The Dark Energy Spectroscopic Instrument (DESI), which involves UCL researchers, has released its most detailed analysis yet of dark energy, the mysterious force driving the universe's accelerating expansion.
Using data from 15 million galaxies and quasars (extremely distant yet bright objects with black holes at their cores), DESI has produced the largest-ever 3D map of the universe, providing the most precise measurements of how cosmic structures have evolved over the past 11 billion years.
These findings strengthen hints that dark energy may not be constant over time, challenging the current standard model of cosmology.
The fate of the universe depends on the balance between matter and dark energy. The simplest model suggests that dark energy is constant over time, described by Einstein's equations in the Lambda Cold Dark Matter model.
However, when DESI's new data is combined with other cosmic observations - including the light leftover 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 lensing) - scientists find growing evidence that dark energy's influence may be changing over time.
DESI is an international experiment with more than 900 researchers from over 70 institutions around the world and is managed by the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab).
The collaboration shared its findings today on the online repository arXiv and in a presentation at the American Physical Society's Global Physics Summit in Anaheim, California.
Professor Ofer Lahav (UCL Physics & Astronomy), a DESI collaborator and a member of its Executive Committee, said: "Tracing the evolution of the dark energy concept over the past century is fascinating. It started with Einstein adding a number to his theory of general relativity to account for a static universe. He dismissed this number, known as a cosmological constant, when it became clear the universe was expanding. Then the idea was reclaimed to represent a constant dark energy driving the universe's accelerated expansion.
"Now, DESI observations, combined with other probes, suggest the intriguing possibility that the density of dark energy may be evolving with cosmic time. If confirmed, this would represent a paradigm shift in our understanding of the universe.
"If dark energy is constant, the universe would continue to expand at an accelerating rate forever. If it evolves with time, the fate of the universe is more uncertain."
Dr Paul Shah (Honorary Research Associate, UCL Physics & Astronomy), a member of the Dark Energy Survey supernovae team, said: "In Einstein's original equation, dark energy is a constant of nature that doesn't change with time or place.
"The pattern of galaxies observed by DESI suggests this isn't true, but on its own this isn't strong enough evidence. However, when this is paired with observations of supernovae from the Dark Energy Survey, in which UCL has also played a key role, we can build a history of how dark energy seems to be evolving with time. Pulling together evidence from multiple surveys in this way reduces the possibility that this is chance or misinterpretation of the data.
"If it is true that dark energy evolves with time, this would be one of the most significant discoveries in cosmology in a generation, as it implies something we don't yet understand is causing the change."
Professor Alexie Leauthaud-Harnett, co-spokesperson for DESI based at UC Santa Cruz, said: "What we are seeing is deeply intriguing. 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."
Taken alone, DESI's data is consistent with our standard model of the universe: Lambda CDM, where CDM is cold dark matter and lambda represents the simplest case of dark energy, where it acts as Einstein's cosmological constant.
However, when paired with other measurements, there are mounting indications that the impact of dark energy may be weakening over time and other models may be a better fit.
Professor Will Percival, co-spokesperson for DESI based at the University of Waterloo, said: "We're guided by Occam's razor [the principle that the simplest explanation is the best], and the simplest explanation for what we see is shifting.
"It's looking more and more like we may need to modify our standard model of cosmology to make these different datasets make sense together - and evolving dark energy seems promising."
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DESI is one of the most extensive surveys of the cosmos ever conducted. The state-of-the-art instrument was constructed and is operated with funding from the DOE Office of Science.
DESI contains 5,000 fibre-optic "eyes", each of which can collect light from a galaxy in just 20 minutes. Professors Peter Doel and David Brooks at UCL Physics & Astronomy helped design, assemble and build the DESI's optical corrector - six lenses, the largest 1.1m across, that focus light on to the "eyes". The optical corrector construction was supported by STFC.
DESI is mounted on the U.S. National Science Foundation's Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory (a program of NSF NOIRLab) in Arizona. The experiment is now in its fourth of five years surveying the sky, with plans to measure roughly 50 million galaxies and quasars by the time the project ends.
The new analysis uses data from the first three years of observations and includes nearly 15 million of the best measured galaxies and quasars. It's a major leap forward, improving the experiment's precision with a dataset that is more than double what was used in DESI's first analysis, which also hinted at an evolving dark energy.
