杏吧原创

KATHERINE Freese and William Kinney don鈥檛 look much like superheroes, but this pair of astrophysicists may just have rescued all life in the Universe. And as in all the greatest comic book stories, deliverance has come before most of humanity even knew they needed saving. A couple of years ago, two other physicists announced that the latest cosmological discoveries meant that life in our Universe was doomed. Now Freese and Kinney tell us that it can go on forever.

The ultimate future of life may seem a strange thing for physicists to be debating, but the behaviour of the Universe can tell us about more than just time and space, galaxies and supernovae. Our future in the cosmos is not just a matter of chance circumstances or our own ingenuity. It is circumscribed by the most fundamental laws of nature, written into the fabric of the Universe during the big bang.

Using physics to work out our fate may be a tad speculative, but it鈥檚 not unprecedented. Freese, of the University of Michigan in Ann Arbor, and Kinney, who is at Columbia University, New York, are building on work done by the Princeton physicist Freeman Dyson.

In 1979, Dyson published a study on the thermodynamics of life, using physics to argue that life could last forever in the Universe. His argument is really about life that has consciousness: life that thinks. At some level, thought must be like computation, he says, involving the physical processing of information. Any practical computation depletes its energy source. Dyson assumed that the Universe doesn鈥檛 hold infinite sources of energy, so any life would eventually face an energy crisis. But he argued that an organism can stretch out a dwindling energy supply by slowing its metabolism, which is equivalent to operating at a lower temperature, and slowing down the rate at which it performs computations. In other words, it can save energy by thinking more sluggishly.

But even cooling down 鈥� and thus slowing down 鈥� can鈥檛 eke out a finite energy resource forever. That鈥檚 because there鈥檚 a limit on how cold an organism can get.

Every computation produces heat that has to be radiated away. In order to do this, the organism must remain warmer than its environment, because heat can only flow from a hot object to a cold one. The cosmological data of Dyson鈥檚 time indicated that the temperature of the cosmos was dropping more quickly than would the operating temperature of any organism trying to keep thinking. So far, so good.

However, Dyson also realised that, because radiating away heat relies in general on the properties of electrons, quantum mechanics dictates a fundamental limit to how fast the heat can be dissipated. If the organism produces heat faster than its electrons can dissipate it, it is doomed to death by overheating.

The answer, said Dyson, is to 鈥渉ibernate鈥�. In Dyson鈥檚 definition, a hibernating organism essentially stops its metabolism entirely, which means it must stop thinking. Yet it continues to radiate away accumulated waste heat. He showed that a judicious combination of periods of ever-slower activity and spells of hibernation make it possible to perform an infinite number of computations with a finite amount of energy: the organism can go on thinking forever. So, the outlook for life 鈥� albeit a strange, sluggish and cold kind of life 鈥� is good. Or so we thought.

But bad news was on the way. Recent observations of distant supernovae have shown that the expansion of the Universe, which has been going on since the big bang, is speeding up (New 杏吧原创, 11 April 1998, p 26). That鈥檚 a problem, as it means our future energy sources may be slipping out of our grasp. In an expanding Universe, the further away an object is from us, the faster it is heading into the distance. At a particular distance, known as the de Sitter horizon, objects are receding at the speed of light. Anything beyond the de Sitter horizon is forever out of reach. So, because of cosmic acceleration, distant parts of the Universe will eventually reach the point of no return. Every galaxy beyond our Local Group 鈥� which gravity keeps bound together 鈥� is moving inexorably towards our de Sitter horizon. Once they pass it, their light can never reach us.

That means the galaxies will wink out one by one. The space beyond our galactic neighbourhood will begin to appear cold and dark, and we鈥檒l no longer be able to find out anything about it (New 杏吧原创, 20 October 2001, p 36). That鈥檚 not just a problem for astronomers of the year two trillion. It means that when a galaxy disappears over the de Sitter horizon, we lose a potential source of energy.

The same is true everywhere. All life, wherever it is in the Universe, faces a dwindling energy supply. As Dyson showed, it seems that life can survive such a setback by slowing down its metabolism and hibernating periodically. But the de Sitter horizon does more than create an energy crisis. In the wrong circumstances, it can set a limit to how cold the Universe can get. And if you are a creature trying to operate at ever-lower temperatures, this is very bad news indeed.

The problem stems from a suggestion first made by Stephen Hawking. Whenever there is a horizon beyond which one cannot see or travel 鈥� be it a black hole鈥檚 event horizon or a de Sitter horizon 鈥� this boundary emits a small but significant amount of radiation. The radiation comes from quantum fluctuations that occur in the vacuum of space. These fluctuations continually create pairs of particles and antiparticles. Ordinarily, these particles annihilate one another immediately. But if a pair pops into being close to a de Sitter horizon, one particle can wander over the horizon and became irrevocably separated from its partner before they can cancel each other out. And so one half of the pair is left, adding a contribution to the heat energy in the accessible Universe on our side of the de Sitter horizon. This means that space in an accelerating Universe can never get cooler than a particular temperature, known as the Hawking temperature.

It鈥檚 hardly a balmy glow: working from what the supernova data reveals, it鈥檒l be something of the order of 10^鈭�29 kelvin. Nevertheless, as we lower our operating temperature to slow down our metabolism, we are eventually going to hit a point where we reach the same temperature as our environment. Then we won鈥檛 be able to radiate away waste heat. In other words, although we might be extremely cold, we鈥檒l still fry the moment we try to think.

Cosmologists John Barrow and Frank Tipler were the first to point out this disastrous scenario in their book The Anthropic Cosmological Principle. And then, in 2000, Lawrence Krauss and Glenn Starkman of Case Western Reserve University in Cleveland, Ohio, published a much more detailed analysis. Their conclusion was stark: given Dyson鈥檚 scenario, nothing 鈥� not mining the Universe for its energy resources, not even hibernation 鈥� could preserve life forever in an accelerating Universe.

But Freese and Kinney have come to the rescue. Krauss and Starkman were looking at what would happen if the Universe鈥檚 accelerating expansion were being driven by a particular form of energy: the vacuum energy created by a 鈥渃osmological constant鈥�. This constant, whose existence was first suggested by Einstein, is a measure of how much energy is released in empty space when the paired particles and antiparticles created by the vacuum鈥檚 quantum fluctuations annihilate each other.

The cosmological constant 鈥� if it exists 鈥� endows empty space with a repulsive force that causes space-time to expand. It鈥檚 the explanation for the Universe鈥檚 accelerating expansion that most physicists favour, but there are problems with it. Einstein only proposed its existence to make his theory of general relativity fit the (mistaken) assumption of his time that the Universe was static. And no one can explain why the cosmological constant has the value it apparently does: according to conventional theories, it should be 10¹²⁰ times larger than astronomical observations suggest.

The cosmological constant is by no means the only way to explain the acceleration. There鈥檚 鈥渜uintessence鈥�, for example. This idea was first proposed in 1987 by a group that included Freese. The quintessence theory suggests that the energy responsible for the acceleration of the Universe comes partly from the vacuum energy and partly from dark matter, the unobservable 鈥� and, as yet, inexplicable 鈥� stuff that astronomers must posit to explain why galaxies rotate as they do. The useful thing about quintessence is that the energy accelerating the cosmic expansion need not be constant. 鈥淨uintessence is a vacuum energy that changes in time,鈥� Freese says.

Our freezing future

The vacuum energy determines the energy of the particle-antiparticle pairs created by quantum fluctuations of empty space. And this, in turn, determines the temperature that an accelerating Universe is destined to reach once all other sources of energy have disappeared over the de Sitter horizon. So if the vacuum energy is steadily falling, as the quintessence idea suggests, the temperature of the Universe keeps falling too. There鈥檚 no temperature limit that would place a limit on life鈥檚 longevity.

Another model that Freese has developed is Cardassian expansion, named after the reptilian bad guys of Star Trek: Deep Space Nine. Like the theory of quintessence, Cardassian expansion allows the temperature of the Universe to keep falling, but unlike quintessence or the cosmological constant it doesn鈥檛 invoke vacuum energy. 鈥淭he only ingredients are ordinary matter and radiation,鈥� Freese says. It might sound like a rather convenient construction, but it is actually a consequence of 鈥渂rane physics鈥�, a new idea that is fast gaining credibility.

Many physicists now think our visible Universe of three spatial and one time dimension is just one membrane, or brane, embedded in a space that has six or more dimensions (New 杏吧原创, 29 September 2001, p 26). When Freese and Daniel Chung of the University of Chicago were looking into this idea, they noticed that the extra dimensions would pull on the brane that we know as our Universe. Working with Michigan graduate student Matthew Lewis, Freese has shown that the pulling of the extra dimensions can in fact cause our Universe to accelerate. The consequences of this Cardassian expansion fit with observational data such as the cosmic background radiation and the age of the Universe.

And Cardassian expansion will also allow us eternal existence. Since there is no vacuum energy in this model, the Universe is simply filled with normal matter and radiation. Our once-hot Universe will cool forever, moving inexorably towards (but never reaching) absolute zero. As long as an organism keeps pace with the background temperature, never cooling below it, it will always be able to radiate away the waste heat generated by computational operations, and keep on going.

So, although the Universe seems to be accelerating, that needn鈥檛 be the life-ending catastrophe that Krauss and Starkman predict. 鈥淲e discovered that any driver of acceleration other than a cosmological constant can probably allow life to persist indefinitely,鈥� says Freese.

Krauss and Starkman鈥檚 pessimism isn鈥檛 beaten yet, though. They say that when things get very cold quantum-mechanical effects suppress an organism鈥檚 ability to radiate away heat, regardless of the background temperature. To lower its energy, an organism must be able to descend onto a lower rung of the ladder of quantum energy states. But, say Krauss and Starkman, the rungs cannot go on forever. Eventually an organism must reach its quantum ground state, and then it has nowhere to go. They raised this as an objection to Dyson, and think it applies to Freese and Kinney鈥檚 arguments too.

Kinney thinks cosmological models that involve an ever-changing 鈥渄ark energy鈥� can provide a way around this difficulty. 鈥淚f you make space itself dynamic, the expansion of the Universe makes new quantum ground states available to the system, and you can beat Krauss and Starkman鈥檚 limit,鈥� Kinney says. He admits it鈥檚 a hand-waving argument, based on the fact that the ground-state energy of quantum particles is lowered as they become less tightly confined in the expanding Universe. The matter is far from settled, Kinney says. 鈥淣obody has yet made these kind of arguments really rigorous, so their validity is still a very open question.鈥�

All the physicists involved are still locked in debate over the issues. And since the discussion is based on cosmological speculation, very simple definitions of life and the slippery nature of the Universe鈥檚 hidden energy, at the moment it would be unwise to lay bets on whether or not we鈥檒l reach eternity. Kinney concedes that it鈥檚 something of a playful diversion from the more ponderous side of cosmology, 鈥渆specially for somebody like me who was raised on science fiction and comic books,鈥� he says.

Some might say SF has even infiltrated the paper Freese and Kinney have submitted to Physics Letters. The pair have gone so far as to suggest a few far-fetched emergency plans to save the human race (see 鈥淭he great escape鈥�). But no one is panicking yet. The possible extinction of life in the Universe is not going to be a pressing issue for at least 10⁴⁰ years. 鈥淭he timescale in question is immense,鈥� says Krauss. 鈥淚t鈥檚 nothing to sell your stocks about.鈥�