What a new map of the universe tells us about dark matter
The most precise map so far of dark matter in our universe was released last week by an international team of astrophysicists.
Even though we can’t see dark matter (as it doesn’t emit light), we have been aware of its effects for more than 80 years, by observing how galaxies cluster and move around. Using a different technique, the Dark Energy Survey charted the distribution of dark matter by measuring how light from background galaxies is distorted as it passes through dark matter’s dark web.
But what does this image show?
It shows the distribution of dark matter in the relatively recent universe. The red patches are spots where the dark matter density is high, the blue parts are areas where its density is low.
Previously the most precise observations of the universe had been made by measurements of the cosmic microwave background, which is the relic radiation left over from the big bang. It gave a snapshot of what the universe looked like about 380,000 years after the Big Bang.
Given such a pristine view of the early universe, we were able to throw that into theoretical calculations and computer simulations to predict what the universe should look like between 7 billion and 13.8 billion years later (from about half the age of the universe until today).
Remarkably, when we observed the dark matter, the observations matched the predictions. That’s like observing the moment of conception and predicting not only how tall a person would be in adulthood, but also what their face would look like.
The dark mystery
Over the past century, observations have revealed there is a lot of the universe we can’t directly see. There are two distinctly different dark components, which we call dark matter and dark energy.
Dark matter is clumpy, it holds galaxies together and is the scaffolding underlying all the structures we see in the universe.
Dark energy, in contrast, seems to be smoothly distributed around the universe, it doesn’t clump, and it pushes, rather than pulls, thus accelerating the expansion of the universe.
Between them they make up 95% of the energy density of the universe. But what are they? That’s still unknown.
Explaining dark matter and dark energy will solve one of the biggest mysteries in modern physics.
So in 2004 a group of researchers got together to try to figure out how to measure the dark side of the universe more precisely. Thus the Dark Energy Survey was born.
They realised they had to build new technology — digital cameras that could rapidly survey large regions of sky, detect thousands of supernovae and millions of galaxies.
The culmination of almost a decade of effort was the Dark Energy Camera (DECam), which is now mounted on the Blanco 4-metre telescope in the Cerro Tololo Observatory in Chile.
DECam is one of the most powerful cameras in existence – 570 megapixels, superb sensitivity, and a huge field of view so that each image captures an area of the sky 14 times the size of the full moon.
Using DECam the Dark Energy Survey has so far scanned 1/30th of the entire sky, and the new map released this month is from an analysis of a whopping 26 million galaxies.
How the map was made
If most of the mass in the universe is dark matter, then the galaxies we can see are like white icing on a dark chocolate cake. So far we’ve been using the icing to detect the cake.
Even though we can only see the galaxies, they fall toward clumps of dark matter just like skydivers fall to earth. So we can use the position and motion of bright galaxies to infer where the dark matter is.
All of their motion confirms what you would expect if the dark matter scaffolding was there to pull them. But we still haven’t seen dark matter.
So it was worth testing in a different way. Everything falls under the influence of gravity, even light. So as light from a distant galaxy passes by clumps of dark matter, its path will be bent slightly.
This distorts the apparent shape of the background galaxy and makes something that would have looked round appear to be somewhat oval. Thus by measuring the shapes of millions of background galaxies, we can infer the presence of dark matter in the foreground and measure how it is distributed.
That’s how the Dark Energy Survey made the map. With 26 million galaxies and very precise measurements of their shape, the team detected the influence of the intervening web of dark matter.
What did we learn?
This map means we now have a new way to test advanced theories of gravity and quantum physics that try to explain what dark energy and dark matter could be.
Many different theories have been proposed including some that predict gravity would affect light differently than how it affects matter.
So if we saw anything unexpected in the warping of light from distant galaxies, we could have lent support to those theories. But the observations are consistent with our standard theory of general relativity, with no indication yet that gravity treats light any differently from matter.
We have found that our understanding of the universe is even more robust than many people had thought. We are really narrowing in on the range of possibilities for what dark matter and dark energy could be.
It looks like dark matter is a type of particle that is not part of our standard model of particle physics, and it seems that dark energy is consistent with being a “cosmological constant”, perhaps an intrinsic feature of the vacuum itself, which is why it appears so smooth and unchanging across the universe.
This week’s data release represents only the first year of data from the Dark Energy Survey. We are about to begin our fifth year of observing, and our final analysis will cover three times as much area with 300 million galaxies.
So watch this space.