Searching for the Stochastic Gravitational-Wave Background with Advanced LIGO

Among the key predictions of Einstein's General Theory of Relativity is the existence of gravitational waves (GW): ripples moving at the speed of light in the geometry of space-time caused by the fast motion of large masses. Although well tested in terms of their indirect effects, the direct detection of gravitational waves incident on Earth poses an outstanding challenge. The scientific rewards from achieving this ability would be enormous - ranging from probing the extreme dynamics of exploding stars to gleaning information about the state of the Universe almost at the moment of the Big Bang itself. The effort to enable this new window on the Universe has occupied several decades of experimental and technological developments that have pushed the boundaries across diverse fields in the physical sciences. The year 2015 will mark a highly-anticipated watershed moment for the goal of direct detection of gravitational waves, as the second generation of gravitational-wave detectors is commissioned at the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO). The sensitivity of the new aLIGO detectors will be ramped up to become about ten times better than that of the first-generation detectors, opening up a spatial volume for observing GW sources that will be 1000 times larger than before. The aLIGO data will be used to search for a variety of astrophysical and cosmological gravitational wave sources. This specific award will support research that searches the aLIGO data to look for the background signals that arise from gravitational-wave signals from many independent sources.

The first observation data from aLIGO is expected in 2015, with the sensitivity and duty-cycle of the detectors steadily increasing over the following three years. This project focuses on the searches for the stochastic gravitational wave background (SGWB), which arises from an incoherent superposition of gravitational-wave signals generated by many independent sources. The SGWB could be of cosmological origin: for example, in inflationary models the SGWB is generated just moments after the Big Bang, implying that the SGWB may contain unique information about the first moments in the evolution of the universe and about the physics of energy scales not accessible in laboratories. Similarly, the SGWB could be of astrophysical origin: for example, summing up contributions from all magnetars or from coalescences of all binary neutron stars and/or black holes in the universe leads to a stochastic gravitational-wave 'foreground' that contains information about these most violent objects and events in the universe.

The SGWB searches with aLIGO data will be conducted using cross-correlation techniques that were developed and successfully used in the SGWB searches with initial LIGO data. The searches for isotropic SGWB are expected to result in a measurement about 1000x more sensitive than the current best upper limits. Similarly, the searches for anisotropic SGWB will result in about 100x more sensitive measurement than the current best upper limits. Such substantial advances will result in a series of interesting cosmological and astrophysical implications, including detection or new constraints on parity violation in the early universe, evolution of the universe during the first minute after the Big Bang, equation of state in neutron stars, rate of compact binary coalescences and its evolution with redshift.