Martin Rees, Baron Rees of Ludlow, has been the Astronomer Royal  since 1995, and is also Emeritus Professor of Cosmology and Astronomy at  the University of Cambridge, and Honorary Fellow of King’s College,  Cambridge. On behalf of the King’s Review, Professor Michael Proctor,  Provost of King’s College, sat down with Professor Rees to talk about  the past and future of cosmology, the big bang, and the possibility of alien life.

Michael Proctor: Within the twentieth and, even more  so, the twenty-first century, we’ve seen an explosion in our  understanding of the universe. How do you see this in the context of  astronomy since Newton in the last few centuries?

Martin Rees:

Since the earliest times, people  everywhere have looked up at the stars and wondered about them. The  night sky is the most universal feature of the human environment. Newton  was the first person to actually quantify what was happening in the  solar system. Of course, right back to the Babylonians, records of the  sky had been taken. They didn’t understand what was happening, but they  discerned regularities and predicted eclipses. After Newton, the  structure of the solar system became better understood. Newton was lucky  to hit on one of the few phenomena in nature which was both  understandable and predictable. The orbits of the planets were  understandable and predictable, and that was really the first exact  science.

The next step forward was in the nineteenth-century when people  realized that the stars were made of the same stuff as the Earth. Of  course, the traditional view had been that the vault of heaven was made  from some kind of ‘fifth essence’. From the mid nineteenth-century,  astronomers were able to take spectra of the brightest stars and of the  sun and realize that they are made of the same stuff. That allowed  astronomers to do not just dynamics, but to do physics. That’s the  process whereby we came to understand so much about the universe, by  applying the physics that we know on Earth to the stars.

What has benefited us more through the last century is the burgeoning  of technology. We’re certainly not as wise as Newton was; arm-chair  theory alone wouldn’t have carried us far. Progress has stemmed from  more powerful telescopes, being able to observe the sky from space, and  so forth. But I think it’s amazing we’ve got as far as we have and that  is because astronomy is based on physics and physics is easier than  biology.

Another point I should have mentioned is that even where we can’t do  experiments on what’s out there, we’ve benefited hugely from computer  simulations – experiments in a virtual world. Computers are now powerful  enough to do realistic simulations of what happens when stars explode  or galaxies crash together – this is a really crucial development in the  last ten or twenty years. Now we can use the kind of programs that  aero-engineers use test aerofoils, as a substitute for wind tunnel  tests. If they have enough confidence in them in that context, we should  have reasonable confidence in the outcome of simulations of stars and  galaxies.

Cosmology used to be described as a science with only two numbers in  it: that was said by those who liked the subject. Those who didn’t like  the subject said it wasn’t a science at all – because it dealt with a  unique entity, and you couldn’t really say much about the Universe as a  whole. But cosmology has now been transformed. Observations made by all  kinds of techniques, both on the ground and in space, have revealed a  great deal about the range of objects that exist in the Universe – from  planets, up to galaxies. Also, we have, at least in outline, a  ‘timechart’ of how our Universe has evolved, from some mysterious  beginning about 13.8 billion years ago to its present structure. One  challenge now is to fill in the details of how the first atoms, stars,  galaxies and planets formed – to understand our ‘cosmic environment’, as  it were. A second challenge it to probe back very close to the  beginning, where there are still mysteries. We’d like to understand why  the Universe contains the ‘mix’ of matter and radiation that we observe,  why the Universe is expanding.

MP: There are a number of new observational devices  available to cosmology – the Planck instrument, for example. Is it  likely that further advances in observations might yield useful  information concerning the challenges that you describe?

MR:

We now, thanks to huge amounts of excellent  data, have a very good understanding of galaxies. We know that they  contain stars and gas, but they also contain dark matter: this is made  up of a swarm of particles that behave as though they are affected by  gravity but no other forces. But we don’t know exactly what these  particles are: we only know their aggregate properties. Another big  challenge is to understand how galaxies and clusters have evolved from  the early Universe, which was hot, dense, and almost smooth. Here we’ve  actually had wonderful success, especially by careful measurements of  the microwave radiation which is an ‘afterglow’ of the big bang – and  how its temperature varies between different directions.

But these advances bring into sharper focus all the things we don’t  understand: one of course is what this dark matter is. We know it  behaves as though it’s a swarm of electrically-neutral particles, but  there’s a variety of possible candidates. In the early Universe there  was not only radiation, and a lot of atoms, but there was this extra  stuff, and we’d like to understand it. There are three lines of attack  on that problem. The first is to hope that physicists will discover new  particles that have properties consistent with the dark matter. That  hasn’t happened yet. The experimenters at the Large Hadron Collider  haven’t found any particles, apart from the Higgs particle. But they are  still looking for so-called super-symmetric particles.

The second method is to actually detect the particles directly. This  can be done because the dark matter pervades the entire galaxy. Some of  these particles would be moving through this room at about 200 km per  second. They mainlygo straight through matter. But occasionally one of  them would collide with one of the atoms in the room; delicate  experiments could detect the ‘recoil’ when this happens. Searches of  this kind have to be done deep underground to get rid of all the  backgrounds, and to seek the rare events when one of these dark matter  particles interacts with an atom in a sensitive detector. Several such  experiments have already been done. So far there are no compelling  detections, but as instruments improve (and involve a larger mass of  detector material) they are reaching the sensitivity level where they  might expect to see something.

The third line of attack is to look up into space, because some ideas  suggest that these particles would collide with each other and  annihilate themselves, converting their energy into high energy photons,  particles of light, or into positrons. There are detectors in space  which are sensitive to energetic photons (gamma rays), and to  fast-moving particles – but here again there are still no more than  tantalising clues. It’s hard work actually pinning down what this dark  matter is. All we can say now is that there’s about five times as much  mass in it, in terms of gravitational effects, as there is in all the  stars and gas we see. That’s why it’s so important.

MP: It’s just detected in the large by its gross gravitational effect?

MR:

In the large, yes. And we can infer something  about its gross properties. We know it’s not like a fluid or a gas by  observing what happens when galaxies or clusters have collided. If it  were a fluid you’d get shocks and compression waves. Whereas if it’s a  swarm of independently moving particles, two swarms go through each  other and ‘phase mix’. The latter option fits the data best.

MP: That’s quite exciting. But the path from  observation to theory is quite indirect in this case. You have this  great effect of dark matter and then you have to find the particles,  before you can begin to understand them. So we are a long way from  reaching such an understanding.

MR:

Fortunately, one can make progress in  understanding the effects of the dark matter even without understanding  exactly what it is. Indeed, we cannot get a consistent picture for the  formation of galaxies without it. Clusters of galaxies are the biggest  self-gravitating structures in the Universe. One can measure the total  mass of these systems in a number of ways. You can calculate the total  mass needed to bind the system and stop it flying apart. And the  temperature of gas in the cluster can be measured by detecting the  energy of the X-rays that very hot gas emits, and that tells you the  depth of the gravitational potential well it’s confined in. The third  thing you can do is to look for the effect of gravitational lensing,  light-bending, by a cluster of galaxies. Distant galaxies being viewed  though a foreground cluster seem magnified, and distorted into streaky  arcs, by the ‘lensing’ due to the cluster’s gravity. The cluster behaves  like a poorly figured converging lens, and from the strength of the  focusing you can infer its mass.

MP: And its distribution as well.

MR:

Yes. And these three very different lines of  evidence all give consistent estimates of how much dark matter exists in  clusters of galaxies. I stress this because there have been conjectures  that dark matter does not exist and that what we’ve got wrong is the  theory of gravity. It’s indeed true that if the inverse square law of  gravity broke down at large distances, the inferences would change. But  you’d then be jettisoning not just Einstein but Newton too, and most of  us are unwilling to do that except as a last resort. I personally don’t  think such ideas should be taken very seriously unless and until we  exhaust all the options for dark matter. Moreover, any alternative  theory has to mimic the effects of dark matter consistently in very  different ways – in light-bending, as well as its effect on galaxies and  gas.

MP: Do you regard String Theory as a science? Are there any experiments that can validate it at this stage?

MR:

The theory hasn’t yet been related to any  phenomena we can observe or measure. Most of its proponents accept this:  it won’t be taken seriously as science until there are such tests or  until it yields formulae for basic numbers which we can’t otherwise  explain. We are not going to have any direct measurements at the huge  energies where its effects dominate because that’s billions of times  higher than the energies we can achieve in the accelerator – indeed the  best hope is cosmological observations which probe, at least indirectly,  the initial instants of the big bang.

But there’s one important point here. One will never have the data to  test every consequence of any theory. But that would be expecting too  much. You have to be able to test a theory robustly enough to gain  confidence in it; having done that, you then take seriously the  predictions that you can’t directly verify or refute. For example, we  believe what Einstein’s theory of gravity says about the inside of black  holes, even though we can’t observe there, because we have been able to  test and corroborate the theory in many other ways, outside black  holes.

MP: You talked about the variations in the cosmic  microwave background as being the starting point for numerical  calculations, but is there any explanation for the anomalies that you  actually observe?

MR:

It’s very important in presenting cosmology,  especially to non-specialists, to keep clear water between the parts  that are well-established and the parts that are still speculative. I  would criticize some popular writers for either over-egging the more  speculative bits, or blurring that distinction. I would claim that we  have great confidence in extrapolating back to when the Universe was one  second old. That’s the era when nuclear reactions turn hydrogen to  helium and deuterium, and the predictions match well with what we find.  Indeed that extrapolation can be made with as much confidence as most  inferences that geologists offer about the early history of our Earth.  Cosmologists have ‘fossils’, and indeed data that are easier to  interpret and more quantitative. So back to one second after the big  bang, cosmology is now no more ‘speculative’ than geophysics.

Indeed, we have reasonable confidence extrapolating back a bit  further – to a nanosecond. That’s the time at which all the particles  are moving around with an energy similar to what is achieved in big  accelerators like the LHC. If we try to extract it back still further,  then of course the conditions are beyond the direct reach of  experiments. When the universe was a nanosecond old, everything we now  see out the limits of our horizon (including billions of galaxies) would  be squeezed to the size of our solar system. But to address fundamental  questions about why the universe is expanding the way it is we have to  go back far further still – to an era when the Universe would be  squeezed down to the size of a tennis ball. That is a huge backward  extrapolation. There is a theory called inflation which would apply at  that very early stage and which has survived some tests. The  fluctuations predicted by this theory are consistent with what we  actually observe. So it has something going for it. Inflation theories  have been studied for 30 years, and there are a huge number of variants  with regard to the detailed physics. We would like to narrow the range  down by further observations. (And some results announced just last  month, from an experiment called Bicep 2 which measured a new kind of  fluctuation, are an important further step). In particular we would like  to know whether our Big Bang the only one or were there others as well.

MP: There are physical quantities, like the fine  structure constant and other physical constants, which obviously have an  effect on the universe. Could these take different values from the  observed ones, and still lead to a sensible universe? Are there any  constraints which apply?

MR:

The first thing to say is that we would not make  any progress in cosmology were it not that these constants do seem to  be ‘universal’, at least over the cosmic domain that we observe. One  could envisage, in principle an ‘anarchic’ universe where different  galaxies, or even different stars, were governed by different physics.  It does seem that the strength of gravity, the mass of the electron, and  other basic numbers of physics are the same everywhere we can measure  them. And we can make that statement with pretty high precision. We can  tell that gravity has not changed very much in the past because it would  make stars evolve differently. If gravity had been stronger in the  past, then the centres of stars would have been squeezed more and  nuclear fusion would have burnt them up more quickly. And the early  universe would have expanded faster. So we infer that gravity has not  changed very much. We can also analyse the light from distant galaxies.  In their spectra you see various features which correspond to different  chemical elements. These spectra indicate that the atoms in distant  galaxies are the same as those in the lab to precision of one part in a  million, and that the mass of the electron has not changed more than  that.

We have fairly good evidence that a basic uniformity prevails  throughout the part of our Universe that we can see. But this opens up a  very important question. Is the observable domain a large part of  overall physical reality, or could it be a tiny (and perhaps  unrepresentative) fragment of some still vaster whole? Even the most  conservative cosmologists would accept that there is a lot of material  beyond what we can see. Our ‘horizon’ is delineated by the distance  light can travel since the big bang. There’s no more reason to believe  that this is a limit to the overall universe, any more than you believe  the ocean ends just beyond your horizon. There are indeed fairly strong  reasons for thinking that the Universe extends thousands of times beyond  the domain we can see. If you look as far as you can in one direction,  and then in the opposite direction, then conditions look just the same.  If there is an overall gradient, it is very slight – less than one part  in a hundred thousand across the distance we can observe, That suggests  that if we’re in a finite ‘island universe’ that has an edge, that edge  is far further away than our horizon

MP: But that’s a philosophical question. If light  can only have travelled as far as we can see since Big Bang, or does it  mean to say there is something beyond that? Does that mean there has to  be another Big Bang in another place?

MR:

No, in fact one has to be wary of  over-interpreting the speed of light because the famous ‘speed limit’  only applies in flat space, and when the speed is measured by a local  and stationary clock. For instance, if we take the present favoured  cosmology seriously, and the Universe is accelerating, what happens if  you watch a particular galaxy? You will only see a finite part of its  future. It gets more and more red shifted and its clock appears to us to  go slower and slower, so however long we watch it for, we only see a  finite part of its future. It’s rather like what happens if you watch  something (or someone) falling into a black hole: the theory says you  only see a finite part of its future, you don’t see the final  spaghettification in the centre of the black hole. So most people would  accept unobservable galaxies beyond the horizon as part of our physical  reality. When we get to the idea of other Big Bangs then it becomes a  bit more speculative.

These may well be unobservable, but I still think they are part of  (albeit speculative) physics and not metaphysics. I like to present this  argument as an exercise in aversion therapy. If you are scared of  spiders, you start off with a little spider a long way away and end up  with tarantulas crawling all over. You start off by noting there are  galaxies which we can’t observe because they are too far away. In a  decelerating universe they’d eventually come into view, but because of  the acceleration they can never be observed even in principle. But that  doesn’t make them less ‘real’. Most of us are happy with that, so why  should we be less happy with the epistemological status of unobservable  galaxies which are the aftermath of a different Big Bang?

If there are other Big Bangs, then would they be governed by the same  physics as ours? This is a very interesting question because many  theorists, like those who work with String Theory, suspect that there is  nothing unique about our physics, and in particular they think that  there could be spaces where the repulsive force that we call lambda  could be much stronger. And the micro physics could be different too:  for instance, electrons could have different masses. Some of these  universes would be sterile or stillborn – the laws may not allow complex  chemistry, emergence of stars and planets, enough time for evolution,  etc. We would then find ourselves not in a typical universe, but a  typical member of the subset that allows complexity to develop. This is  an instance of what’s called ‘anthropic reasoning’ – something that  makes some physicists foam at the mouth, but which I think may be  unavoidable.

I often give talks about this theme and tell the audience that if you  don’t like the idea of multiverses, then just regard this as an  exercise in counter-factual history – rather as you might conjecture,  say, what would have happened if the dinosaurs had not been killed off,  or if the Brits had fought better in 1776. So in the same spirit, even  those who don’t like the idea of multiple universes can develop their  intuition by asking what the universe would be like if key parameters  were a bit different. One definite requirement for a complex universe is  a force of gravity – but it’s best if it’s very weak. Gravity is about  40 powers of 10 weaker than electric forces on the atomic scale. It  ‘wins’ on big scales because, whereas positive and negative electrical  charges are almost in balance in any large object, everything has the  same sign of ‘gravitational charge’. And it is only because gravity is  so weak for that reason that our cosmos can be so big, and entities like  us can exist without being crushed by gravity. On the microscopic  scale, of atoms and molecules, gravity is negligible.

Imagine a set of objects of increasing mass – sugar lumps, asteroids,  planets and stars. For the sugar lumps, gravity is ineffective, for the  asteroids, ditto, though for objects bigger than asteroids gravity is  powerful enough to make it round – like a planet. Planets heavier than  Jupiter would actually be crushed, and you then get into the realm of  the stars. Because gravity is so weak, there are many powers of ten  between the microworld and the cosmos. And that is essential for our  existence, because structures like human beings are very large compared  to atoms, with have layer upon layer of complex structure, but we can  stand up rather than being crushed by planetary gravity.

So there has to be one large number in cosmology, a large number that  reflects the weakness of gravity compared to the micro physical forces.  Because gravity is weak, stars are big and live a long time. But there  are other requirements for a complex cosmos. The existence of a periodic  table of elements requires a rather delicate balance between the two  most important forces in the microworld: the electrical force, which  repels protons from each other, and the nuclear force which binds them  together in a nucleus. It’s this same delicate balance which allows  nuclear fusion to release the energy that powers the stars. Otherwise  there would be just hydrogen – no fuel for the stars, no complex  chemistry and no ‘us’.

MP: That is a very good explanation of why we are as  we are. Of course, you could have a different large number and you get  something similar. There must be a range.

MR:

Yes, gravity isn’t fine-tuned. It just has to be  weak. If it were still weaker that might be even better, insofar as  there would be even more time, and an even larger object could assemble  before gravity crushes it. In contrast, some of the other numbers have  to be tuned more carefully to get the periodic table. The balance  between the nuclear and electric forces has to be fairly close. The  other number which is important is the number lambda which measures the  cosmic repulsion. If that were much bigger then galaxies would never  have formed because the Universe would have started to accelerate before  they had had a chance to form and gravity would be overwhelmed.

MP: You’ve talked about very small intervals after  the Big Bang. Is the Big Bang a singularity in time or only in space? If  it’s not a singularity in time what happened beforehand?

MR:

It’s in a sense a singularity in both, but we  would suspect a singularity would be smoothed over if we had the right  physics. The singularity is a signal that the physics is broken – that  known physics has been applied beyond the range where it’s valid. As you  go beyond the scales of the everyday world, either up or down, then  you’ve got to jettison more and more common-sense notions. We’re used to  the idea that the microworld confronts us with counterintuitive quantum  effects. In the still more extreme conditions of the ultra-early  universe then the idea of three spatial dimensions and time ticking  away, may be oversimplified. Space and time may get screwed up and  intermingled; extra dimensions may come into play and the idea of before  and after might not be clear-cut. The idea of what’s before the Big  Bang is a mystery at the moment. But I think what is remarkable is that  we can talk with a straight face back to a nanosecond and say something  about it and that’s progress in the last 40 years. When I was a research  student back in the 1960s, even the idea of the Big Bang was  controversial.

MP: Obviously, you think that the Big Bang is the  correct explanation for the origins of the universe, as opposed to, say,  the steady state theory. Do you feel there is a place for any sort of  external force or creator?

MR:

We don’t know about the very beginning. The Big  Bang may be embedded in some grander structure where there are many,  many Big Bangs. I think all bets are off with regards to how it started  and whether there even was a beginning. I think the aim of studying the  cosmos is to push back the causal chain. Newton explained why the  planets move in ellipses, but he wrote in a couple of places that he  found it mysterious that the planets were moving on more or less the  same plane, what we call the ecliptic, whereas the comets come from more  random directions. He thought that must be providence. We understand  now why planetary orbits are more or less coplanar – it’s a consequence  of their origin in a dusty protostellar disc. And we’ve pushed back the  causal chain to understand the formation of atoms, stars and galaxies,  but there’s always a further step as well.

MP: Is there ‘life out there’?

MR:

That’s one of the most fascinating questions of  all. But it’s a biological question. And biology is a more difficult  subject than astronomy in that it deals with more complicated phenomena.  We don’t even understand how life began on Earth. We understand how  Darwinian evolution led from simple life to our complex biosphere. But  people don’t understand the transition from complex chemistry to the  first metabolising and reproducing systems. It’s gratifying that some  really serious biochemists are now addressing this question When we  understand that, it will tell us two things: whether it’s likely or  unlikely that extraterrestrial life is widespread; and whether there is  something particularly special about the chemistry on which terrestrial  life is based. In other words, would we expect any other life to have  the same DNA? So we don’t know that. Even the most firmly Earth-bound  biologists would be fascinated by this issue. Another exciting prospect  is that observations will tell us whether some of these planets have a  biosphere. That will be do-able within ten or twenty years by the next  generation of giant telescopes, powerful enough to provide sharp images,  and collect enough light to identify the spectrum of a planet even if  it’s millions of times fainter than its parent star.

MP: It’s all very exciting. Like the Universe, the discoveries are accelerating.

MR:

Astronomy is a fundamental science, but it’s  also an environmental science. If you look up at the stars, they’re the  most universal part of the human environment. And what we’re discovering  makes this environment seem more fascinating. I’m sometimes asked  whether being an astronomer has any impact on my attitudes to everyday  life. The one thing I certainly know, having lived among astronomers, is  it doesn’t make them any more serene and relaxed about everyday matters  (and you must know that as well). But there is one special perspective  which they probably do have, and that’s an awareness of the far future.  Most people are aware that we’re the outcome of about 4 billion years of  evolution. But I’d guess that they tend to feel that we humans are the  culmination – the endpoint of the process. No astronomer could think  that way. We know the future is at least as long as the past: the sun is  less than halfway through its life – the universe may even have an  infinite future. So to us, it seems natural to suppose that humans are  just a step on the way, maybe not even a halfway stage in the emergence  of ever greater complexity. And this post-human evolution could happen  here on Earth or somewhere beyond the Earth; it may be silicon-based  rather than organic. This realisation gives us a different perspective  on humans, but also perhaps an extra motive for concern about the  future, because we realise that a catastrophe could foreclose an immense  post-human era.

MP: It seems most unlikely considering the short time we’ve existed.

MR:

Any creatures who will be alive to witness the  death of the sun won’t be human – they could be as different from us as  we are from protozoa, because the time between now and then is longer  than the Earth’s resent age. Indeed future evolution is going to take  place not on the Darwinian time scale, of natural selection, but on the  technology time scale, because we’re obtaining the capacity to modify  the genome. We might try to constrain such developments on Earth, but if  there are communities, a few centuries from now, living on other  planets or on asteroids we’d surely wish them good luck in deploying all  known science to adapt their progeny to an alien and hostile  environment

MP: Do you think that’s very likely?

MR:

I don’t think manned space flight should be  prioritized now, but I think it’s going to happen. One day there will be  groups of pioneers living on Mars or on asteroids. I personally think  the initial stages will involve crazy adventurers, bankrolled via  independent, private funds. But I think it’s very important not to kid  ourselves that we can solve Earth’s problems by mass emigration into  space. There’s nowhere in our solar system even as clement as the top of  Everest or the south pole, so it’s only going to be a place for  pioneers – on cut-price private ventures and accepting higher risks than  a western state could impose on civilians.

MP: you have had an enormously productive career as  an astronomer, during a very exciting period. But do you think that if  you’d been 30 years younger, you might have been a biologist instead?  Biology has become much more dynamic in recent decades.

MR:

I don’t know what I would’ve done. I think I am  the kind of person who is better at writing the first paper on a subject  than the last definitive one. I prefer to work on topics that are just  opening up – trying to get the general picture, rather than tidying  things up. We know that there are people of both those types in every  science. And I prefer a more synoptic style of thinking – trying to make  sense of fragmentary data, rather than pursuing elaborate mathematical  modelling or deductions.

I’ve been quite lucky in that most of the work I’ve done has been on  newly discovered phenomena and one can make some advances by applying  general principles in a simple way and so I would’ve wanted to choose a  subject of that kind. The advice I give to all students is to pick a  subject where new developments are happening – either new observations  or more powerful techniques. Otherwise you’re stuck trying to do the  problems that the old guys got stuck on – and unless you’re cleverer  than them, you won’t succeed either. So you’ve got to try to tackle a  problem that they didn’t have a chance to do.

Looking back over my research, I was lucky because when I started,  the subject was opening up: the first evidence of black holes, the first  quasars, the first evidence of the Big Bang, the first pulsars, all  came in the late 1960s. And so it was good to be a young astrophysicist  then, because the experience of the old guys was at a discount.