blog

By: Bela Abolfathi

June 1st, 2021

On May 17th,  DESI officially began a 5-year survey of the cosmos to study dark energy and its role in the accelerated expansion of the universe. Over the course of its survey, DESI will collect spectra from over 30 million galaxies across 11 billion years, a feat that will result in the largest 3D map of the universe ever created.

Prof. David Kirkby at UC Irvine created an interactive 3D visualization that helps put into perspective exactly how ambitious of an undertaking this is. The visualization linked to the image below shows around 30,000 galaxies observed during the survey validation phase — less than 0.1% of the galaxies DESI will eventually catalog — jam-packed into the space behind your palm held at arm’s length.

Each of the four types of galaxies specifically targeted by DESI are denoted by a separate color and correspond to bright galaxies (BGS), luminous red galaxies (LRG), emission-line galaxies (ELG), and quasars (QSO). These types populate different redshift regimes, as seen from the progression of colors over the course of the movie. That’s not to say that types are confined to select regions of redshift-space. Rather, they are targeted in this way because, once they’ve been redshifted to our telescope, their unique signatures fall into the 360-980 nanometer window registered by our detectors. In the case of high-redshift quasars, these objects are selected because they help reveal the distribution of matter in the intergalactic medium via the shadows they cast along their journey.

Like the opening crawl at the beginning of every Star Wars movie, the visualization starts in the nearby universe and takes us back in time to distances far, far away. Exactly how do we determine these distances? We start with each galaxy’s redshift, which we can measure from its observed spectrum. Both the composition and curvature of the universe will determine how fast it’s expanding, and by extension any notion of distance. We therefore need to assume a particular cosmological model in order to then transform from redshift to comoving distance.

The visualization defaults to using the fiducial values of 31% matter, 69% dark energy, and zero curvature. Changing these parameters will have a noticeable effect on the distance traveled as well as the age of the universe.

One of DESI’s main cosmological probes is baryon acoustic oscillations (BAO), the imprints of relic sound waves from the early universe. BAO is a standard ruler used to measure the scale at which galaxies tend to cluster. The concentric circles emanating from the center of the 3D visualization show the comoving 150 Mpc BAO scale at which we expect galaxies to cluster slightly more often compared to a random distribution of galaxies. 

Michael Levi, DESI Director

May 19, 2021

On the eve of the official start of the DESI science survey, I wanted to take a minute to reflect on how we arrived at this exciting moment.   We’ve now proven that we can obtain 100,000 science-quality spectra in a single night, so we are ready!  It has been quite a long and at times bumpy road to get here, but it has always been an honor to be part of this team.  Of course, the ground was prepared by SDSS-BOSS, which established BAO as a precision probe of dark energy and created a desire to go beyond plug-plates.  The seeds of this project germinated in 2008 out of the DOE/NASA JDEM space mission competition, when Saul Perlmutter and I were co-PI’s of the SNAP/JDEM proposal.  The competing JDEM concepts led to the realization that a lot of the proposed BAO science could be performed from the ground, and this in turn led very quickly to a new concept for a ground-based spectroscopic survey.  A rapid-fire set of ad hoc proposals and public presentations to DOE and NSF agency committees ensued as a core formed around this idea.

 At the 2009 Particle Astrophysics Scientific Assessment Group (a subpanel of the NSF/DOE HEPAP Federal Advisory Committee), Nikhil Padmanabhan (then an LBNL Chamberlain Fellow), gave a presentation, which resulted in a recommendation to support the R&D.  David Schlegel presented the BigBOSS concept to the 2010 Decadal Survey, which recommended mid-scale projects, explicitly calling out ground-based spectroscopic surveys as a compelling area of interest.  In early 2011, two unsolicited proposals were submitted to DOE, for BigBOSS led by LBNL, and for DESpec a competing concept led by FNAL.  This prompted the formation in mid-2011 of a dark energy community planning panel, chaired by Rocky Kolb, to advise the agency.  The panel identified a wide-field spectroscopic survey as a key project that would meet the needs of the dark energy community.  DOE agreed and, in the year following, approved “Mission Need” (aka CD-0) for a Mid-Scale Dark Energy Spectroscopic Instrument (MS-DESI) on Sept 18, 2012. 

DOE didn’t make a down-select between BigBOSS and DESpec.  Instead, they assigned the management of the new MS-DESI project to Berkeley Lab on Dec 12, 2012 and charged us to form a new collaboration.  I was appointed Project Director the next week and given the task to figure it out and come back with a plan.  Fortunately, we had just received a $2.1M grant from the Gordon and Betty Moore Foundation for a spectroscopic survey (followed a bit later by a similar grant from the Heising-Simons Foundation) and this provided the financial resources for us to get started building the first spectrograph and initiating the long-lead acquisition of the corrector lenses.   At the same time, we began to hold collaboration meetings and created a “big tent” where all were welcome, and the DESI collaboration grew to include most of the institutions of the original proposals, plus many additional recruits.

 A year later, in 2013, I presented the new plan and the newly formed science goals to the Particle Physics Project Prioritization Panel (P5), a subpanel of the NSF/DOE HEPAP Federal Advisory Committee.  The P5 report, issued in mid-2014, recommended MS-DESI only in an optimistic budget scenario.  Fortunately, budgets did align sufficiently for us to eke out a start in 2015 (albeit financially constrained) with CD-1 approval (site selection, and establishment of cost range) in March 2015 and with Congressional authorization in the appropriations language that year.  DESI was then baselined in Sept 2015, construction started, and the rest is history as they say.  Although there were plenty of remaining bumps in the road during construction, I will save that for another time.

 It is a privilege for me to represent the collaboration and contribute to its continuing success, and I hope all our collaborators feel equally privileged.  The DESI collaboration has come together in this common quest to understand dark energy and the mysteries of the accelerating Universe.  Now the telescope and instrument are humming, twilight and science awaits.

by: Paul Martini, DESI Instrument Scientist

May 17, 2021

The discovery of cosmic acceleration over twenty years ago captured the imagination of the general public and professional scientists alike, and the cause of this acceleration remains one of the greatest, unsolved questions in science. Something mysterious, such as a new particle, a new force of nature, or some property of space itself, is overcoming the gravitational attraction of all of the mass in the universe. While we commonly call the origin of this acceleration ‘dark energy,’ naming it did not solve the mystery, nor have the last two decades of progressively larger and more sophisticated sky surveys. 

Enter the DESI survey. For the last decade, hundreds of scientists and engineers have worked to build a new instrument and design a survey capable of constructing the most precise measurement of cosmic expansion history ever. On Friday, May 14 this work came to fruition with the formal launch of survey observations. And over the next five years, DESI will measure approximately 35 million galaxies and quasars that extend across about 12 billion years of cosmic history. These observations will measure an order of magnitude more targets than the largest survey to date, and complete these observations in a small fraction of the time. 

DESI aims to survey about 14,000 square degrees of the night sky, which corresponds to most of the sky visible from Arizona. We will do this with over 15,000 unique observations that we call tiles. Each of these tiles is a unique configuration of the 5000 robotically-positioned fiber optic cables designed to collect the light of specific galaxies and quasars distributed across the 3.2 degree diameter field of view of the instrument. 

The image below shows a small fraction of this field of view that includes the nearby Coma cluster of galaxies, one of the largest concentrations of galaxies in the local universe. The circles in the bottom panel illustrates the high density of the spectroscopic observations obtained with a single tile. Nearly 10,000 of these tiles are designed for good to great conditions, such as when the Moon does not appreciably affect the darkness of the night sky. In these cases, we observe our faintest targets, including luminous red galaxies, emission-line galaxies, and quasars. The remaining tiles target brighter galaxies and Milky Way stars, sources that can be observed when the moon is brighter and there may be moderate cloud cover.

As the DESI survey will be so much larger than previous surveys, we anticipate measuring cosmic acceleration and other parameters with substantially smaller statistical errors. Yet in order to fully realize these gains, and not be limited by systematic effects, it is especially important for us to carefully plan all aspects of the experimental design and maximize the reproducibility of our work. One aspect of this is that we have built substantial software and hardware systems to help us to achieve the same sensitivity with all of our tiles, in spite of the fact that they will be observed on nights spread over five years, and obtained under nights with varying amounts of atmospheric turbulence or seeing, and varying amounts of cloud cover. Other aspects include superb and repeatable algorithms that handle every aspect of observing, ranging from the selection of potential targets, to the assignment of targets to the individual fibers, to the measurement of their properties. 

We have spent the last many months building, testing, and refining all of these aspects of the survey with increasingly sophisticated and extensive observations. During this Survey Validation phase, we have obtained over 1 million unique redshifts, which has already made DESI the second largest spectroscopic redshift survey in the world before it formally begins. Over the coming months, we expect that the number of new galaxies and quasars we measure will continue to expand, much like the universe itself!

by: Daniel Eisenstein

April 28th, 2021

On April 5, 2021, DESI turned to the second and final phase of its Survey Validation (SV) work, the so-called “1% survey”.  Whereas the first phase of SV was aimed at getting long and definitive observations of a broad superset of the planned spectroscopic targets, the 1% survey aims to demonstrate that we can operate the facility in a model closely matched to that planned for the 5-year survey.

The 1% survey is being conducted with target selection similar to what is planned for the main survey and with exposure times only mildly longer.  Importantly, we aim to observe each region of sky more times than in the main survey so that we achieve a higher fiber-assignment completeness, the fraction of targets that are assigned a fiber.

A single exposure with DESI provides about 600 fibers per square degree, but the survey target lists contain about 3500 targets per square degree for the dark/grey conditions (when the moon is small or set) and about 2000-2500 targets per square degree for bright conditions (when the moon is fuller).  In the main survey, we will return to a point on the sky numerous times to observe about 80% of the targets, but allowing some incompleteness so that we can efficiently move on to new regions.  In the 1% survey, we plan to return to each region at least 10 times for dark-time targets and 8 times for bright-time targets, each with a distinct target list, so as to observe all but a few percent of the targets.  

So far, we have started observations on 16 regions, each 7 square degrees and each including both dark-time and bright-time targets.  We expect to finish this program in May.  With this data set, we will be able to get a first estimate of the 3-dimensional clustering of the galaxy and quasar samples, which will then allow the collaboration to tune the models of simulated galaxy positions in our cosmological mock catalog.

With the observations moving at normal survey pace from one target set to the next, the rate of gathering new redshifts has increased dramatically.  In the first 8 nights, we acquired over 400,000 separate successful redshift measurements of 350,000 unique sources.

The plot shows a visualization of one of the 16 regions, which has 13 dark-time visits, showing a map of the galaxy locations in Mpc on the plane of the sky.  Only galaxies between redshift 0.90 and 0.95 are shown, with the luminous red galaxy targets plotted in red and the emission-line galaxy targets in blue.  One can see the cosmic web of large-scale structure, with walls, filaments, and voids, as well as the tendency of red galaxies to cluster together more than blue galaxies.  This is about 1 part in 20,000 of the final survey size!

by: Daniel Eisenstein

April 19th, 2021

Since mid-December, DESI has been intensively performing its Survey Validation (SV) observations.  With SV, we seek to optimize the 5-year survey design using on-sky observations of our target classes.  We of course based our designs on what was known before DESI, but DESI is a big enough step forward that one can’t be sure until one  verifies the performance with the instrument itself!

One of the challenges for Survey Validation is that we want to verify our survey plans with targets that are fainter and spread over wider areas of the sky than have previously been observed.  So we need to make our own truth tables by observing our targets with much longer exposures than we plan for the survey, so that the correct answers become obvious.  We then can split the observations into portions to see what a survey-length exposure would yield. 

From mid-December through the end of March, DESI observed 161 separate tiles as part of the first phase of its survey validation program, collecting over 1600 separate spectroscopic exposures.  We combined 4.3 million separate observations of 465,000 distinct examples of our target classes, with an average of 95 minutes per target, obtaining redshifts for 412,000 objects.  This is already one of the largest extragalactic spectroscopic data sets ever collected, including 108,000 redshifts above 0.8.

The whole DESI collaboration has mobilized to analyze these on-sky data, because rapid answers are needed to define the main survey.  The results have been very encouraging; we’ll share more examples in future blog posts.

By: Brian Bauer, Daniel Allspach, and Noah Franz

April 16th, 2021

We are a working group of three undergraduate students at Siena College. We started with DESI under Dr. John Moustakas in February 2020. We were intrigued by the opportunity to work with such a large collaboration with so many scientific possibilities. Since then, Noah Franz and Brian Bauer have focused on identifying strong gravitational lenses in DESI spectra by using Python to separate the source and lens galaxy spectra then analyzing the success. Daniel Allspach has been working to fit stellar templates to model DESI spectra continua and determine galactic demographics and outflow classifications. Contributing to these projects has provided us with invaluable insight and experience into working with large collaborations.    

After working with DESI, attending Zoom telecons, and learning how a collaboration functions, what we enjoyed most of all is the welcoming nature of everyone involved. Without this feeling of acceptance we would have found it difficult to integrate into a project effectively. Although receiving emails in the wee hours of the morning can be a little odd at first, we were able to quickly adapt and become accustomed to the practices and methods employed by DESI. Being included in any aspect of the project, especially as an undergraduate, is an honor, yet we were continually challenged to dive deeper and join as much as we can. A welcoming environment provides the perfect jumping off point for new members and that is truly the best part about the DESI collaboration.

With over 700 contributing members, DESI is a large collaboration, and, as undergraduate researchers we found entering the community of mostly graduate students and PhDs intimidating and overwhelming. While some of the DESI updates and zoom meetings can be daunting, after spending time reading the literature and learning the acronyms we became accustomed to the jargon. Once we surpassed this learning curve, we were able to start our own research projects. In many ways this was our introduction into large scale collaborative astronomy and cosmology: many things are new to us.  Having the DESI environment to help us orient and navigate this research environment has been incredibly helpful. DESI has given us a fantastic first experience not just with research but also working with a large collaboration. As a result, we all were inspired to continue research going into the future. 

By: John Moustakas 

February  3, 2021

 

Now, the typical temperature of these star-forming regions is a balmy 10,000 degrees Kelvin, hotter than the surface of the Sun! So how does the region get rid of this extra energy and cool down? Well, as atoms like oxygen whip around in random directions, they will occasionally crash into one another. When this happens, some of the kinetic energy of motion goes into “exciting” one of the atom’s electrons into a higher energy state. But the electron doesn’t stay excited very long!

After about a second, the excited electron spontaneously “jumps down” to a lower energy level, simultaneously emitting a photon of energy (i.e., light) in the process. Subsequently, this photon escapes from the star-forming region, robbing it of the original energy pumped in by the hot young star and helping it cool down. In the actively star-forming emission-line galaxies which DESI is hunting, we observe oxygen “shining” in this way at two close but distinct energies or wavelengths, 372.71 and 372.98 nanometers. (One nanometer is one billionth of a meter, so the wavelength of this light is about 170 times smaller than the width of human hair!)

This pair of lines is called the “oxygen two doublet” (the “two” confusingly means that the oxygen atom is missing one electron) and it is written using the symbol “[O II].” In December 2020, DESI began observations as part of its “Survey Validation” phase, and one important question this phase aims to answer is, “How efficiently will DESI be able to pre-select (or “target”) emission-line galaxies?” So far, the answer to this question has been a resounding, “Really well!” To illustrate the kind of tremendous data DESI is obtaining, Dr. Julien Guy of Lawrence Berkeley National Lab has created an animation of the [OII] doublet in a sample of roughly 3400 emission-line galaxies which DESI observed in December, a tiny fraction of the more than 35 million galaxies DESI will observe during its 5-year survey.

credit: DESI Collaboration

 

This movie shows the strikingly clear [OII] doublet in these galaxies, which brings DESI one step closer to being able to uncover the mystery of dark energy.

 

By: Jeremy Tinker and Zheng Zheng

January 19th, 2021

DESI member and noted rapper David Weinberg, from Ohio State University, was awarded the Dannie Heineman Prize for Astrophysics. He shares the award with Robert Lupton of Princeton University. The award celebrates the massive contributions both researchers have made to ushering in the era of large-scale three-dimensional mapping of the universe through the spatial distribution of the galaxies within it, primarily through their work with the Sloan Digital Sky Survey (SDSS).

As a project, DESI owes a debt of gratitude to SDSS. SDSS was the first truly large-scale galaxy redshift survey, using a CCD camera and fiber-fed spectrographs. With first light in 2000, its goal— realized in 2007 with the completion of SDSS-II— was to map nearly a quarter of sky by obtaining distances (redshifts) of the million brightest galaxies and quasars in the universe. In late 1980s, when the SDSS was merely an idea being batted around the Paris Conference Room of the Chicago O’Hare Hilton, David was a graduate student at Princeton, working with SDSS’s visionary founder Jim Gunn. He both literally and figuratively got started on the ground floor of large-scale spectroscopic surveys.

David became a full SDSS member in 1992, and went on to serve as the Scientific Spokesperson for SDSS-II, a position that demands a myriad of critical tasks related to the design, organization, promotion, and execution of the project. When the SDSS-III collaboration was created in 2008, David was chosen to be the Project Scientist. In recent years David has brought his expertise to the DESI collaboration, being one of the chief architects of the Bright Galaxy Survey component of the project as well as serving as the inaugural BGS working group co-chair. 

While a member of SDSS, David made invaluable contributions to survey design, galaxy target selection, and eventual analysis of the maps of cosmic structure. David was an early adopter and developer of the halo occupation model to describe the distribution of galaxies in space, who introduced the now commonly used term Halo Occupation Distribution (HOD). The model relates galaxies to clumps (halos) in the matter distribution in the universe. The SDSS data led to the pioneering application of the HOD framework to interpret the clustering of galaxies in space, with clustering trend naturally explained and galaxy-halo connection informatively inferred. The HOD model has ever since been widely adopted to analyze galaxy clustering data, becoming a powerful tool in learning about galaxy formation and cosmology and in creating simulated galaxy catalogs for various purposes in large galaxy surveys. 

In addition to mapping the universe with galaxies, David was also a pioneer in a novel method of determining the matter distribution of the universe: the Lyman-alpha forest. This method uses bright quasars as cosmological flashlights. Cool gas along the pathway to our telescopes absorbs some of the quasars’ light, leaving wiggles (Lyman-alpha forest) in the quasar spectra. Such wiggles reveal the spatial distribution of cool gas, which tracks dark matter. Thus, the spectra of quasars taken in SDSS and in DESI provide complementary maps of cosmic structure. 

As a scientist, David wears many hats in addition to survey astronomy. He has produced highly influential work on hydrodynamical simulations of galaxy formation, he has worked on the life cycles of active galactic nuclei, and more recently he has used SDSS’s spectra of stars to perform “chemical cartography” within our own Milky Way Galaxy. His 169-page review article on “Observational Probes of Cosmic Acceleration”, with nearly 900 citations, has become the standard reference in guiding our observational efforts toward revealing the nature of cosmic acceleration.

In addition to his research-oriented scientific pursuits, David has a long-standing collaboration with the MacArthur Award winning sculptor Josiah McElheny. Together, they have created cosmologically-inspired glass sculptures that have been exhibited all over the world. David’s role is in making the designs representative of the physics of the universe. For example, the piece An End to Modernity depicts the history of the universe from the Big Bang to the present day, emphasizing the evolution of cosmic structure and the epoch of galaxy formation.

Since starting at Ohio State in 1995, David has been advisor and mentor to 17 graduate students. Many of his former students, including the two of us, are themselves members of SDSS and DESI, and are using these data to mentor and train their own students, drawing on the lessons learned while working with David. Needless to say, David’s profound influence on cosmological redshift surveys will be felt for many academic generations. Congratulations to David for this well-deserved recognition of his continuing impact on astronomy, cosmology, and its presentation to the public.