Martes 7 de Mayo de 2002, Ip nš 14

Out with the big bang, and in with cosmic crunch
WASHINGTON. What if the big-bang theory is wrong? What if the universe never began and will never end, driven forever to expand in a series of monster explosions and contract every eon or so in a cosmic crunch?

Princeton University physicist Paul Steinhardt suggested just that in a report published on Thursday that even he called "mind-bending."

The big-bang theory, accepted by many scientists for decades, holds that the universe was born some 14 billion years ago when an unimaginably small, dense entity blew up, sowing the seeds of every bit of matter and energy.

Soon after that first explosion, the universe expanded rapidly, in a phenomenon astronomers call inflation, and then continued to spread out at varying speeds until the present day, according to the big-bang theory. Under this theory, time would begin but never end.

But the model of the universe envisioned by Steinhardt and Neil Turok of Cambridge in the journal Science sees the big bang as merely a turning point on an infinite road: an endless series of big bangs make the universe expand and an equally endless series of subsequent crunches make it contract.

The current estimated age of the universe according to the big-bang theory would seem like the blink of an eye under the cyclic universe theory, which assumes the universe waxes and wanes in cycles lasting as long as trillions of years.

"Time does not have to have a beginning," Steinhardt said in a telephone interview. He said that what scientists theorize as the dawn of time might, in fact, be "only a transition or a stage of evolution from a pre-existing phase to the present expanding phase."

Exotic dark energy

Scientists who favor the big-bang model see the expansion of the universe as governed by the amount and kinds of energy that comprise it. If the energy is the kind earthlings know -- gravitationally self-attractive energy that clumps into galaxies, stars and planets and also makes a set of keys fall off a table -- it tends to slow down the expansion.

But if it is a mysterious kind of gravitationally self-repulsive energy, known as dark energy, that would tend to speed expansion up.

Astronomers and others who ponder this question have been at pains in recent years to explain why the universe's expansion has been accelerating over the last several billion years after a long slowdown.

Dark energy's strange ways could be responsible.

"We can see, both directly and indirectly, that most of the stuff in the universe is not composed of ordinary matter, nor of dark matter, but of some third species," he said. "And we can see that the ratio is roughly 70-30 -- 70 percent exotic stuff, 30 percent ordinary stuff."

What Steinhardt calls ordinary stuff is what allows the slower expansion of the universe, which permits gravity to create galaxies, stars and planets, including Earth. The accelerated expansion driven by dark energy would blow all that away before it could coalesce.

"This stuff, once it takes over the universe, it pushes everything away at an accelerating pace," he said. "So the universe will double in size every 14 to 15 billion years so long as there is this gravitationally self-repulsive energy that dominates the universe."

The big crunch comes when dark energy changes its character, according to Steinhardt. He likened it to a ball rolling down a hill that picks up speed as it goes along.

"This field of dark energy is picking up more and more energy as it rolls down the hill, the nature of the force that controls it causes it to rebound and go back to where it started, back and forth in a very irregular fashion," he said.

"When it's changing slowly, it's gravitationally self-repulsive and when it's changing fast, it picks up speed, it's gravitationally self-attractive," Steinhardt said.

Steinhardt admitted this made dark energy sound capricious.

"It is capricious but it's no more capricious than the standard picture," he said. "It's just different."


  26/04/2002. CNN.