Beyond the cosmic horizon
Dark energy and galactic birth with astronomer Martin Rees.
Baron Martin Rees of Ludlow is Royal Society Professor at Cambridge University, Fellow of King’s College, the UK’s Astronomer Royal, and a Fellow of the Royal Society. Professor Rees has recently (Dec. 2005) been appointed the president of the Royal Society, the UK’s National Academy of Science. He is the author of about 500 research papers, seven books (five for the general public), and widely acknowledged as one of the world’s leading astronomers and cosmologists. Professor Rees spoke to Earth & Sky’s Jorge Salazar about the frontiers of cosmology.
Salazar: Thank you for taking time today to speak with me, Dr. Rees. I understand your visit to the States in November of 2004 was to address the American Astronomical Society with the Russell Prize Lecture called Scanning Cosmological Horizons.
Rees: The point I want to emphasize in my talk is that we’ve made immense progress in cosmology in the last few years, but this progress has brought into focus a new set of mysteries that we never envisioned before. We’ve learned about what the universe is made of, but that’s given us surprises. We’ve learned that it’s only 4% made up of ordinary atoms, about 25% is made up of dark matter, which is completely unknown, but more than 70% is even more mysterious – energy latent in space itself. So we’ve got these deep mysteries, even though we’ve made progress. And so, the challenge in the future is to understand the nature of dark matter, to understand the nature of space, and also to understand the way our universe has changed, from a Big Bang, which is very dense and very hot, to our complex cosmos nearly 14 billion years later. And so that’s the challenge for the next ten years.
We’ve also learned that perhaps our universe contains a lot more than astronomers can see. Not only is 96% of this stuff invisible, but there may be a lot of material far beyond the horizon of our observations, far further away than our telescope can see, because what we see when we look through our telescope may be limited by a horizon which may be just as artificial as the horizon which you see when you are in the middle of the ocean, and that there may be, beyond that still more galaxies that we can never yet see. Our universe may even be infinite. Still, more than that, it’s possible that our Big Bang may not be the only one. We are the aftermath of a Big Bang, but there may be other Big Bangs which led to other universes, quite separate to our own. That, I think is a fascinating idea. We won’t be able to put that idea on a firm footing until we have a better theory of the very beginning of our universe. We don’t have a theory which gives us confidence that we can talk about the first tiny fraction of a second, the first formative stages of our universe. And that’s a challenge to physicists and to cosmologists for the 21st Century.
Salazar: Would you describe what this dark matter is a little more?
Rees: We’ve known for two decades that our universe contains dark matter, and the question over those decades was whether that dark matter contributed enough material to bring our cosmic expansion to a halt – to provide a universe that would eventually halt it’s expansion. It turns out that there’s only about 30% as much as will be needed to bring the expansion to a halt. So, that suggested either that the Universe was destined to expand forever and was a rather complicated universe that had not the most likely density, or there was something else. And in the last few years, we’ve discovered that there was indeed something else, which is energy latent in space itself. This energy is very different than anything than we’re used to, for reasons which come out of Einstein’s famous equations, this energy causes the universe to speed up in its expansion, not to slow down. So, we’re faced with a universe which is far from the most simple and natural which we might have imagined, universe in which only 4% of the stuff is in ordinary atoms, about 25% is in dark matter, but more than 70% is in space itself, in the form of this energy which causes the expansion to speed up. So, the far future of our universe is likely to be a very cold and empty one, because the universe may continue expanding forever, but the galaxies will not only get further and further away, but get more and more red shifted, and eventually disappear from view, to the extant that if you came back a trillion years later, we would find a universe where there was nothing visible at all except the remnants of our own galaxy and Andromeda and a few of their near neighbors.
Salazar: What are some of the unexpected turns that astronomers have taken in pursuit of the understanding of dark matter?
Rees: There have been two important surprises in the study of cosmology in the last few years. The first is, that the expansion of the universe has been found to be speeding up. This discovery was made by looking at the expansion rate of the universe in the past by using supernova explosions as standard candles. And this surprised everyone, because gravity was the force we knew about which operated on a large scale, and gravity tends to slow the expansion down because everything in the universe exerts a gravitational pull on everything else. So, it was a big surprise when people found that the expansion was speeding up. It implied that there was some other force, apart from ordinary gravity, which was overwhelming gravity on the scale of the universe.
This force is something which goes by the name of dark energy. It’s a force that’s present in space itself, and gives rise to a kind of antigravity, which causes distant galaxies to be speeding up in their recession from us. That is the first line of evidence, but there’s been corroboration in the last three years from a different type of operation. If there had been nothing in our universe apart from dark matter and atoms, we could have predicted from Einstein’s equations that our universe would contain a space that was curved. What that means is that the angles of a big triangle would not add up to 180 degrees.
However, when observations were made of the background radiation that fills the universe, the structure of that radiation allows astronomers, by a rather clever technique, to measure whether the universe had this property of being curved or of being flat. And what they found was that the micro–background radiation had the properties you would expect if the universe was flat, in the sense that the angles of a triangle added up to 180 degrees exactly. That therefore meant that there must be something else other than the dark matter and the atoms, that flattened the universe. And that something else is this deep mystery the dark energy that’s latent in space itself. Now most physicists hope that within a few years we’ll have clearer idea about what the dark matter is. We think that they’re some kind of particles made in the Big Bang along with the atoms and the radiation. But as to what the dark energy, everyone is completely flummoxed. We won’t understand the dark energy until we’ve got a theory about the nature of space at the very deepest level, because we believe that if we were to study space on a tiny, tiny scale, a scale far smaller than single atoms, we would find that it had a complicated, grainish structure. We would find that just as you can’t subdivide a solid indefinitely, because you get down to single atoms eventually, likewise you can’t subdivide space and time. You get down to some tiny scale, where space and time are themselves quantized. And until we understand what happens on that scale, you won’t be able to explain why space has an energy associated with it, an energy that is the dominant force in the large–scale universe and causes the cosmic acceleration.
Salazar: What lies ahead for cosmologists as they peer into the cosmological frontier?
Rees: I think that there are two very different frontiers in the coming decade. One will be to understand the first tiny fraction of a second after the Big Bang to understand why the universe has expanded the way it is, why it contains a peculiar mixture of atoms, dark matter, and dark energy. That’s a challenge where astronomers and cosmologists have to work together with theoretical physicists. But there’s another challenge, and that’s to understand how our universe evolved from this hot, dense beginning, into the cosmos that we see around us, and of which we are part, the cosmos containing stars and galaxies. And we are starting to understand that by being able to observe the universe back to very large red shifts. We can observe objects now back to redshifts of more than six. What that means is that we can observe objects whose lights set out when the universe was 1/10 or so of its present age. We can probe 90% of cosmic history with our telescopes, and we can therefore not only look at not only what our universe is like today, study the galaxies that are nearby, but we can study populations of galaxies as they were five billion, ten billion, even 12 billion years ago. Thereby, we can learn how galaxies first formed, and how they evolved. And we can test these theories against computer simulations where we simulate, as it were, a virtual universe on our computer and understand how the force of gravity, and the feedback effects from stars causes galaxies to form and evolve. So, what I call environmental cosmology, understanding how a simple Big Bang evolved into our complex cosmos is one of the undoubted frontiers which we’re going to be working on in the coming decades.
For quite a long time we’ve understood the basic reasons why stars exist, and why they have the sizes and brightness they have. But we haven’t, until recently, had anything like that in our understanding of galaxies, why galaxies are these agglomerations of about a hundred billion stars, with sizes of about 10,000 or more light years.
Salazar: How did an understanding of galactic evolution come about?
Rees: We’re getting this by having much better observation and also by being able to do computer simulations, where we can simulate what happens to a young galaxy, how the stars evolve, and what feedback process they cause. We also benefit enormously from observations, because observations can now probe back further into the past by probing deeper into space. A few years ago, we couldn’t see normal galaxies all that far back, in time. But now we can see galaxies so far back that we’re almost certainly seeing, as it were, embryonic galaxies, galaxies which are just about 1 billion years old, which is less than 10% of their present age. So, we’re getting a snapshot, as it were, of all the stages in the evolution of a galaxy — from its formation from the first primordial clouds that condense out under gravity, to the present structures that we see around us, and of which we are a part in our own Milky Way.
Our universe started off very hot and very dense, but in a sense very simple, because it was expanding in a very uniform way. Every part of it was like every other part. But, for reasons we understand quite well now, the density contrast started to grow – regions that were slightly denser than average lagged behind in their expansion, whereas regions that were below average density expanded faster. The result was that density contrast grew, and the overdense regions condensed out to make the first precursor of galaxies. That, we believe happened when the universe was maybe 200 million years old, about 2% of its present age. Those first structures were much smaller than present day galaxies. They probably contained a million solar masses of material. But, as time went on, more and more stars formed in those systems, and those systems, each weighing more than a million suns, merged together to make progressively larger structures. So, the formation of galaxies is a hierarchical process, whereby starting with smaller systems, where the first star condensed, a process of successive mergers leads progressively to galaxies with the dimensions and masses that we observe. In other words, up to a hundred billion times the mass od the sun.
In the case of our own galaxy, we have evidence, rather like the fossil evidence which the geologist has about the past history of the earth. We can look at the populations of star, some of which are old, up to 12 billion years old, some of which are young. And, of course the sun, which is a middle aged star, it’s about 4 1/2 billion years old. By comparing the properties of these stars by different ages, by comparing in particular the proportions of elements like carbon and oxygen in them, we can work out how the galaxy evolved. We can work out how the galaxy started out from just pristine hydrogen and helium, eventually built up the elements like carbon and oxygen of which planets and we ourselves are made.
We can understand how this happened by studying stars of different ages. We believe that our galaxy formed by a process of smaller galaxies sticking together. And that made the so–called halo of our galaxy. But then a lot more gas fell in to this already assembled halo to make the huge disk which we call the Milky Way. This happened probably between five and eight billion years ago. And we can understand how this happened by doing computer calculations and of course by looking at other galaxies which are at the earlier stages of this process. So, we are coming to understand how our galaxy formed, and what its made of. We know that it also contains, in the outer parts, small satellite galaxies, as it were. The Magellanic clouds are the best known of these. But when our galaxy was young, it probably had a lot more of these small satellite galaxies, but most of those have now spiraled inwards and merged to become part of our entire galaxy. That’s part of the merger process that’s an ongoing feature of all galaxies. And, our galaxy is also in a group of galaxies, a small cluster of which Andromeda is the other dominant member. But there are 30 or more small members which are associated with it. In the far future of the universe, if we were to come back in a trillion years, the only thing you’d be able to see, however big your telescope was, would be this local group of galaxies. All of the rest of the universe would, through the acceleration and expansion, have disappeared from view beyond the kind of horizon.
They not only move away from us, but their red shift would have gotten so large that none of their light would be able to reach us. That’s a special feature of an accelerating universe. Not only do distant galaxies get further away, but their red shift gets bigger and bigger. So, what happens to a distant galaxy, as it moves towards the horizon, is rather like what happens to something which you see falling into a black hole. It goes faster and faster, and eventually it gets so red–shifted that you can’t see it any more.
Salazar: Thank you for the generous gift of your time. Is there anything else you’d like to share with the public or the listeners of Earth and Sky?
Rees: Let me just say one more thing, we’d like to be able to look even farther back into the past than we now can – this would involve seeking objects which have redshifts much larger than six. And, the question is, is there any chance of doing this in the near future? Some people are pessimistic – some people think that objects at red shifts much larger than six will be intrinsically faint, and therefore not detectable. But there’s a chance that there might be additional stellar explosions which we could detect even further away than the most distant galaxy. There are objects that explode much more violently than supernovae – these are called gamma–ray bursts, where a single star, in a few seconds, puts out more energy than the sun will radiate over its entire ten billion year lifetime. And, many of us believe that the very first stars are even more likely to become gamma ray bursts than any present day star would. And that therefore means that there may be the possibility of detecting the very first stars, because in their death throes, they will produce these colossal explosions, manifesting themselves by a burst of gamma rays. And, detectors now up in orbit are now able to detect these gamma ray bursts, and perhaps some of them will prove to be the most distant objects known in the universe, and therefore crucially important probes for how the very first stars in our universe formed.
We know that stars that weigh more than ten times as much as the sun explode to make supernovae. We’re familiar with supernovae from the famous remnants like the Crab Nebulae, and from the fact that these can be observed in distant galaxies. But many people believe that stars that are above, say 30 solar masses, they end their lives in a still more spectacular way. They may produce not just the optical brightening that gives us supernovae, but a very sudden flash lasting only a few seconds, which produces huge amounts of energy in the form of x–rays and gamma rays. This is a phenomenon that has been observed in the last ten years, called gamma–ray bursts. And these gamma–ray bursts, they fascinate me for two reasons. First, they are a challenge for theorists to explain, because they are the most extraordinary explosions that we know about in the universe. They display very extreme physics. But also, they’re important because owing to their brightness, they can be detected out to immense distances, and it could be that the way we will detect the very first stars, the stars that form before the first galaxies assembled, will be if some of those stars, end their lives in this cataclysmic way, giving rise to gamma–rays, which can be detected, even though those gamma–rays were emitted when the universe was only 2–3% of its present age.
The most distant things we can see in the universe now set out their lights when the universe was perhaps 1/10 of its present age. That means that if we say that the universe is 14 billion years old, those objects set out their lights when the universe was maybe 12.8 billion years old. But it could be that we’ll be able to detect the first stars that set out their lights even earlier, when the universe was only 2–3% of its present age, rather than 10% of its present age. So thereby, we’d be probing back closer to the Big Bang, to the time when the very first structures destroyed the primordial smoothness that came out of the Big Bang.
