��������

Key points:

An international team of researchers, including scientists from the ��������, has made the largest 3D map of the Universe and measured its expansion over 11 billion years.

The researchers used the Dark Energy Spectroscopic Instrument (DESI) to create the map and measure the effects of dark energy, the mysterious cause of the Universe’s accelerating expansion.

DESI uses 5,000 tiny robots mounted within a mountaintop telescope, near Tucson, Arizona, and allows researchers to look billions of years into the past.

Using light from far-flung objects in space, DESI’s scientists mapped the cosmos as it was in its youth and traced its growth to what we see today.

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.

To study dark energy’s effects over the past 11 billion years, DESI has created a map from over six million galaxies, the largest 3D map of our cosmos ever constructed, with the most precise measurements to date.

This is the first time scientists have measured the expansion history of the young Universe with a precision better than one per cent, giving the best view yet of how the Universe has evolved.

Researchers shared the analysis of their first year of collected data in multiple papers that will be posted today (Thursday 4 April 2024) on the arXiv open-access archive and in talks at the American Physical Society meeting, in the United States, and the Rencontres de Moriond, in Italy.

Dr Seshadri Nadathur from the Institute of Cosmology and Gravitation, at the ��������, who led parts of the analysis, and is one of the scientists presenting the results at the Moriond conference, said: “It’s been a major team effort and a huge amount of work to produce these amazing results. We’re already using data on over six million galaxies and quasars in just a single year of DESI operations – more than double what the previous largest survey managed over two decades!

“There is a lot more data still to come too, and seeing these tantalising results on dark energy is a good motivator to start those analyses soon.

The leading model of the Universe is known as Lambda CDM. It includes both a weakly interacting type of matter (cold dark matter, or CDM) and dark energy (Lambda).

Both matter and dark energy shape how the Universe expands – but in opposing ways. Matter and dark matter slow the expansion down, while dark energy speeds it up. The balance between the two determines how the Universe evolves.

The Lambda CDM model does a good job of describing results from previous experiments and how the Universe looks throughout time.

However, when DESI’s first-year results are combined with data from other studies, there are some subtle differences with what the model would predict.

Dr Nadathur added: “These results are very exciting, because there are some hints that the data don’t agree as well with the Lambda CDM model as we were expecting, which may be telling us something important about dark energy. It is too early to give a definitive answer just yet, but we’ve only analysed a small part of the DESI dataset so far! We’re looking forward to finding out if these hints of tensions become more severe when we add more data.”

As DESI gathers more information during its five-year survey, these early results will become ever more precise, tightening the current results and revealing whether there is a need to revise the standard model of our Universe.

More data will also improve DESI’s other early results, which weigh in on the Hubble constant (a measure of how fast the Universe is expanding today) and the mass of tiny fundamental particles called neutrinos.

The UK DESI institutions are key members of the collaboration and helped design and build the instruments – Durham University, the fibre optic cable system which funnels light onto the spectrograph and UCL the telescope’s mirror optical corrector. UK researchers have played a key role in the analysis, modelling and interpretation of the data. Professor Carlos Frenk, Ogden Professor in the Institute for Computational Cosmology, Department of Physics, Durham University, and a member of the DESI team and its International Advisory Board, said: “The first results from DESI are hugely exciting. You can feel the buzz at our regular telecoms where colleagues from around the world meet regularly to discuss the data.

“Never before has mankind measured the basic properties of our Universe with such precision.

“On the whole, the elegant model of our Universe that has emerged over the past four decades gets a clean bill of health. And yet, there are hints in the data that perhaps the simplest possible form of dark energy – Einstein’s cosmological constant, Lambda – may not be the whole story. We will know for sure in the coming three years.”

DESI is an international collaboration of more than 900 researchers from over 70 institutions around the world. The instrument was constructed and is operated with funding from the DOE Office of Science, and sits on the US National Science Foundation’s Nicholas U. Mayall 4-metre Telescope at Kitt Peak National Observatory, a program of NSF’s NOIRLab.

It is the first spectroscopic experiment to perform a fully “blinded analysis,” which, at first, conceals the real data from the scientists to avoid any subconscious confirmation bias. Researchers work in the dark with modified data, writing the code to analyse their findings. Once everything is finalised, they apply their analysis to the original data to reveal the actual answer.

Dr Eva-Maria Mueller, Ernest Rutherford Fellow at the University of Sussex, who led part of the cosmological interpretation of the DESI data, said: “When we finally unblinded the data, I couldn't believe how fascinating the results turned out to be. It was a moment I'd been eagerly anticipating since the start of my PhD. The findings were not just interesting - they were captivating, sparking fresh insights into the fundamental nature of our Universe. It's moments like these that remind me why I'm passionate about cosmology.” 

DESI is managed by the US Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab).

It includes the ��������, Durham University and, UCL and the ��������, along with individual researchers at the universities of Cambridge, Edinburgh, St Andrews, Sussex and Warwick from the UK.