New cosmic map supports Einstein’s prediction about gravity

KEY POINTS 

• Albert Einstein’s prediction about how gravity behaves has been supported by an international team of researchers who studied how the force acts on cosmic scales.
• Dark Energy Spectroscopic Instrument (DESI) researchers found that the way galaxies cluster is consistent with our standard model of gravity and the predictions from Einstein's theory of General Relativity.
• A complex analysis of the first year of data from DESI provides one of the most stringent tests yet of General Relativity and how gravity behaves at cosmic scales.
• Looking at galaxies and how they cluster throughout time reveals how cosmic structure grows, which lets DESI test theories of modified gravity – an alternative explanation for our universe’s accelerating expansion.
• DESI is managed by the US Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab). UK involvement in DESI includes the University of Portsmouth, Durham University, and UCL as full member institutions, together with individual researchers at the universities of Cambridge, Edinburgh, St Andrews, Sussex and Warwick.  

Albert Einstein’s prediction about how gravity behaves has been backed by an international team of researchers who studied how the force acts on cosmic scales.

Scientists, including astrophysicists from the University of Portsmouth, used the Dark Energy Spectroscopic Instrument (DESI) to map how nearly six million galaxies cluster across up to 11 billion years of time.

Their complex analysis of DESI’s first year of data provides one of the most stringent tests yet of Einstein’s famous theory of General Relativity and how gravity behaves at cosmic scales.

Looking at galaxies and how they cluster throughout time reveals how the universe’s structure has grown.

This allowed DESI’s scientists to test theories of modified gravity – an alternative explanation for our universe’s accelerating expansion typically attributed to dark energy.

They found that the way galaxies cluster is consistent with our standard model of gravity and the predictions made by Einstein.

The result validates the leading model of the universe and limits possible theories of modified gravity, which have been proposed as alternative ways to explain unexpected observations such as the expansion of the universe.

Several UK universities were involved in DESI’s latest research findings including the University of Portsmouth, Durham University and University College London. 

The DESI collaboration shared their results in a number of papers posted to the online repository arXiv today.

Dr Seshadri Nadathur, Associate Professor at the University of Portsmouth’s Institute of Cosmology and Gravitation, led the group producing the new analysis. 

Dr Nadathur said: “The data we have gathered with DESI allows us to measure the subtle patterns in how galaxies cluster together. What is really exciting is that we can use these patterns not only to measure how fast the Universe has been expanding, but even test our understanding of gravity itself! So far General Relativity is holding up well, but we have seen some surprises with dark energy.”

Nathan Findlay, a PhD student at the University of Portsmouth, also led part of the work on quantifying some of the uncertainties in the analysis. He said: “The fact that we can learn about dark matter, dark energy, the history and fate of the Universe, even the correct theory of gravity – all these fundamental questions in physics – using this data from DESI is mind-blowing, really. It’s very exciting to be part of it.”

Dr Pauline Zarrouk, a cosmologist at the French National Center for Scientific Research (CNRS) working at the Laboratory of Nuclear and High-Energy Physics (LPNHE), co-led the new analysis. 

Dr Zarrouk, who was a postdoctoral researcher at Durham University’s Institute for Computational Cosmology, and is now an academic visitor in the institute, said: “General relativity has been very well tested at the scale of solar systems, but we also needed to test that our assumption works at much larger scales.

 “Studying the rate at which galaxies formed lets us directly test our theories and, so far, we’re lining up with what General Relativity predicts at cosmological scales.”

A detailed analysis of the DESI data, co-led by Durham University researchers Dr Willem Elbers and Professor Carlos Frenk, provided new upper limits on the mass of neutrinos, the only fundamental particles whose masses have not yet been precisely measured in the laboratory. 

Neutrinos influence the clustering pattern of galaxies ever so slightly but this can be measured with the quality of the DESI data. Neutrino laboratory experiments set a floor on the neutrino mass; remarkably, the distribution of galaxies in DESI sets a ceiling on this mass which is now very close to the floor, with a value of about a ten millionth of the mass of the electron. 

Durham University is a key member of the DESI collaboration and also designed and built the fibre optic system which funnels light onto DESI’s spectrograph. Durham scientists also carried out supercomputer simulations of the Universe, crucial for the interpretation of DESI’s data.

DESI team member Professor Carlos Frenk, of the Institute for Computational Cosmology, Durham University, said: “General Relativity is one of the most elegant and profound theories in Physics. That the Universe seems to conform to its precepts is truly remarkable, a testament to Einstein’s talent and to that of the astronomers who have devised methods to test it. 

“Equally remarkable is the insight that DESI has brought to the long-standing mystery of the neutrino mass. These are tiny elementary particles with very small masses but the force of gravity that they collectively produce affects how galaxies move and cluster in space. The unprecedented size and quality of the DESI dataset has made it possible to detect this tiny effect and this is very exciting for both cosmologists and particle physicists.”  

DESI contains 5,000 fibre-optic “eyes”, each of which can collect light from a galaxy in just 20 minutes. Researchers at UCL, also a key member of the DESI collaboration, helped design, assemble and build DESI’s optical corrector – six lenses, the largest 1.1m across, that focus light on to the “eyes”.

Today’s latest results also provide an extended analysis of DESI’s first year of data, which in April made the largest 3D map of our universe to date and revealed hints that dark energy might be evolving over time. 

The April results looked at a particular feature of how galaxies cluster known as baryon acoustic oscillations (BAO). The new analysis, called a “full-shape analysis”, broadens the scope to extract more information from the data, measuring how galaxies and matter are distributed on different scales throughout space. 

Like the previous study, today’s results used a technique to hide the result from the scientists until the end, mitigating any unconscious bias. 

DESI is a state-of-the-art instrument that can capture light from 5,000 galaxies simultaneously. It was constructed and is operated with funding from the DOE Office of Science. 

DESI sits atop the US National Science Foundation’s Nicholas U Mayall 4-metre Telescope at Kitt Peak National Observatory (a programme of NSF’s NOIRLab), in Arizona, USA. The experiment is now in its fourth of five years surveying the sky and plans to collect data on roughly 40 million galaxies and quasars by the time the project ends.

The collaboration is currently analysing the first three years of collected data and expects to present updated measurements of dark energy and the expansion history of our universe in spring 2025. DESI’s expanded results released today are consistent with the experiment’s earlier preference for an evolving dark energy, adding to the anticipation of the upcoming analysis. 

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