New Low-Frequency Gravitational Wave Detector­

Terrestrial gravitational wave (GW) detectors are mostly based on Michelson-type laser interferometers with arm-length of a few km covering 10 Hz to a few kHz with strain sensitivity up to 10-23 Hz-1/2.  These detectors are optimized for detection of compact binary coalescence events that produce strong signals typically at ~1 kHz.   Several astrophysical processes generate GWs below 10 Hz that will not be observed by the terrestrial laser interferometers.  Two serious obstacles in constructing terrestrial GW SOGRO 30 m square strutsdetectors below 10 Hz are seismic and Newtonian noises (NN).

Due to the transverse nature of GWs, a detector that measures all the components of the curvature tensor could distinguish GWs from near-field Newtonian gravity.  By combining six magnetically levitated superconducting test masses, one could construct a full-tensor detector.  Such a detector would have uniform sensitivity to GWs for all incident angles and be capable of determining the source direction and wave polarization.  We name this detector SOGRO (Superconducting Omni-directional Gravitational Radiation Observatory).  With a baseline of 30 m cooled to 1.5 K, SOGRO could reach a sensitivity £ 10-20 Hz-1/2 at 0.1-10 Hz.  A procedure of removing the NN has been investigated. 

Advanced SOGRO (aSOGRO) with a baseline of 100 m cooled to 0.1 K could reach a sensitivity £ 10-21 Hz-1/2 at 1-10 Hz.  Such a detector would be sensitive enough to detect not only intermediate-mass black hole (IMBH) binaries but also the low-frequency precursor of stellar mass BH binaries like GW150914 and be able to alert advanced interferometers days before merger.  A major technical challenge is the construction of a large (30 100 m and 100 ~ 500 tons including the test masses), rigid enough (³ 10 Hz), platform with high Q (³ 106) that can be cooled to the liquid helium temperature (£ 4 K).  Detailed engineering studies need to be performed to find a platform design which reduces the weight while providing sufficient rigidity.

An interesting application of SOGRO is mitigation of the NN for advanced interferometer GW detectors.  Since SOGRO is a very sensitive gravity strain gauge, one could employ scaled-down SOGROs, in place of a large array of seismometers, to directly measure and subtract the NN affecting the interferometer test masses.  A mini-SOGRO with 4-m arm-length cooled to 4.2 K and located underneath each test mass could help mitigate the NN of aLIGO by a factor of 5 to 2 10-23 Hz-1/2 at 10 Hz.