Cosmologists shed light on dark matter distribution across the universe

Image credit: UCL Mathematical and Physical Sciences via flickr.

For the last few decades, researchers in Astronomy and Cosmology have been trying to understand how the universe has evolved at its very early stage, by chasing the oldest light around us. To understand the procedure, imagine that we are given an empty field and have been told to turn it into a forest. We plant seeds across its surface at the places that we want the trees grow, and then at some point in the distant future, we go back, only to see that the forest is exactly how we planned it! By observing the location of the trees, we can estimate where the seeds were planted, all those years ago. A similar process can be followed with matter: by observing how matter is distributed across the length of the universe, we can form a picture of how it was at its early beginning.

This is what Dark Energy Survey (DES) has been doing for the last 4 years. They have created the most accurate map of how mass was distributed in the universe so far, that shows the places in the early universe (about 14 billion years ago) with the tiniest fluctuations in temperature and density. Those are the places where the concentration of mass would result in the creation of massive galaxies and stars.

The DES mission is to detect cosmic patterns in the universe that would reveal the nature of dark energy and dark matter, two “invisible” forms of matter that are theorised to make up 96% of the universe’s total mass. DES Survey is a collaboration of 400 scientists across the globe, including researchers from the University of Edinburgh.

Their map covers about 12% of the sky and agrees perfectly with the current theory about the structure and distribution of dark matter (26%) and dark energy (70%) in the universe. For their latest measurements, researchers used data recorded with a 570-megapixel dark energy camera that traced the exact positions of galaxies and density of dark matter. Additionally, by measuring how 26 million galaxies bend light, by gravitational lensing, they could measure the galaxies’ masses and map the patterns of the hidden dark matter’s gravity. New techniques had to be developed in order to record the tiny lensing formed by the gravitational fields, so that the measurements were precise.

Why is this important though? According to Einstein’s theory of general relativity, gravity should lead to a slowing of the cosmic expansion. Nevertheless, in 1998, astronomers studying distant supernovae (the explosions of massive stars), discovered that the expansion is instead accelerating! To explain this, we either accept that there is an exotic form of mass – dark energy – whose gravity does not behave in the same, attractive way as gravity of ordinary matter, or we must replace general relativity with a new theory of gravity.

The Dark Energy Survey has already delivered some remarkable discoveries and measurements, and they have still barely scratched the surface of their data. And these are the results of just the first out of five years of research. As DES researchers phrased it “In the age of precision cosmology, and the alleged tension between various measurements of the Hubble’s constant and subsequent cosmological models, DES has been successful in staking a claim as a mediator.


This article was written by Christos Kourris and edited by Bonnie Nicholson.

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