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 detectors 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.