Gravity wave 'smoking gun' fizzles, says Case Western Reserve University physics researchers

CLEVELAND—A team of researchers from Case Western Reserve University has found that gravitational radiation—widely expected to provide “smoking gun” proof for a theory of the early universe known as “inflation”— can be produced by another mechanism.

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 in Physical Review Letters this week.

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.