Jul 17, 2007

more habitable islands in the multiverse?

Cosmologists are trying to test the limits of the weak Anthropic Principle by tweaking some fundamental assumptions about physical laws, NewScientist reports:
It all comes down to numbers. Harnik argues that there will be countless more universes with myriad properties different from our own. By varying just one property, cosmologists have been too conservative. Harnik, Kribs and Perez decided to highlight this flaw in anthropic reasoning by taking a radical measure: they switched off the weak nuclear force, one of the four fundamental forces in nature. In practice, this means changing a multitude of parameters and constants simultaneously.

The weak force is responsible for the radioactive beta decay of atomic nuclei and is considered essential for a complex universe like ours. Take it away, and you might expect the "weakless" universe to be wildly different from our own.

Only Harnik, Kribs and Perez have discovered it isn't. They considered what would happen to crucial processes in the history of the universe - the forging of elements in the big bang, the powering of stars and supernovae explosions. By examining the equations that describe these processes, they made an astonishing discovery: the weakless universe is still capable of supporting observers....

It is not the only evidence to suggest that we need to broaden our horizons when it comes to testing the anthropic principle. In 2001, Anthony Aguirre of the University of California, Santa Cruz, found another island in the multiverse....

The crucial parameter that determines whether the big bang is hot or cold is the number of photons per baryon. In our universe it is about a billion. Aguirre wondered what would happen if it was in the range 0.1 to 100 - much, much cooler.

Aguirre's universe started off quite unlike our own (Physical Review D, vol 64, p 083508). After our hot big bang, the universe took tens of millions of years to cool to the point where matter could clump into stars. "But in the cold big bang universe, stars can begin to form within 100 years of the big bang," says Aguirre.

He even modelled an extreme cold big bang universe where the cosmological constant was 1017 times what it is in our universe. By rights, this strong repulsive force ought to fling matter apart, preventing the formation of galaxies. However, in the cold big bang universe, stars form so quickly that they are in place before this cosmological repulsion takes hold. "The stars then rush away from each other," says Aguirre. "It's a pretty dull universe with each star isolated in a vast ocean of space. Nevertheless, there is nothing to prevent such stars having planets and observers."
As computing power increases, expect to see more viable simulations of alternate universe formations. Also, expect to be sucked in by the eerie logic of Nick Bostrom's Simulation Argument. As I've summarized it, "If we grant that a sufficiently advanced civilization could create a workable simulation of existence, we have every right to suspect we inhabit that simulation."

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