joannajita

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.

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