The Shape of the Universe

27 May 2007

Interview with

Dr Carolin Crawford, University of Cambridge

Chris - A lot of people probably look up into the void of space and think "how did all this get here?" And wonder what the black bits between the stars are... So Carolin; what is the fabric of space?

Carolin - Straight into the deep questions! What is the fabric of space? Well there's so much in space; there are stars, there are planets, there are galaxies... All of this started right at the point of the big bang, so all of it starts there and it expands outwards, and it's all devolved into these wonderful things we see. Hubble_deep_field_image

Chris - But is space a vacuum? Is it some kind of material like that which we refer to as the 'ether'? What is space?

Carolin - Well space is never truly a vacuum, you have these enormous voids in space and even those might have a few particles in, but generally we regard it as a vacuum. But that's in the enormous spaces between the stars and dust and gas clouds and the galaxies.

Chris - So when you say that everything started with the big bang, and that was about 14 billion years ago...

Carolin - 13.7 thousand million years ago...

Chris - That's a very precise answer...

Carolin - Oh yes, I looked it up before I came in...

Chris - But how do we know that that's when it happened?

Carolin - Well we can track the motions of the galaxies, they're all expanding outwards after this initial explosion and we can work out how long they've taken to get there travelling at that speed. You back track it and find that they all come together about 13.7 thousand million years ago.

Chris - So when you say galaxies, can you just define what is a galaxy and how did they come into being?

Carolin - Galaxies are giant aggregations of stars. In something like our own galaxy you have maybe 100 thousand million stars held together by gravity. Galaxies come in shapes that are like frisbees, footballs, rugby balls, and they're just big collections of stars all held together. All of these are scattered throughout the universe.

Chris - But why would they form a galaxy? Why would they cluster together in that way, why wouldn't they just spread out? If you had something that kicks out in a massive explosion like we think the big bang was, why didn't it just shower everywhere with material? What causes it to come together in this way?

Carolin - That's because the universe was never exactly the same density throughout, even in these very early stages just after the big bang there were some bits that were just slightly more dense than others. If there's more matter, there's more gravity, so that pulls more matter towards it... You can regard these as the seeds, which pull stuff together and it starts to accumulate things that are eventually going to grow into the galaxies that we see today.

Chris - Why would they be different sizes and shapes, why wouldn't they end up as a fairly consistent size and aggregation? They have different structures, don't they?

Carolin - They have got different structures, and they key things might be: Whether they formed near another galaxy; whether you've got two galaxies forming in the same area, or they formed in isolation; whether the gas cloud was originally spinning; as things that are spinning tend to condense down into flat discs, so maybe that is how you get a spiral galaxy as opposed to the ball shaped ones which maybe came from a much more static cloud.

Chris - Andrew Gaspar has emailed and asks, "I understand that there are two types of galaxies. Some have spiral arms and others an elliptical shape..."

Carolin - That's right.

Chris - "So what processes would form those two different types?"

Carolin - Well, we think with galaxy formation that everything was very hot after the big bang, so you start off with this enormous cloud of very hot gas that begins to cool. As the gas cools, it starts off by radiating x-rays, it cools down more and eventually it condenses down into the stars that form a galaxy. If you have a large cloud of gas which is quite static, then maybe it will condense into an elliptical shaped galaxy. If it's rotating ever so slightly, the things that condense from it will end up disc shaped such as the spiral shaped galaxies like we think our Milky Way is.

Chris - What's in the middle of our galaxy that keeps us all spinning around it?

Carolin - Well our galaxy is kind of like two fried eggs stuck back to back. The white forms a frisbee shape but the yoke represents a massive ball of stars. Right at the centre of the big ball of stars is a supermassive black hole.

Chris - Presumably we can't see it, so is that inferred that there must be something big and heavy there? Is that what's attracting and holding everything in the shape that it's got?

Carolin - Well, it's not the supermassive black hole that holds our galaxy together, but we know it's there because it affects the motion of the stars that are close to it. We can see them responding to a mass that's about 3 million times the mass of our sun within a tiny, tiny volume. The only thing we know that can be like that is a black hole. But it's not that which keeps the whole galaxy together, you've got the stars that we see in the bulge and in the disc and there's also the stuff that you can't see, the dark matter. It doesn't radiate in any waveband, but we know it's there because of the motions of the stars. That's crucial in holding our galaxy together.

Chris - Why doesn't the black hole just eat the ball of stars that are around it, why don't they just fall in?

Carolin - Because it's probably eaten anything that came close to it, the galaxy's been around for a long time, a thousand million years, so it's eaten everything within it's vicinity. There are stars that come close to it and orbit around it, but it's probably cleared out it's immediate surroundings and things further out orbit around it, but they are so far away and the gravity is such that it doesn't pull things into it, just into orbit around it.

Chris - It's said, and I think Edwin Hubble discovered, that the universe is expanding. We know that because if you look at distant objects they seem to be getting further away from us, they're so called 'red-shifted'. Why is it that when you look at some galaxies they appear to be coming towards us, because if space is getting bigger wouldn't they all be moving away from each other?

Carolin - On the whole, everything is moving away from each other. Whichever galaxy you do the measurements from you find that, on the whole, all other galaxies are moving away from you. That's the general motion of space-time moving out, this is the expansion of the universe. But of course, if you've got a galaxy that's actually quite close to you, you might start feeling the gravity of that galaxy. For example, the Milky Way is moving towards our nearest big neighbour, the Andromeda galaxy. Even though we're both moving out, as space-time expands, we're still moving towards each other. So sometimes you get these little local peculiarities where galaxies pull together under gravity even though they're still moving outwards.

So if you get things very close to you then sometimes due to these little peculiarities they can seem to be moving towards you, but in general everything else is moving out

Chris - I've got a couple of questions from listeners here. Keith in Watford asks, "what determines the orbital distance from a star at which planetary material will coalesce? Is it just gravity?"

Carolin - That's a difficult question. It also depends on what your star is made of, how big it is and how much stuff is left behind. If you have a planet that's made of gases and volatiles, like Jupiter or Saturn, they're only going to condense much further out from the sun. They tend to get swept out by the winds from the young star forming in the nebula and condensing down to form the planetary system. If they're made of solid rock material, like Earth, Mercury, Venus or Mars, they tend to coalesce much closer in, so it does depend what kind of planet you're forming.

Chris - Fred and Scott have asked - "The Earth orbits the Sun in a non-circular orbit, if outer space is really cold, and the Sun is really hot, how close would you have to get to the Sun in order to be about room temperature?"

Carolin - Room temperature where on the Earth?

Chris - They don't specify, so I guess you have full degrees of freedom, if you'll pardon the pun.

Carolin - It's difficult, because as you go up from the surface of the Earth, you get colder, also, on the side of the Earth facing away from the Sun it will be different from the day side. It's actually a very difficult question to answer. As there's so many different ways to approach it.

Chris - Recently there was a Swedish space walk, and they were talking about how when people are in space, they must wear reflective space suits because if they're in the full glare of the Sun their body will quickly heat up to boiling point. In the dark side though, it's very cold because you radiate the heat straight away again. So it is a difficult question to answer, because if you're having sunlight hitting you, you soak up a lot of radiation, and if you're out of direct sunlight, you're not.

Carolin - Also, if you think about aeroplanes, you can see the condensation freezing on the outside of the aeroplane, and although you're above the clouds, you're still nowhere near space.

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