Centuries ago, maps of the Earth had blurred boundaries, where the cartographers wrote "there be dragons." But after the pioneer navigators had traversed the globe there was no expectation of a new continent, nor that we would ever drastically revise our estimates of the Earth's size and shape. At the end of the 20th century we have, remarkably, reached this stage in mapping our universe-its spatial extent, its structure, its main constituents, and its huge span in time. For the first time the big cosmic picture is coming into focus. This story-a collective achievement of thousands of astronomers, physicists and engineers-can now be presented (in outline) with conviction.
Our own star
How did our own star, the sun, begin; and how will it end? The proto-sun condensed from a cloud of dusty gas in our Milky Way. Gravity pulled it together until its centre got squeezed hot enough to trigger nuclear fusion-the process that makes hydrogen bombs explode. Even though it is already 4.5 billion years old, less than half the sun's central hydrogen has been used up. The sun will keep shining for a further 5 billion years. It will then swell up to become a red giant, large and bright enough to engulf the inner planets, and to vaporise all life on Earth. After this "red giant" phase, some outer layers are blown off, leaving a white dwarf-a dense star no larger than the Earth, which will shine with a dull glow on whatever remains of the solar system.
Not everything in the cosmos happens slowly. Stars more than ten times as heavy as our sun expire violently, by exploding as supernovae. The nearest supernova of modern times was in 1987. On 23rd-24th February, a new bright "star" appeared that had not been visible before. Astronomers have studied this supernova closely, especially how it fades and decays. When a heavy star has consumed all its available hydrogen, its core contracts and heats up, releasing energy via a succession of reactions involving progressively heavier nuclei, of helium, carbon, oxygen, silicon and so on. The star then faces an energy crisis when it cannot draw on any further nuclear sources. The consequences are a supernova explosion, throwing back into space a mixture of atoms, in roughly the proportions observed on Earth. Why are carbon and oxygen atoms so common here on Earth, but gold and uranium so rare? The answer involves stars that exploded before our solar system formed.
Our galaxy is like a huge ecosystem, recycling gas through successive generations of stars, gradually building up the entire periodic table. Before our sun even formed, several generations of heavy stars could have been through their life cycles, transmuting hydrogen into the basic building blocks of life-carbon, oxygen, iron and the rest. Everything on Earth is the ashes of long-dead stars.
Are there other planets like earth?
There are about 100 billion stars in our own galaxy-the Milky Way-but it is only in the past few years that we have confirmed the existence of planets and planetary systems around other stars. These formed, we believe, from spinning discs of gas encircling their parent stars. Present techniques can only detect giant planets like Jupiter, but it is likely that these stars are orbited by retinues of planets, of which some may resemble Earth.
The actual layout of our own solar system is the outcome of many "accidents." Our moon was torn from Earth by a collision with another protoplanet, and the craters on the moon testify to the violence of Earth's early history.
Planets on which life could evolve, as it did here, must be quite special. Their gravity must be strong enough to stop the atmosphere from evaporating; they must be neither too hot nor too cold, and thus the right distance from a long-lived and stable star. There may be other conditions required: some claim that Jupiter was essential to life, because its gravity reduced the rate of catastrophic asteroid impacts on Earth; also, the tides induced by our large moon may have stimulated evolution. Even if there are such extra requirements planets are, it seems, so common in our galaxy that Earth-like ones could number in the millions.
The search for life on Earth-like planets is now a main thrust of Nasa's space programme. This is a long-range goal-it will require huge telescopes in space-but will stimulate much good science along the way. We still don't know whether life's emergence is "natural," or involves a chain of accidents so improbable that nothing like it has happened on another planet anywhere else in our galaxy. That's why it would be so crucial to detect life, even in simple forms, elsewhere in our solar system-on Mars, or under the ice of Europa. If it had emerged twice, quite separately, within our solar system, this would suggest that the entire galaxy would be teeming with life.
But even if simple life exists, we don't know how likely it is to evolve towards intelligence. A manifestly artificial signal from out there-even if we couldn't make sense of it-would convey the momentous message that "intelligence" (although not necessarily consciousness) was not unique to the Earth, and that concepts of logic and physics were not peculiar to the "hardware" in human skulls. The nearest potential sites are so far away that signals would take many years in transit. For this reason transmission would have to be primarily one-way-there would be time to send a measured response, but no scope for repartee.
Absence of evidence would not be evidence of absence-intelligent life may be doing nothing to reveal itself. But perhaps there is just a great silence. Some find it depressing to feel alone in a vast inanimate cosmos. I don't. If Earth were the sole abode of life in our galaxy, we could view it in a less humble cosmic perspective than it would merit if our universe teemed with advanced life-forms.
A short history of the universe
Our Milky Way, with its hundred billion stars, is just one galaxy among billions, which are visible with large telescopes. One amazing picture taken with the Hubble space telescope shows a small patch of sky, less than 1 per cent of the area covered by a full moon. It is densely covered with faint smudges of light-each a billion times fainter than any star that can be seen with the unaided eye. But each is an entire galaxy that appears so small and faint because it is several billion lightyears away. What is most fascinating about this picture is not the record-breaking distance in itself, but the huge span of time that separates us from these remote galaxies. Most are being viewed when they have only recently formed.
But what about still more remote epochs, before galaxies had formed? Did everything really start with a so-called "big bang"? The phrase was coined by Fred Hoyle, as a derisive description of a theory he didn't like. But the name stuck, and two discoveries-what James Peebles calls the two "golden moments in cosmology"-have confirmed the hypothesis.
The first of these was Edwin Hubble's realisation that our universe was expanding. This was based on his observation that the more distant a galaxy was from Earth, the faster it seemed to be receding from us. The further away it was, the more its light shifted to the longer, red end of the spectrum. The second "golden moment" was the detection by Arno Penzias and Robert Wilson of the "afterglow of creation"-of cosmic background radiation. This warmth is a relic of the original "fireball" phase; the microwaves an echo of the explosion that initiated the universal expansion.
During the first millisecond of the universe, everything would have been squeezed denser than an atomic nucleus or a neutron star. Even though we know a great deal about the nature of the "big bang," its earliest phases confront us with a condition so extreme that we do not know enough physics to solve its riddles. The foundations of 20th-century physics-quantum physics, crucial within atoms, and Einstein's theory of relativity, governing gravity and the cosmos-are still disjointed. To understand the very beginning we need a new synthesis of cosmos and microworld. The most promising current ideas involve "superstrings,"-vibrations on scales far tinier than atoms, in a space with six extra dimensions.
The history of our universe as we now understand it divides into three parts. The first part was a brief but eventful moment when the key features of the universe were imprinted during the first millisecond. This is the intellectual habitat of mathematical physicists. The relevant physics is speculative and counterintuitive. The second part is from the first millisecond to when the universe was about 1m years old. This is an era where cautious empiricists (like me) feel more at home. There is good quantitative evidence and the relevant physics is well-tested in the lab. The densities are far below nuclear density, but everything is still expanding smoothly.
The third era began when the first gravitationally bounded structures condensed, when the first stars and quasars formed and lit up. This is the era studied by most astronomers and the one in which we can witness complete manifestations of basic laws. Gravity, gas dynamics and feedback effects from early stars initiate the complexities around us.
Big bang to big crunch
In about 5 billion years the sun will die; and the Earth with it. At about that time the Andromeda galaxy will crash into our own Milky Way: our galaxy will surely end in a great crash. Will the rest of the universe go on expanding for ever? Or will the entire firmament eventually recollapse to a "big crunch"?
The answer depends on how much the cosmic expansion is decelerating. Everything in the universe exerts a gravitational pull on everything else. The expansion could eventually be reversed if there were, on average, more than about five atoms in each cubic metre. That does not sound like much. But if all the galaxies were dismantled, and their constituent stars and gas spread uniformly through space, they would make an even emptier vacuum-one atom in every ten cubic metres-like one snowflake in the entire volume of the Earth. That's 50 times less than the "critical density," and seems to imply perpetual expansion.
But things are less straightforward. We know that galaxies, even clusters of galaxies, would fly apart unless they were held together by the gravitational pull of about ten times more material than we can see. There is probably enough "dark matter" to contribute 20 per cent of the critical density. Until recently, we couldn't rule out that several times this amount might exist in the space between clusters of galaxies. That now looks unlikely. It seems that expansion will be never-ending-our universe will become ever-emptier and darker, as the galaxies recede from each other and their constituent stars exhaust their fuel.
People sometimes wonder how our universe can have started off as a hot amorphous fireball-and ended up intricately structured. Temperatures now range from the blazing core of stars to the night sky only three degrees above absolute zero. This seems contrary to the second law of thermodynamics-CP Snow's touchstone of scientific literacy-but it's actually a natural outcome of cosmic expansion and the workings of gravity.
If one had to summarise, in just one sentence, what's been happening since the big bang the best answer might be: ever since the beginning, gravity has been building up structures, and enhancing temperature contrasts-a prerequisite for the emergence of the complexity that lies around us ten billion years later, and of which we are a part. The way slight initial irregularities in the cosmic fireball evolve into galaxies and clusters is in principle as predictable as the orbits of the planets, which have been understood since Newton. We have pushed the causal chain far further back than Newton did, but we still reach a stage when we have to say "things are as they are because they were as they were."
Many universes not one?
Intricate complexity has unfolded from simple laws-we wouldn't be here if it hadn't. Our universe couldn't have become structured if it were not expanding at a special rate. And there are other prerequisites for complexity. If nuclear forces were a few per cent weaker, only hydrogen would be stable: there would be no periodic table, no chemistry and no life. Or the residue of the big bang might be entirely dark matter-no ordinary atoms at all. Or gravity could be so strong that any large organism was crushed. Or the number of dimensions might even be different.
This apparent "tuning" of the universe could be just a brute fact. But I prefer another interpretation. It is that many universes exist. The seemingly "designed" features of our universe need then occasion no surprise-just as, in a clothes shop with a large stock, you are not surprised to find one suit that fits. Perhaps, then, our big bang wasn't the only one. This speculation dramatically enlarges our concept of reality. The history of our universe would be just an episode, one facet, of the infinite multiverse. Some universes may resemble ours; most would be "stillborn" because they would recollapse after a brief existence, or because the laws governing them wouldn't permit complex consequences. Such ideas will remain speculative until we understand the physics that prevailed when our universe was squeezed smaller than an atom.
How did a fireball evolve, over ten billion years, into our complex cosmos? How did atoms assemble-on Earth, and perhaps on other worlds-into living beings intricate enough to ponder their origins? Physicists may eventually uncover the equations governing physical reality. But no scientist will be able to tell us what breathes life into the equations-why there is something rather than nothing is a mystery beyond physics.
