Map of the cosmos 'sees' the dark universe

3 August 2017

Blanco telescope

Blanco Telescope (centre), Milky Way (left), Large Magellanic Cloud (top)
(Credit: Fermilab)

The most accurate map to date of the structure of dark matter in the universe is being released today, supporting the theory that dark energy and dark matter make up most of the universe.

For the first time, scientists from the international Dark Energy Survey (DES) collaboration can see the structure of the current universe in contrast to the universe at its very beginning – and therefore can answer many questions about how it evolved.

The UK has been involved in this international project from its conception, back in 2004, with the Science and Technology Facilities Council (STFC) providing funding on behalf of the UK.

The DES collaboration is led by Fermilab in the US and is made up of more than 400 scientists from 26 institutions in seven countries, with UK astronomers playing a key role. Scientists at the universities of Cambridge, Edinburgh, Manchester, Nottingham, Sussex, Portsmouth and UCL have led work central to today’s results

The results from this survey could help to provide answers to one of the most pressing questions of the universe – why the universe is expanding at an ever-increasing rate. Crucially, these findings support the theory that about 4% of the present composition of the universe is ordinary matter, 26% comes in the form of mysterious cold dark matter, and 70% is in the form of an elusive dark energy, possibly causing the accelerating expansion of the universe.

“The DES measurements support the simplest version of the dark matter/dark energy theory,” said Joe Zuntz, of the University of Edinburgh, who worked on the analysis. “This is a huge part of the picture showing how the cosmos has evolved over the last 14 billion years. It’s the dark Universe made visible on an unprecedented scale.”

Surprisingly, it is easier to measure the large-scale distribution of matter in the universe in the distant past than it is to measure it today.

In the first 400,000 years following the Big Bang, the universe was filled with a glowing gas, the light from which survives to this day. The European Space Agency’s orbiting Planck observatory created a map of this cosmic microwave background radiation, giving us a snapshot of the universe at that very early time.

Since then, the gravity of dark matter has pulled mass together over time, while dark energy has been pushing matter apart. Using the Planck map as a start, cosmologists can calculate precisely how dynamic plays out as the Universe evolves over 14 billion years.

Ofer Lahav of UCL, chair of the DES:UK Consortium and chair of the DES Advisory Board, said: “It is remarkable that dark energy, proposed in its simplest form by Einstein 100 years ago, is now measured so accurately by DES. However, the nature of dark energy is still a big mystery.

“This result is the culmination of 13 years of dedicated work.”

Map of dark matter

Map of dark matter made from gravitational lensing measurements of 26 million galaxies in the Dark Energy Survey. The map covers over 1300 square degrees of the entire sky and spans several billion light years in extent. Red regions have more dark matter than average, blue regions less dark matter.
(Credit: Chihway Chang of the Kavli Institute for Cosmological Physics at the University of Chicago, and the DES collaboration)

The primary instrument for DES is the 570-megapixel Dark Energy Camera on the Blanco telescope in Chile, one of the most powerful cameras in existence, able to capture digital images of light from galaxies eight billion light years from Earth. Part of the DES Camera, the optical corrector, was assembled at UCL, with the support of STFC.

The new DES results draw from data collected during the DES first year, which covers over 1,300 square degrees of the sky, equivalent to the area covered by about 6,600 full moons.

DES scientists used two methods to measure dark matter. First, they created maps of galaxy locations to trace its density, and second, they precisely measured the shapes of 26 million galaxies to directly map the patterns of its gravity over billions of light years, using a technique called gravitational lensing.

To make these ultra-precise measurements, the DES team developed new ways to detect the tiny lensing distortions of galaxy images, an effect not even visible to the eye, enabling revolutionary advances in understanding these cosmic signals. In the process, they created the largest guide to spotting dark matter in the cosmos ever drawn using galaxy distortions.

"The size of these maps and the amount of data going into them makes them incredibly powerful" said Jack Elvin-Poole, a PhD student at the University of Manchester, who led work building the catalogue of galaxies used in the analysis. "Bringing maps of galaxies and dark matter together gives us a whole new way to understand how the dark side of the Universe behaves."

David Bacon of the University of Portsmouth added: "Dark matter is invisible to us, but we can see its effects - its gravity is bending the light from distant galaxies, so they look distorted to us. We can work back from measuring the distortions to mapping the cause. It's so exciting to see this truly vast map of the dark Universe at last.”

The new results released today draw only from data collected during the survey’s first year, which covers one thirtieth of the sky. Scientists on DES will map an eighth of the sky in unprecedented detail over five years. The fifth year of observation will begin in August.

These results and others from the first year of the Dark Energy Survey will be released today online.

Notes to editors

More information on the DES collaboration.

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