What is the Universe made of?

02 June 2015

Interview with

Dr Andrew Pontzen, UCL

Science has been grappling with a problem that's been around for nearly 100 Under the Milky Wayyears - what is our universe made of? When physicists look out into space, they're confronted with a mystery: the material or matter we can see makes up only about 5% of the mass that we know must be out there. The other 95% is totally invisible to us, and so scientists refer to it as dark. At least 20% of the mass out there is what we call Dark Matter. But if it's invisible, how do we know it's out there? University College London cosmologist Andrew Pontzen explained to Chris Smith... 

Andrew - Well, we can kind of take a census of what's out there in the universe and we do that by making indirect measurements. So for instance, you might measure how fast things are  moving around and one of the famous measurements of this was made by Fritz Zwicky. He was looking in clusters of galaxies and measuring how fast the galaxies within that were moving around relative to each other.

He found that they were zooming around really fast. So fast, that that galaxy cluster would simply fall apart unless there was something sticking it together. And so, his explanation for that was, there must be some extra stuff in there with an extra pull of gravity, serving that purpose of gluing the galaxy clusters together. And since then we've seen this phenomenon all over the place

Chris - Because we know how things should move when gravity acts on them. When we make observations of things in the universe, we're seeing them moving at the wrong speed for they're not to be more mass there, more gravitational acting material than we can actually see and that's what we're calling dark matter.

Andrew - That's right. So, it builds on how we know gravity works and thanks to Newton and then later Einstein, we think we kind of have that pinned down. We think we know how gravity works and then this idea that there has to be extra stuff follows.

Chris - And when you take the estimates that people have made it, looks like it sums to about a fifth of the mass.

Andrew - Yeah. It's got to be something like that.

Chris - So, it's pretty prolific. Where is it distribute in the universe? Is there dark matter in this room with us?

Andrew - Yup! If we're right, there certainly is dark matter in this room, zipping through really fast., it has to be said. It doesn't clump together in the same concentrated waya normal matter does. So, the Solar System and everything within it - the Earth and the sun, and everything in this room is stuck together by forces that dark matter doesn't seem to feel. So, as a result, it's much more spread-out through the universe and it's zipping through the room at the moment maybe at hundreds of kilometres every second.

Chris - So, what physically might it be?

Andrew - Well, we think it's got to be some kind of extra fundamental particle. Particles are the things that make up normal matter. In fact, we talk about particles when we're talking about say, protons and neutrons, and electrons, the things that we know make up atoms and then make up the familiar world. So, we think there's an extra type of particle that as yet, hasn't been directly discovered but is out there in the universe.

Chris - Are these the WIMPs that physicists are fond of talking about?

Andrew - That's right. So, that's one particular type of WIMP or weakly interacting massive particle.

Chris - And how might we be able to detect the stuff that we can't see?

Andrew - Well, there are a number of different ways. One way is to try and make it: if you take something like the Large Hadron Collider for example, you can smash particles of normal matter together. We think if you could get to high enough energies in those collisions, occasionally, you'd actually manufacture one of these mysterious new particles. And then actually, you think, how would you find out? How would you know that you manufactured it if it's invisible? And the answer is, you would look for things going missing. You would put in a certain amount of energy and you would sum up everything that you saw come out and realise, something has gone missing. So, you'd know you made something extra. That's one way you could do it.

Another way is to actually go all out and just say, "Well surely, we must be able to detect these things. We must be able to see directly that they're there one way or another." That's what we call direct detection and people are certainly trying to do that as well. It's just really, really challenging because you need to be incredibly sensitive. If we're right about these particles, they so rarely and so weakly interact with normal matter that we're made out of and any detector that we could build would be made out of, that it's a real challenge just to build something that can do that.

Chris - And that's the reason they're invisible because they just don't interact with stuff.

Andrew - Exactly. In fact, when we say dark matter, it's almost a bit of a bad name because the stuff isn't actually dark. It's just transparent because exactly like you say, simply not interacting with stuff, except through its pull of gravity.

Chris - Does that mean that they're interacting with the other thing that's been big news in recent years, the Higgs Boson, which CERN made a great climax about announcing? They think they have discovered this particle that appears to add mass to things. Does that mean that these dark matter particles are in some way bound up with Higgs Bosons?

Andrew - Certainly what's true is that the Higgs Boson would feel the gravitational pull of a dark matter particle and vice versa. That's got to be true, but what we don't know is just how bound up are these two things. Could it be the case, for instance, that the Higgs Boson - when they start to probe its properties in more detail - the Large Hadron Collider - could it be that it points the way to understanding of where dark matter is coming from? Well, it could be but we just don't know at the moment.

Chris - Because I was going to say, the standard model, which is this jigsaw puzzle of how we think matter is constructed and what the particles are that give rise to matter, when the Higgs Boson came along, people said, "right. Look, that's the missing piece." Well now, we've got this extra piece in the puzzle, the dark matter particle, where does that go in a puzzle that's full then?

Andrew - . Within the standard model, there are certainly conundrums...

Chris - Then it's conundra.

Andrew - I'm sorry. I forgot that that's in Cambridge. Yes, there are certainly conundra within the puzzle and those conundra, as we try to start to answer them, for instance, how does - what we know about particle physics on very small scales, fit together with how we know the universe works on very large scales - this just doesn't fit together at the moment. It's as people try to work out how all that fits together, that they realise, "Okay, looks like we've got to have these extra particles."

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