Professor Martin Rees, Trinity College
Chris - Another guest on our punt is Professor Martin Rees. Martin is president of the Royal Society. He is also the Master of Trinity College. Heís the Astronomer Royal and heís come to talk to us about life, the universe and everything. Martin, thank you very much for coming to talk to us.
Martin - Great to be here on this sunny day.
Chris - Letís put some numbers on things, first of all. How old is the universe?
Martin - The universe is nearly 14 billion, 14 thousand million years old. We know that number with a precision of about 5 per cent I would guess. The Earth itself is about four and a half billion years old. The first life started not much after that. When we think about the origin of the sun and the planets we have to realise that when they formed already the universe had been expanding for about nine billion years.
Chris - 5% is pretty accurate. How do you know the universe is that old?
Martin - We know the universe is expanding. If we know how fast an object is moving away from us and how far away it is then we can work out, roughly speaking, how long it has taken to get to that distance assuming everything started close back together. Then you have to make corrections because the present speed is not the average speed depending on whether the universe is accelerating or decelerating. That argument and some others has given us this picture of how long it was. Itís everything squeezed together in a very hot, dense state which we call the aftermath of the Big Bang.
Chris - How long did that go on for? The big bang obviously occurred in a fraction of a second but then things have been evolving since.
Martin - Well, the first microsecond is still shrouded in mystery because the conditions then were rather extreme. From then onwards we do have a fairly good general picture of how the universe evolved. After one second it would have been at a temperature of ten billion degrees. Soon after that hydrogen or helium atoms or nuclei of the atoms formed. After half a million years the radiation left over from the early universe cooled to a temperature of about three thousand degrees. Thatís important because thatís a low enough temperature, lower than the surface of a star, where the atoms become neutral and they can start clustering together. After about half a million years the atoms are clustering together to make the first galaxies and the first stars.
Chris - Do we know what the anatomy of those first galaxies was? Were they similar to what we see today or were they very different?
Martin - We donít know quite when the first stars and galaxies formed. We know that after the first half million years the universe because literally dark because the primordial light diluted and shifted it into red. The universe became literally dark until the first stars formed and lit it up again. We do believe that the first star to form not in isolation but in what I would call sub-galaxies: objects which are maybe about a million times as big as a star. These sub-galaxies then agglomerated and merged together until systems the scale of present-day galaxies built up.
Chris - What keeps galaxies together? Why donít they just spread out and all the matter and the material just get dispersed through space evenly?
Martin - Well, the galaxies are held together by gravity. But the gravity of the stars and gas that we see is not enough to stop their disruption. We know how fast theyíre moving and therefore how much kinetic energy has to be counterbalanced by gravity. The important conclusion we draw from this is that galaxies must consist of not just gas and stars but also of some other ingredient, that we call a dark matter. This material is of some uncertain nature. Itís probably some kind of particle made in the Big Bang along with the atoms and the radiation which is rather like heavy neutral atoms as it were. They donít emit or absorb light but they feel gravity and the cluster together in a sort of swarm. We believe that every galaxy contains not just is and gas but also a swarm of dark matter whose total mass is probably five times as big as the mass of all the stars and galaxies we see.
Chris - If they were produced in the Big Bang and they like to cluster together how did they get separated in the first place only then to come back together again at the hearts of the galaxies we have today?
Martin - The early universe was very smooth, almost uniform. If it had been completely uniform then it would now, after 40 billion years, be just a cold, very dilute hydrogen: no galaxies, no stars and no people. The early universe wasnít completely smooth. It had small fluctuations: some regions denser than others, some expanding slower than others. During the expansion the density contrasts grow under the action of gravity. Thatís because if a region is slightly denser than average gravity exerts a bigger pull and slows it down more. The density contrast grows. Starting from very tiny non-uniformities one part in a thousand or thereabouts on can end up in the theoretical simulations of galaxy formation with structures forming at a late stage in the universe. We believe thatís what happened. There were these fluctuations, one part in a thousand from place to place. As the universe expanded the density contrast grew and the dense regions eventually separated out to make the first galaxies.
Chris - Dark matter which is intensively gravitationally positive and pulls things towards itself explains one aspect of what youíve been saying. One other thing that you mentioned is that the universe is expanding. If youíve got everything pulling together whatís driving the opposite? Whatís pushing everything apart to make it expand?
Martin - Even now if we look at the expansion of the universe it seems that it is speeding up, not slowing down. This is rather surprising because you would expect that the gravitational pull that everything exerts on everything else would cause the expansion to slow down. But it does seem that there is in the universe now, to everyoneís surprise, an extra force which is unimportant on the scale of everyday life; unimportant in the solar system, even in the galaxy. On the scale of the entire universe it exerts a push which overwhelms the pull of gravity and causes the expansion to be accelerating. This tells us the long-range forecast for the universe is to become ever colder, ever emptier, ever more dilute. We suspect also, although we donít know this, that in the early stage of the universe there was a repulsive force rather like the one operating now but much, much stronger. That gave the universe its initial impetus, as it were.
Chris - Looking at the shorter range, closer to home, in our own neighbourhood Ė this galaxy, the Milky Way Ė does that mean the space between us and our next near neighbours is getting bigger to?
Martin - No, thereís what we call a local group of galaxies. Thereís us plus Andromeda plus a few smaller galaxies which is a system held together by its own gravity. Thatís not participating in the expansion of the universe. If we imagine what the universe would be like 50 billion years from now then it would look very empty indeed and almost everything that we now see with our telescopes will have disappeared. What will be left will be just the remnants of our galaxy, Andromeda and a few others which by then will have merged into an amorphous galaxy consisting of dark matter and stars which will then mainly have died except the very faint slow-burning ones.