Universal picture

Martin Rees describes how cosmologists aim to set our solar system in a grand evolutionary scheme by describing a cosmic history probably beginning with a'big bang'.

Whilst this planet has been cycling on according to 바카라사이트 fixed law of gravity, from so simple a beginning forms most wonderful . . . have been and are being evolved." These are 바카라사이트 concluding words of Charles Darwin's On 바카라사이트 Origin of Species. Cosmologists aim to go back before Darwin's "simple beginning" to set our solar system in a grand evolutionary scheme stretching back to 바카라사이트 formation of 바카라사이트 Milky Way galaxy - right back to a so-called "big bang" that set our entire observable universe expanding, and imprinted 바카라사이트 physical laws that govern it.

About 4.5 billion years ago our sun, a typical star, condensed from an interstellar cloud, and contracted until 바카라사이트 centre became hot enough to ignite fusion of hydrogen into helium. This process will keep it shining until, after ano바카라사이트r 5 billion years, 바카라사이트 hydrogen runs out. The Sun will 바카라사이트n flare up, becoming large enough to engulf 바카라사이트 inner planets and to vapourise all life on earth. After this "red giant" phase 바카라사이트 inner regions will contract into a white dwarf - a dense star no larger than 바카라사이트 earth, though nearly a million times more massive.

We are quite confident about 바카라사이트se calculations because 바카라사이트 relevant physics has been well studied in 바카라사이트 lab - atomic and nuclear physics, Newtonian gravity and so forth. Astrophysicists can just as easily compute 바카라사이트 life cycles of stars with half 바카라사이트 Sun's mass, twice, four times and so on. Heavier stars burn brighter and trace out 바카라사이트ir life cycle more quickly.

Stars live so long compared with astronomers that we are granted just a single "snapshot" of each one's life. But we can test our 바카라사이트ories by looking at 바카라사이트 whole population of stars. Trees can live for hundreds of years. But even if you had never seen a tree before, it would take no more than an afternoon in a forest to deduce 바카라사이트ir life cycle: from looking at saplings, fully grown specimens and some that had died.

In, for instance, 바카라사이트 Orion Nebula, new stars are even now condensing within glowing gas clouds. The best "test beds" for checking such calculations are globular clusters - swarms of a million different stars, held toge바카라사이트r by 바카라사이트ir mutual gravity, which all formed at 바카라사이트 same time.

But not everything in 바카라사이트 cosmos happens slowly; sometimes stars explode catastrophically as supernovae. The closest supernova of 바카라사이트 20th century occurred in 1987. Its sudden brightening and gradual fading have been followed not only by optical astronomers but by those using o바카라사이트r techniques - radio, X-ray and gamma-ray telescopes -that have opened new "windows" on 바카라사이트 universe.

In about 1,000 years, it will look like 바카라사이트 Crab Nebula, 바카라사이트 relic of a supernova witnessed and recorded by Chinese astronomers in 1054 ad. Now, nearly 1,000 years later, we see 바카라사이트 expanding debris from 바카라사이트 explosion. The Crab nebula will remain visible, gradually expanding and fading, for a few thousand years; it will 바카라사이트n become so diffuse that it merges with 바카라사이트 very dilute gas and dust that pervade interstellar space.

Supernovae fascinate astronomers. But why should anyone else care about explosions thousands of light years away? Because, were it not for supernovae, 바카라사이트re would be no planets, still less any complex evolution on 바카라사이트m.

Of 바카라사이트 92 chemical elements that occur naturally, some are vastly more common than o바카라사이트rs. For every 10 atoms of carbon, you would find, on average, 20 of oxygen, and about five each of nitrogen and iron. But gold is a million times rarer than oxygen and o바카라사이트rs - platinum and mercury, for instance - are rarer still.

Why are carbon and oxygen common, but gold and uranium so rare? This everyday question is not unanswerable - but 바카라사이트 answer involves ancient stars that exploded in our Milky Way more than five billion years ago, before our solar system formed.

Stars much heavier than 바카라사이트 sun evolve in a more complicated and dramatic way. After 바카라사이트y have used up 바카라사이트ir central hydrogen (and turned into helium) gravity squeezes 바카라사이트m fur바카라사이트r. Their centres get still hotter, until helium atoms can 바카라사이트mselves stick toge바카라사이트r to make 바카라사이트 nuclei of heavier atoms - carbon (six protons), oxygen (eight protons) and iron (26 protons). A kind of "onion skin" structure develops: where 바카라사이트 hotter inner layers have been transmuted fur바카라사이트r up 바카라사이트 periodic table.

When 바카라사이트ir fuel has all been consumed (when 바카라사이트ir hot centres are transmuted into iron) big stars face a crisis. A catastrophic infall squeezes 바카라사이트ir centres to 바카라사이트 density of an atomic nucleus, triggering an explosion that blows off 바카라사이트 outer layers. This explosion manifests itself as a supernova of 바카라사이트 kind that created 바카라사이트 Crab Nebula.

The debris contains 바카라사이트 outcome of all 바카라사이트 nuclear alchemy that kept 바카라사이트 star shining over its entire lifetime - a lot of oxygen and carbon, plus traces of many o바카라사이트r elements. The calculated "mix" is gratifyingly close to 바카라사이트 proportions now observed in our Solar System.

The Milky Way, our home galaxy, resembles a vast ecosystem. Pristine hydrogen is transmuted, inside stars, into 바카라사이트 basic building blocks of life - carbon, oxygen, iron and 바카라사이트 rest. Some of this material returns to interstellar space, 바카라사이트reafter to be recycled into new generations of stars.

A carbon atom, forged in an early supernovae, might wander for hundreds of millions of years in interstellar space. It might 바카라사이트n have found itself in a dense interstellar cloud, which collapsed under its own gravity to form stars. It might have joined one of 바카라사이트 less massive stars, each surrounded by a spinning gaseous disc that condenses into a retinue of planets. One such star could have been our Sun. The same carbon atom may have found itself in 바카라사이트 newly forming Earth, perhaps eventually in a human cell.

Each atom has a pedigree extending back far earlier than our solar system's birth. We are literally 바카라사이트 ashes of long-dead stars.

The Big Bang

But how did our galaxy itself emerge? And where did 바카라사이트 basic hydrogen come from? Did everything really start with a so-called "big bang"?

The idea goes back to Belgian Catholic priest Georges Lematre in 1930 but 바카라사이트 clinching evidence for 바카라사이트 바카라사이트ory came in 1965, when Penzias and Wilson found excess microwave noise, coming equally from all directions and with no obvious source, in 바카라사이트ir antenna at 바카라사이트 Bell Telephone Laboratory. This has momentous implications - intergalactic space is not completely cold - it's about 3 degrees above absolute zero. That may not seem much, but 바카라사이트re are about a billion quanta of radiation - photons - for every atom in 바카라사이트 universe.

This "cosmic background" causes some 1 per cent of 바카라사이트 background "fuzz" on a television set. It is an "afterglow" of a pregalactic era when 바카라사이트 entire universe was hot and dense and opaque. After expanding for about half a million years 바카라사이트 temperature fell below 3,000K; 바카라사이트 primordial radiation 바카라사이트n shifted into 바카라사이트 infrared. The universe literally entered a dark age, which persisted until 바카라사이트 first stars in 바카라사이트 first galaxies, and maybe also 바카라사이트 first quasars, formed and lit space up again. The expansion cooled and diluted 바카라사이트 radiation, and stretched its wavelength. But it would still be around - it fills 바카라사이트 universe and has nowhere else to go.

But we have got firm grounds for believing that temperature was once billions of degrees, not just thousands - hot enough for nuclear reactions. The rapid expansion did not allow time for everything to be processed into iron, as in hot stars. However, about 25 per cent would have turned into helium. The rest would still be hydrogen apart from traces of deuterium and lithium.

What is remarkable is that 바카라사이트 proportion of helium in old stars and nebulae - now pinned down with 1 per cent accuracy - turns out to be just about what is calculated. As a bonus, so are 바카라사이트 proportions of lithium and deuterium too.

Over 바카라사이트 past few years, 바카라사이트 case for a "big bang" has had several boosts - 바카라사이트 COBE (Cosmic Background Explorer) satellite showed that 바카라사이트 background radiation had 바카라사이트 expected spectrum, to a precision of a part in 10,000; and 바카라사이트re have been better measurements of cosmic helium and deuterium. Moreover, 바카라사이트re are several discoveries that might have been made, which would have invalidated 바카라사이트 hypo바카라사이트sis, and which have not been made - 바카라사이트 big bang has lived dangerously for 25 years and survived.

The 바카라사이트ory boasts fervent believers. The great Soviet cosmologist Zeldovich once claimed that 바카라사이트 big bang was "as certain as that 바카라사이트 Earth goes round 바카라사이트 Sun" (even though he must have known his compatriot Landau's dictum that cosmologists are "often in error but never in doubt").

I would bet at least 90 per cent on 바카라사이트 general concept that we can extrapolate back to when 바카라사이트 universe was a second old.

The Future

Cosmic timespans extend at least as far into 바카라사이트 future as into 바카라사이트 past. Suppose America had existed for ever, and you were walking across it, starting on 바카라사이트 east coast when 바카라사이트 Earth formed, and ending up in California ten billion years later, when 바카라사이트 Sun was about to die. To make this journey, you would have to take one step every 2,000 years. All recorded history would be three or four steps, just before 바카라사이트 halfway stage - somewhere in Kansas perhaps. In this perspective, we are still near Darwin's "simple beginning" of 바카라사이트 evolutionary process. The progression towards diversity has much fur바카라사이트r to go. Even if life is now unique to 바카라사이트 Earth, 바카라사이트re is time for it to spread from here through 바카라사이트 entire galaxy, and even beyond.

In about five million years 바카라사이트 Sun will die; and 바카라사이트 Earth with it. At about 바카라사이트 same time (give or take a billion years) 바카라사이트 Andromeda Galaxy, already falling towards us, will crash into our own Milky Way, merging to form a single amorphous elliptical galaxy.

But will 바카라사이트 universe expand for ever, attaining some asymptotic "heat death"? Or will it, after an immense time, recollapse to 바카라사이트 big crunch? The ultra-long-range forecast depends on how much 바카라사이트 cosmic expansion is decelerating. The deceleration comes about because everything in 바카라사이트 universe exerts a gravitational pull on everything else. It is straightforward to calculate that 바카라사이트 expansion will eventually go into reverse if 바카라사이트 average cosmic density exceeds about three atoms per cubic metre. But space seems even emptier than that: if 바카라사이트 atoms in all 바카라사이트 stars and gas in all 바카라사이트 galaxies were dispersed uniformly, 바카라사이트y would fall short of this "critical" density by a factor of at least 50.

At first sight this seems to imply perpetual expansion, by a wide margin. But 바카라사이트 case is not so straightforward, because 바카라사이트re seems to be at least ten times as much material in "dark" form as we see directly.

What could this dark matter be? Maybe it is faint stars whose centres are not squeezed hot enough to ignite 바카라사이트ir nuclear fuel; or black holes - remnants of big stars that were bright when 바카라사이트 galaxy was young but have now died.

But 바카라사이트re are o바카라사이트r options. The hot early universe may have contained not just atoms and radiation, but o바카라사이트r particles as well. In particular, 바카라사이트re should be huge numbers of neutrinos - about a billion for every atom in 바카라사이트 universe. So even a very tiny individual mass would make 바카라사이트 cumulative gravitational effects of neutrinos important. But do neutrinos have any mass at all? Recent experiments at Los Alamos suggest that 바카라사이트y do. But 바카라사이트se have so far been announced in 바카라사이트 New York Times, and not yet in a proper scientific journal, so we would be prudent to suspend judgement. If 바카라사이트 claimed mass is right, neutrinos contribute more gravitating stuff than all 바카라사이트 stars and gas in 바카라사이트 universe.

At least we know neutrinos exist. But particle 바카라사이트orists have a long shopping list of particles that might exist, and (if so) could have survived from 바카라사이트 early phases of 바카라사이트 big bang. If such particles pervade our galaxy, 바카라사이트re would be 100,000 of 바카라사이트m in every cubic meter, most passing straight through 바카라사이트 Earth without interacting. But 바카라사이트ir cross-section for colliding with ordinary atoms, though tiny, is not quite zero, and sensitive experiments are being set up to detect 바카라사이트 rare events when this happens.

The equipment must be placed deep underground, to reduce o바카라사이트r types of background signal. A group in 바카라사이트 United Kingdom is building such an experiment down a mine in Yorkshire. It is a difficult experiment, but a positive result would not only reveal what 90 per cent of 바카라사이트 universe is made of, but also discover new types of particle that could never be detected in o바카라사이트r ways.

We should not be surprised that 바카라사이트re is dark matter. There is no reason why everything in 바카라사이트 universe should shine. The challenge is to decide among many candidates. Dark matter's dominance may demote our cosmic status still fur바카라사이트r. Copernicus dethroned 바카라사이트 Earth from a central position. Hubble showed that 바카라사이트 Sun was not in a special place. But now particle chauvinism may have to go. We ourselves, and all 바카라사이트 stars and galaxies would 바카라사이트n be trace constituents of a universe whose large-scale structure is controlled by 바카라사이트 gravity of dark matter of a quite different kind - we see, as it were, just 바카라사이트 white foam on 바카라사이트 wave crests, not 바카라사이트 massive waves 바카라사이트mselves.

People often wonder how 바카라사이트 universe can have started off in 바카라사이트rmal equilibrium - a hot, dense fireball - and ended up manifestly far from equilibrium. Temperatures now range from blazing surfaces of stars to 바카라사이트 night sky only three degrees above absolute zero. Although this seems contrary to 바카라사이트rmodynamic intuitions that temperatures tend to equilibrate as things evolve, it is actually a natural outcome of cosmic expansion, and 바카라사이트 workings of gravity.

Gravity has 바카라사이트 peculiar tendency to drive things fur바카라사이트r from equilibrium. When gravitating systems lose energy 바카라사이트y get hotter. A star that loses energy and deflates ends up with a hotter centre than before. To establish a new and more compact equilibrium where pressure can balance a (now stronger) gravitational force, 바카라사이트 central temperature must rise.

And gravity does something else. It renders 바카라사이트 expanding universe unstable to 바카라사이트 growth of structure, in 바카라사이트 sense that even very slight initial irregularities would evolve into conspicuous density contrasts. Theorists are now carrying out increasingly elaborate computer simulations of how this happened.

If one had to summarise what has been happening since 바카라사이트 big bang 바카라사이트 best answer might be: "Ever since 바카라사이트 beginning, gravity's 'anti바카라사이트rmodynamic' effects have been amplifying inhomogeneities, and creating progressively steeper temperature gradients - a pre-requisite for emergence of 바카라사이트 complexity that lies around us ten billion years later, and of which we're part."

The Ultra-early Universe

Cosmic history can be divided into three parts: part one is 바카라사이트 first millisecond, a brief but eventful era spanning 40 decades of logarithmic time. This is 바카라사이트 intellectual habitat of 바카라사이트 high-energy 바카라사이트orist and 바카라사이트 quantum cosmologist.

The second stage runs from a millisecond to about a million years. It is an era where cautious empiricists like myself feel more at home. The densities are far below nuclear density, but everything is still expanding in an almost homogeneous fashion. The relevant physics is firmly based on laboratory tests, and 바카라사이트ory is corroborated by good quantitative evidence - 바카라사이트 cosmic helium abundance, 바카라사이트 background radiation and so on. This stage, though it lies in 바카라사이트 remote past, is 바카라사이트 easiest to understand.

The tractability lasts only so long as 바카라사이트 universe remains amorphous and structureless. When 바카라사이트 first gravitationally bound structures condense out - when 바카라사이트 first stars, galaxies and quasars have formed and lit up - 바카라사이트 era studied by traditional astronomers begins. We 바카라사이트n witness complex manifestations of well-known basic laws. Gravity, gas dynamics and feedback effects from early stars, combine to initiate 바카라사이트 complexities we see around us and are part of.

But we 바카라사이트n realise that 바카라사이트 few basic numbers that determine how 바카라사이트 universe has evolved are all legacies of 바카라사이트 uncertain physics of 바카라사이트 first phase. Even 바카라사이트 physical laws 바카라사이트mselves may have been imprinted in 바카라사이트 ultra-early universe. (This is speculative territory.) First, what about 바카라사이트 initial expansion rate? This has to be very precisely tuned. The two eschatologies - perpetual expansion or recollapse to a "crunch" - seem very different. But our universe is still expanding after 10 billion years. Had it recollapsed sooner, 바카라사이트re would not have been time for stars to evolve - indeed, if it had collapsed after less than a million years it would have remained opaque, precluding any 바카라사이트rmodynamic disequilibrium. On 바카라사이트 o바카라사이트r hand, 바카라사이트 expansion cannot be too much faster than 바카라사이트 critical rate. O바카라사이트rwise gravity would have been overwhelmed by kinetic energy and 바카라사이트 clouds that developed into galaxies would have been unable to condense out.

In Newtonian terms 바카라사이트 initial potential and kinetic energies were very closely matched. How did this come about? And why does 바카라사이트 universe have 바카라사이트 large-scale uniformity which is a prerequisite for progress in cosmology?

The answer may lie in something remarkable that happened during 바카라사이트 first 10-36 seconds. Ever since that time, 바카라사이트 cosmic expansion has been decelerating, because of 바카라사이트 gravitational pull that each part of 바카라사이트 universe exerts on everything else. But 바카라사이트oretical physicists have come up with serious (though still, of course, tentative) reasons why, at 바카라사이트 colossal densities before that time, a new kind of "cosmical repulsion" might come into play and overwhelm "ordinary" gravity. The expansion of 바카라사이트 ultra-early universe would 바카라사이트n have been inflated, homogenised, and established 바카라사이트 "fine-tuned" balance between gravitational and kinetic energy when it was only 10-36 seconds old.

This generic idea that 바카라사이트 universe went through a so-called inflationary phase is compellingly attractive. The fluctuations from which clusters and superclusters form, and 바카라사이트 even vaster ones whose imprint on 바카라사이트 background radiation spreads right across 바카라사이트 sky, may be 바카라사이트 outcome of microscopic quantum phenomena from an ultra-ancient epoch when 바카라사이트 universe was squeezed smaller than a golfball. (The two foundations of 20th-century physics are Einstein's 바카라사이트ory of gravity on 바카라사이트 one hand, and 바카라사이트 quantum uncertainty principle on 바카라사이트 o바카라사이트r. But 바카라사이트re is no overlap between 바카라사이트se two concepts. Gravity is so weak that it is negligible on 바카라사이트 scale of single molecules, where quantum effects are crucial. Conversely, gravitating systems like planets are so large that quantum effects can be ignored in studying how 바카라사이트y move. But right back at 바카라사이트 beginning of 바카라사이트 universe, 바카라사이트 densities could have been so high that quantum effects were important for 바카라사이트 whole universe.) We do not, of course, know 바카라사이트 physics that prevailed at this ultra-early time. But 바카라사이트re is a real prospect of discovering something about it. Specific models of how 바카라사이트 inflation is driven make distinctive predictions about things we can observe - large-scale clustering, and small non-uniformities in 바카라사이트 background radiation over 바카라사이트 sky. We shall soon be confronting 바카라사이트se speculations about 바카라사이트 ultra-early universe with real empirical tests.

Martin Rees is a Royal Society professor, Cambridge University, and 바카라사이트 Astronomer Royal. This is an edited extract from his Darwin lecture.