The way that nearly six million galaxies have clustered over 11 billion years lines up with what Einstein's theory of general relativity predicts, according to a new study of data from the Dark Energy Spectroscopic Instrument (DESI), involving UCL researchers.
The analysis, using the first year of data from DESI, an instrument on a mountaintop in Arizona, in the US, traces the growth of cosmic structure over most of the Universe's history, providing the most precise test to date of how gravity behaves at very large scales.
The result validates the leading model of the universe and limits possible theories of modified gravity, which have been proposed to explain the observed accelerated expansion of our universe.
The DESI collaboration involves more than 900 researchers from over 70 institutions around the world and is managed by the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab).
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. (Four other UCL staff and their early career associates, as well as scientists from six other UK universities, are also involved in DESI.)
With just one year of data, DESI has made the most precise overall measurement of the growth of the Universe's structure, surpassing previous efforts that took decades to make.
Professor Ofer Lahav (UCL Physics & Astronomy), a DESI collaborator and a member of its Executive Committee, said: "The clustering of DESI galaxies on huge cosmological scales spectacularly validates Einstein's theory of gravity, as well as telling us about dark matter and dark energy. Remarkably, the galaxy clustering also sets upper limits on the yet unknown mass of tiny particles called neutrinos, as their presence affects the growth of structure."
Previous neutrino experiments found that the sum of the masses of the three types of neutrinos should be at least 0.059 eV/c2. (For comparison, an electron has a mass of about 511,000 eV/c2.) DESI's results indicate that the sum should be less than 0.071 eV/c2, leaving a narrow window for neutrino masses.
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), who co-led the new analysis, 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."
The collaboration, including UCL co-authors, shared the results in several papers posted to the online repository arXiv. The complex analysis used nearly 6 million galaxies and quasars and lets researchers see up to 11 billion years into the past.
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The new results 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. The study required months of additional work and cross-checks. Like the previous study, it used a technique to hide the result from the scientists until the end, mitigating any unconscious bias.
Professor Dragan Huterer of the University of Michigan, co-lead of DESI's group interpreting the cosmological data, said: "Both our BAO results and the full-shape analysis are spectacular. This is the first time that DESI has looked at the growth of cosmic structure. We're showing a tremendous new ability to probe modified gravity and improve constraints on models of dark energy. And it's only the tip of the iceberg."
DESI was constructed and is operated with funding from the DOE Office of Science. DESI is mounted on the US National Science Foundation's Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory (a programme of NSF NOIRLab). The experiment is now in its fourth of five years surveying the sky and plans to collect 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.
DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Additional support for DESI is provided in the UK by the Science and Technology Facilities Council.
The DESI collaboration is honoured to be permitted to conduct scientific research on I'oligam Du'ag (Kitt Peak), a mountain with particular significance to the Tohono O'odham Nation.
