What's in our universe?

14 August 2017

Interview with

Ofer Lahav, University College London

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The Universe contains a lot more mass than we can actually see. In fact, we think that physical matter, like the stuff we’re made of, accounts for less than 5% of what’s out there. The remainder is what we call Dark Matter and Dark Energy. We don’t know what these entities are, but we are beginning to work out where they are, because 400 scientists across the world are working together on a project called the Dark Energy Survey. They’re using a telescope in Chile to image distant astronomical objects, and the first year’s results were released this week. Izzie Clarke spoke with Ofer Lahav from University College London who is one of the founders of the project...

Ofer - We’re really looking for the unseen. It might sound a bit contradictory that we are actually observing light and trying to figure out the dark matter component of the universe. The current picture we have which is very much supported by the data just released by the Dark Energy Survey suggest only 4% of the cosmic budget at present is made of ordinary matter, the stuff we see around us in everyday life including trees, and waterfalls, and ice cream. The remaining 96% is dark and we think about 26% is in the form of a cold dark matter and 70% in the form of dark energy.

Izzie - So, can you tell me, what exactly is dark matter and what is dark energy?

Ofer - Dark matter is probably due to some particle which we haven’t found yet. The problem has been around for over 80 years, and we see it because when we look the way stars move around the galaxy, it turns out that we cannot explain the rotation of stars in the galaxy just with ordinary matter, we have to evoke this extra component.

Similarly, with dark energy, we cannot explain what we see. The pattern of galaxies and the distortion of distant galaxies, we cannot explain without putting in this component of dark energy.

It’s still a big mystery, what are these components? So that’s why it’s so important to map those ingredients.

Izzie - So you’ve mapped the sky, you’ve mapped all these distant galaxies, what do they actually look like? How does dark energy and dark matter affect that image?

Ofer - I should also mention the area covered in that map of the dark matter. It’s only 3% of the sky but, in fact, it’s quite big. In fact, you can accommodate more than 6,000 full moons in that area. This map was produced by looking at the images of 26 million distant galaxies. For each of them, the collaborators actually measured the shape. Now what this shape tells you is, in part, it tells you what the actual galaxy looks like in reality. But also, this shape gets distorted by a very small amount due to the intervening matter between us and that galaxy and, from the amount of distortion, you can infer what the dark matter between us and those distant galaxies. That’s exactly what this Dark Energy Survey collaboration has done and pictures are better than a thousand words, so I encourage people to look at the pictures.

Izzie - What can we do with these new findings?

Ofer - Those finding provide us with some of the most accurate measurements of the composition of the universe at present. 94% of the universe is dark. 26% dark matter, 70% dark energy, and only this 4% of ordinary matter. The universe is expanding; we’ve known that a long, long time since the discovery by Hubble but, since the discovery by the Supernova team, it seems now the universe is actually accelerating, and these results we got from Dark Energy Survey support that.

The point is that you can think about different hypothetical universes, and then we take the one we map with that telescope and compare it to all these hypothetical universes and see who the winner is. And then we say well, that’s probably the universe we live in. The results published today are based on the first year - well, hopefully, the best is still to come.

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