Life can go on forever
An accelerating universe does not have to fry life.27 May 2002
Life can carry on indefinitely. Physicists in the United States have come to the comforting conclusion that just because the universe is accelerating as it expands, this does not necessarily sound the death knell for life in the far future1, as some have claimed2.
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In the right kind of accelerating universe "life can go on indefinitely", say Katherine Freese of the University of Michigan and William Kinney of Columbia University in New York. We don't know whether ours is the right kind, so the doomsday scenario is still possible, but at least there's hope.
An accelerating universe limits how much energy is available to life because distant regions get too far away to be reached by organisms confined to travelling below the speed of light.
Faced with finite energy resources, organisms can use up energy more slowly by slowing down their metabolism. In principle there's no limit to how sluggish life can get, so it could eke out an existence indefinitely.
But there's a snag. To have any sort of metabolism - to do anything, in other words - an organism has to get rid of heat.
This can only happen if the organism is hotter than its surroundings, because heat can flow only from hot to cold. An organism that is cooler than its environment will eventually fry.
There's a limit to how cool an accelerating universe can get. As life slows to conserve its limited energy supplies, it too will cool. Eventually, though, it will hit this temperature limit and stop being able to shed heat.
Freese and Kinney now suggest that this lower temperature limit need not be fixed, but might itself decrease over time. It all depends on why the universe is accelerating.
Constant worry
One reason why a universe might expand is that empty space actually exerts a pressure that counteracts the force of gravity. This pressure is quantified by the so-called cosmological constant, inserted into Einstein's equations of relativity, which describe the structure of space and time.
A universe with a cosmological constant acquires a background radiation. It is this warm vacuum that some believe will eventually fry ultra-sluggish life.
Freese and Kinney reckon that two alternative explanations for the acceleration of the expanding universe - quintessence and cardassian expansion - give life a rosier prognosis.
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The quintessence model invokes a kind of vacuum energy like that produced by a cosmological constant. But crucially, this energy, and thus the background temperature, decreases with time. The same is true of cardassian expansion.
This means that a life form could conceivably slow its metabolism down to avoid running out of energy, while keeping pace with the cooling universe so that it can always radiate heat into a cooler environment.
Even if the universe does turn out to be dominated by a cosmological constant, Freese and Kinney are reluctant to give up hope. One day, they speculate, we might figure out how to synthesize a new universe in a laboratory, set off a Big Bang, and move into it, abandoning our present universe as a lost cause.
- Freese, K. & Kinney, W.H. The ultimate fate of life in an accelerating universe. Preprint.(2002).
- Krauss, L. M. & Starkman, G. D. Life, the universe, and nothing: life and death in an ever-expanding universe. Astrophysical Journal, 531, 22 - 30, (2000).
© Nature News Service / Macmillan Magazines Ltd 2002