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The skies of the far future will be dark and lonely, but our descendants will have one telltale clue to help them decipher the cosmos

ONCE, long ago, we shared the universe with 100 billion galaxies. Now only a single island of stars remains, floating in an unutterably vast, unutterably empty ocean of space. This is a vision of the Milky Way when the cosmos is 10 times older than it is today.

The perpetrator of this desperately lonely future is dark energy, the mysterious force that is accelerating the expansion of the cosmos. Billions of years from now, the expansion will drive all the other galaxies over the cosmic horizon where their light can never reach us. Ours will be the only galaxy left in the observable universe.

So disturbing is this image that some researchers have been toying with the idea of moving the galaxy to a more hospitable neighbourhood. Others are more sanguine about our fate and are exploring what, if anything, cosmologists of the future will be able to tell about the universe they live in.

We live at a fortunate time in cosmic history because we can gaze through telescopes at a night sky ablaze with the beacons of billions of galaxies. By charting how fast these galaxies are receding we have inferred that the universe was smaller in the past and that it burst into being 13.7 billion years ago in an explosion called the big bang.

Had we come on the scene in the year 100 billion or so, how would we figure out where the universe had come from with no other galaxies to help us? Surely the science of cosmology would be impossible.

Or would it? Last October, of the Centre for Astrophysics at Harvard University gave a public lecture about the long-term future of the universe. He pointed out that the laws of physics and cosmology are so accurate, we can predict what will happen to the universe billions of years from now.

After his lecture, many people asked whether intelligent life 鈥� if it still exists 鈥� would be able to figure out as much about the universe as we have. Loeb was intrigued. So, on a day free of teaching, he sat down with a pen and paper to explore the idea.

He quickly concluded that most astronomers have been too pessimistic. Incredibly, there will be a way to do cosmology beyond AD 100 billion. Admittedly, it will not be as easy as today. But it will not be impossible. 鈥淲hen there are no longer galaxies to use as indicators of cosmic expansion, another type of celestial body steps up to the plate,鈥� says Loeb.

To find out what it is, you first need to follow what will happen to the universe and, in particular, our galaxy over hundreds of billions of years. Over this enormous span of time, two principal things will happen to the Milky Way 鈥� one dramatic and one not so dramatic but equally profound in its consequences.

The drama will come courtesy of our neighbouring galaxy, Andromeda, which is currently plunging towards us in a journey orchestrated by gravity. Over the next few billion years, the giant spiral galaxy will loom larger and larger in our night sky, until we can see two bands of stars instead of just the one making up the Milky Way.

In 2.3 billion years鈥� time, Andromeda will fly past the Milky Way. Its gravity will do little more than ruffle the stars in our galaxy but this is a mere dress rehearsal for the main event. Like a pendulum overshooting its lowest point and swinging back, in 5 billion years鈥� time, Andromeda will return. And this time it will collide catastrophically with the Milky Way.

As a result, the supermassive black holes at the centre of the two galaxies will merge and the motion of the stars surrounding them will be thrown into chaos. 鈥淲here once there were two spiral galaxies, there will now be a spherical swarm of stars,鈥� Loeb says. 鈥淚n the turmoil, the sun will be kicked from its position 26,000 light years from the centre out to about 60,000 light years.鈥�

Loeb and others have dubbed the merged elliptical galaxy Milkomeda. It is here that the massive, bright stars will burn out and die, including our very own sun. By 5 billion years from now, it will have exhausted all its nuclear fuel and be unrecognisable as our life-giving star. It will have swollen into a red giant, possibly swallowed the Earth, then dwindled to an Earth-sized stellar ember, slowly fading from view. 鈥淏y this time, our descendants, if they still exist, will have long left the planet,鈥� Loeb says.

Fortunately, there is no shortage of alternative accommodation. About 70 per cent of stars are cool low-mass stars called red dwarfs. So miserly with their fuel are these red dwarfs that they easily burn for 10,000 billion years. 鈥淎nd they have habitable zones where planets might support life,鈥� says Loeb.

The view from a planet orbiting a red dwarf in AD 100 billion would be very different from our night sky. Whereas several thousand stars are visible from Earth with the naked eye, not a single one would be bright enough to pierce the darkness.

Even worse would be the view of the universe through telescopes, or rather the lack of it. Dark energy has a repulsive gravity, which is speeding up the expansion of the universe. Remarkably, dark energy gets stronger as the universe expands. If the universe doubles in volume, the dark energy doubles too, and so on, with dark energy controlling the universe rather than gravity.

Such expansion will drive away every other galaxy, eventually causing them to rush away from us at the speed of light. When this happens, light from other galaxies can never reach us. So astronomers in the year 100 billion will look out on a vast dark wasteland utterly devoid of galaxies. Cosmology would appear to be a hopeless pursuit.

However, Loeb鈥檚 work shows this needn鈥檛 be the case. One of his other research interests is a remarkable group of objects called hypervelocity stars. These first came to light in 2005 when Warren Brown, also at Harvard, spotted a young, hot, blue star streaking outward through the halo of the Milky Way at the astonishing speed of 850 kilometres a second. 鈥淣o new stars are being born in the halo, so it stuck out like a sore thumb,鈥� says Loeb. 鈥淚t was not from there.鈥�

The star, already 250,000 light years from the centre of the galaxy, is moving so fast it will escape the gravity of the Milky Way altogether. The only thing that could have boosted it to such a high speed is a close encounter with the supermassive black hole lurking at the centre of our galaxy.

Since the first hypervelocity star came to light, 14 more have been discovered. According to Loeb, our supermassive black hole has been flinging a star out of the galaxy roughly every 100,000 years since the galaxy was born about 10 billion years ago. 鈥淎round 100,000 stars have already been ejected,鈥� says Loeb. 鈥淭he earliest ejected ones are already 50 million light years away.鈥�

It is these stars, kicked into touch by Milkomeda鈥檚 supermassive black hole, which Loeb says can play the role of cosmological indicators. Detecting such super-faint stars racing away from Milkomeda will not be easy but, as Loeb wryly observes, the astronomers of the far future will not be short of time. The key is to measure the Doppler shift 鈥� the amount by which light from these stars is stretched to longer, redder wavelengths on its journey through space-time. This tells you how much the universe has expanded since the light was emitted. By charting this Doppler shift for hypervelocity stars at different distances, Milkomeda鈥檚 cosmologists will be able to tell that something is accelerating the universe鈥檚 expansion 鈥� in exactly the same way we found the cosmic acceleration using supernovae explosions in galaxies.

As the universe expands, matter becomes ever more diluted. So future astronomers will be able to infer that matter was far denser in the past and they will be able to work out exactly when dark energy鈥檚 repulsive force won out over gravity.

Hypervelocity stars will also provide them with important information about Milkomeda itself. Before ejected stars get so far from the galaxy that they are accelerated away by the cosmic expansion, their speed will be braked by the gravity of all the stars and matter in Milkomeda. By observing the deceleration of such stars, it will be possible to deduce the mass and density of Milkomeda.

Will these cosmologists also be able to work out how the universe came into being? Today we can observe the dim afterglow of the big bang fireball. This cosmic microwave background radiation has a characteristic wavelength. 鈥淏ut 1000 billion years from now, it will be stretched from its current value of about 2 millimetres to larger than the observable universe,鈥� says Loeb. 鈥淚t will effectively cease to exist as a cosmic probe.鈥�

Incredibly, all is not lost. Loeb points to the light elements 鈥� helium, deuterium, lithium and so on. Low-mass stars, which fuse hydrogen into helium in their cores, can at most convert a few per cent of the universe鈥檚 hydrogen into helium. Yet 25 per cent of the mass of the universe is helium. The only way so much could have formed is if the universe as a whole went through a super-dense, super-hot phase in which there was an orgy of fusion to make helium. So the abundances of light elements in Milkomeda will tell cosmologists that the universe experienced a hot big bang, says Loeb. 鈥淐ontrary to expectations, astronomers of the future will be perfectly able to piece together the history of the universe,鈥� he says.

Fred Adams of the University of Michigan is intrigued by Loeb鈥檚 idea of using hypervelocity stars as probes of cosmic expansion. However, he points out a flaw in the strategy. 鈥淓ven the smallest, longest-lived stars will burn out after 10,000 billion years or so,鈥� he says. 鈥淭hen we are out of business again.鈥�

Not everyone is as relaxed about the future cosmos as Adams and Loeb. of the Institute for Advanced Study in Princeton is appalled by the prospect of living in a future universe with only one, super-dull galaxy. 鈥淚t presents a dismal picture of the future awaiting our descendants,鈥� he says in correspondence with Loeb.

Instead, Dyson speculates whether such a future could be changed by the intervention of intelligent life. He wonders whether a super civilisation might actually be able to move galaxies about by harnessing their gravitational energy. By concentrating them in one giant super-cluster, the inhabitants of these galaxies will at least be able to huddle together as other galaxies disappear over the horizon. 鈥淥ur descendants would stay in communication with 100 million galaxies instead of only one,鈥� says Dyson.

鈥淎dvanced civilisations capable of moving galaxies will be able to huddle them together as others disappear鈥�

Loeb points out, drolly, that moving galaxies will be very hard indeed. It would be far simpler, he maintains, to identify a large concentration of galaxies, such as the Coma cluster, and go there 鈥� although the 320 million-light year journey would be a very long one. 鈥淭ravelling is a lot easier than cosmic engineering,鈥� he says.

Even if our descendants stay within the confines of the galaxy, it will be far from lonely, says Greg Laughlin of the University of California at Santa Cruz. 鈥淢ilkomeda will contain upwards of 100 billion red dwarf stars and well over 100 billion planets,鈥� he says. 鈥淭hat鈥檚 a lot of worlds to explore, so the future 鈥� at least in the 100-billion year 鈥榥ear-term鈥� 鈥� doesn鈥檛 seem too bleak at all.鈥�

Loeb has a final, rather bizarre thought. 鈥淢y paper is the only one I have ever written that has a chance of being cited in 1000 billion years.鈥�

See gallery: 鈥�Colliding galaxies鈥�