Stars ablaze in other skies

Calculations find that many universes could sustain stars

Fred Adams sees stars in the most unlikely places.

His calculations suggest that, contrary to some previous claims, stars are not only common in our cosmos but are also ablaze in myriad other universes, where the laws of physics may be drastically different. Even in a cosmos where balls of gas and dust never collapse and ignite to make conventional stars, radiation produced by black holes and clumps of invisible material called dark matter may play the same role as stars, says Adams, a theorist at the University of Michigan in Ann Arbor.

“In fact, all universes can support the existence of stars, provided that the definition of star is interpreted broadly,” notes Adams in the August online Journal of Cosmology and Astroparticle Physics.

Adams embarked on his study because he wanted to find out if other possible universes, in which the constants of nature might have different values, could support the existence of stars. Multiple universes, or the idea of a multiverse, is envisioned by some modern versions of the Big Bang theory.

According to inflation, a leading theory of the birth of the universe, the cosmos underwent a tremendous growth spurt in its first tiny fraction of a second, enlarging from subatomic scale to the size of a grapefruit. This rapid expansion may also have occurred in other patches of space remote from our cosmos, creating a multitude of pocket universes, or multiverses, with different physical laws.

In his analysis, Adams simulated conditions in other universes by simultaneously varying three parameters: the gravitational constant, which determines the strength of gravity; the fine structure constant, which sets the strength of the electromagnetic force; and a composite number that determines the rate of nuclear reactions, which keep stars shining.

Other researchers, he notes, have considered a broader class of questions in exploring the multiverse, including not only the possibility of star formation and stellar structure, but also that of galaxy formation and the existence of life.

“I did a specific approach that is much more detailed on the particular topic of ‘can there be stars?’ ” says Adams.

By allowing all three of the parameters, rather than a single parameter, to vary, Adams created a simulation that may embrace a larger number of possible universes, he says. He finds that stars are stable entities in roughly one-fourth of the universes he considered. “That’s a sizable amount of real estate.”

He cautions, however, that his calculations assume that all possible values of the each parameter are equally likely. For instance, it may be more likely for a universe to have a smaller nuclear reaction rate than a larger one. “We simply do not know,” Adams says.

The results are not “particularly surprising, as stars are both fairly simple and fairly robust objects that essentially require [only] a heat source and gravity,” says Anthony Aguirre of the University of California, Santa Cruz.

“But Adams has done an elegant job of working through the problem to find out exactly how different the universe could be while supporting stars,” Aguirre says.

The findings have several intriguing implications, he adds. Had Adams found that the range of parameters that allowed for stars was very small, that would have suggested that the laws of physics in our universe have been “fine-tuned” to allow for star formation, Aguirre notes. Instead, Adams’ study shows that our universe doesn’t seem particularly special in that regard.

“The paper nicely points out that when considering whether other, different universes can sustain life, it is very important to carefully consider how the known universe could change, but also to consider all sorts of things that don’t really exist here, such as black-hole-powered solar systems, or dark-matter stars,” Aguirre says. “This open-minded approach can serve, in some cases, as a counter-argument to claims that our universe is fine-tuned for life.”

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