Universo tem mais entropia do que se pensava antes

UNIVERSE HAS MORE ENTROPY THAN THOUGHT
New calculations suggest that the cosmos may be a bit closer to heat death

By Ron Cowen Web edition : Friday, October 2nd, 2009

For all its tumult — erupting stars, colliding galaxies, collapsing black holes — the cosmos is a surprisingly orderly place. Theoretical calculations have long shown that the entropy of the universe — a measure of its disorder — is but a tiny fraction of the maximum allowable amount.

A new calculation of entropy upholds that general result but suggests that the universe is messier than scientists had thought — and slightly further along on its gradual journey to death, two Australian cosmologists conclude.

An analysis by Chas Egan of the Australian National University in Canberra and Charles Lineweaver of the University of New South Wales in Sydney indicates that the collective entropy of all the supermassive black holes at the centers of galaxies is about 100 times higher than previously calculated. Because supermassive black holes are the largest contributor to cosmic entropy, the finding suggests that the entropy of the universe is also about 100 times larger than previous estimates, the researchers reported online September 23 at arXiv.org.

Entropy quantifies the number of different microscopic states that a physical system can have while looking the same on a large scale. For instance, an omelet has higher entropy than an egg because there are more ways for the molecules of an omelet to rearrange themselves and still remain an omelet than for an egg, notes cosmologist Sean Carroll of the California Institute of Technology in Pasadena.

A black hole is the entropy champ because there are myriad ways for all the material that has fallen into it to be arranged microscopically while the black hole retains the same numerical values for its observable properties — charge, mass and spin.

Researchers who previously calculated the cosmic sum of black hole entropy had assumed that, on average, each galaxy houses a 10 million solar-mass black hole at its center. Under this assumption, researchers had determined that supermassive black holes contribute an entropy of about 10102, in units derived from a quantity known as Boltzmann’s constant.

In contrast, Egan and Lineweaver relied on new data that included a fuller range of the masses of supermassive black holes rather than just using the average mass. “The upshot was that much more entropy is contributed by a smaller population of much larger, 1-billion-solar-mass black holes,” Egan says.

Carroll says that the team’s calculation looks sensible. “I see no reason to doubt their numbers,” he says.

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In the case of the universe, Egan says, “we'd like to know [when and] if the entropy will eventually reach a maximum value, marking the end of all dissipative processes, including life.” Physicists have dubbed that maximum entropy “heat death.”

Egan and Lineweaver’s new value for the entropy of the universe is still a billionth of a billionth the maximum possible entropy that researchers have estimated. Nonetheless, the new value “indicates that that the universe is a bit closer to the heat death than previously computed,” comments theorist Paul Davies of Arizona State University in Tempe.

Not everyone agrees that the higher entropy contributed by supermassive black holes puts the universe closer to heat death. Theorist Ned Wright of the University of California, Los Angeles says that because the extra entropy is locked inside the black holes, the rest of the universe should have lower entropy and be further away from heat death.

The new entropy calculation also highlights a cosmic puzzle, Carroll says. The entropy was relatively small in the early universe (10 88), bigger now (10 104), but still falls far short of the maximum (10 122). No known physical principle can explain why the cosmic entropy is so low. But it’s a good thing because the low value “is responsible for everything we experience about the [unidirectional] flow of time — breaking eggs, growing older and dying, remembering the past but not the future,” notes Carroll. “The universe is incredibly more orderly than it has any right to be. Egan and Lineweaver have shown that it's just a bit more disorderly than we thought.”

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Read here/Leia aqui.

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Interessante este artigo Eternal Inflation and Its Consequences, de Alan Guth.

Alan H. Guth
Center for Theoretical Physics, Laboratory for Nuclear Science, and Department of
Physics, Massachusetts Institute of Technology, Cambridge, MA 02139
E-mail: guth@ctp.mit.edu

Abstract.

I summarize the arguments that strongly suggest that our universe is the product of inflation. The mechanisms that lead to eternal inflation in both new and chaotic models are described. Although the infinity of pocket universes produced by eternal inflation are unobservable, it is argued that eternal inflation has real consequences in terms of the way that predictions are extracted from theoretical models. The ambiguities in defining probabilities in eternally inflating spacetimes are reviewed, with emphasis on the youngness paradox that results from a synchronous gauge regularization technique. Although inflation is generically eternal into the future, it is not eternal into the past: it can be proven under reasonable assumptions that the inflating region must be incomplete in past directions, so some physics other than inflation is needed to describe the past boundary of the inflating region.

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PDF gratuito do artigo aqui.