly.

Try this yourself. Fix the matter content, then compare two universes: one with a plain cosmological constant (ΛCDM, the standard picture in which dark energy never changes), and one with a strongly evolving dark energy (w₀w_a, in which dark energy’s strength changes over time). Try to look closely. The honest result is that they come out looking nearly identical. You have to push the parameters to extremes before your eye can reliably tell them apart. That is not a flaw in the tool, but the whole reason DESI exists. The real differences are so small that we need the extraordinary precision of DESI to measure them.

The BAO Scale

Unlike that small difference, there is one genuine signal you can bring into view. Hidden in the distribution of galaxies is a faint, preferred distance. Pairs of galaxies are very slightly more likely to be found at one particular separation than at neighboring distances, an echo of pressure waves from the hot early universe. This is the baryon acoustic oscillation (BAO) signal, and it is the standard ruler that DESI uses to measure the history of cosmic expansion. In the actual data it is small and easy to miss. So in the Explorer we added a dedicated ‘BAO’ mode that sculpts this preferred distance into the starting conditions, with an artificial amplification strong enough to see. Of course this distance depends on the composition of our universe, and now you can watch what DESI is chasing in its measurements.

Cosmic Voids

DESI measures far more than just the BAO scale: how strong galaxies cluster, how fast that clustering grows, and more. Among these probes are cosmic voids, where my own research sits. These voids are enormous underdense regions enclosed by clusters, filaments, and walls. They contain very little matter, yet they fill most of the volume of the cosmos. That near-emptiness turns out to be quite useful. With so little matter inside, gravity is gentler, which keeps void statistics within reach of simpler theoretical models. And because voids are so empty, faint effects that are hidden inside dense regions stand out in them: the way dark energy shapes the expansion, and even the small imprint left by massive neutrinos, whose speed keeps them from clustering the way ordinary matter does.

In our simulation we find voids the way we do in surveys: we locate the emptiest points and let each region grow outward until it runs into high-density walls and filaments, so every void has its own irregular shape, defined by the surrounding structures rather than by an assumed sphere. In the Explorer you can then watch voids appear, grow across cosmic epochs, and see how they change when you alter the physics of the simulated universe.

Built to Connect and Foster Intuition

There is a personal reason the Cosmic Web Explorer ended up this detailed. Voids did not spark the creation of the tool, but they shaped where it went. Wanting to see them form properly is what pushed me toward local gravity, adjustable tracer numbers, and most of the realism it now has. Getting all of this to run smoothly in a browser was the biggest challenge: every new feature or more accurate physics implementation had to be balanced against its computational cost. However, seeing the first realistic-looking cosmic web assemble in front of my own eyes was thrilling. Watching voids carve themselves out of the large-scale structure confirmed we had built something that felt real and that was genuinely fun to use. That mix of realism and fun is also the point: the Explorer exists to bring cosmology closer to people, for outreach, teaching, and giving a feel for what we work on and why we find these questions so exciting.

It is also worth being clear about what it is not. Like the amplified BAO rings, the whole visualization represents a thin, flattened slice of the universe with short-range gravity. The equations describing the large-scale movement of tracers are directly implemented from theoretical models, but to keep it running in real time in the browser, we had to make certain simplifications. The tool is therefore intended for intuition, not for absolute precision. For a more detailed picture, the code and notes are on GitHub, and we describe how it was built in our paper.

A final, less serious feature: with the right settings the cosmic web can be made to spell out a word as it forms (as in the video below), a deliberate fiction, albeit an enjoyable one. The best way to grasp any of this and to get closer to the science we do is to experience it yourself, so open the Cosmic Web Explorer, build your own universe in your browser, and see what happens when you change it.

About the author: Nico Schuster is a postdoctoral researcher at the Centre de Physique des Particules de Marseille (CPPM) in Marseille, France. In DESI he studies cosmic void statistics within the Alternative Clustering Methods Group. He is also involved in creating preliminary forecasts for cosmic voids with DESI-R2.

Edited by Joan Najita

Follow up of Rubin Alerts Characterizes the Transient Sky

4 May 2026

Joan Najita (NOIRLab)

With its large field of view and ability to acquire a lot of spectra simultaneously, DESI is well suited to many investigations beyond its primary mission of charting out the role of dark energy in the expansion history of the Universe. One example is its ability to characterize phenomena discovered by today’s large time domain facilities such as the Vera C. Rubin Observatory. In a major development, Rubin Observatory recently issued its first “alerts,” announcements that report changes in the night sky. Each alert documents a new source of light, a source that brightened or dimmed, or an object that moved, with 800,000 alerts issued the night of 24 February. The Rubin Observatory’s Legacy Survey of Space and Time (LSST), set to begin later this year, will scan the Southern sky for ten years and is expected to generate up to seven million alerts each night. Understanding the nature of these events often relies on spectroscopy, and a highly multiplexed spectroscopic system like DESI, with its large field of view, is a good match to the large volume of alerts.

To learn what DESI is up to in this area, we sat down with Xander Hall, a graduate student at Carnegie Mellon University who is working with Antonella Palmese. They use DESI to study extragalactic transients, i.e., objects or events that rapidly change their brightness.

Q: Can you tell us about DESI’s Rubin Transients project and what it aims to do?

Xander: The DESI Rubin Transients project is a special part of the broader DESI Transients Survey we have been running. We originally proposed to carry out the Transients Survey through DESI’s call for proposals to use its “spare fibers”, fibers that can’t be matched to target from the main DESI program on a given pointing. We started the Survey with transients discovered by other telescopes — ATLAS (the Asteroid Terrestrial-impact Last Alert System robotic survey network in Hawaii) and ZTF (the Zwicky Transient Facility, a high-cadence survey in California) — observing them with spare fibers as DESI targets of opportunity. This allows us to spectroscopically classify a large number of transients. We have taken spectra of about 1000 transients this way.

Now that Rubin is issuing alerts, we’ve started the DESI-Rubin part of the project, where we are pointing DESI specifically at transients from the Rubin Deep Drilling fields (DDF). The DDFs are regions of the sky that Rubin’s LSST is studying intensively in the first few years of the survey. Somewhat perfectly, DESI and Rubin have very similar fields of view. As a result, DESI can almost entirely cover a Rubin DDF, such as the COSMOS field, in one pointing.

Q: How do you identify your targets? And how do you schedule them for DESI observations?

Xander: We identify targets through the Babamul broker, one of the services that is processing the LSST stream of alerts. We apply custom filters and use various machine learning scores and star/galaxy catalogs to filter out variable stars, since we’re not interested in those. Our goal is basically to select every extragalactic transient within the Rubin field and put a DESI fiber on it. We’ve been able to target as many as 300 transients in a single DESI pointing.

Q: How long does it take to receive the reduced spectra? And what happens next?

Xander: In theory, we can get reduced spectra throughout the night, but in practice we get them the next day. To classify the transient, we run the Next Generation Superfit (NGSF) software on the spectrum. NGSF can identify supernovae of all major types based on their spectra. We find that most of the transients are AGN but each pointing has a few supernovae.

Q: How do you share your results with the world?

Xander: Currently we publish our supernova classifications and spectra to the Transient Name Service (TNS). There are tentative plans to make all the transient spectra public.

Q:  What have you found so far?

Xander: We’ve followed up 400 Rubin transients to date and found about 10 supernovae. So far, they are all classical supernovae. One advantage we have with DESI is that due to the quantum efficiency of the instrument and the 4m telescope aperture, we can study targets as faint as those accessible to 8-10m facilities. As LSST begins full-time observations, we hope to spectroscopically probe higher redshift transients.

Q: I read that your team made the first public report of a spectroscopic classification of a Rubin-discovered transient. That sounds like an achievement!

Xander: As far as I can tell we were the first to publicly classify a Rubin transient with public data! Being first is cool and demonstrates the ability of our survey operations team to make this happen. But I think the more interesting thing will be the long-term spectral sequence of various supernovae and hopefully TDEs (tidal disruption events) we will obtain. We are hoping to hit the COSMOS field every 2-3 weeks. This will give us a very nice spectroscopic sequence that normally would be impossible to achieve in a reasonable time with a traditional single-object slit-based instrument. DESI also offers much higher spectral resolution than instruments traditionally used for supernova follow-up. We traditionally obtain spectra with a spectral resolving power of only R = 100–1000, so DESI’s R = 5000 resolution opens up the possibility of finding new narrow-line features. That’s exciting because, for example, infant supernovae can have young CSM (circumstellar medium) interaction lines that are only visible with a high-resolution spectrograph.

Q: How did you become engaged in this endeavor?

Xander: I’ve been doing extragalactic transient science since high school. In high school, I mainly did variable stars, some machine learning classification of stars versus galaxies, and worked on Zooniverse projects. When I started my PhD at CMU last year, I joined DESI and started leading the spare fiber program. As part of that effort, I’ve worked on selecting targets, classifying them, eventually leading to a population analysis, which we wrote about here. Now, that effort has expanded to also include this DESI-Rubin project.

Q: How long will you keep this up? And what do you hope will happen?

Xander: So far, our plans are to keep going until the end of the DESI-I survey. We’ve been working to incorporate transient follow-up more broadly into the goals of the follow-on DESI-II survey, including more dedicated plans to catch more Type Ia supernovae, which are a critical rung on the distance ladder. My personal interest is in Tidal Disruption Events (TDEs) and probing them to higher redshifts. I’m hoping we can identify a couple of bright high-z TDEs within these deep drilling fields and then take an excellent DESI spectral sequence.

With DESI having mapped the sky faster than anticipated, and studied with more galaxies and stars than planned, four collaborators reflect on the achievement

15 April 2026

Joan Najita (NOIRLab) and Sindhu Satyavolu (IFAE)

Today the DESI dark energy experiment reached the remarkable milestone of having successfully mapped all of the sky it originally planned to study, accomplishing the feat in less time than expected, while also mapping vastly more galaxies and stars than planned. Designed to measure, over a 5-year period, spectra of 34 million galaxies and quasars spread out over two-thirds of the northern sky, DESI has now successfully mapped more than 47 million galaxies and quasars, as well as 20 million stars in the Milky Way, over this area, completing the observations ahead of schedule. 

The results have produced the largest high-resolution 3D map of the Universe ever made and raise new questions about the nature of dark energy and the expansion history of the Universe.  Analyses of DESI’s first three years of data hint that dark energy, once thought to be a “cosmological constant”, may instead evolve over time, sparking great excitement in the cosmology community. The full survey dataset will allow these results to be measured with much greater precision. The collaboration will now begin processing the completed data, with the first dark energy results from DESI’s full five-year survey expected in 2027.

Building on this remarkable success, DESI is now set to continue mapping the sky through 2028, expanding its survey area by about 20%, from 14,000 square degrees to 17,000 square degrees and aiming to gather 63 million total extragalactic redshifts. The extended map will cover parts of the sky that are more challenging to observe: areas that are closer to the plane of the Milky Way and areas further south. The experiment will also revisit the current survey area to map out more distant and fainter “luminous red galaxies,” in order to produce a more detailed map that gives researchers a clearer picture of the Universe’s history. DESI will also study nearby dwarf galaxies and stellar streams in the Milky Way and the neighboring Andromeda galaxy to better understand dark matter, the invisible form of matter that accounts for most of the mass in the Universe but has never been directly detected. 

Raising the bar for future surveys

Beyond its impact on cosmology, DESI has also been quietly raising community expectations for what astronomical surveys can aspire to and accomplish, particularly in survey efficiency. Reaching its current milestone well ahead of schedule is the result of DESI’s high efficiency. Within 2 minutes of measuring the spectra of one set of 5000 objects, DESI is ready to observe the next 5000 objects in a different part of the sky. DESI also uses an innovative, dynamic observing strategy, in which exposure times are adjusted in real time based on atmospheric conditions and sky brightness. As a result, DESI produces uniform data across the sky that meet expectations for spectrum quality, while also expending no more observing time than necessary. Data are reduced quickly, with redshifts ready for analysis the next day. These innovations have raised the bar and are likely to have a major impact on how future spectroscopic surveys are carried out. 

DESI collaboration members reflect on the milestone

The achievements are the result of a global effort. The DESI experiment brings together more than 900 researchers (including 300 PhD students) from over 70 institutions around the globe. To get a sense of how DESI collaborators “in the trenches” view the current milestone, we asked a few collaborators about their thoughts and concerns at the start of the survey compared to how they see the project today. 

Adam Myers (U. Wyoming, Joint Manager for DESI Survey Operations), recalled: 

“At the outset, I was most excited about working with a phenomenal new instrument that would produce an unprecedentedly large map of the sky. I knew the vast number of spectra would yield unexpected  discoveries, although I wasn’t sure what those discoveries would be. I was concerned that our BAO result would be underwhelming — that we would discover that dark energy was a cosmological constant and just reduce the error bars on that measurement. At the very back of my mind, I was also concerned that the instrument would underperform in some unexpected way. 

“Things didn’t work out at all like I expected! Our finding that dark energy seems to evolve is far from underwhelming! It is a huge and wonderful surprise. While the survey did, indeed, encounter a number of roadblocks, like the Contreras Wildfire, which shut down the telescope, we nevertheless finished the bright-time survey 6 months earlier than expected, and the dark-time survey ahead of schedule. As it turns out, the instrument is so robust we will continue to make exciting observations for many, many years to come. I also learned how superb the DESI collaboration is. The reason we finished early despite various roadblocks was, I think, that everyone from the observing teams, to the Kitt Peak engineers, to the hardware and software teams, to the scientific analysis groups, to the core managers, helped to eke out fractions of a percent improvement in the experiment. Those multiple different fractions added up to a significant gain in DESI efficiency. Looking back, I realize how incredibly proud I am to be part of this dedicated, creative and industrious team.”

Dick Joyce (NOIRLab, Mayall Telescope Scientist), said:

“Back in 2011, when I first started working on the project that would become DESI, it was a welcome opportunity for me, as a longstanding instrumentalist, to participate in the design, development, construction, and operation of an exciting project that would address one of the key issues in modern astrophysics. The project was also serendipitously timed to provide a future for the aging but robust Mayall Telescope, which was one of the few platforms capable of hosting DESI. At the time, some of the project goals seemed impossibly ambitious (30 million redshifts! 5000 robotic fiber actuators! 2 minute reconfiguration time!), but the fact that these goals have been achieved speaks to the quality of the collaborative effort to design, build, and operate this exciting experiment.

“While it is probably unusual for a scientist to wax enthusiastic about management, I have to recognize that the project management made it possible for the complex components of DESI to be built by far-flung international collaborators, shipped to Kitt Peak, and successfully assembled into a functional instrument that has been carrying out a scientific program that has met or exceeded the design goal, is being completed ahead of schedule, and whose recognized success has yielded a three year extension.  It is personally very satisfying to have contributed in a small way to this top-notch collaboration, and I suppose it is comforting to know that if the conclusions that Dark Energy may be decreasing are true, we do not face the prospect of the Universe being ripped apart at the subatomic level trillions of years from now.”

Claire Poppett (LBL, DESI Lead fiber scientist, DESI observing operations lead): 

“I remember feeling an incredible sense of excitement as DESI was finally getting underway. We had spent years carefully planning the instrument, then more years building and testing each subsystem, before bringing everything together at Kitt Peak. By the time we reached the start of operations, it felt like we were standing at the culmination of an enormous collective effort. Some of the earliest moments with the fully assembled instrument were unforgettable. The first time we took an image of a galaxy through the new corrector and saw 0.6-arcsecond seeing, which showed that the system was performing beautifully. And then the first night we successfully placed targets onto fibers and watched the detectors fill with beautiful spectra. In that moment it became clear that DESI wasn’t just a concept anymore—the instrument was truly working, and now we could begin doing what DESI had been built for—to build the largest 3D map of the universe ever constructed.

“Today, one of the strongest impressions I’m left with is just how much can be accomplished with a dedicated team working toward a shared goal. DESI was designed to be an incredibly efficient, highly optimized redshift–gathering machine, and in many ways it performed exactly as we hoped. But what stands out most to me now is how much human effort and commitment went into making that success possible, from people answering the phone in the middle of the night to troubleshoot an instrument issue, to spending long days refining procedures and improving data quality. The survey depended on the expertise, responsiveness, and persistence of the team to keep things running at the level needed to deliver high-quality science.

“We’ve also demonstrated how much can be achieved by pairing the right instrument with an existing platform—one that many had considered too old and ready for decommissioning. Looking ahead, I’d like to see us continue building on that approach to deliver world-class science.” 

Klaus Honscheid (OSU, Co-Instrument Scientist and Instrument Operations Lead)

“Thinking back to the beginning, I remember being really focused on how the Instrument Control System (ICS)—which my group at Ohio State developed—would handle everything in practice. A lot has to happen in the short window between exposures. While the previous data is digitizing, the telescope is slewing, the hexapod is aligning, and we are moving 5000 fiber-positioning robots. At the same time, we use DESI’s six guide cameras to lock in our exact on-sky position, take a backlit fiber image, and send final tweaks to the robots. Our requirement was to do all of this in under two minutes, which felt like a pretty tall order at the time. I was excited to see it all come together by the end of the commissioning phase.

“The Contreras wildfire in 2022 threw us a major curveball. It threatened the whole observatory and took out the power and internet infrastructure for months. But honestly, it’s one of the things I’m most proud of—we managed to get DESI back on sky much sooner than anyone expected. Luckily, we had set up an emergency Starlink communication backup just before the fire. We switched to satellite for monitoring and literally invented a ‘sneaker net’: every day, someone drove a hard drive with the night’s data down the mountain to Tucson, where it was transferred to the NERSC computing facility in Berkeley. The following year, when a cybersecurity attack hit a number of observatories, we quickly disconnected from the public internet to protect the system, then fired up Starlink and the sneaker net again and basically didn’t lose any observing time.

“Over time, our definition of success has evolved. While we originally managed to get that complex inter-exposure sequence down to just over a minute, which was fantastic for survey efficiency, today we have two minutes between exposures. It sounds like a step backward, but it’s actually a great technical compromise. Some of the CCDs developed charge transfer inefficiencies, and we didn’t have enough spares to replace them all. LBNL electrical engineer Armin Karcher came up with a clever new readout scheme using just two of the four amplifiers to avoid the bad regions. When integrated into the ICS, it doubled our readout and digitization time but fully restored the science quality of the data.

“As a final reflection on where we are today, Ann Elliott and I initially set up acoustic alarms to alert observers to specific errors using different barnyard animal sounds. The chicken and the cow were definitely the most feared because they usually meant observing had to stop and an expert had to be called. Over the years, we’ve worked out the bugs, and now the ‘barnyard’ is mostly quiet. The instrument often runs for days without a single alert, which is incredibly rewarding to see.”

For fun, check out this visualization, which shows how DESI created its map over five years, connecting the 2D tiling of the observations to the build up of the 3D map.

Read the Berkeley Lab press release for further details.

9 March 2026

Joan Najita (NOIRLab) and Andrea Muñoz-Gutiérrez (UNAM)

Two DESI scientists were recently awarded first place in the 2025 Annual Review of Nuclear and Particle Science Video Contest for their contribution, “Neutrino masses, from ghost particles to cosmological tensions,” which explains DESI’s results on the neutrino mass. We sat down with the winners, Hernán Noriega and Bernardita Ried Guachalla, to learn more about their research interests and their contest experience.

Q: Can you tell us a little about your research interests?

Hernán: My research focuses on the large-scale structure of the Universe. I work on perturbation theory and effective field theory methods to model galaxy clustering, with a particular emphasis on how massive neutrinos shape cosmic structure formation. More recently, I’ve also been exploring modified gravity scenarios.

Berni: My research interests scrutinize fundamental questions about our Universe:  What is the physical origin and evolution of the cosmos? What astrophysical knowledge can be inferred from astronomical surveys? What are dark matter and dark energy? And more specifically, what can we learn from the cosmic microwave background data when combining it with observations of the large-scale structure?

Q: Can you tell us about the video contest you entered?

Hernán: The contest was organized by Annual Reviews as part of the Annual Review of Nuclear and Particle Science (ARNPS). The goal was to encourage early-career researchers to translate cutting-edge nuclear or particle physics into a short, engaging, and accessible video for a broad audience. Participants submitted brief videos explaining a concept they have expertise in, and the winning entries were selected to be featured by Annual Reviews to reach researchers, educators, and science enthusiasts. More details about the contest are available on the ARNPS Video Contest page.

Q: What ideas and messages were you hoping share with the public? The idea of a negative neutrino mass seems crazy!

Berni: The video explores one of the most intriguing questions in modern physics: the total neutrino mass. Once thought massless, neutrinos are now known to have mass, but recent results from cosmology suggest limits so tight they even hint at “negative” values. We explain what neutrinos are, how oscillation and direct experiments like KATRIN measure their mass, and why cosmology offers a powerful but indirect probe. Current results from DESI, Planck, ACT, and SPT push constraints to unprecedented levels, exposing tensions between particle physics and cosmology. Could this mean that neutrinos hold the key to new physics, or even a revision of the ΛCDM model?

Q: What was DESI’s role in these results?

Hernán: DESI provides extremely precise measurements of large-scale structure through galaxy redshift surveys. These data tighten cosmological constraints by improving our handle on the expansion history and the growth of structure, which in turn helps constrain parameters like the sum of neutrino masses when combined with CMB datasets. Importantly, DESI can also place competitive neutrino-mass constraints on its own, with minimal external inputs, providing an independent cross-check that can be directly compared against other probes.

Q: …And you won first place! What was your reaction?

Hernán: I was truly thrilled…. especially because we are proud of the final result. We had a strong feeling it could do well, and after the final edit we thought the video was genuinely compelling. Winning first place was incredibly exciting.

Berni: Since I saw the contest I thought that we could win it. Hernán is a great colleague who knows a lot about neutrinos and he was visiting Stanford when we decided to do it. It was an amazing experience which allowed us to further explore ways of communicating DESI results in a different format.

To learn more, see Bernadita and Hernán’s 5-minute prize winning video, available here.

6 October 2025

Seshadri Nadathur (University of Portsmouth)

To coincide with the official publication, today in Physical Review D, of the first DESI papers on cosmology results from baryon acoustic oscillations from Data Release 2 (DR2), the DESI collaboration has also released the full set of cosmology chains and other results supporting the papers. This set of data products derived from the DR2 data is being released in advance of DR2 itself, and will allow researchers outside DESI to reproduce our results and explore new avenues.

The published “cosmology chains” contain valuable statistical information about the cosmological parameters and how well they are known. To infer the parameters, cosmologists often use methods that draw random samples from complex, multi-dimensional probability distributions. In the Markov Chain Monte Carlo (MCMC) method, a current set of randomly selected parameter values is used to generate the next random sample, resulting in a “chain” of sampled parameter values, a “cosmology chain”.   

The DR2 cosmology data released today are available from this webpage, and are based on the BAO results announced in March 2025 (see here for a guide to those results). They can also be accessed directly from the NERSC supercomputing facility. The dataset includes all the MCMC chains posterior sampling runs for cosmology inference, as well as information on the “best-fit” parameter values that maximize the posterior obtained through optimization. 

These data are being provided in advance of the full DR2 release in recognition of their very high significance to the cosmology community. When the full DR2 is made public in some months, these data will eventually form one of the “value-added catalogues” or VACs that accompany it.

Alongside this release, we have also released two new VACs supplementing the older Data Release 1 announced in March: these contain the corresponding cosmology chains and results based on the DR1 BAO cosmology papers from April 2024 (available here, see the paper guide here), and the DR1 full-shape cosmology papers from November 2024 (available here, see the paper guide here). Both of these new datasets are now included in the full list of DR1 VACs.

We invite interested scientists to explore these datasets, and hope that they prove to be useful for your research. In case of questions, please use the contact information provided on the landing pages of the dataset in question.

25 August 2025

The American Astronomical Society (AAS) has announced its selection of the DESI collaboration as the recipient of the 2026 Lancelot M. Berkeley–New York Community Trust Prize. Established in 2011, the prize is awarded annually for highly meritorious work advancing astronomy in the previous year. Previous recipients have included both individuals and teams for their work in diverse areas of astronomy.

The 2026 prize recognizes the DESI collaboration for its work on two papers published this year. The first, published in February, presents baryon acoustic oscillation (BAO) results from the first year of DESI data, while the second, released in March, reports BAO results from observations of 14 million galaxies and quasars, obtained in the first three years of DESI operations.

The observations not only constitute the largest 3D map of the Universe ever made, they also provide new insights into the nature of dark energy and the evolution of the Universe. When combined with other cosmological constraints, the DESI results provide strong hints that dark energy evolves over time, challenging our current leading model of the Universe, Lambda CDM.

The prize comes with a monetary award and an invitation to present a prize lecture at the AAS winter meeting. Daniel Eisenstein will accept the award on behalf of the collaboration and give the closing plenary lecture of the 247th AAS meeting on 8 January 2026 in Phoenix, Arizona.

21 August 2025

Joan Najita (NOIRLab)

DESI recently reported compelling evidence that dark energy is dynamic and evolves with time. What drives this evolution? In a recent paper, a group of DESI scientists propose that the evolution is driven by the creation of non-singular black holes, which converts ordinary (baryonic) matter into dark energy. The black holes form as a consequence of the evolution of massive stars and grow with the expansion of the Universe over cosmic time. The scientists’ model for this process not only provides a good fit to the existing constraints on dark energy, it also relaxes the cosmological constraints on neutrino masses, allowing for positive neutrino masses. We sat down with a few of the coauthors — Kevin Croker, Greg Tarlé, and Steve Ahlen — to learn more about this intriguing result.

Q: How is baryonic matter converted into dark energy in this scenario?

Greg: We don’t yet know how this works exactly. We do know that in the very early universe, inflationary vacuum energy was eventually converted into the matter of our universe. If you ask yourself the simple question, “Where in the current universe are conditions of high density, intense gravity, and curvature comparable to those in the very early Universe,” there is only one place, near the center of a forming black hole. Something must happen to stop an unphysical singularity from forming. We believe that it is the formation of dark energy from matter, like a little Big Bang played in reverse.

Steve: The standard form of our model involves the conversion of matter to dark energy inside black holes during gravitational collapse. If I allow myself to speculate, one possible mechanism for this is the reverse of the electro-weak phase transition whereby the Higgs scalar field acquires energy from the collisions of high energy massive particles (the inverse of the “Higgs mechanism”).

Q: What is a “non-singular” black hole? And why is it the right kind of black hole to create dark energy?

Kevin: Technically, it’s a solution to Einstein’s equations that is identical to the Schwarzschild black hole solution outside the Schwarzschild radius, but contains dark energy inside the Schwarzschild radius. The Schwarzschild black hole solution is empty on the inside, and the mathematical description of the inside breaks where you’d expect the object’s center to be. That’s the singularity part. But if you have dark energy inside your black holes, the object is stabilized and there are no singularities. Some solutions even lack one-way layers (horizons), so the dark energy on the inside can influence, and be influenced by, the Universe outside.

Q: Earlier DESI papers seemed to push us toward negative (unphysical) neutrino masses. What is the physical argument for this interpretation? And how does your new model ease the tension and allow for positive neutrino masses?

Kevin: In interrogating the universe’s energy density budget, the DESI data (from the late-time Universe) prefer a smaller total matter contribution than do data from the early universe (from cosmic microwave background experiments). Because neutrinos contribute to the total matter budget, when other types of matter are assumed to remain unchanged as the universe grows, the data push the neutrino contribution to negative values in order to reconcile the two distinct measurements.

Under the Cosmologically Coupled Black Hole (CCBH) hypothesis, matter is converted into dark energy because stars (made of matter) collapse to non-singular black holes (made of dark energy). This naturally decreases the amount of matter today, relative to the early universe. It turns out that this decreases the amount of matter in the late universe by just the right amount for neutrinos to contribute as we know they must from terrestrial neutrino experiments.

Q: Fascinating! A couple more questions about how the CCBH picture works. As a human, the idea that dark energy continues to increase as the Universe expands seems to violate some kind of conservation law, like we’re somehow getting a free lunch: dark energy causes the Universe to expand, which creates yet more dark energy. How is this explained in your picture?

Greg: In the CCBH model, the dark energy interiors of black holes are coupled to the external expanding universe. Dark energy is characterized by negative pressure, so the dark energy interiors of black holes do negative pressure-volume work on the universe as it expands. To balance this negative work, the interior dark energy must grow as the universe expands. To an outside observer, the mass of the black hole will therefore grow over cosmic time. This is all consistent with Einstein’s General Theory of Relativity, and remarkably, there are enough black holes formed from heavy stars in the Universe for CCBHs to account for the amount of dark energy we observe. So there is no free lunch.

Kevin: Regarding the apparent free lunch, the catch here is that, when your universe isn’t empty, energy is no longer conserved in Einstein gravity. You can lose energy to the gravitational field, like photons do (that’s why they redshift). But you can just as easily gain energy from the gravitational field too. In the Cosmologically Coupled Black Hole (CCBH) hypothesis, non-singular black holes “tap into” the gravitational field and gain energy in this way.

Q. How does the dark energy created in the black hole lead to the acceleration of the expansion of the universe? In the usual picture in which dark energy pervades all of space, it seems easy to imagine it pushing the Universe everywhere to expand. But here, I’m guessing that the dark energy created inside a CCBH doesn’t escape out into the Universe (because nothing escapes a black hole). If that’s the case, how does the concentration of dark energy in small regions (the insides of black holes) cause the Universe’s expansion to accelerate in a uniform way?

Kevin: You’re right that dark energy doesn’t escape from black holes. It sits inside, stabilizing them. As to how it influences the expansion of the Universe: Friedmann’s equation, the one that determines the cosmological behavior, does not care where the dark energy is. My thesis work with the differential geometer Joel Weiner established this back in 2018. This is the counter-intuitive result, and seven years later, it is still not widely understood. The result is technical, but it can be understood from symmetry as follows: the only gravitational degree of freedom in the Robertson-Walker cosmological model is the scale factor. The scale factor is defined to have only dependence in time. This means that its dynamics cannot have any spatial awareness. But without spatial awareness, the scale factor cannot discriminate between the inside or outside of anything. So cosmological dynamics are determined only by what is in the universe, not where it is.

Q. As another angle on that question: most CCBHs will reside in galaxies (i.e., where stars have formed). Does the higher density of CCBH in galaxies affect galaxy dynamics, e.g., cause them to expand? Or if not, why do CCBHs drive the expansion of the Universe more than their local environment?

Kevin: Great question! How CCBHs act on scales below galaxy cluster scales is not yet computed. We’ve only done the calculations through linear order. Observationally, they cannot cluster as strongly as cold dark matter, otherwise certain observed dwarf galaxies like Eridanus II would likely have been shredded. Local mass growth (CCBH mass proportional to scale factor cubed), if present, helps to evade these constraints, so we’ve not had cause to visit a detailed calculation yet. Above cluster scales, we demonstrated a scenario (published in ApJ back in 2020) where the CCBHs disperse toward genuine uniformity. This was more a proof of concept that you could build the model so that galaxies didn’t get destroyed. We’re currently working on a more comprehensive picture, including continual input from star formation. Whether or not CCBH will pass this important first-order test remains to be seen!

As far as local vs. global dynamics, I feel it important to emphasize my position that Friedmann’s equations and the linear perturbation equations, at late-times, only make sense as effective theories. In the early universe, everything was so hot and dense, that it really was a uniform soup with small perturbations. But 13 billion years later, the universe is most certainly not isotropic and homogeneous (look out the window). Modeling the late-time universe as a Robertson-Walker cosmology in a mathematically consistent way then requires that a stress-energy inventory determine the global expansion rate. Not “where,” only “what.”

Q: What drew you to this problem?

Kevin: The connection to neutrino masses was serendipitous. We were working with the DESI public year-one data, and saw that the CCBH hypothesis gave dark energy which tracked the best-fit DESI models using the star formation rate alone. In passing, Greg mentioned that there was some sort of trouble with the neutrinos. I forgot about this for a few days and then things just sort of clicked: we’re consuming more matter to make things work at later times, so that might open up space for the neutrinos. So providing a way to fix the neutrino mass problem just dropped out of the wash.

Greg: My connection to this line of research was also serendipitous. In 2018, when I took a sabbatical at the University of Hawaii Institute for Astronomy, this passionate young theorist named Kevin Croker came into my office, handed me some dense and obscure General Relativity papers he wrote and wanted to know whether DESI (which hadn’t started taking data yet) could measure the effect he predicted. I hadn’t touched a General Relativity calculation in over 44 years since graduate school.  Because he was so enthusiastic, I didn’t have the heart to tell him I didn’t have the time to read his papers. It took me five full passes through the papers before I could understand them and follow the equations. I couldn’t find anything wrong with the theory but, at the time, I thought the effect would be too small to see with DESI. Nevertheless, I joined Kevin and a small group of physicists and astronomers who searched for places where the impact of CCBHs could be measured. In the last three years we found three independent observational signatures; SMBH growth without accretion or merger in passively evolving elliptical galaxies, DESI DR1 dark energy density tracking star formation history and now, the easing of the DESI neutrino mass tension with DESI DR2 data. It took the amazing data that only DESI can now provide to observe the effects of CCBHs on cosmology.

Q: Was the result what you expected to find? Were there any surprises?

Steve: The result that neutrino masses could be constrained and found to be consistent with the experimental particle physics results — this is the result I hoped for. What was surprising and unexpected was how well the cosmology neutrino mass agreed with the particle physics mass if we allowed for the possibility that some matter is lost in being converted to dark energy.

Kevin: While the result was consistent with my expectations, it was a bit surprising how “flexible” the standard model, Lambda CDM, is. The neutrino masses push up against zero in LCDM when you allow them to do so. But if you only allow them to float down to physically measured lower-bounds, the LCDM model will adjust itself in various ways so that there is very little statistical penalty. So it’s really going to take a confluence of observational constraints to definitively pin things down.

Q: What’s next for you?

Steve: I am personally interested in applying some of the same principles of CCBH/Dark Energy to study the early Universe. In particular, it seems possible to me that “Cosmologically Coupled Topological Defects” may provide a mechanism for the accelerated expansion (known as “Inflation”) in the first fraction of a second after the beginning of the Universe during the so-called Grand Unified Theory phase transition. I find it appealing to contemplate that the very early and the very recent Universe could be explained by similar physical mechanisms.

Kevin: The success of such a simple model has drawn excellent critical attention. While things work well at cosmological scales, investigations by other groups at the scale of individual black holes paint an observational picture far less coherent. For example, if you know a black hole’s birthday and mass, you can figure the smallest its mass could be today. Some clever studies figured out ways to get birthdays, but made typical assumptions about birth masses. If you do that, black holes don’t grow fast enough to contain dark energy. But we don’t know the smallest birth mass for black holes without horizons, and if you set it to 25% of the typically assumed value of 2 suns, then things stay consistent. There are many assumptions that have to be carefully revisited when studying non-singular black holes. This is my primary focus these days.

Greg: I plan to continue developing this picture through a more detailed analysis of its cosmological implications. I am thrilled that many DESI collaborators have decided to join me on this exciting adventure.

Read more in the University of Michigan press release.

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, a first-of-its-kind scientific and research cloud computing platform that provides low/no-code “science engines” for researchers to run simulations, analyze large data sets, and train AI models. They 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 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.