Technology Development for Mid-Frequency Gravitational-Wave Detector

University Of Maryland, College Park, College Park MD

Investigators

Abstract

This award supports research in cryogenic gradiometry and it addresses the priority areas of NSF's "Windows on the Universe" Big Idea. Direct detection of gravitational waves is a breakthrough research area in astrophysics and fundamental physics, thanks to long-term technology development which led to the successful detection of gravitational waves by NSF's Advanced LIGO observatories beginning in 2015. However, the Advanced LIGO detectors are only sensitive at frequencies above about 20 Hz, while the LISA space detector, to be launched in the 2030s, will only cover frequencies below 0.1 Hz. SOGRO is a design for a new mid-frequency gravitational-wave (GW) detector to explore the frequency range in between LIGO and LISA. SOGRO employs magnetically levitated test masses and utilizes the exotic properties of superconductors and enhanced stability of materials at cryogenic temperatures. Unlike LIGO, SOGRO would be equally sensitive to GWs coming from anywhere in the sky, and would be capable of resolving the source direction and wave polarization. The goal of the present research project is to demonstrate key technologies of SOGRO by modifying an existing superconducting gravity gradiometer (SGG). The work will test the technical soundness of the detector concept and help identify areas that will require further studies to establish the feasibility of SOGRO while training the next generation of scientists in precision superconducting measurement techniques. In addition to GW science, an improved SGG can benefit geophysics, especially the newly emerging field of gravity-aided earthquake early warning. By detecting transient gravity signals arising from ground rupture in the early stages of earthquakes, which travel at the speed of light, SGGs could reduce the size of the 'blind zone' of early warning systems and increase their lead times, which should reduce losses due to earthquake shaking. By combining the motions of six symmetrically distributed test masses at equidistant points on a sphere, the SOGRO detector design has the full-tensor characteristics of a spherical GW antenna. A three-axis diagonal-component SGG with a 19-cm baseline and mechanically suspended test masses (TMs) reached a strain sensitivity level of 5 x 10^(-13) Hz^(-1/2) at 1 Hz by the early 1990s, which is three orders of magnitude more sensitive than demonstrated to date by other gradiometers, including atom interferometers and torsion bars. SOGRO is a greatly enlarged version of the tensor SGG with a 50-m baseline. To reach its target sensitivity for GW signals, SOGRO must meet several demanding technical requirements. Two of them are: (1) the levitated superconducting TMs must have differential-mode frequency as low as 0.01 Hz and quality factor as high as 10^8 at the temperature of 0.1 K, and (2) the platform must be suspended in such a way to isolate the detector from the ground angular motion to one part in a million. The objective of the present research project is to modify an existing SGG with magnetically-suspended TMs and a 13.5-cm baseline (that was constructed for a NASA-funded project) and demonstrate (1) a differential mode frequency as low as 0.3 Hz with Q as high as 3x10^5 at T = 4.2 K, and (2) isolation of the ground angular motion by a factor of 10^3 by suspending the platform as a pendulum. Successful demonstration of these features would advance two key technologies of SOGRO to within two to three orders of magnitude from the levels required for a full-scale SOGRO GW detector. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.