From: LARRY KLAES (ljk4_at_msn.com)
Date: Wed Jun 11 2003 - 11:17:51 PDT
http://www.nature.com/nsu/030609/030609-7.html
Universe can surf the Big Rip
Alternative proposed to dark energy's cosmic doomsday.
11 June 2003
PHILIP BALL
The end of the world is not so nigh. A Spanish scientist has found a loophole in the suggestion that there might be a Big Rip in the universe about 22 billion years from now1.
Earlier this year, US researchers showed how the recent discovery of an accelerating universe raises the possibility that in future everything may rend asunder, starting with clusters of galaxies and ending with the smallest of subatomic particles2.
Now Pedro González-Díaz of the Consejo Superior de Investigaciones Científicas in Madrid is arguing that, even if the universe is built the way Big Rip proponents suggest, a cosmic doomsday is not inevitable. The universe might just go on expanding, he says.
Given the timescales involved, we needn't start fretting too much either way. But there's no denying how terrible the Big Rip sounds. It is a kind of breakdown of all the fundamental forces of nature, as empty space becomes so full of energy that it overwhelms them. When that happens, everything falls apart.
Phantom menace
The destruction begins, say Robert Caldwell of Dartmouth College in New Hampshire, USA, and his coworkers2, about a billion years before it ultimately ends in a Big Rip. First, gravity loses its grip at cosmic scales, allowing clusters of galaxies to drift apart.
Sixty million years before doomsday, our own galaxy, the Milky Way, fractures as stars slip from each other's grasp. A few months before the end, planetary systems like the solar system will be dismembered, and 30 minutes before the Big Rip, the planets and stars themselves disintegrate.
In the split-second before the end, atoms and molecules are torn apart, then the particles that constitute them. Finally, space itself flies open.
All of this is driven, the argument goes, by something known as phantom energy, which fills all of space. The density of phantom energy increases with time, like a bomb that grows ever bigger.
Energy bar
No one knows if phantom energy exists at all. But recent astronomical observations hint that it might.
Five years ago, astronomers found that the universe is expanding at an ever-accelerating rate. The cosmic speed-up suggests that space is permeated by dark energy, creating a kind of pressure that opposes the pull of gravity.
One explanation for this dark energy reinstates the idea of a cosmological constant, which Albert Einstein first proposed and then rejected in the early twentieth century. According to this hypothesis, the universe will merely expand forever, with distant galaxies gradually winking out of sight.
An alternative possibility is that the dark energy takes the form of so-called phantom energy. This is more pathological than the dark energy supplied by a cosmological constant, Caldwell and colleagues say. They point out that phantom energy will become ever more dominant over other kinds of matter and energy as time progresses.
Or perhaps not. González-Díaz points out that some kinds of phantom energy can be well behaved, avoiding the blow-ups and instabilities that lead to a Big Rip.
Dark energy can be thought of as a kind of gas filling all of space, the density of which is proportional to its pressure. González-Díaz shows that if one assumes that this 'gas' has certain properties - specifically, that the speed an oscillation passes through it decreases with time - then there is no longer a Big Rip. This might sound contrived, but actually, González-Díaz reckons it is a more realistic kind of behaviour than the alternatives.
To settle the debate over what's in store for the Universe, astronomers will have to probe deeper into how it looked soon after the Big Bang, and how it is expanding now. Questions like this are being investigated by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite operated by NASA.
References
González-Díaz, P. F.You need not be afraid of phantom energy. Preprint, http://xxx.arxiv.org/abs/astro-ph/0305559, (2003). |Article|
Caldwell, R. R., Kamionkowski, M. & Weinberg, N. N. Phantom energy and cosmic doomsday. Preprint, http://xxx.arxiv.org/abs/astro-ph/0302506, (2003). |Article|
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