LOST property doesn’t come any bigger than this. We are about to lose most of the visible universe. Billions of sparkling galaxies are flying away from us into regions we will never see, leaving only a few thousand of them in our sky. Future Earthlings will have no notion that anything ever existed beyond the Milky Way and its close neighbours. “After that, there won’t be anything we can learn about the distant universe,” says Harvard University astrophysicist Abraham Loeb.
It’s not going to happen for 100 billion years or so, but Loeb has calculated that distant objects, including nearly all the other galaxies in the universe, will have vanished for ever. They will have flown out beyond a truly final frontier, one that is patrolled by nature’s speed cops.
The point is that we can only see a star or galaxy if its light can reach us. That means the light must have taken no longer than the 14 billion years or so since the beginning of the universe to travel from the distant galaxy to Earth. But what’s the problem? According to the big bang theory, everything started out at a single point in time, and nothing can travel faster than light, so how could any galaxy now be so far away that its light hasn’t got time to reach us?
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Well, expansion doesn’t mean simply that galaxies are travelling outwards; the very fabric of space is changing. It is possible for space to expand so fast that some regions of the universe fly apart faster than light. On top of that, the distances light has to travel are changing too. Light might set out from a galaxy with only a billion light years to travel to Earth, but find that space is stretching out the distance before it so it takes 10 billion years to reach its destination. The net result is that there are places so far removed from us that we have never seen their light.
The outlook is so bleak because of a “dark energy”, sometimes referred to as Lambda or the cosmological constant (see “From Lambda to infinity”) that pushes galaxies apart. Dark energy’s existence has long been suspected, but observations made at the Cerro Tololo Inter-American Observatory in Chile have set it in stone. On 5 March 1997, astronomers using the telescope got a shock when they saw the light from a distant supernova. The brightness of a supernova’s light is dimmed only by the distance it has had to travel to Earth, so this light provided a measure of how much the expansion of the cosmos has slowed since the big bang.
Speeding up
The shocking discovery was that the supernova showed the expansion was speeding up. And this is bad news for anyone wanting to observe the universe.
If you take the measured values of the rate of acceleration of the universe and the amount of matter it contains, and assume that the acceleration is indeed caused by a kind of constant vacuum energy, you can calculate that the visible universe is losing territories – and fast.
Loeb did the calculation using the astronomical measure of distance known as red shift. This reflects how much space has expanded and stretched out the light from a galaxy or other source. Loeb calculated that galaxies we see with a quite modest red shift of 1.8 are now disappearing over our horizon. While we can still see them by the light they emitted about 10 billion years ago, the light they are emitting now will never reach us.
Light that is already on its way from these distant galaxies will continue to reach us for several billion years yet. But after that, we’ll start to see weird things happening as the horizon-crossing becomes visible. Einstein’s general theory of relativity implies that the galaxies won’t simply disappear. Instead, each will leave behind a gradually fading snapshot of itself sitting on the horizon.
These galaxies are astronomical Dorian Grays. Beyond this horizon – the de Sitter horizon – they’ll be growing older naturally, but all we’ll see is fading pictures of them in their youth. The light from these ghosts will become fainter and stretch to ever-longer wavelengths, until they are impossible to detect at all.
Eventually, in around 100 billion years, Earthlings will only be able to see what’s now in our galactic backyard – the merger product of the Milky Way galaxy and its neighbour, the Andromeda galaxy, which will remain fairly tight knit because gravity gasps them together. “Beyond that, everything will be dark,” says Loeb. “There won’t be any sense in doing extragalactic astronomy then.”
The same problem will afflict the cosmic microwave background, the relic radiation of the big bang fireball. The radiation carries ripples corresponding to density fluctuations in the early universe, and watching these ripples gives astronomers crucial information that helps them measure the size, shape and weight of the universe. But in around 100 billion years, the apparent source of the cosmic radiation will also cross our horizon, freeze and fade. After that, there’ll be no way of extracting its secrets.
If so, we are beneficiaries of a happy coincidence, treated to information that will only be visible for a brief period in the possibly infinite life of the universe. Such apparent good luck makes some astronomers uneasy. “It seems too weird,” says Alexei Filippenko of the University of California, Berkeley. It may be that our understanding of dark energy is incomplete; there is growing evidence, for instance, that the vacuum energy is not constant but changes with time. We’ll only find out for sure by surveying more astronomical objects to get a sense of the acceleration.
One possibility is a new satellite called SNAP (Supernova/Acceleration Probe), a telescope fitted with a billion-pixel camera, the largest digital astronomical imager ever constructed. That could watch 2000 supernovae flare up in a single year – 20 times as many as we’d expect to see in a whole decade if we were observing them from the ground – and would tell astronomers just how the expansion rate of the universe changed in the past. That would in turn shed light on what kind of force drives the acceleration of space, and whether it will change in future. It may also answer the burning question: is the cosmos about to turn out the lights for good?