Published online 28 July 2008 | Nature | doi:10.1038/news.2008.985

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Stars may not be so fine-tuned after all

A change in nature's fundamental constants could still allow star formation.

Would stars light up the sky in other universes? It's often claimed that the fundamental constants of physics in our own Universe are exquisitely tuned to permit stars – and therefore life - to exist. But Fred Adams, an astrophysicist at the University of Michigan in Ann Arbor, now suggests otherwise.

PleiadesCould inhabitants of other universes marvel at their own starry nights?NASA/ESA/AURA/Caltech.

In a paper soon to be published in the Journal of Cosmology and Astroparticle Physics1, Adams says that the three most relevant physical constants that determine star formation can have very different values, yet still permit stars to appear. In other words, there is nothing obviously 'special' about their values in our Universe at all.

That's certainly not the prevailing view2 3, which holds that a slight tweak to the strength of the electromagnetic force, for example, would disrupt stars so much that they could not create the materials and conditions necessary for life.

Universes with fundamental constants different from our own may actually exist, according to the most favoured current cosmological theory, called inflation. So would these universes, damned by a poor draw in the cosmic lottery, really be devoid of stars, and therefore barren?

Crucial constants

To answer the question, Adams looked at the three fundamental parameters that are most crucial to the formation of stars: the gravitational constant G; the fine-structure constant α, which fixes the strength of the electromagnetic force; and a parameter C that determines the rates of the nuclear reactions responsible for the fusion process that makes stars shine.

He calculated the ranges of different values of G, α and C that would support stars that would burn for long enough to give life a chance of evolving on surrounding planets – a billion years or so to judge from our own experience.

One way to work this out is by looking at how these parameters affect the minimum and maximum possible stellar masses. If stars have too low a mass, they cannot get dense and hot enough to spark fusion. If they have too high a mass, the 'radiation pressure' created by emission of light won't be big enough to prevent the star from collapsing under its own gravity into a superdense, dark body such as a black hole.

Adams estimates that all the constants G, α and C could have values different from those measured in our Universe by a factor of a hundred greater or smaller, and still allow stars to exist.

Live fast, die crushed

"This is a very interesting paper," says Mario Livio, an astrophysicist at NASA's Space Telescope Science Institute in Baltimore, Maryland. "It shows that it is certainly not impossible in principle for other universes to develop stars for a relatively wide range of values of the constants of nature."

Martin Rees, a cosmologist at the University of Cambridge, and Britain's Astronomer Royal, says that we shouldn't be too surprised by the result, as other astronomers have shown that universes in which gravity is stronger could support stars — although they would have much shorter lives. "This would not be a propitious universe because there wouldn't be enough time for complex evolution," he adds, "and objects as big as us would be crushed by gravity."

But stars aren't the only way to power life. For example, black holes are thought to radiate energy called Hawking radiation, in a kind of evaporation process that eventually consumes the hole itself. Adams shows that there is a wide range of values of α and G that permits the formation of black holes that would radiate strongly enough, and for long enough, to power a planet for several billion years.

Adams emphasizes that his study is only the beginning of a more thorough understanding of how finely tuned our Universe is. Even if the laws of physics allow for stars to exist and to burn through fusion, he says, other fundamental constants may determine whether or not they can actually form in large numbers – let alone whether or not they can support life. 

  • References

    1. Adams, F. C. J. Cosmol. Astropart. Phys. In the press; preprint at http://arxiv.org/abs/0807.3697 (2008).
    2. Davies, P. The Goldilocks Enigma: Why the Universe is Just Right for Life (Penguin, 2006).
    3. McMullin, E. in Fitness of the Cosmos for Life (eds. J. D. Barrow, S. Conway Morris, S. J. Freeland & C. L. Harper, Jr. (Cambridge University Press, 2008).
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