Dark Matter And The Big Bang

The Naked Scientists spoke to Professor Gerry Gilmore, University of Cambridge Institute of Astronomy
10 December 2006

Interview with 

Professor Gerry Gilmore, University of Cambridge Institute of Astronomy


 The Andromeda Galaxy

Chris - We're joined this evening by Gerry Gilmore who is the Deputy Director of the Institute of Astronomy at Cambridge University. Hi Gerry! Thank you for coming in.

Gerry - Hi Chris.

Chris - I have to say hello to everyone who's listening on radio Europe, REM FM. That means that one of the places we're being broadcast is the Canary Islands and that's a place where you do a lot of your observations because you have telescopes there, I gather.

Gerry - Yes that's right. The UK plus Spain and various other countries have really big observatories on top of the Canary Islands. They're nice dry sunny places.

Chris - Now Gerry, your interest is obviously in space science, but can we wind the clock back about 14 billion years ago to the time when the Universe was just beginning. Can you just sort of orientate us? What was the big bang? We all talk about it, but what actually was it?

Gerry - In a very real sense, it was the beginning. It's impossible to say what there was before, because there was nothing before the beginning. Our current understanding of the concepts is that one has just nothing. And nothing is a bit like the number zero. You can think of the number zero as being at the bottom of all the numbers and you count one, two, three off, for as far as you go. But it's also at the top of all the negative numbers, minus one, minus two, minus three and so on. And so having nothing, having zero, isn't actually the same as not having anything at all. When you have just vacuum, just energy, you actually have this really tense balance between all the positives and all the negatives. And occasionally, when you have that much tension pulling against itself, you get a little imbalance somewhere or another. And one of those little imbalances which just happen by chance, when it gets unbalanced enough, brings into something. And that something becomes the universe. And that something creates space, and it creates time. And so a universe, space and time, just flash into existence out of literally nothing.

Chris - Didn't people at one time say that studying space was anti-religion? They tried to argue that this was trying to de-cry the efforts of God, and Creation. I'm just saying that because Gilian in Wilburton said "is there any way to relate the big bang to God and Religion? Could we say that this is perhaps the moment of Creation?" Because we can literally put a time on when the Big Bang happened, can't we?

Gerry - Yes, a time that we know quite accurately, actually. We're able to measure it more and more accurately as the experiments get better. So, yes we know almost precisely when the universe first came into existence. You need to be a little bit careful to call that Creation, the word "Creation" is used slightly differently in physics than it is in some other subjects, and whether or not it's related to a cause, is of course one of the big questions of physics. And that's the fundamental distinction between a physics explanation for the universe and a religious explanation for the universe.

Chris - Now when you say you know exactly when it happened, how do we know exactly when the big bang happened? How can we put a time point on that?

Gerry - Well what we see today is the universe expanding, it's getting bigger and bigger all the time. And so we can just measure that speed of expansion directly, in fact we can measure it directly all the way back to the time when the universe was only about 270,000 years old. We have quite accurate measurements all the way back to then. And it's very easy then to extrapolate the last little bit, the last quarter of a million years, and so from that we can actually measure directly. Now that's because, there's a special thing about astronomy, it's totally unlike biology or any other subject. When we look out on the universe, because light is travelling at a finite speed, what we're actually doing is looking back in time. So when you look at the sun, you see the sun as it was seven or eight minutes ago, or whatever the number is. When you look out to stars that are light years away we see them as they were years ago. When we look back to the earliest galaxies, we see them as they were 10, 12 billion years ago. So the light left them, it's like a video picture coming through space towards us, that we just see today. And so in a real sense we don't know what those things are like now. But we do know precisely what they were like 12 or 13 billion years ago. So it's very easy for us to measure the past. It's just incredibly difficult to tell you about now.

Chris - So how do you know the universe is expanding? What are you watching to get those clues?

Gerry - All you need to do is measure a distance and distance is hard of course, but there are some objects whose brightness is known. And so we call them standard candles. They're particular types of star or exploding star, or types of galaxies or whatever. And when you see something that you know how bright it really is, and you measure how bright it seems to be to you, you can very easily work out how far away it is. It's just the old inverse square law. And so that's most of what cosmology is about. You measure things that you know how bright they really are, you measure how bright they seem to us and you work out how far away they are. And we measure their doppler shift. We measure how fast they're moving away from us. And so we see experimentally that as we look further and further away, things are moving faster and faster. And that is the definition of an expansion.

Chris - So what's actually driving the expansion? Obviously there was a big explosion for the big bang but something must be continuing 14 billion years later, to be pushing these things away. What's doing that?

Gerry - Well that is a non-trivial question actually. To start with, the universe just had enough energy from the bang. "Bang" was a term invented by Fred Hoyle here in Cambridge. It was a rather derogatory term but it turns out to be more accurate than he realised. It really was an explosion. And the energy of that explosion just threw space time out at a very high speed. It's slowly slowed down under it's own weight, which was pulling it back. And it in principle would keep slowing down and slowing down.

Chris - So is it growing at the speed of light? Or faster?

Gerry - That's another slightly non-trivial question. Yes, is the answer. The outer parts of the universe are always growing at the speed of light. But you need to be careful there because there's two sorts of universe. One is the universe that we can see. The observable universe, the stuff that we know about. And that is definitely growing at the speed of light. But there's also whatever else that might be out there that's beyond our horizons. And so one needs to be a little careful. People tend to confuse those two concepts, as do most of my colleagues I must say. And myself, quite frequently. But the universe is expanding. And the outer limits of it, the bits that we can observe, are expanding at the speed of light. But the local space time part is expanding much slower than that.

Chris - What's actually pushing it? What's making it do that?

Gerry - What it should be doing is just slowing down slowly. Inertia is just whatever's left over from the bang.

Chris - But you said it's speeding up.

Gerry - Yes that's right. It isn't doing that, which became apparent about 5 years ago. There was a hint a decade ago but it became clear only 5 years ago. And there was a very accurate determination of this only about 3 months ago. The universe is actually speeding up. We don't know what the physical cause of this is. What we know is the process. There is something that is sometimes called dark energy. It's a particularly unhelpful label, it's no more than a known unknown in the common sense of the term. But there is some intrinsic property of space time, which is it acts like a pressure. And it's pushing apart space-time itself. So it's not a force in the simple sense of the word, it's like a pressure.

Chris - I had heard though that if you just take that side of the equation, the equation would be rather unbalanced. The universe should be blowing itself to pieces so something is acting to negate that effect, again something we can't measure. Dark Matter. So what's that all about?

Gerry - Well dark matter only partially negates that effect. Based on the current numbers the universe is actually blowing itself apart. And it will continue to blow itself apart until eventually even something like the earth, or us, gets ripped apart by the expansion of the universe. And you end up with an entire universe containing nothing but one particle in it. So the universe will become a very lonely place. It really is blowing itself apart in a very ridiculous way. But, you're right. There's an awful lot of mass slowing it down and pulling against it. And in fact about 10 times more mass than you would guess from just looking at all the stars and galaxies and biologists in the world. And that matter, we know it's matter but it doesn't shine and so we call it dark matter.

Chris - Is there any way to find it? Is there any way to easily detect it? Presumably not or we would have found it by now.

Gerry - No, it's very easy to detect, actually. Matter has only one fundamental property, and that is that it knows about gravity. So matter both generates and responds to gravity. And that's the definition of matter. There are in fact three other forces in nature, which the sort of matter that we're made of respond to as well. But dark matter responds to probably none of those three. Certainly it doesn't respond to electromagnetic force which is what makes things shine, and therefore we call it dark. But it has matter and we can weigh it in fact. It's very easy to find.


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