Dark Energy May Evolve in Surprising Ways, Scientists Say

Key highlights:

• The latest analysis from the Dark Energy Spectroscopic Instrument (DESI), which uses three years of data, indicates that dark energy could be changing over time
• This DESI data analysis challenges previous scientific thinking that the Universe's dominant dark energy component can be described as a "cosmological constant"
• The DESI collaboration has publicly released the first 13 months of data from its main survey - a treasure trove that will help other researchers investigate big questions in astrophysics
• DESI is an international collaborative experiment involving more than 900 researchers from over 70 institutions around the world, including astrophysicists from the University of Portsmouth

New results from the Dark Energy Spectroscopic Instrument (DESI) collaboration hint that dark energy, widely thought to be a "cosmological constant", might be evolving over time in unexpected ways. Dark energy is responsible for the accelerating expansion of the Universe, but the new results show its impact might now be weakening.

Several UK universities were involved in DESI's latest research findings including the University of Portsmouth , Durham University and University College London, along with individual researchers at the universities of Cambridge, Edinburgh, St Andrews, Sussex and Warwick.

The researchers used data collected from 2021 to 2024 to make the largest 3D map of the Universe ever, which they have then used to track dark energy's influence over the past 11 billion years.

Seshadri Nadathur , from the University of Portsmouth's Institute of Cosmology and Gravitation (ICG) and co-chair of DESI's Galaxy and Quasar Clustering working group, said: "This is a very important and exciting result. It's not just that the data continues to show a preference for evolving dark energy, but that the evidence is stronger now than it was.

"We've performed many additional tests compared to the first year, and they're making us confident that the results aren't driven by some unknown effect in the data that we haven't accounted for."

Understanding how the Universe has evolved is tied to how it ends, and to two of the biggest mysteries in physics: dark matter, which makes up most of the mass of the Universe and dark energy, the unknown ingredient causing the Universe to expand faster and faster.

Alexie Leauthaud-Harnett , co-spokesperson for DESI and a professor at UC Santa Cruz, said: "It is exciting to think that we may be on the cusp of a major discovery about dark energy and the fundamental nature of our Universe."

"We're in the business of letting the Universe tell us how it works, and maybe the Universe is telling us it's more complicated than we thought it was," added Andrei Cuceu , a postdoctoral researcher at Berkeley Lab and co-chair of DESI's Lyman-alpha working group. "It's interesting and gives us more confidence to see that many different lines of evidence are pointing in the same direction."

"We're guided by Occam's razor, and the simplest explanation for what we see is shifting," said Will Percival, co-spokesperson for DESI and a professor at the University of Waterloo, who was also formerly professor at the University of Portsmouth.

DESI is a state-of-the-art instrument that can capture light from 5,000 galaxies simultaneously. It uses 5,000 tiny robots mounted within a mountaintop telescope, near Tucson, Arizona, and allows researchers to look billions of years into the past.

The DESI collaboration's findings have been shared in multiple papers that will be posted on the free distribution service arXiv and in a presentation at the American Physical Society's Global Physics Summit in Anaheim, California.

The DESI collaboration experiment is now in its fourth of five years surveying the sky, with plans to measure roughly 50 million galaxies and quasars. (the blazing centres of active galaxies powered by a supermassive black hole feeding on huge quantities of gas).

Alongside unveiling its latest dark energy results from Data Release 2, the DESI collaboration also announced that its Data Release 1 (DR1) is now available for anyone to explore. The dataset includes millions of galaxies mapped during the first 13 months of DESI's main survey.

DR1 contains a staggering 270 terabytes of information, surpassing all previous 3D spectroscopic surveys combined.

Professor Robert Crittenden , from the University of Portsmouth's Institute of Cosmology and Gravitation , said: "This is a remarkable milestone which could help us to discover the nature of dark energy, as well as enable researchers all around the world, including here in Portsmouth, to test many other theories about how the Universe came to be."

The full set of DR1 files is free and available to access through the National Energy Research Scientific Computing Center ( NERSC ), a facility at Berkeley Lab where DESI processes and stores data. Space fans can also explore some of DESI's data through an interactive portal: the Legacy Survey Sky Browser.