Big Bang or Big Bounce?

reposted from:SciAm (pdf) - click for complete Scientific American article

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 infinite 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 [see “The Myth of the Beginning
of Time,” by Gabriele Veneziano; Scientific American, May 2004].

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 current
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 Inflation Deflation
Several new alternatives to cosmic inflation posit a cycle of birth and death for the universe. But not all alternatives
demand such reincarnations. Robert Brandenberger of McGill University
and his colleagues conjecture that the universe began with a hot, dense gas of strings, energy strands whose vibrations generate fundamental
particles and forces. Thermal fluctuations
of this gas then led to galaxy clusters and other cosmic structures. This model is “agnostic” as to whether anything existed before the big bang, Brandenberger says. It should, if correct,
lead to gravitational signatures that future telescopes could detect.

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 acquires
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 support
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.”

Charles Q. Choi is a frequent
contributor.