23 October 2008

End of days: a universe in ruins

By
COSMOS magazine
When will the universe end? And will the ultimate apocalypse arrive with a bang, or a whimper? A look into the far, far future - to the day the cosmos decays into a frozen, featureless void.
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PREDICTING HOW HUMAN SOCIETY will evolve is fraught with difficulty. Our accelerating science and technology hint at unprecedented wealth and leisure, while the danger of natural disasters, epidemics, wars and countless other unpredictable factors could lead to a much darker future.

When it comes to purely physical systems – the Sun and its planets, the galaxy, the universe – we can see more clearly what lies ahead. The Solar System, for example, turns out to be a fairly simple physical system, and astronomers have understood stellar physics well enough for several decades now to predict, with fairly high confidence, the fate of our home star and its family of planets.

Our planet’s fate is inexorably tied to that of our Sun, which has been shining for five billion years, with at least five billion to go. The Sun shines by burning hydrogen in its core, fusing it into helium. As the Sun’s nuclear fuel supply starts to run out, it will begin some peculiar contortions.

Gravity will at first cause it to shrink in size – but this will make the Sun’s core hotter, which will actually cause its outer layers to expand significantly. At this stage, five to seven billion years from now, the Sun will loom in our sky as a blazing ‘red giant’.

A few hundred million years later – a short period in terms of the Sun’s lifetime – it will undergo yet another phase of heating and expansion, shedding much of the material in its outer layers, and finally collapsing into a so-called ‘white dwarf’. By this time, its mass will still be about three-quarters of its current value, but compressed into a sphere the size of the Earth.

Click here to read the end of the universe timeline that goes with this feature.

Endofdays_deadearth

A dead Earth in the far, far future.

The Sun’s initial swelling during the onset of the red giant phase will destroy our blue planet. The additional sunlight reaching our atmosphere will cause global warming beyond Al Gore’s worst nightmares. The oceans will evaporate into space, leaving only deserts; life as we know it will not be able to sustain itself. As astronomer Fred Adams of the University of Michigan, Ann Arbor, put it: “Within a few billion years, our world – nowgreen and flowering with life – will closely resemble present-day Venus, with a hellish atmosphere fuelled by a runaway greenhouse effect.”

According to recent calculations by Klaus-Peter Schröder, at the University of Guanajuato, Mexico, the Sun’s diameter will eventually swell from its current 1.4 million km to as much as 358 million km. The inner planets, Mercury and Venus, will be swallowed outright by the raging Sun.

Given that the diameter of the Earth’s orbit is only about 300 million km, our own prospects don’t seem much better. But it’s not quite that simple: because of the Sun’s weakening gravitational attraction, the Earth’s orbit will have expanded to about 370 million km. So we won’t be engulfed by the swelling Sun – not yet.

What is left of our planet, however, will be scorched beyond recognition, baked by a crimson Sun that takes up half the sky. At its brightest, the Sun will shine with an intensity more than 4,000 times greater than today. As Adams put it in a recent paper: “Current estimates indicate that our biosphere will be essentially sterilised in about 3.5 billion years, so this future time marks the end of life on Earth.”

The outer planets in our Solar System will fare somewhat better. Mars, for example, will become distinctly more hospitable over the next few billion years. In about six billion years, Adams says, the Red Planet will absorb about as much sunlight as the Earth does today. Will the next chapter in human history unfold on the fourth rock from the Sun?

Our planet, by this time devoid of life, will linger a bit longer. Although Earth will have moved to a wider orbit, Adams explains that it will meet with more resistance as it passes through matter from the Sun. This will ultimately cause Earth’s orbit to decay, dragging the planet closer to the Sun, where it will meet its demise. In his paper, Adams describes the end of planet Earth in two terrifyingly concise sentences: “Earth is thus evaporated, with its legacy being a small addition to the heavy element supply of the solar photosphere. This point in future history, approximately seven billion years from now, marks the end of our planet.”

Happily, this billion-year time scale is inconceivably long compared to the 200,000 years or so that our species has been around, let alone the few millennia in which we’ve been using technology. So perhaps we can dare to imagine that we will have spread out across the galaxy – or at least beyond our doomed planet – before the Earth’s demise. So let us turn, then, to the long-term prospects for the universe itself.

ISAAC NEWTON IMAGINED a static cosmos of infinite space and time. In such a universe, it would be reasonable to conceive of an infinite future for our species or its remote descendants. But the discoveries of 20th century physics changed that picture, and after the Big Bang model began to solidify, astronomy textbooks typically described two possible fates for our universe.

First, if the average density of the universe were great enough, the universe would be ‘closed’: gravity would eventually halt its expansion and the universe would start to contract, ultimately collapsing in a kind of reverse Big Bang, or ‘Big Crunch’.

Alternatively, if its average density were lower than this threshold, the universe would be ‘open’: it would expand forever and all processes in the universe would gradually cease, in accordance with the second law of thermodynamics (which says, essentially, that the amount of disorder in the universe must always increase, and that systems tend towards equilibrium). The universe would become ever darker, colder and less hospitable to life.

The 20th century American poet Robert Frost captured the essence of the two possibilities in 1923 with a famous poem ‘Fire and Ice’: “Some say the world will end in fire | Some say in ice”. Until the final decades of the 2oth century, this was the best we could do: the universe would suffer one of these fates, but we could not say which one.

But the universe had more surprises in store, and as the century drew to a close, it delivered a whopper. In the late 1990s, astronomers were studying the properties of distant supernovae when they found something remarkable: the universe wasn’t just expanding – the rate of expansion was accelerating. The rate of expansion had been decelerating until sometime around seven billion years ago, at which time we entered a new era of ever-faster expansion.

What could possibly be causing the expansion of the universe to accelerate? The Big Bang explosion would have given everything an outward push – but astronomers and physicists thought that the force of gravity ought to be slowing that expansion. They concluded that there must be some kind of intangible energy that works against gravity– some energy that is literally pushing all of the galaxies away from each other. No one knows exactly what that entity is; for now it has been labelled ‘dark energy’.

One thing we do know about dark energy: the extra ‘push’ it delivers would seem to guarantee an open, ever-expanding cosmos. Today astronomers look out across the universe and see galaxies lumped together in clusters, with those clusters grouped together in superclusters. The superclusters, in turn, appear to be strung out in vast string-like filaments that stretch for hundreds of millions of light years across the cosmos. Gravity has crafted these structures – but dark energy will tear them apart.

The discovery of dark energy came at an awkward time for Adams, who had just published his book on the long-term fate of the cosmos, The Five Ages of the Universe (with co-author Greg Laughlin). How does the presence of dark energy affect his forecast? “Perhaps the most important update is that we now ‘know’ that the universe is accelerating,” he told me by e-mail (using quote marks to emphasise the fact that, in science, no result is ever 100 per cent certain). “Since the expansion of the universe is speeding up, essentially no more cosmic structure will form.”

In other words, those clusters and superclusters and stringy filaments are the end of the line in terms of cosmic evolution. “The things that we have now in the universe will be all that you get – ever,” Adams said.

Thanks to dark energy, those large-scale structures will gradually disintegrate, and the universe will eventually look very different from what we see today. Things will appear fairly normal for the first few trillion years; stars will continue to shine and any planets they may harbour could be reasonably hospitable places. Adams calls this the ‘stelliferous era’ (meaning “filled with stars”); it is the era we now inhabit. (The first of Adams’s eras is the ‘primordial era’, which covers roughly the first million years of cosmic history, from the Big Bang to the creation of the first stars.)

endofdays_deadsun

A burned out Sun.

Eventually, the stars will exhaust their nuclear fuel, and – perhaps 100 trillion years from now – no new stars will be able to form. The stelliferous era will have come to an end, and we will enter what Adams calls the ‘degenerate era’. The most prominent objects in the universe at this stage will become ‘degenerate stellar objects’ – essentially, the wasted cores of stars that no longer shine. Ordinary stars will have evolved into white dwarfs, while heavier stars will become ultra-dense neutron stars or black holes.

Ultimately, we shouldn’t get too attached to these stellar remnants either, Adams cautions. After a mind-numbing period of time, white dwarfs and neutron stars will disintegrate through a process called proton decay, in which solid matter gives way to radiation. The lifetime of a proton is not yet known, but theory suggests that they last for 1030 to 1040 years. These are very large numbers indeed; the age of the universe at the moment is only about 1010 years.

After that point, the only sizable objects left in the universe will be black holes, and we enter the aptly-named ‘black hole era’. Black holes are the most enduring objects that our universe and the laws of physics are able to craft. And yet they, too, must succumb to the endless time of an expanding universe. Black holes will ultimately disappear, evaporating by a process known as Hawking radiation – a quantum-mechanical process first described by British physicist Stephen Hawking in the 1970s. A black hole with the mass of the Sun may last for 1065 years; a supermassive black hole may endure for 10100 years.

After the last black hole has disappeared in a puff of Hawking radiation, the universe will be nearly empty. All that will remain will be a sparse flotilla of fundamental particles, drifting endlessly across a frozen, featureless void. Adams calls this final epoch the ‘dark era’.

If we could somehow transport ourselves to this distant era, what would we see? “Very little,” says Adams. “The universe would be very, very dark, very diffuse.” All that will remain is a thin ‘soup’ of particles “and perhaps other things that we don’t know about.”

Not much will happen in this rarefied environment, Adams explains. Occasionally, an electron will bind with a positron to form an atom of ‘positronium’ – but even these will eventually disintegrate. Electrons and positrons can also directly annihilate each other. “Except for these low-level annihilation events,” says Adams, “the universe is a very low-energy, low-key kind of place … A sea of darkness.”

Perhaps T.S. Eliot was close to the mark in his poem, “The Hollow Men”: “This is the way the world ends | Not with a bang but a whimper.”

It is hard to think of anything more depressing than this slow decline of the cosmos into eternal darkness. But here goes: because of dark energy’s unforgiving push, the night sky of the remote future will be far less rich than the one we see today, and astronomers of that era – if they exist and can exist – will have no inkling of the vast and complex cosmos that once existed.

Our galaxy, the Milky Way, and our closest neighbour, the Andromeda Galaxy, are bound together by gravity; together with a sprinkling of dwarf galaxies, they make up the so-called Local Group. The billions of other galaxies beyond the Local Group are not gravitationally bound to us, and the expansion of the universe, driven by dark energy, will eventually push them out of view. The most distant objects will be the first to disappear – “cloaked behind a cosmological horizon,” as Adams puts it. Nearer galaxies will follow, slipping away one by one.

By 100 billion years from now, give or take, even the Virgo cluster – the next-closest cluster of galaxies beyond the Local Group – will have disappeared over the cosmic horizon. We will be then completely isolated from the rest of the universe: beyond the handful of galaxies that make up our Local Group, our telescopes will reveal only blackness. All of those other clusters suffer the same fate; each of them will be similarly isolated from their neighbours. Should astronomers exist in those other realms, their telescopes, too, will reveal nothing.

Our Local Group will still see some action: our galaxy and the Andromeda Galaxy are currently moving toward each other, and they are expected to collide – or rather, merge – in about six billion years. (The merger will not directly affect most stars: stars are very far apart compared to their individual diameters, so a typical star in our galaxy will not undergo a collision with a star from Andromeda.) In the long run, the Milky Way, Andromeda and the other smaller galaxies of the Local Group will merge into one large conglomeration.

When our Local Group becomes a universe unto itself, astronomers will have things to aim their telescopes at locally but will be ignorant of the universe’s overall structure. According to Lawrence Krauss of Case Western Reserve University in Ohio, astronomers of this distant era will be hard pressed to infer that anything like a Big Bang had ever occurred.

With those distant galaxies out of view, it will no longer be possible to measure their recession speeds, as U.S. astronomer Edwin Hubble did in the 1920s. The same goes for the other main piece of evidence in support of the Big Bang – the cosmic microwave background radiation (or CMB), sometimes described as ‘the echo of the Big Bang’, discovered in the 1960s. As the CMB radiation gets stretched out to longer and longer wavelengths, it will be harder and harder to detect.

Krauss has said that astronomers living at that time will be misled by their observations: “It will lead them to the wrong conclusion about what the universe is doing,” he said in a recent interview. “The universe will look static, and that’s vastly wrong, because the universe is expanding so fast they can’t see it.”

This is troubling on many levels. It is naturally disheartening to think of knowledge that we have today no longer being available in the remote future; perhaps it will make us strive to preserve that knowledge at all costs. It may also make us wonder just how confident we should be in our interpretations of what we see in the sky right now.

WE HAVE SEEN HOW the universe is destined to end in darkness; what, then, is the fate of life? In an open universe, it would seem that every entity, every being, every thought, must come to an end. As philosopher Bertrand Russell once put it: “All the labours of the ages, all the devotion, all the inspiration, all the noonday brightness of human genius, are destined to extinction … The whole temple of Man’s achievement must inevitably be buried beneath the debris of a universe in ruins.”

The Roman poet Lucretius must have had a similar vision in mind two millennia earlier when he wrote:

Again, perceivest not,
How stones are also conquered by Time?
Not how lofty towers ruin down,
And boulders crumble? Not how shrines of gods
And idols crack outworn?

In the late 1970s, however, physicist Freeman Dyson from the Institute for Advanced Study in Princeton, New Jersey suggested an alternative. Dyson defines life as any entity that can process information. Because this requires energy and generates heat, it would seem that an expanding universe offers less and less usable energy to keep such a system functioning. But Dyson imagined a way out: he suggested that life could, in effect, ‘hibernate’ for ever-increasing periods of time. By lengthening the span of the hibernation periods – effectively lowering their ‘metabolism’, so to speak – life could endure more or less forever, Dyson asserted.

The discovery of dark energy, however, may spoil Dyson’s idea. Krauss, together with colleague Glenn Starkman, has argued that in a universe containing dark energy, life is doomed. Life requires energy, and in an ever-expanding universe it becomes more and more difficult to collect and harness that energy. “The cosmic dilution of energy is truly dire,” they wrote recently.

As we become isolated in our respective island universes, the resources at our disposal become strictly limited. With finite resources, any living creatures (or equivalent machines) would have a finite memory. Finite information, they argue, implies a finite number of thoughts.

“Eternity would become a prison, rather than an endlessly receding horizon of creativity and exploration,” they assert. In the long run, “life, certainly in its physical incarnation, must come to an end.”

THIS IS NOT A PARTICULARLY HAPPY outlook for life, the universe and everything. But perhaps we can take away something positive from our speculation. First of all – and I seem to recall the late astronomer and author Carl Sagan saying something like this toward the end of his TV series Cosmos back in the 1980s: all those billions of years that lie ahead offer the opportunity to do a great deal of good. Further, it is quite impressive that with our finite hominid brains we have been able to peer so far ahead, with at least some degree of confidence.

And isn’t it intriguing that the fate of the universe – trillions upon trillions of years from now – is clearer to us today than the fate of our own civilisation just a few centuries ahead?

Dan Falk is a science journalist based in Toronto, Canada, and the author of In Search of Time: The Science of a Curious Dimension. This story for COSMOS won the prestigious 2009 Science Writing Award, handed out by the American Institute of Physics.
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