dimanche 16 mars 2008

New Beginnings

Ideas for a time before the big bang—which might be testable :

The big bang is often thought of as the beginning of everything, including time, making any questions about what happened beforehand nonsensical. Now exotic theories that suggest the existence of an era before the big bang are growing in number. They indicate that imprints of this era might exist and that an upcoming generation of telescopes could detect them.
According to conventional big bang thinking, the universe emerged from a point of in finite energy and density, a singularity where the laws of physics break down. The universe then underwent “inflation,” briefly expanding much faster than the speed of light. By smearing the cosmos out fairly evenly and smoothing out the early universe’s curves, inflation solved a number of puzzles, including why spacetime is “flat,” whereby light commonly travels in straight, not warped, lines. Ripples occurring during inflation could also explain the overall pattern, or structure, of galaxies seen now.
Observations of the cosmic microwave background radiation—the leftover heat from the big bang—have confirmed several broad predictions of the inflationary model. Still, inflation should have caused powerful gravitational waves that in turn should have distorted cosmic microwaves in detectable ways. The telescopes have not seen such distortions yet, ruling out several inflationary models. Moreover, critics say that the theories underlying inflation should mean that inflation is an eternal process; it should generate an infinite number of pockets of space with different properties, requiring more complex theories for why we live in a pocket that has the flatness and structure we see.
In the past 15 years, challenging theories arose that conjectured an era before the big bang, during which our universe contracted and then rebounded. Researchers say that the ekpyrotic scenario proposed in 2001 could successfully generate the current universe’s structure, flatness and other features. (The name comes from the ancient Stoic notion of ekpyrosis, a fire in which the universe continuously gets reborn.) The cyclic model, derived from the ekpyrotic model in 2002, also accounts for the dark energy posited to be now causing universal expansion to accelerate.
Still, these bouncing models did not convince many theorists. These scenarios posit that ripples before the big bang successfully passed the daunting barrier of a singularity to initiate structure in the cur-rent universe, an idea “most cosmologists are extremely skeptical of,” admits Princeton University cosmologist Paul Steinhardt , who with University of Cambridge theoretical physicist Neil Turok helped to develop the ekpyrotic and cyclic models. In addition, the models were originally couched in terms of string theory, which many scientists disdain, because it calls for undetected extra dimensions of reality beyond those of space and time.
A flurry of new bouncing models has just burst out in the past few months. Strikingly, they come in a variety of different flavors, many of which avoid a singularity and all of which require no dimensions beyond those of space and time. “There’s a lot of skepticism against bouncing, due perhaps to string theory,” Steinhardt says. “These new results use more familiar physics and should convince most cosmologists—even those who don’t want to consider extra dimensions—that there are real alternatives to inflation.”
For instance, to prevent a singularity at the big bang, two models suggest that, essentially, a strong push kept the past universe from collapsing to a point. This force comes from a “ghost condensate,” a fluid of exotic particles that can theoretically exert more pressure than even dark energy. These scenarios originated independently from theoretical physicist Burt Ovrut of the University of Pennsylvania and his colleagues and cosmologist Paolo Creminelli of the Abdus Salam International Center for Theoretical Physics in Italy, in partnership with Harvard University cosmologist Leonardo Senatore.
Another way to evade a singularity could be the intrinsic nature of spacetime. Relying on loop quantum gravity, an alternative to string theory, Pennsylvania State University theoretical physicist Martin Bojowald calculates that at extremely tiny scales, spacetime can become repulsive, preventing it from collapsing. A consequence of his scenario is what he calls “cosmic forgetfulness,” in which the universe after the big bang forgets some of its past properties and ac-quires new ones independent of what it had before.
The new bouncing models should have resulted in post–big bang gravitational waves far weaker than inflation would generate, by 50 orders of magnitude. If more sensitive future telescopes, such as the Planck Surveyor, still fail to spot the distortions in the microwave background that inflation and its gravitational waves were supposed to have created, then such null results could sup-port the idea of an era before the big bang.“At the moment I think it fair to say that inflation is more compelling,” Creminelli says. “At the end, however, experimental data will decide between the alternatives.”

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