Martin Rees earned his degrees in mathematics and astronomy at the University of Cambridge , where he is currently professor of cosmology and astrophysics and Master of Trinity College. Director of the Institute of Astronomy at Cambridge, he has also been a professor at Sussex University. He has been Britain's Astronomer Royal since 1995.

He has modeled quasars and has made important contributions to the theories of galaxy formation, galaxy clustering, and the origin of the cosmic background radiation. His early study of the distribution of quasars helped discredit the steady state cosmological theory.

He was one of the first to propose that enormous black holes power the quasars. He has investigated the anthropic principle, the idea that we find the universe the way it is because if it were much different we would not be here to examine it, and the question of whether ours is one of a multitude of "universes." He has written nine books .

Through his public speaking and writing he has made the Universe a more familiar place for everyone.

Helen Matsos (HM): Last year the big "science event" was measuring the cosmic microwave background and dating the big bang to 13.8 billion years ago, within an 8 to 10 percent margin of error. Can you give us some idea of the boundaries of the big bang - what was it like in the first seconds, and how far will the universe expand in the future?

Martin Rees (MR): It is remarkable that in the last two years we have been able to firm up some of the basic cosmic numbers about the age of the universe, the way it's expanding, and also what it's made of.

What it's made of turns out to be rather surprising because atoms are only 4 percent of the total, another 25 percent is so-called dark matter - probably some particles made in the big bang that have no electric charge but just swarm around.

And there's also some energy latent in empty space itself, something we call dark energy, and that's what's controlling the expansion of the universe. So we've learned that the universe has these rather mysterious ingredients.

The long-range forecast is that the universe will go on expanding forever. Stars will eventually burn out, the atoms they're made of will eventually decay, and the stars will erode away. Distant galaxies will not merely fade but will get further and further apart and disappear from view because of the red shift.

So the long-range future is a universe that is a very cold and empty place. Nonetheless it will go on for an infinite time.

That's the best guess, but I think we can't have great confidence in that forecast because it depends on the nature of dark energy, which at the moment is making the expansion of the universe speed up. If it continues that way then we can forecast an infinite future, but the dark energy may be more complicated than we know, so we can't be sure about the future.

As regards the past, we can trace things back to the initial instant of the big bang. When the universe had been expanding for one second, at a temperature of about 10 billion degrees, the density of atoms still was not very high.

But when we go back to the first microsecond, the first nanosecond, the first tiny fraction of a second, then things become slightly more uncertain because conditions were more extreme. If we go back to times earlier than of a trillionth of a second, then the conditions were so extreme that we don't have any confidence in explaining the physics.

In the first trillionth of a second, every particle in the universe was moving with more energy than can be produced in the biggest possible accelerator on Earth, and the density was far higher than the density of the atomic nucleus.

So the very early universe is a matter or conjecture rather than consensus, because we don't understand the basic laws. Nonetheless, there are many fascinating ideas about what happened in the very early universe in that first tiny fraction of a second. Certainly the key features of the present-day universe were imprinted at that time.

The fact that the universe contains matter but not antimatter, the way it is expanding, the fact that it is fairly smooth but has these fluctuations which were the seeds for galaxy formations - all those features were determined at very early stages by physics.

HM: So here it comes Professor Rees, my favorite slumber party question: What happened before the big bang?

MR: (laughs) People always ask, "What happened before the big bang?" We certainly can't answer that question, because we have to worry about what the question might actually mean.

One of the most popular ideas by physicists is that when you extrapolate back to the very beginning, we have to jettison many of our common sense ideas about space and time. Maybe it's no longer the case that space has just three important dimensions and time just ticks away.

That makes the early universe more complicated to analyze. If you don't have a clear idea of clocks ticking away, the idea of a direction of time - a "before" and "after" - doesn't have any clear meaning.

There are lots of ideas of what might have happened at the very beginning, but we can't say whether there are other big bangs apart from ours.

If there are, we can't say whether they are before or after or alongside ours, because to make such a statement implies that you can have a single coordinate system covering them all and a single clock that can be coordinated and synchronized between the different universes.

So we can't trace things right back to the beginning, we can't say whether our universe is the only one, and we can't even say whether there are only three dimensions of space.

HM: Are you alluding to string theory? Does this theory shed new light on multiple universes?

MR: One feature of string theory is it requires six extra-spatial dimensions. The debate is about whether those dimensions all are so tightly wound that they manifest themselves on a microscopic scale. Each point in our ordinary space would be like origami, tightly wound to six other dimensions.

But the more exciting possibility is that not all the extra dimensions are tightly wound together. There could be other universes that are separate three-dimensional spaces, separated from us because we are all embedded in four-dimensional space.

We are unaware of them in the same way that bugs crawling around on a sheet of paper might be unaware of bugs on a different sheet of paper. Each think they are in a two-dimensional universe, and have no concept of a third. So there could be another universe just a millimeter away from us.

That's one of the many ideas opened up by string theory. The ideas are very speculative because there's no direct measurement we can make, but they have made people more open-minded about different possibilities. Physical reality is much more complicated than we can observe with our telescopes.

Indeed, some extreme versions of this idea suggest that physical reality might be as complicated as biology, and that what we call our "observable universe" may be, in the perspective of cosmic reality, no more than one twig on one tree in some enormous forest.

HM: Almost like a fractal analogy.

MR: Yes, but on a vast scale.

HM: Do you personally believe in string theory?

MR: When it's so uncertain, it's best to remain agnostic and open-minded about all these new ideas. I certainly think it's good that people are seriously exploring these ideas in the hope that there will be some way of firming them up.

It's an inspiring conception: a physical reality even grander than the part people can see. Just as we regard our Earth as a rather special oasis in our galaxy, so we might regard our whole observable universe as some friendly oasis within a huge multi-verse.

The Martin Rees interviews on cosmology and biology are serialized in three parts: Our Cosmic Patch (1), Before the Beginning (2), and Our Cosmic Self-Esteem (3).

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