Scientists release the most detailed analysis yet on the expansion of the universe

Scientists at the Dark Energy Survey (DES) collaboration, have released their latest and most detailed picture yet on how the universe has expanded over the last six billion years.

A century of discovery

Around 100 years ago, scientists discovered that distant galaxies appeared to be moving away from Earth. They found that the further away a galaxy is, the faster it recedes, providing the first key evidence that the universe is expanding.

Researchers initially expected that this expansion would slow down over time due to gravity.  However, in 1998, observations of distant supernovae revealed that the universe’s expansion is accelerating rather than slowing down.

To explain this surprising result, scientists proposed the idea of dark energy, which is now thought to drive the universe’s accelerated expansion.

Astrophysicists believe dark energy makes up about 70% of the mass-energy content of the universe, yet we still know very little about it.

Combining four cosmic probes

These recent findings combine results from 18 separate studies and, for the first time, bring together four major techniques for studying dark energy within a single experiment, a milestone envisioned when DES was conceived 25 years ago. These techniques are:

• weak gravitational lensing (distortions in galaxy shapes)
• galaxy clustering
• supernovae
• galaxy clusters

The combination of these techniques enabled scientists to cross-check their measurements and gain a more robust understanding of how the universe behaves.

International collaboration

The Dark Energy Survey is an international collaboration of more than 400 astrophysicists, astronomers and cosmologists from over 35 institutions. The international research team is led by the US Department of Energy’s Fermi National Accelerator Laboratory, with UK support from the Science and Technology Facilities Council (STFC) and six UK universities, including the University of Edinburgh.

Through STFC, the UK is also supporting research programmes that will advance DES science in the next generation of astronomical surveys, including the Vera C. Rubin Observatory, currently under construction in Chile.

Professor Joe Zuntz, Personal Chair of Cosmology, Institute for Astronomy, University of Edinburgh, said:

As well as needing a fantastic telescope, research like this needs supercomputers to tell us what the data actually means. That’s one reason why the UK’s next supercomputer, being hosted in Edinburgh, is so valuable and why I’m proud to develop the programming to help scientists understand what DES’s measurements can tell us about the Universe.

Far reaching science

To study dark energy, the DES collaboration carried out a deep, wide-area survey of the sky between 2013 and 2019, using a specially constructed 570-megapixel Dark Energy Camera mounted on a telescope at the US National Science Foundation’s Cerro Tololo Inter-American Observatory in Chile.

Over six years, scientists collected images and data from hundreds of millions of distant galaxies, billions of light-years from Earth, mapping about one-eighth of the sky.

For the latest results, scientists refined how they use subtle distortions in galaxy shapes, known as weak gravitational lensing, to reconstruct the distribution of matter in the universe over six billion years. They did this by measuring both how galaxies cluster together and how similarly their shapes are distorted by gravity.

By reconstructing the universe’s matter distribution across 6 billion years, these measurements reveal how dark energy and dark matter have influenced the universe’s evolution.

A mystery remains

The team compared their observations with two main theories, one in which dark energy remains constant over time (the standard model of cosmology), and another in which dark energy changes as the universe evolves.

DES found that although the data mostly align with the standard model, broadly agreeing with the most widely accepted theory of the universe, there remains a long-standing discrepancy in how matter clusters in the universe, and this has become more pronounced with the inclusion of the full dataset.

Paving the way

Looking ahead, DES will combine these latest findings with results from other dark energy experiments to explore and test alternative ideas about gravity and dark energy.

The work also helps prepare the ground for future breakthroughs at the upcoming Vera C. Rubin Observatory in Chile to do similar work with its Legacy Survey of Space and Time (LSST).