THE future’s not what it used to be. Cosmologists were once confident they knew how the universe would end: it would just fade away. An ever colder, ever dimmer cosmos would slowly wind down until there were only cinders where the stars once shone. But that’s history.
The latest research suggests many different possible futures. Cosmic cycles of death and rebirth might be on the cards, or a very peculiar end when the vacuum of space suddenly turns into something altogether different. The universe might collapse back in on itself in a big crunch. Or we could be in for an even more violent end, called the big rip. The slow drift into darkness is still a contender, but fear not: that long night could be a lot more interesting than you might think – imagine the cosmos filled with giant diamonds and you’ll have an inkling of where we could be headed.
Why this wealth of possibilities all of a sudden? Well until recently, the dominant force in the universe seemed to be the gravity of stars and other matter, and that meant there were only two options. Either the universe was dense enough for gravity to halt the expansion from the big bang and pull everything back together in a “big crunch”, or else it wasn’t, in which case the expansion would carry on forever.
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Most researchers thought the latter possibility was the more likely. Then, in 1998, astronomers got a shock: they found that the universe’s expansion isn’t slowing down at all, but accelerating. Studies of the light of distant supernovae revealed that something stepped on the gas around 6 billion years ago. The finding seemed to seal our fate, condemning us to a headlong rush to oblivion. Acceleration means a universe that will become cold and boring much faster than we’d thought.
But researchers have now realised this gloomy outlook was premature,because no one knows what is causing the acceleration. Astronomers have dubbed this mysterious force “dark energy”, but its origin and nature are a mystery. So how can anyone say what it is going to do in the future? “People started realising that as long as we have no clue what dark energy is, we can’t be so arrogant,” says Max Tegmark of Harvard University.
Although the long-term forecast is still open to debate, astronomers do at least agree on what will happen to our neighbourhood in the near future. Things will become rather uncomfortable in about 6 billion years, when the sun swells to become a red giant, boiling the oceans away and possibly even swallowing the Earth. Then our star will exhaust its nuclear fuel and shrink into a white dwarf roughly the size of Earth, leaving our old planet (if it still exists) cold enough to be covered in nitrogen ice. At least the view will be lovely: gas blown into space by the red giant will be energised by ultraviolet rays from the white dwarf, so for a while the Earth will be surrounded by a glowing multicoloured nebula.
Our galaxy is in for a rough time too. We are heading towards another, larger spiral called Andromeda, and we could collide in as little as 3 billion years. For a while, the merger will create a brilliant, elaborate hybrid galaxy, as streamers of stars are flung outwards, and most of the loose gas in both galaxies is compressed to form bright new stars.
After only a couple of billion years more, those stellar tentacles will subside and the two delicate spirals will have merged into one great blob, an elliptical galaxy. Most of the free gas will have been used up, so relatively few new stars will form, except when small nearby galaxies in our local group are swallowed, each giving up its gas in a little puff of star formation.
“The universe might collapse in on itself in a big crunch, or it could end more violently in a big rip. A slow drift into darkness is still on the cards, but even that could get pretty exciting”
Our collision with Andromeda will have a spectacular climax. At the centre of the Milky Way is a giant black hole more than 3 million times the mass of the sun. Another in Andromeda is probably 10 times the size. These two black holes will settle towards the centre of the new galaxy, and there they will spiral together and eventually merge. The energy released will be tremendous, sending out a blast of light and X-rays, and a pulse of gravitational waves that will squeeze and stretch every star and planet.
Looking out into the universe we will see other galaxies moving away from our elliptical home, dragged ever faster by the hand of dark energy. But for how long? That depends on the nature of dark energy. Perhaps, for instance, its energy density will decrease with time. That could have radical consequences: some researchers have come up with model universes where the dark energy becomes negative. And as positive dark energy has repulsive gravity, negative dark energy would have attractive gravity, like ordinary matter.
If that happens in our universe, the consequences will be extreme. First acceleration will slow, and then dark energy will begin to really put the brakes on. Expansion will eventually halt, and then reverse, so that galaxies rush back towards each other and soon start colliding at ferocious speeds. Eventually, everything will be crushed together in a big crunch, unimaginably dense and hot, like the big bang in reverse.
That won’t happen for a while, though. Those observations of distant supernovae, which trace the expansion of space over time, show that if dark energy is fading, it can’t be doing so very fast. Andrei Linde of Stanford University in California has calculated that we are safe from a big crunch for at least 25 billion years, almost twice the age of the universe today.
Phantom menace
But an even more grisly end could be in store. In 2003, Robert Caldwell of Dartmouth College in New Hampshire explored the opposite idea: that dark energy could become stronger. This exotic flavour of dark energy is called phantom energy. The expansion of space makes phantom energy increase, and the phantom energy makes space expand even faster, setting up a devastating positive-feedback loop he calls the big rip.
If Caldwell is right, then a crisis could come as little as 40 billion years from now. It would be perhaps the most watchable doomsday that cosmologists have imagined, strangely similar to the spectacle laid on in Douglas Adams’s The Restaurant at the End of the Universe.
Roughly 60 million years before the end, the phantom repulsion becomes strong enough to tear our galaxy apart. Then, just months before the end, the real show begins. Let’s assume that by this point we have found ourselves a new home in a solar system not unlike this one. We will first see the outer planets fly away one by one. Then our adopted Earth will be torn from its sun. Less than an hour from the end, the sun will explode, and minutes later the Earth will be ripped apart too. We might just be able to keep watching until a fraction of a second before the end, but presumably not long enough to see the destruction of molecules and atoms at around t-minus 10-19 seconds, when the phantom overpowers all electromagnetic forces. Neither will we see the subsequent shredding of nuclei, protons and neutrons. Pity.
“In a big crunch, everything will be squashed into a super-hot, super-dense sea of radiation. It is certainly not going to be healthy for human life”
But there doesn’t have to be anything exotic about dark energy. The most conservative theory – what cosmologists call its vanilla flavour – suggests that a given volume of vacuum has an inherent fixed energy, often called the cosmological constant. Many experts would bet that this kind of dark energy is what’s causing the universe’s expansion to accelerate, and particle physicists even have a partial explanation for it: according to quantum mechanics, countless ephemeral subatomic particles are constantly popping in and out of existence even in a vacuum, and their energy might add up to something. The only problem is that physicists struggle to explain the observed value of about 1 nanojoule per cubic metre. They can see how the particles’ energy might cancel to zero or add up to a huge value, but not to next to nothing.
Nevertheless, this remains the most popular flavour of dark energy among cosmologists. If dark energy stays constant, our universe will steer carefully between crunch and rip. This middle-of-the-road future may be long and lonely.
Acceleration will soon steal most of the universe from view, as the increasing expansion of space carries other galaxies beyond our view: their light will no longer reach us because it is being dragged back over our cosmological horizon like a tortoise on a treadmill. According to Fred Adams of the University of Michigan, every other galaxy will have been pulled out of sight in a couple of hundred billion years.
“At least the view will be lovely: for a while we will be surrounded by a glowing multicoloured nebula”
Then we will be all alone, the observable universe reduced to our one elliptical galaxy, and a dingy one at that. There will be only a trace of free gas left to make new stars. Adams has calculated that even that will be used up after about a hundred trillion years, and all nuclear-powered stars will have gone out. A little faint infrared radiation will come from stars called brown dwarfs, which are too small to ignite fusion in their cores.
Other stars will be reduced to dense, dead remnants – black holes, neutron stars and ageing white dwarfs, slowly dimming to black. Our sun will become one of these black dwarfs: a single crystal of carbon, like an ultra-dense diamond, with a surface cool enough to touch.
Occasional flares will lift the gloom, when brown dwarfs collide to form a new star, or a black hole shreds a stellar carcass. Once in a trillion years, two relatively heavy black dwarfs will collide and explode as a supernova.
Every now and again, a star will be thrown out of the galaxy after a close encounter with another star. The whole galaxy will dissipate in about a hundred quintillion (1020) years.
Now our observable universe is reduced to a diaspora of dead stars, loosely centred on a massive black hole surrounded by a cloud of dark matter. If there is a remnant of the Earth, then for a while it might trail after the black dwarf that was once our sun. But the system will slowly lose angular momentum by emitting gravitational waves, and Earth’s cinder will eventually spiral in to hit the sun’s.
Meanwhile, dark energy will still be at work. Each star will see all its old companions disappear over the horizon one by one. Our black dwarf will be in a universe of its own.
After that, it gets a lot more speculative, but here’s the best guess. Particle physicists suspect that protons are unstable and probably only last between 1033 and 1045 years. As protons decay into their constituent quarks, all the black dwarfs, neutron stars and planets will crumble away, leaving behind nothing but loose photons, neutrinos, electrons and positrons. Even black holes eventually evaporate, by a process called Hawking radiation, although that takes even longer – more than 1086 years for our central black hole.
And then? Dark energy continues working, even on these ashes. One day, every single particle will find itself alone inside its own horizon.
Fade out
Aside from crunches, rips and this lonely death, there is a fourth possibility, a path almost parallel to that of the cosmological constant but fractionally less bleak. In the cosmologists’ models, some kinds of dark energy gradually fade in strength, but never become negative. Among them are defects in space-time that might be left over from the big bang, and a kind of energy field called quintessence. Just like the cosmological constant, these flavours would give us a chilly future where no stars shine and all solid bodies eventually dissipate into a cloud of fundamental particles. However, the acceleration will eventually tail off, so it won’t isolate every particle within its own horizon. Particles could still interact, albeit at a glacial rate, and some kind of chilly life might just be able to cling to existence (Â鶹´«Ă˝, 3 August 2002, p 28). Cold comfort, perhaps.
To find out which of these paths we will take, astronomers are examining the nature of dark energy. If they can pin down how space has expanded in the past, and learn what dark energy is really made of, we should have a clue to the future (see Graphic).
The favoured oracles are distant stellar explosions known as type Ia supernovae. These supernovae are all of about the same power, so measuring both their apparent brightness and their distance tells us how much space has expanded since they went off. Astronomers are gathering more and more observations from the ground, and a proposed space telescope called SNAP could spot thousands of type Ia supernovae (Â鶹´«Ă˝, 5 April 2003, p 30), although its progress is being delayed by President Bush’s moon and Mars mission.
To complement the supernova observations, other astronomers are using galaxies to peer into dark energy’s past. Because dark energy counteracts matter’s tendency to clump together, its strength will have affected the number of galaxy clusters that formed at different stages of cosmic history. The best hope of tracing enough ancient clusters to pin down this history is a device called the Large Synoptic Survey Telescope, which could be running by 2011.
The results could tell us what the future holds. It may seem like a poor set of options – to be crushed, ripped apart or evaporated away. But in fact none of these scenarios need be the uttermost end. The universe, and perhaps even life, could survive any one of these disasters.
In a big crunch, everything will be squashed into a super-hot, super-dense sea of radiation. It is certainly not going to be healthy for humans. But nobody knows what physics does when stuff gets that hot, so it’s hard to predict what would happen to the universe itself. “Re-expansion is a possibility,” says Linde. Since as far back as the 1930s, physicists have played with this idea, and if the universe can bounce, then maybe our own big bang was preceded by a crunch. It could happen again and again, big crunch leading to big bang and so on.
Black hole disease
It’s an enticing idea, but there’s a catch. Oscillating universes are vulnerable to a fatal disease: a plague of black holes. Holes survive the crunch, and in each cycle they grow. “They keep getting bigger and bigger till they swallow the whole universe,” says Katherine Freese of the University of Michigan. But she may have a cure. With Matthew Brown at Michigan and William Kinney at the University of Buffalo, New York, Freese has concocted a new kind of oscillating universe that is immune to black hole disease.
Oddly, the prescription is a big rip. A dose of phantom dark energy tears everything apart – even black holes, effectively making them boil away. The cure may sound worse than the disease, but in fact this big rip can be repaired. This crazy-sounding idea is based on a respectable speculation, the “braneworld” model, in which our universe of three space and one time dimensions is like a membrane floating in higher-dimensional space (Â鶹´«Ă˝, 29 September 2001).
In the new model, when phantom energy skyrockets to create the rip, it disturbs fields in the higher dimensions outside our brane. They then transform the phantom energy, turning it negative and making our universe start to recollapse. Although all stars, planets and other structures from before the rip are gone, new objects might form during the collapse. If astronomers are among them, they will look back in time and discover a kind of big mend.
Then, as the universe reaches a big crunch, the energy density of ordinary matter and radiation soars. Fields in the higher dimension react again, making the contraction bounce back to become the expansion of a new big bang. All this may be rather contrived, but at least it shows there is a possibility that neither crunch nor rip need be the end of everything.
“Eventually, anything could spontaneously pop into existence. Most of these things will be senseless messes, but a vanishingly small proportion will be people, planets, galaxies, and five-mile-high models of your left arm made from gold”
Freese suspects that her model universe would eventually run out of steam and stop bouncing. In contrast, the “ekpyrotic” universe (the name derives from the Greek word for conflagration) devised by Paul Steinhardt of Princeton University and Neil Turok of the University of Cambridge, is supposed to be eternal. It’s another braneworld model, with the twist that our brane is not the only one. Just a fraction of a millimetre away along the fifth dimension there is another universe. “They can collide from time to time like a pair of cymbals,” says Turok, and when they do their kinetic energy turns into a blast of radiation that we call a big bang.
Critics point out that when the branes collide, everything becomes infinitely dense, so the equations break down and the theory doesn’t make much sense. Turok and his collaborators have now published a paper using M-theory – string theory’s big brother – to show that that doesn’t happen, but their idea remains controversial.
So what are your chances of surviving the cymbal-crashing big splat? Well, all the particles in any object would briefly become massless and fly apart at the speed of light, so you’d get rather scrambled, but it is possible that life could survive. “We would have to figure out how to preserve all our memories and information in the form of radiation. If you could imagine making a computer out of light, you could transmit it through the big bang and recover it on the other side.”
There might be a similar escape route through Freese’s rips and crunches – at a fundamental level information is preserved, so conceivably there is a way to encode ourselves into the next cycle of creation. Even in the long, slow decline of a constant dark energy there’s a chance the universe could reinvent itself (see “Quantum resurrection”). Who says you can’t live forever?
It’s time for a confession. All of these forecasts are only local: they apply to the bit of our universe that lies within our cosmological horizon. But it is quite possible that the universe is truly infinite. Far beyond the horizon, conditions may be very different. Even the constants of physics may be different, and perhaps some of those regions may be more durable. In some models an infinite cosmos is constantly spawning new big bangs.
None of this can affect us, or have any bearing on our future – unless, perhaps, we somehow learn to manipulate wormholes in space-time and tunnel to freedom, moving to a fresh region of the cosmos whenever the old one gets tired. But even if we can’t escape our local universe, at least it might be reassuring to think that the cosmos itself is immortal.
Instant revolution
THE fact that empty space has some energy hints that it could be unstable – and could leap into an entirely different state at any moment. If there is a lower energy state that the vacuum of space could adopt, then decay is only a matter of time. We would get no warning. The vacuum would go twang in one place, and a bubble of the new state will grow at the speed of light to engulf the whole universe.
No one knows exactly what space would be like if it does change its state, but things would probably be very different. For example, Andrei Linde of Stanford University has developed a model based on string theory, which posits six extra dimensions of space curled up so tightly that we can’t see them. In Linde’s model, these extra dimensions unfurl when the vacuum decays. We would suddenly find ourselves in a universe with nine large space dimensions. Such a highly multidimensional universe would almost certainly be dead, as no stable structures could exist, but perhaps other kinds of vacuum decay would lead to a more interesting future.
Quantum resurrection
EVEN if we face a future in which the cosmological constant reduces us all to a set of isolated particles, there is some hope. Quantum mechanics tells us that there are always fluctuations in any system. Energy fields waver at random, and particles can appear out of the vacuum. Large fluctuations are very rare, and you’d have to wait an extraordinary length of time for something big to appear – a whole atom or molecule, say.
But if our future is infinite, time is not a problem. Eventually, anything could spontaneously pop into existence. Most of these things will be senseless messes, but a vanishingly small proportion will be people, planets, galaxies, and five-mile-high models of your left arm made from gold. “In an infinite amount of time, I will reappear. A crazy thought, but true,” says Katherine Freese of the University of Michigan.
How about a whole new universe? Sean Carroll of the University of Chicago thinks that random fluctuations could spark a new big bang (Â鶹´«Ă˝, 21 August 2004, p 34). He’s even worked out how long we might have to wait for it, something in the region of 101056 years, or a 1 followed by 1056 zeros.
This dwarfs all the timescales we have met so far – it’s even impossible to write down in the conventional longhand notation. It is hard to imagine how any kind of life could survive long enough to take advantage of the new universe – unless, perhaps, it can find a technology that will trigger the new big bang, restarting the cycle of cosmic life and death.
