Is our universe fine-tuned for life?

02 August 2008 by 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.

http://www.newscientist.com/article/mg19926673.900