The most precise measurement of our expanding universe

April 4, 2024
Written By:
Lauren Biron, Lawrence Berkeley National Laboratory
DESI has made the largest 3D map of our universe to date. Earth is at the center of this thin slice of the full map. In the magnified section, it is easy to see the underlying structure of matter in our universe. Image credit: Claire Lamman/DESI collaboration; custom colormap package by cmastro
DESI has made the largest 3D map of our universe to date. Earth is at the center of this thin slice of the full map. In the magnified section, it is easy to see the underlying structure of matter in our universe. Image credit: Claire Lamman/DESI collaboration; custom colormap package by cmastro

With 5,000 tiny robots in a mountaintop telescope, researchers can look 11 billion years into the past.

The light from far-flung objects in space is just now reaching the Dark Energy Spectroscopic Instrument, or DESI, enabling us to map our cosmos as it was in its youth and trace its growth to what we see today.

Now, using the largest 3D map of our cosmos ever constructed, the DESI collaboration has made the most precise measurements to date of how fast the universe has expanded throughout its history. Understanding how our universe has evolved is tied to how it ends, and to one of the biggest mysteries in physics: dark energy, the unknown ingredient causing our universe to expand faster and faster.

Researchers, including University of Michigan scientists, shared the analysis of their first year of collected data in multiple studies published on the open-access repository arXiv and in talks at the American Physical Society Meeting in the United States and the Rencontres de Moriond in Italy.

“DESI has mapped out the expansion history of the universe over the past 11 billion years to unprecedented accuracy. In doing so, it has provided new insights about the behavior of dark energy that causes the accelerated expansion today and whose physical nature remains a key mystery,” said U-M physicist Dragan Huterer. “In the years to come, DESI will be providing invaluable new information about how our universe works.”

DESI’s Hubble diagram plots a characteristic pattern – baryon acoustic oscillations, or BAO “bubbles” – at different ages of the universe. The amount of dark energy determines how fast the universe grows, and therefore the size of the bubbles. The solid line is how big Lambda CDM predicts the bubbles will be, while the dashed line shows the prediction from a different model where dark energy evolves with time. DESI will gather more data to determine which model is a better description of the universe. Image credit: Arnaud de Mattia/DESI collaboration

Our leading model of the universe is known as Lambda CDM. It includes both normal and dark matter (“cold dark matter,” or CDM) and dark energy (Lambda). Both matter and dark energy shape how the universe expands—but in opposing ways. Matter slows the expansion down, while dark energy speeds it up. The amount of each influences how our universe evolves. This model does a good job of describing a wide variety of cosmological observations.

However, when DESI’s first-year results are combined with data from other studies, there are some subtle differences with what Lambda CDM would predict. As DESI gathers more information during its five-year survey, these early results will become more precise, shedding light on whether the data are pointing to different explanations for the results we observe or the need to update our model. More data will also improve DESI’s other early results, which weigh in on the Hubble constant (a measure of how fast the universe is expanding today) and the mass of particles called neutrinos.

​​DESI’s overall precision on the expansion history across all 11 billion years is 0.5%, and the most distant epoch, covering 8 billion to 11 billion years in the past, has a record-setting precision of 0.82%. That measurement of our young universe is incredibly difficult to make, says U-M physicist Gregory Tarlé.

A simplified explanation of the different parts of DESI’s Hubble diagram. Image credit: Claire Lamman/DESI collaboration

“After over a decade of effort building DESI, it is gratifying to see the first cosmology results emerge,” he said. “I was taken by surprise by just how significant and intriguing the results are at this early stage of the project.”

Tarlé led the construction project for the 5,000 robots that placed optical fibers on the galaxies of interest while Huterer is co-coordinating the effort to produce constraints on the cosmological model from DESI data. U-M research scientist Michael Schubnell, the project’s focal plane scientist, is responsible for ensuring that the instrument performs optimally.

“Compared to most instruments on telescopes, DESI is extremely complex with many moving parts,” Schubnell said. “Because DESI is all about statistics, every minute counts.”

Multiple times a night, DESI’s 5,000 robotic positioners maneuver optical fibers into place to capture light from galaxies and extremely distant objects called quasars. The light is split into its colors (or wavelengths) by equipment called spectrographs, and features in those “spectra” can be used to find the speed and distance of galaxies rushing away from us.

Traveling back in time

DESI is an international collaboration of more than 900 researchers from over 70 institutions around the world, with the project managed by DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab). The instrument was constructed and is operated with funding from the DOE Office of Science, and sits atop the U.S. National Science Foundation’s Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a program of NSF’s NOIRLab.

Looking at DESI’s map, it’s easy to see the underlying structure of the universe: strands of galaxies clustered together, separated by voids with fewer objects. Our very early universe, well beyond DESI’s view, was quite different: a hot, dense soup of subatomic particles moving too fast to form stable matter like the atoms we know today. Among those particles were hydrogen and helium nuclei, collectively called baryons.

Tiny fluctuations in this early ionized plasma caused pressure waves, moving the baryons into a pattern of ripples that is similar to what you’d see if you tossed a handful of gravel into a pond. As the universe expanded and cooled, atoms formed and the pressure waves stopped, freezing the ripples in three dimensions and increasing clustering of future galaxies in the dense areas.. Billions of years later, we can still see this faint pattern of 3D ripples in the characteristic separation of galaxies, a feature called Baryon Acoustic Oscillations.

Researchers use the BAO measurements as a cosmic ruler. By comparing the apparent size of these bubbles, they can determine distances to the matter responsible for this extremely faint pattern in the sky. Mapping the BAO bubbles both near and far lets researchers slice the data into chunks, measuring how fast the universe was expanding at each time in its past and modeling how dark energy affects that expansion.

Using galaxies to measure the expansion history and better understand dark energy is one technique, but it can only reach so far. At a certain point, light from typical galaxies is too faint, so researchers turn to quasars, extremely distant, bright galactic cores with black holes at their centers. Light from quasars is absorbed as it passes through intergalactic clouds of gas, enabling researchers to map the pockets of dense matter and use them the same way they use galaxies.

Researchers used 450,000 quasars, the largest set ever collected, to extend their BAO measurements all the way out to 11 billion years in the past. By the end of the survey, DESI plans to map 3 million quasars and 37 million galaxies.

Beyond dark energy

DESI is the first spectroscopic experiment to perform a fully “blinded analysis,” which conceals the true result from the scientists to avoid any subconscious confirmation bias. Researchers work in the dark with modified data, writing the code to analyze their findings. Once everything is finalized, they apply their analysis to the original data to reveal the actual answer.

U-M physics research fellow Uendert dos Santos Andrade helped develop and validate a methodology designed to mitigate confirmation bias. He co-led the paper validating this approach.

“Our strategy, which involves ‘blinding’ our cosmological analysis to experimenter biases, has proven successful in maintaining the integrity of our findings,” Andrade said. “The essence of our scientific findings revolves around the enigmatic role of dark energy in the universe, i.e., as the driver of cosmic acceleration. Participating in DESI, which gathers unprecedentedly powerful data, both in terms of amount and detail, has been a profoundly inspiring experience for me as I am a part of a project that may unravel one of the greatest mysteries in science: dark energy.”

Sikandar Hanif, U-M graduate student in physics and a co-author of the studies, used machine learning methods to help create realistic simulations or mock data used to test the DESI analysis pipeline that is then used on real data.

“As a real, physical instrument, DESI is subject to tangible constraints, such as the size of its 5,000 fibers and the fact that two or more galaxies in the sky might be closer together than two fibers to observe them both can realistically be placed,” Hanif said. “Machine learning gives us a fast way of taking such ‘fiber assignment’ into account for realistic mock data by quantifying how the observation of each target is affected.”

By cataloging the cosmos so precisely, DESI is a powerful tool for studying dark energy across time. That fidelity is also useful for studies of dark matter, the mass of neutrinos, and how individual galaxies develop over time. It also provides essential information about the dust surrounding our own Milky Way galaxy, which influences many of the other astronomy measurements.

“One of the most exciting things is how close we are to learning something new about neutrinos. These particles are so light and interact so weakly with everything else, but still, in cosmic scales they can affect the expansion of the universe,” said Otávio Alves, U-M graduate student in physics and co-author of the studies. “DESI is characterizing that expansion with such accuracy that it helps us set tight limits on how massive neutrinos can be based on cosmological data, and that information is complementary to what particle physics experiments can currently provide.”

Alves helped estimate the uncertainty of the group’s measurements.

“Correctly characterizing the instrumental and observational limitations that lead to those uncertainties is crucial, and most of my work has been showing that we can do that in a robust way,” Alves said. “With a proper assessment of our precision, we can now compare our results to others and judge how compatible they are.”

The Dark Energy Spectroscopic Instrument (DESI) making observations in the night sky on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory in Arizona. Image credit: KPNO/NOIRLab/NSF/AURA/T. Slovinský

U-M physics research fellow Johannes Lange co-leads a large working group within DESI that explores what researchers can learn from DESI in combination with other state-of-the-art cosmology experiments.

“What’s so exciting to me is to see years of work coming together and DESI delivering on its promise to give us new insights into the properties of dark energy,” Lange said. “These additional ways to investigate this amazing data set together with many years of DESI data still to be analyzed gives us a lot to be excited about.”

U-M graduate students Jiaming Pan and Tianke Zhuang and research fellow Minh Nguyen contributed to the DESI cosmological analysis as well, using cutting-edge statistical analysis techniques in order to tease out cosmological constraints from DESI data.

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

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