http://space.newscientist.com/article/mg19926673.900
Is our universe fine-tuned for life?
02 August 2008
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Michael Brooks

DON'T take our starry skies for granted. If you were unlucky enough to be living
in some other universe, you might have nothing to stare at but black holes.

At least, that's the view of a new study that examines the nature of other
universes that might support life and suggests that our cosmic habitat is
nothing special after all - wondrously starry skies apart.

The idea that certain aspects of our universe make it uniquely suited to life
has never been properly tested, says Fred Adams of the University of Michigan in
Ann Arbor. "You hear people say our universe is fine-tuned for life, that stars
are rare and couldn't form if certain things were different," he says. "The
truth is, no one has done the calculations." Adams has now rectified that
situation and found that it is not unusual for stars to form that can support
life.

Claims of fine-tuning have generally been based on what happens when you vary a
single characteristic of the universe, say the strength of gravity, while
holding all others constant. That, says Adams, is too artificial a scenario to
tell you anything about whether there are other universes that can support life.
"The right way to do the problem is to start from scratch," he says. "You have
to turn all the knobs and find out what happens."

To do this, Adams started with a simple definition of a star: a massive body
held together by its own gravity that is stable, long-lived and generates energy
through nuclear processes. Just three constants are involved in the formation of
such stars. One is the gravitational constant. The second is alpha, the fine
structure constant that determines the strength of interactions between
radiation and matter. The third is a composite of constants that determines the
reaction rates of nuclear processes.

Adams selected a range of possible values for each of these constants, then put
them into a computer model that created a multitude of universes, or a virtual
"multiverse". Each universe within the multiverse used different values for the
three constants and was subject to slightly different laws of physics.

About a quarter of the resulting universes turned out to be populated by
energy-generating stars. "You can change alpha or the gravitational constant by
a factor of 100 and stars still form," Adams says, suggesting that stars can
exist in universes in which at least some fundamental constants are wildly
different than in our universe.

And though some universes were filled with things we might not usually think of
as stars - radiating black holes or bodies formed of dark matter - they all gave
out enough energy to power some form of life, and lasted long enough for life to
evolve.

That may not necessarily be life as we know it, however. Since the simulations
didn't rely on the stars producing carbon, Adams points out that very different
life forms to ours might be better suited to some of the universes. Because life
depends on chemistry, and chemistry depends on alpha, varying alpha changes the
nature of life. "You have no idea what life would be like in a universe with
different constants," Adams says.

Adams reckons his results, which will be published in the Journal of Cosmology
and Astroparticle Physics, suggest that the "specialness" of our universe could
well be an illusion. And this is only the very beginning of what can be probed
to undermine the idea that our universe is fine-tuned for life. There are plenty
more constants and processes that can be tinkered with, he says.

Adams's approach is "extremely interesting", says Michael Murphy of Swinburne
University of Technology in Melbourne, Australia. "I've long had a suspicion
that this talk of fine-tuning needs constant questioning and re-examination," he
says. "It's sometimes hard to recognise that living somewhere else in a
different way might be just as easy."

Sean Carroll of the California Institute of Technology in Pasadena is also
impressed, and intrigued by the idea of unusual life forms that, say, feed off
black holes. "I don't know what it would look like or how it would work, but
black holes radiate, just like stars do. Why couldn't you have life arise in the
'atmosphere' of a gently radiating black hole?" he wonders.