Abstract

Gravitational waves as probes of the early moments of our Universe

Just like the cosmic microwave background radiation (CMBR) that bathes our Universe, an isotropic gravitational-wave background is also expected to permeate through it. The former is characterized by a Planckian distribution with a temperature of 2.73K, and was discovered serendipitously by Penzias and Wilson in 1964. The CMBR has been crucial in providing us with a snapshot of how the universe looked about 380,000 years after its birth. The CMBR, however, can not provide a picture of a younger universe owing to its opacity, arising from Thomson scattering of photons by free electrons in the hot primordial plasma. This is where gravitational waves, which are ripples in space-time curvature and which interact with matter much more weakly, can prove useful since they can probe the universe when it was younger than a minute.

In this talk, I will describe how using interferometry gravitational-wave detectors, such as LIGO and Virgo, are hunting for this gravitational-wave background (GWB). Since multiple models of the primordial universe have been proposed, the discovery of the GWB and the unraveling of its spectrum will help us understand the early moments of our Universe. Specifically, I will present recent results from LIGO and Virgo [Nature 460, pp. 990-994 (2009)], which have not detected the GWB yet, but have succeeded in ruling out some models that predict a strong enough GWB that should have been picked up if nature were consistent with those models. I will end with a discussion of planned and proposed experiments that will have a better shot at detecting the GWB.