“OFTEN in error but never in doubt.” This accusation has sometimes been levelled against cosmologists – and justifiably so. They have frequently embraced poorly grounded speculations with irrational fervour, and been led by wishful thinking to read too much into vague and tentative evidence.
However, although this still happens, even the more cautious among us are confident that we now know some of the key cosmic numbers, and are grasping at least the outlines of how stars and galaxies emerged. Astronomers can trace the evolutionary story back long before our solar system formed 4.5 billion years ago; we can now observe galaxies that are so far away that their light set out 12 billion years ago.
We also have precise measurements of the microwaves that are, we believe, an afterglow of an intensely hot “genesis event” about 14 billion years ago. The first microsecond is still shrouded in mystery, but everything that happened since then – how our complex cosmos has unfolded from simple beginnings – is the outcome of laws that we can understand, even though the details still elude us.
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This astonishing recent progress could not have been achieved by armchair theorising. It is primarily a product of technology: ingenious advances in telescopes, detectors, computers and spacecraft have transformed cosmology into a quantitative science.
However, despite this progress – or maybe because of it – it seems there is more to understand than ever. We have extended the frontiers of our knowledge, and new mysteries have come into focus; we’re still perplexed, but at a deeper level than before.
As we compiled a more complete inventory of what is out there in space, for example, something very surprising has emerged: 96 per cent of the universe is unaccounted for. Everything that shines – stars, galaxies and glowing gas – amounts to just 4 per cent of all that’s there.
Cosmic afterthought
Ever since the astronomer Copernicus, we have known that we are not located at any special central place in the cosmos. Cosmic modesty now has to be taken a step further: we are not even made of the dominant stuff – atoms seem a mere afterthought. The other 96 per cent, the “dark side”, controls our universe – how it began, its eventual fate, and whether it is finite or infinite.
Most of the gravitational force that holds each galaxy together comes not from the stars and gas we see, but from “dark matter” – probably a swarm of particles, whose nature remains unknown, left over from the big bang. The most likely candidates are heavy, electrically neutral “supersymmetric” particles, whose properties may be pinned down by experiments at the new Large Hadron Collider at CERN, the European centre for particle physics near Geneva.
But there are many other ideas too. The search for these dark matter particles is forging new links between the very large and the very small – the “inner space” of the subatomic world and the “outer space” of the cosmos. They exemplify how the cosmos offers a chance for us to study experiments that we could never do here on Earth.
Indeed, there is a symbiosis between cosmology and physics. Astronomers obviously need to know how atoms behave in order to understand the stars; they learn this via laboratory experiments. But now, in a complementary way, astronomers are discovering things that tell physicists something new. We can see where nature has done experiments for us – galaxies crashing together, stars exploding, immensely strong gravity. If experimenters succeed in their delicate searches for dark matter, they will not only find out what our universe is mainly made of, but as a bonus may discover an important new kind of particle.
And the mysteries don’t stop with dark matter. If there were no forces other than the gravitational pull of dark matter, then the expansion of the universe would be slowing down slightly (though it would never halt), and the geometry of the universe would be hyperbolic – curved so that the three angles of a large enough triangle would add up to less than 180 degrees.
However, neither of these things is true. The expansion is not slowing down at all: studies of distant supernovae show that it is speeding up. And the universe is “flat” – angles of triangles add up to exactly 180 degrees. The most compelling evidence for the latter conclusion comes from the WMAP experiment, aboard a spacecraft orbiting the sun more than a million and a half kilometres from Earth. Even if we had no other information at all, data from this little craft alone would have told us that we lived in a flat universe that contains something else apart from atoms and dark matter.
These unexpected discoveries – firmed up only in the last two years – have remarkable implications. There seems to be some kind of latent energy and tension even in empty space, which exerts a push rather than a pull. This “dark energy” overwhelms gravity on the cosmic scale and affects the geometry of the universe.
Optimistic future
What are the implications? Will our cosmos grow ever colder and emptier? Will it collapse to a big crunch? Will the dark energy eventually decay, or could it even mount up until it’s strong enough to tear apart stars, planets and even atoms? We don’t have the answers, but that doesn’t stop optimists from grappling with the rapidly improving data and fast-developing concepts.
And there is plenty to grapple with. One of my favourite magazine covers showed a red circle beneath the caption “the universe when it was a trillionth of a trillionth of a trillionth of a second old – actual size”. According to a popular theory, our universe “inflated” from a hyper-dense blob under the influence of a cosmic repulsion. However, that repulsion is no longer as strong: it now seems to be 120 powers of 10 weaker.
One could say it is a fortuitous situation: if the force had weakened by only, say, 114 powers of 10 rather than 120, the cosmic repulsion would overwhelm gravity even within galaxies (where the density is about a million times higher than the cosmic average). The galaxies could then never have condensed out of the expanding universe, and any prospect of stars or life would have been quenched or foreclosed.
Nevertheless, it seems remarkable that this force could have switched off, or somehow been neutralised, with such amazing precision. Why should this be so precise that it leads to a row of 119 zeros after the decimal point, but not 120 or more?
We still don’t have an answer. But sometimes in science, a new way of looking at a problem turns it on its head. The question used to be: why should empty space exert a force? Now we ask: why is the force so small? The one conclusion we have reached is that empty space is anything but simple. Any particle, together with its antiparticle, can be created by a suitable concentration of energy. On an even tinier scale, space may be a seething tangle of strings. From this perspective, the puzzle is why all the complicated processes going on in empty space don’t have a net effect that is much larger.
We may only find the answers through new developments in fundamental physics. When a theory breaks down, or confronts a paradox, we need a new unifying idea that transcends what went before. For example, Einstein’s theory and the quantum theory are both superb within limits – indeed they are the foundation of 20th-century physics – but these theories cannot be meshed together: at the deepest level they are contradictory. Until there has been a synthesis between them, we cannot tackle the overwhelming question of what happened right at the very beginning: what banged and why it banged. Still less can we attach any meaning to the question of what happened before the big bang.
The physical laws that we can study in the lab seem to apply throughout the entire domain we can survey with our telescopes, but that domain – what we have traditionally called “the universe” – may be just an infinitesimal part of reality. Just as space may have a rich structure on scales a trillion trillion times smaller than an atom, so it may also on scales far larger than the entire universe we know.
There may have been an infinity of big bangs, not just one. Each cooled down differently, ending up governed by different laws. Just as Earth is a very special planet among zillions of others, so – on a far grander scale – perhaps our big bang was also a very special one. What we have traditionally called the fundamental physical laws – those of Einstein and the quantum – could, from this stupendously enlarged perspective, be mere parochial bylaws. Our universe could be just one island, just one patch of space and time, in a vast and varied cosmic archipelago.
This is certainly a remarkable time to be working in cosmology. Possibilities once firmly believed to be in the realms of science fiction have shifted to the centre of respectable scientific debate. From the very first moments of the big bang to the mind-blowing possibilities for alien life, parallel universes and beyond, scientists are now led towards worlds even weirder than anyone ever imagined.