The first direct GW detection by the two giant optical interferometers of Advanced LIGO in September 2015 opens a new area for physics: in the future, GW detectors will reveal new information about massive astrophysical objects such as neutron stars, black holes, pulsars and their dynamics. The transient signal of this first observation, a chirp in frequency from 35 to 250 Hz lasting about 150 ms with a peak strain amplitude of 10−21 , corresponds to the merging phase of a black hole binary system. Due to their bandwidth limited to the frequency range 10 Hz-10 kHz, only the last evolution phase of binary systems is observable with current GW detectors. Before their coalescence,the same sources emit at lower frequencies quasi-continuous GW signals in their “inspiral” phase. A new class of low frequency detectors would enable to observe the signal of such sources years before they enter in the bandwidth of ground-based optical detectors. For example, this first observed source, GW150914, was emitting at a frequency of 16 mHz 5 years before coalescence with a characteristic strain amplitude of the order of 10−20. Low frequency GW detectors would therefore open the possibility of multi-band GW astronomy with long term observation of all evolution phases of binary systems.
Such observatories would also enable to precisely predict the event of coalescence in time and space, which would ease coincident observations in the electromagnetic domain. Multi-band GW astronomy then offers a great scientific payoff with the perspective of multi-messenger astronomy, but also promises new gravity and cosmology tests. Low frequency detectors would therefore open a completely new area for GW astronomy.
MIGA will extend the concept of correlated interferometry from the laboratory scale to that of a geological site, the LSBB at Rustrel, using an underground array of atom sensors distributed along a 200 m horizontal arm. Several techniques based on correlated atom interferometry will be implemented to characterize the gravitational field of the site, such as the simultaneous measurement of gravity acceleration and gradient and the measurement of gravitational curvature. It will be thus possible to investigate several geological phenomena, like the non-invasive detection of underground density anomalies, the gravity perturbations due to local density changes caused by fault evolution as proposed in, and the characterization of gravity-gradient noise, also called Newtonian Noise (NN).