Making the Largest 3D Map of the Universe with the Dark Energy Spectroscopic Instrument

April 4, 2024

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Researchers can now explore 11 billion years into the past using a mountaintop telescope equipped with 5,000 tiny robots. This is possible as the light from distant astronomical objects just now reaches the Dark Energy Spectroscopic Instrument (DESI), allowing us to depict the universe as it was in its early stages and track its evolution to the present day.

This knowledge about the universe's evolution directly influences the understanding of its possible end, and dark energy, an unknown factor causing the increasing acceleration of the universe's expansion.

DESI, to probe the impacts of dark energy over 11 billion years, has generated the most comprehensive 3D map of the universe, boasting the most accurate measurements ever recorded. The existence of the universe's early expansion has been verified with an improved precision rate of less than 1%, offering the finest representation of the universe’s evolution till now.

Researchers will present the assessment of collected data from the initial year via various papers on the arXiv preprint server, alongside discussions at key scientific gatherings in the United States and Italy.

According to Michael Levi, the DESI director and a scientist at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), their data sets have enabled top-tier cosmology results and are among the first outputs from the new wave of dark energy experiments.

He states that the data has lined up with their best model of the universe and suggest some intriguing differences implying that dark energy could be evolving over time. This new anomaly could dissipate with further data refining, hence they are keen to analyze their set of data for the next three years soon.

The prevalent model of the universe, Lambda CDM, incorporates both weakly interacting matter (cold dark matter) and dark energy (Lambda). Both factors manipulate the universe's expansion in different ways. Matter and dark matter contract the expansion whereas dark energy fuels its acceleration. Each element's presence affects the universe's evolution. This model depicts the results from previous tests and the universe's change over time effectively.

However, blending the first-year DESI findings with other studies indicates subtle deviations from the Lambda CDM predictions. As DESI accumulates more data through its five-year survey, the preliminary results will gain increased precision, revealing whether this data suggests an anomaly in the observable results or if our model needs to be updated.

Additional data will aid in improving other initial DESI results, such as determining the Hubble constant (the current rate of the universe's expansion) and weighing neutrino particles.

Nathalie Palanque-Delabrouille, a Berkeley Lab scientist and co-spokesperson for the project, mentioned that no spectral study has had this amount of data before, and the data collection process, concerning over a million galaxies monthly, is continually going on.

She added that their first-year data readings enable them to measure seven different segments of the universe's expansion history, each with 1-3% precision. A significant effort was given to resolving the complexities of instrumental and theoretical modeling, which now assures them about the robustness of their primary results.

DESI plasticized the entire 11 billion years of the universe's expansion history with only a 0.5% error rate, and a record-setting 0.82% precision for the epoch ranging from 8-11 billion years ago.

Within a year, DESI doubled its measuring capabilities at these early timelines compared to its predecessor, the Sloan Digital Sky Survey's BOSS/eBOSS, which had taken over ten years.

High Energy Physics associate director at DOE, Gina Rameika, expressed her delight over DESI's first-year performance, as it has produced an impressive output and significantly contributed to understanding the universe.

Operating at U.S. National Science Foundation's Nicholas U. Mayall 4-meter Telescope at the Kitt Peak National Observatory under NSF's NOIRLab, DESI has facilitated an international partnership of more than 900 researchers from over 70 institutions worldwide.

When examining the map created by DESI, one can easily discern the fundamental structure of the universe: clusters of galaxies intertwined like threads, punctuated by gaps where fewer celestial objects are located. This contrasts heavily to the early stages of our universe, which is out of DESI's observational range, characterized by a dense, warm mishmash of swift subatomic particles, too quick to settle into the stable atoms we are familiar with in the present. Among such particles were nuclei of hydrogen and helium, also referred to collectively as baryons.

Small variances in the early ionized plasma sparked off pressure waves, arranging the baryons in a ripple pattern akin to the water ripples one would observe after tossing gravel into a pond. As the universe cooled and expanded, neutral atoms started to form and the pressure waves ceased, thereby solidifying the ripples in three dimensions and augmenting the clustering of future galaxies in denser areas.

Even billions of years later, one can still observe this faint three-dimensional ripple or bubble pattern in the typical spacing between galaxies, a phenomenon termed as Baryon Acoustic Oscillations (BAOs).

BAO measurements serve as a cosmic yardstick for researchers. By determining the apparent size of these bubbles, they can gauge the distance to the matter that forms this extremely light pattern in the sky. Plotting BAO bubbles both in close and farther proximity allows researchers to segment the data and figure out the rate at which the universe expanded at each point in its history and form models on how such expansion is impacted by dark energy.

Hee-Jong Seo, a professor at Ohio University and the co-founder of DESI's BAO analysis claims that, 'We’ve clocked the expansion history over a remarkably broad timeline with a level of precision that exceeds the collective precision of all prior BAO surveys. These new measurements will likely enhance and redefine our understanding of the cosmos. The fascination with our universe is age-old and deeply entrenched in the human psyche, sparking a curiosity about its composition and future.'

Although using galaxies to document the universe's expansion history and explore dark energy better is a technique, it has its limitations. Beyond a certain point, the light emitted by typical galaxies is too weak, hence researchers switch to quasars, which are highly luminous galactic cores located extremely far away, harboring black holes at their nuclei. The light from quasars is absorbed as it traverses intergalactic gas clouds, allowing researchers to chart dense matter pockets and use them in the same way as galaxies, a technique recognized as using the 'Lyman-alpha forest.'

Andreu Font-Ribera, a scientist at Spain's Institute for High Energy Physics (IFAE) and a co-leader of DESI's Lyman-alpha forest analysis, explained that, 'We utilize quasars akin to backlights to essentially observe the shadow cast by the intervening gas that lies between the quasars and us. This allows us to observe farther back to when the universe was in its infancy. This is indeed a difficult measurement to make and we find it incredibly thrilling to see it succeed.'

The researchers harnessed a record-breaking collection of 450,000 quasars for these Lyman-alpha forest measurements, enabling them to extend their BAO measurements till 11 billion years into the past. DESI aims to map 3 million quasars and 37 million galaxies by the end of the survey.

Regarding state-of-the-art science

DESI is the inaugural spectroscopic experiment to carry out a fully 'blinded analysis,' which safeguards against subconscious confirmation bias by not revealing the real result to the scientists. Researchers operate in the dark, using modified data and coding the analysis for their findings. Once everything is cemented, they apply the analysis to the authentic data to uncover the actual answer.

'The methodology we used for the analysis instills confidence in our results, particularly demonstrating the efficacy of the Lyman-alpha forest as a tool to measure the expansion of the universe,' expressed Julien Guy, a scientist at Berkeley Lab and co-leader responsible for processing information from DESI's spectrographs. 'The dataset we have is incredible, and it is being amassed at an impressive speed. This is the most precise measurement I have ever accomplished.'

Data from DESI will complement future sky surveys like the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope, and lay the groundwork for a potential DESI upgrade (titled DESI-II) that was recommended in a recent report by the U.S. Particle Physics Project Prioritization Panel.

Arnaud de Mattia, a researcher from the French Alternative Energies and Atomic Energy Commission (CEA) and co-leader of DESI’s team interpreting the cosmological data, commented, 'We are in the golden age of cosmology, with large-scale surveys in progress or about to commence and new techniques being fine-tuned to optimally utilize these datasets. We are all strongly motivated and excited to see whether fresh data will affirm the features we noted in our initial year sample and provide a richer understanding of the dynamics of our universe.'

Journal information: arXiv

Provided by Lawrence Berkeley National Laboratory