From: LARRY KLAES (ljk4@msn.com)
Date: Thu Oct 31 2002 - 06:44:22 PST
OUR EARTH MAY HAVE A SPECIAL PLACE IN THE UNIVERSE AFTER ALL
>From Nature Science Update, 30 October 2002
http://www.nature.com/nsu/021028/021028-4.html
No time like the present: Intelligent life might be more likely in a
Universe in flux.
PHILIP BALL
Ever since Copernicus put the Sun, rather than Earth, at the centre of the
Universe, scientists and philosophers have suspected that there's nothing
special about our cosmic time and place. But two physicists now suggest
otherwise.
Only galaxies about the age of our Milky Way have the right conditions for
intelligent life to develop, argue Jaume Garriga of the University of
Barcelona, Spain, and Alexander Vilenkin of Tufts University in Medford,
Massachusetts1. And that age, they say, might coincide with a fundamental
change in the Universe.
What's more, the search for other planetary systems could tell us whether
they're right or not.
Four years ago, astronomers discovered that the Universe's expansion is
speeding up. Some think that this is because a dark energy is opposing
gravity and driving all matter apart.
One idea is that this dark energy comes from the constant creation and
annihilation of particles and antiparticles in 'empty' space. But the
mystery is then why this 'vacuum energy' seems to be so small - physics
predicts that it should dominate the way in which the Universe behaves.
One day it will, say many cosmologists. The observations of the accelerating
Universe, combined with studies of the early Universe, suggest that in
several billion years vacuum energy will dwarf all other forms, such as the
energy in light and stars.
But today vacuum energy accounts for about 70% of the Universe's total
energy, whereas around 11 billion years ago, when the Universe was young, it
accounted for about 10%.
Dark forces
We seem to be living at the point when vacuum energy is starting to
dominate. But in cosmic terms this rise of vacuum energy has been quite
quick - so it seems surprising that we just happen to be around at this
special time in the Universe's history.
But Garriga and Vilenkin think that they might have worked out why
intelligent life is likely to develop during this transition period. The
conditions for civilizations to emerge, they suggest, will be found mostly
in galaxies close to and rather like our own.
Distant, newly formed galaxies won't contain as much of the heavy elements
such as silicon and iron needed for planets to form, they say. This is
because they are smaller, and so their weak gravity is less able to hold on
to the heavy elements spewed out by exploding stars.
Small galaxies are also more crowded - and so planets there will be more
vulnerable to collisions or close encounters between stars. If vacuum energy
was weaker in the distant past, it's unlikely that anyone would have been
around to measure it.
Based on this reasoning, say Garriga and Vilenkin, the Universe's current
dark-energy component should be roughly as large as observations suggest. In
fact, they say, it should be a little larger, closer to 90%.
So considering dark energy in terms of the conditions needed for intelligent
life leads not just to hand waving, but to testable predictions. For
example, as ever more planets are found outside our Solar System, it may be
possible to see whether only nearby galaxies house habitable planets.
References
Garriga, J. & Vilenkin, A. Testable anthropic predictions for dark energy.
Preprint, (2002).
© Nature News Service / Macmillan Magazines Ltd 2002
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