The Advanced LIGO and Advanced Virgo detectors have commenced observations. Gravitational waves from the merger of binary black hole systems and a binary neutron star system have been observed. A major goal for LIGO and Virgo is to detect or set limits on a stochastic background of gravitational waves. A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved cosmological and/or astrophysical sources. A cosmologically produced background would carry unique signatures from the earliest epochs in the evolution of the Universe. Similarly, an astrophysical background would provide information about the astrophysical sources that generated it. The observation of gravitational waves from compact binaries implies that there will be a stochastic background from these sources that could be observed by Advanced LIGO and Advanced Virgo in the coming years. The LIGO and Virgo search for a stochastic background should probe interesting regions of the parameter space for numerous astrophysical and cosmological models. Presented here is an outline of LIGO and Virgo's search strategies for a stochastic background of gravitational waves, including the search for gravitational wave polarizations outside of what is predicted from general relativity. Also discussed is how global electromagnetic noise (from the Schumann resonances) could affect this search; possible strategies to monitor and subtract this potential source of correlated noise in the global detector network are explained. The results from Advanced LIGO's observing runs O1 and O2 will be presented, along with the implications of the gravitational wave detections.

Alex Amato, Silvana Terreni, Vincent Dolique, Danièle Forest, Gianluca Gemme, Massimo Granata, Lorenzo Mereni, Christophe Michel, Laurent Pinard, Benoit Sassolas, Julien Teillon, Gianpietro Cagnoli, Maurizio Canepa

Michela Mapelli, Nicola Giacobbo, Filippo Santoliquido