Gravity Wave 'Smoking Gun' Fizzles: Gravitational Radiation Can Be Produced More Than One Way

According to physics scholars, inflation theory proposes that the universe underwent a period of exponential expansion right after the big bang. A key prediction of inflation theory is the presence of a particular spectrum of "gravitational radiation"—ripples in the fabric of space-time that are notoriously difficult to detect but believed to exist nonetheless.

"If we see a primordial gravitational wave background, we can no longer say for sure it is due to inflation," said Lawrence Krauss, the Ambrose Swasey Professor of Physics and Astronomy at Case Western Reserve.

At the same time the researchers find that gravitational waves are a far more sensitive probe of new physics near the highest energy scale of interest to particle physicists than previously envisaged. Thus their work provides strong motivation for the ongoing quest to detect primordial gravitational radiation.

Krauss, along with Case Western Reserve colleagues Katherine Jones-Smith, a graduate student, and Harsh Mathur, associate professor of physics, present these findings in an article "Nearly Scale Invariant Spectrum of Gravitational Radiation from Global Phase Transitions" published recently in Physical Review Letters.

Inflation theory arose in the 1980s as a means to explain some features of the universe that had previously baffled astronomers such as why the universe is so close to being flat and why it is so uniform. Today, inflation remains the best way to theoretically understand many aspects of the early universe, but most of its predictions are sufficiently malleable that consistency with observation cannot be considered unambiguous confirmation.

Enter gravitational radiation—the key prediction of inflation theory is the presence of a particular spectrum of gravitational radiation. Detection of this spectrum was regarded among physicists as "smoking gun" evidence that inflation did in fact occur, billions of years ago.

In 1992 Krauss, then at Yale, argued that another mechanism besides inflation could give rise to precisely the same spectrum of gravitational radiation as is predicted by inflation. The argument given by Krauss in 1992 provided a rough estimate of the spectrum.

Last year Krauss teamed up with Case Western Reserve colleagues, Jones-Smith, a graduate student in physics, and Mathur, associate professor of physics, to do a more complete calculation. They found that the exact calculation predicts the signal to be much stronger than the rough estimate.

Describing their results, Krauss said, "It is shocking and surprising when you find the answer is 10,000 times bigger than the rough estimate and could possibly produce a signal that mimics the kind produced by inflation."

Gravitational radiation is a prediction of Einstein's Theory of General Relativity. According to the theory, whenever large amounts of mass or energy are shifting around, it disrupts the surrounding space-time and ripples emanate from the region where the mass/energy shift.

These space-time ripples, known as gravitational radiation, are imperceptible on the human scale, but highly sensitive experiments (such as the Laser Interferometer Gravitational Wave Observatory (LIGO) in Livingston, La.) are designed precisely to look for such radiation and are the only hope for detecting them directly.

However, gravitational radiation from the early universe can also be detected indirectly through its effect on the cosmic microwave background (CMB) radiation (relic radiation from the Big Bang which permeates all space). The radiation from the CMB would become polarized in the presence of gravitational radiation. Detecting such polarized light is the mission of a satellite based experiment (Planck) set to launch in 2009.

The gravitational radiation produced by either inflation or the mechanism proposed by Jones-Smith, Krauss and Mathur would imprint itself on the CMB and be detected as polarization. Until now it was widely believed that a detection of polarized light from the CMB was a "smoking gun" for inflation theory. But with the publication of their recent paper in Physical Review Letters, Krauss and co-workers have raised the issue of whether that polarized light can be unambiguously tied to inflation.

The mechanism proposed by Krauss and coworkers invokes a phenomenon called "symmetry breaking" that is a central part of all theories of fundamental particle physics, including the so-called standard model describing the three non-gravitational forces known to exist. Here, a "scalar field" (similar to an electric or magnetic field) becomes aligned as the universe expands. But as the universe expands each region over which the field is aligned comes into contact with other regions where the field has a different alignment. When that happens the field relaxes into a state where it is aligned over the entire region and in the process of relaxing it emits gravitational radiation.