Searching for Long-Lasting Gravitational-Wave Signals with Second-Generation Gravitational-Wave Detectors

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Predicted by Einstein's Theory of General Relativity, gravitational waves have evaded direct detection for nearly a century due to their exquisitely small amplitudes. However, with the arrival of the second-generation gravitational-wave detectors, such as Advanced LIGO and Advanced Virgo, first detections of gravitational waves produced in astrophysical and cosmological processes are expected to be made over the next decade. This project focuses on gravitational waves emitted on long time scales, such as the stationary stochastic gravitational-wave background (SGWB) generated by an incoherent superposition of signals from many independent sources. The project also targets transient gravitational-wave signals on the time-scales of minutes, hours or longer, generated by a variety of astrophysical processes such as dynamically formed black-hole binaries or fragmentation in accretion disks around black holes. Cross-correlation techniques will be applied to the first data to be acquired by the second-generation detectors in search for these long-lasting signals. With over 1000 times better sensitivity than the latest published results, these searches will probe and constrain a number of proposed SGWB models. Furthermore, the SGWB models will be systematically studied to precisely identify the open science questions within these models that could be pursued by the second-generation detectors. Examples of such open questions include: What is the equation of state in neutron stars? What is the coalescence rate of binary neutron stars? Which cosmological models of SGWB are accessible to the second-generation detectors?This project is expected to have a strong impact on cosmology and astrophysics. For example, the SGWB is expected to be generated just moments after the Big Bang, and hence may contain unique information about the first moments in the evolution of the universe and about the physics of highest energy scales, which is otherwise not accessible in laboratories. Similarly, the SGWB could be of astrophysical origin: for example, summing up contributions from all binary neutron stars, binary black holes, magnetars, or Supernovae in the universe leads to a stochastic gravitational-wave "foreground" that contains information about these most violent objects and events in the universe. The long-lasting transient gravitational-wave signals will also provide information about the astrophysical objects and processes that generated them. For example, direct detection of gravitational-wave signals produced by dynamically formed black hole binaries would shed light on the population of black holes that lurk in the inner regions of nearby galaxies, revealing their density and velocity distribution.