A new analysis of data collected over three years by the Dark Energy Spectroscopic Instrument (DESI) collaboration provides stronger evidence that dark energy evolves in unexpected ways.
Long thought to be a ‘cosmological constant’, it now seems dark energy evolves over time, something that was debatable in previous datasets.
Dr Mustapha Ishak-Boushaki, professor of physics at The University of Texas at Dallas, is co-chair of the DESI working group that interprets cosmological survey data gathered by the international collaboration, which includes more than 900 researchers from over 70 institutions around the world.
In April 2024, at a meeting of the American Physical Society, Ishak-Boushaki presented the collaboration by presenting analyses of its first year of data, which revealed the first hints that dark energy might be evolving over time.
Is dark energy weakening over time?
The new DESI data analysis, paired with other measurements, contributes to mounting indications that the impact of dark energy may be weakening over time.
This means that the Standard Model of how the Universe works might need to be updated.
These other measurements include the light left over from the dawn of the Universe (the cosmic microwave background, or CMB), exploding stars (supernovae), and how light from distant galaxies is warped by gravity (weak gravitational lensing).
The significance of new discoveries for the scientific community
The researchers said that so far, the preference for an evolving dark energy has not yet risen to a statistical significance of 5 sigma, the gold standard in physics that represents the threshold for a discovery.
However, different combinations of DESI data mixed with the CMB, supernovae and weak lensing measurements set the range from 2.8 sigma to 4.2 sigma.
“With a 4.2 sigma significance, I think we are getting to the point of no return,” Ishak-Boushaki said.
“In this new analysis, not only have we confirmed our previous findings that dark energy is likely evolving over time, but we are increasing their significance. The point that I find the most exciting is that the evidence is coming from different datasets.
“I’ve worked on the question of cosmic acceleration for 25 years, and my perspective is if the evidence continues to grow, and it is likely to, then this will be huge for cosmology and all of physics.”
Andrei Cuceu, a postdoctoral researcher at Berkeley Lab and co-chair of DESI’s Lyman-alpha working group, added: “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.
“It’s interesting and gives us more confidence to see that many different lines of evidence are pointing in the same direction.”
The role of DESI in advancing astrophysics exploration
DESI is one of the most extensive surveys of the cosmos ever conducted. The state-of-the-art instrument can capture light from 5,000 galaxies simultaneously.
The experiment is now in its fourth year surveying the sky and aims to measure roughly 50 million galaxies and quasars by the time the project ends.
DESI tracks dark energy’s influence by studying how matter is spread across the Universe. Events in the very early Universe left subtle patterns in how matter is distributed, a feature called baryon acoustic oscillations (BAO).
That BAO pattern acts as a standard ruler, with its size at different times directly affected by how the universe was expanding. Measuring the ruler at different distances shows researchers the strength of dark energy throughout history.
The new analysis uses data from the first three years of observations of nearly 15 million galaxies and quasars.
