DESI's 3D map more precisely measures the expanding universe

Understanding how our universe has evolved is tied to one of the biggest mysteries in physics: dark energy, the unknown ingredient causing our universe to expand faster and faster. To study dark energy’s effects over the past 11 billion years, the Dark Energy Spectroscopic Instrument (DESI) has created the largest 3D map of our cosmos ever constructed, with the most precise measurements to date. This is the first time scientists have measured the expansion history of the young universe with a precision better than 1%, giving us our best view yet of how the universe evolved.

DESI is the result of an international collaboration of more than 900 researchers around the world and analyses the spectra, or colours, of light from galaxies and extremely distant objects called quasars. The instrument, which sits on a mountaintop telescope in Arizona, contains 5000 fibre-optic ‘eyes’, each of which can image a galaxy in just 20 minutes. Researchers shared the analysis of their first year of collected data in multiple papers and presentations earlier this month; by the end of its five-year survey, DESI plans to map 3 million quasars and 37 million galaxies.

“We’re incredibly proud of the data, which have produced world-leading cosmology results and are the first to come out of the new generation of dark energy experiments,” said DESI Director Michael Levi, a scientist at the US Department of Energy’s (DOE) Lawrence Berkeley National Laboratory, which manages the project. “So far, we’re seeing basic agreement with our best model of the universe, but we’re also seeing some potentially interesting differences that could indicate that dark energy is evolving with time. Those may or may not go away with more data.”

The leading model of the universe, known as Lambda CDM, includes both a weakly interacting type of matter (cold dark matter, or CDM) and dark energy (Lambda, in its simplest version). It posits that both ordinary matter and cold dark matter slow the expansion of the universe down, via the attractive force of gravity. While the nature of dark energy is still a mystery, its presence leads to a repulsive force, speeding up the expansion of the universe. The amount of each influences how our universe evolves. This model does a decent job of describing results from previous experiments and how the universe looks throughout time.

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 observed or the need to update the current 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.

“No spectroscopic experiment has had this much data before, and we’re continuing to gather data from more than a million galaxies every month,” said Nathalie Palanque-Delabrouille, a Berkeley Lab scientist and co-spokesperson for the experiment. “It’s astonishing that with only our first year of data, we can already measure the expansion history of our universe at seven different slices of cosmic time, each with a precision of 1 to 3%. The team put in a tremendous amount of work to account for instrumental and theoretical modelling intricacies, which gives us confidence in the robustness of our first results.”

DESI’s overall precision on the expansion history across all 11 billion years is 0.5%, and the most distant epoch, covering 8–11 billion years in the past, has a record-setting precision of 0.82%, which is incredibly difficult to make. Within just one year, DESI has become twice as powerful at measuring the expansion history of these early times as its predecessor (the Sloan Digital Sky Survey’s BOSS/eBOSS), which took more than a decade.

Here in Australia, researchers at The University of Queensland (UQ) were responsible for developing a key piece of software for the project that models the size and shape of the Baryon Acoustic Oscillation (BAO), a remnant of soundwaves from the early universe. As explained by UQ’s Dr Cullan Howlett, “The BAO’s size acts as a standard ruler, and by comparing its size at different distances from Earth to how big it should have been in the early universe, we can measure the expansion rate of the universe.”

The 3D map comprises the spatial coordinates and distances of millions of galaxies. Researchers can measure the longitudinal and latitudinal position of each galaxy, as well as its unique light ‘fingerprint’ — observed by measuring the presence of chemical elements like hydrogen, oxygen and nitrogen. Howlett stated, “We decoded that fingerprint, identified the individual elements, and compared the measured frequencies to those in a lab on Earth to get the distance from us.

“Once we had millions of sky positions and distances, we put each galaxy at its location relative to earth and built a literal 3D map to analyse.”

Collaborator Dr Ryan Turner, from Swinburne University of Technology, meanwhile played a critical role developing statistical tools for measuring the motions of galaxies in the local universe. He developed software that can capture the information hidden in galaxy motions, to learn about the laws of gravity and calculate the rate at which the universe’s largest structures form.

“This is like going from a hand-drawn map of the universe to a satellite image,” Turner said.

“With our much more detailed map, we can picture a larger area and now have significantly more detail in those farther away places and a much more detailed understanding of the structure in our patch of the universe.

“DESI’s new data greatly surpasses all previous surveys of its kind, and with better maps comes better understanding of some of the universe’s most enduring questions.”

Emmanuel Schaan, a staff scientist at the SLAC National Accelerator Laboratory who works on both DESI and the Atacama Cosmology Telescope (ACT), said he is looking forward to collaborations made possible by the amount of data coming in and the large area of the sky DESI covers.

“DESI and ACT both observe large parts of the sky, resulting in a large overlap; we can therefore look for correlations between the galaxy catalogues of DESI and the microwave maps of ACT,” Schaan said. “These correlations reveal a wealth of new information that wouldn’t be seen in either DESI or ACT alone: the whole is greater than the sum of its parts.”

DESI’s data will similarly be used to complement future sky surveys such as the Vera C. Rubin Observatory and Nancy Grace Roman Space Telescope, and to prepare for a potential upgrade to DESI (DESI-II) that was recommended in a recent report by the Particle Physics Project Prioritization Panel.

“I am very excited to see the most precise measurements of spectroscopic galaxy clustering ever made, and what they tell us about cosmology,” Schaan said. “In particular, as hints of mild tensions have emerged between different cosmological observables, the DESI first-year measurements will be helpful in assessing consistency of different cosmological methods.

“Beyond the cosmological parameter measurements, the unprecedentedly large galaxy catalogues themselves will be extremely valuable to the whole astrophysics community.”

Image caption: Earth is at the centre of this thin slice of the full 3D map; the magnified section shows the underlying structure of matter in our universe. Each dot is a different galaxy similar in size to our own Milky Way. Image credit: Claire Lamman/DESI collaboration; custom colourmap package by cmastro.