New DESI Data Shed Light on Gravity’s Pull in the Universe

A University of Texas at Dallas physicist and his international colleagues in the Dark Energy Spectroscopic Instrument (DESI) collaboration are engaged in a multiyear data-gathering mission to try to answer one of the most puzzling observations in astrophysics: Why does the expansion of the universe appear to be accelerating?

Competing theories have attempted to explain this observation. One is that dark energy is somehow pushing galaxies apart. A second theory posits that gravity, the attractive force that in local environments like the solar system draws objects together, works differently at large cosmological scales and needs to be modified to explain cosmic acceleration.

The DESI collaboration, which includes more than 900 scientists from over 70 institutions, has released a new analysis of its data that weighs in on the second theory. The data reveal that the way in which galaxies cluster is consistent with the standard model of gravity: Albert Einstein's general theory of relativity, which also describes how objects fall under gravity and planets orbit the sun.

The analysis provides the most precise test to date of how gravity behaves at very large scales by tracing how cosmic structure grew over the past 11 billion years.

Scientists in the collaboration shared their results in several papers posted Nov. 19 to the arXiv, an online repository of scientific articles not yet peer reviewed.

Dr. Mustapha Ishak-Boushaki, a professor of physics in the School of Natural Sciences and Mathematics at UT Dallas, co-led the DESI working group that interpreted the cosmological data, and he is lead author of the paper that presents a detailed analysis of testing gravity at cosmic scales.

"For this round of DESI results, I focused my efforts at UT Dallas on conducting a large part of the analysis on gravity, which puts constraints on how matter in the universe moves and how large-scale structures, such as clusters of galaxies, evolve," said Ishak-Boushaki, an astrophysicist whose research career has focused on questions in cosmology. "The results from DESI, combined with datasets from other experiments, are consistent with general relativity theory operating at cosmic scales, although they do not completely exclude other theories of modified gravity."

Ishak-Boushaki is slated to present the cosmology results with other researchers on behalf of the DESI collaboration in January at a meeting of the American Astronomical Society in National Harbor, Maryland.

The new results provide an extended analysis of DESI's first year of data, which in April contributed to the largest 3D map of the universe to date and revealed hints that dark energy might be evolving over time.

"The latest analysis is also consistent with our previous findings that give preference to the theory that dark energy is not constant, but dynamic, which is a very important result for cosmic acceleration," Ishak-Boushaki said.

The DESI experiment can capture light from 5,000 galaxies simultaneously. The latest analysis used data from nearly 6 million galaxies and quasars, and lets researchers see up to 11 billion years into the past. With just one year of data, DESI has made the most precise overall measurement of the growth of structure in the universe, surpassing previous efforts that took decades to make.

The latest analysis also provided new upper limits on the mass of neutrinos, the only fundamental particles whose masses have not yet been measured precisely.

The new analysis broadens the scope to extract more information from the data, measuring how galaxies and matter are distributed on different scales throughout space. Like the previous study, it used a technique to hide the result from the scientists until the end, mitigating any unconscious bias.

The DESI experiment is now in its fourth of five years surveying the sky, and researchers plan to collect data from roughly 40 million galaxies and quasars by the time the project ends. The collaboration is currently analyzing the first three years of collected data and expects to present updated measurements of dark energy and the expansion history of the universe in March at a meeting of the American Physical Society.

"The results from DESI, combined with datasets from other experiments, are consistent with general relativity theory operating at cosmic scales, although they do not completely exclude other theories of modified gravity."

Dr. Mustapha Ishak-Boushaki, a professor of physics in the School of Natural Sciences and Mathematics

DESI was constructed and is operated with funding from the Department of Energy (DOE) Office of Science and sits atop the National Science Foundation's (NSF) Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, which is operated by the NSF's NOIRLab. The DOE's Lawrence Berkeley National Laboratory manages the DESI experiment.

DESI is also supported by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Additional support for DESI is provided by the NSF; the Science and Technology Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the French Alternative Energies and Atomic Energy Commission; the National Council of Humanities, Sciences, and Technologies of Mexico; the Ministry of Science and Innovation of Spain; and by the DESI member institutions.

The DESI collaboration is honored to be permitted to conduct scientific research on I'oligam Du'ag (Kitt Peak), a mountain with particular significance to the Tohono O'odham Nation.