joannajita

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.

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.

Joan Najita (NOIRLab)

30 May 2024

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

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

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

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

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

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

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

Joan Najita (NOIRLab) and Luke Tyas (LBL)

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

World Science Festival: Is Dark Energy Decaying? 

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

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

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

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

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

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

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

Joan Najita (NOIRLab)

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

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

The new DESI results shake that foundation.

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

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

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

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

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

Read other news reports on the recent DESI results here.

Joan Najita (NOIRLab)

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

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

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

Joan Najita (NOIRLab)

16 February 2024

BaoBan, DESI’s ambassador for Education and Public Outreach, made a recent appearance at the 85th Annual Tohono O’odham Nation Rodeo and Parade, featuring prominently in the parade entry from Kitt Peak National Observatory (KPNO). A coyote from the wilds of Arizona, BaoBan has previously appeared in comic strips and other DESI-related public engagement activities. 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.

At this year’s rodeo parade, held 3 February 2024, KPNO entered a float decorated with images of BaoBan, planets, stars, and telescopes. Sarah Logsdon, one of the NOIRLab volunteers who accompanied the float along the parade route, was thrilled to see spectators drawing their grandchildren’s attention to the pictures on the float and connecting them to the written words for these, which were posted on the float in both O’odham and English.

This was the first year Kitt Peak has entered a float in the rodeo parade. BaoBan brought good luck and the KPNO team was happy to learn that their float received first place in the business category. Congratulations to the team and thank you BaoBan!