In the early twentieth century, Einstein changed our understanding of space and time from a static stage on which the universe plays out to a deformable space-time fabric. The theory that describes this, General Relativity, allows insight into a wide variety of mind-boggling phenomena from black holes to the expanding universe. General Relativity also predicts new type of radiation: gravitational waves. Gravitational waves are emitted any time there is a variation of a mass distribution. Even shaking your fist back and forth emits gravitational waves. However, the only systems that emit them in appreciable amounts are cataclysmic astrophysical events: black hole binaries or binary neutron star merging, supernova, the Big Bang, etc.
I am part of the LIGO Scientific Collaboration which detects these waves using a pair of 4-km long dual-recycled Fabry-Perot Michelson interferometer, one located in Hanford, Washington and the other in Livingston, Louisiana. After the first detection of gravitational waves in 2015, the field has blossomed into an novel method to observe the universe. We have observed both mergers of binary black holes and binary neutron stars (GW170817 shown below). These observations have furthered our understanding of a variety of phenomena including the nature of the neutron star composition, the source of heavy elements, and black hole populations. Additionally, they have allowed model independent measurements of Hubble’s constant and have set restrictive constraints on alternative theories of gravity.
In addition to my work with LIGO, I have been exploring the possibility of detecting gravitational waves with the geophysical satellite mission GRACE Follow-On. This mission consists of two satellites separated by 220 km whose distance is continuously monitored with 1 nm/√Hz accuracy. Their primary purpose is to monitor the gravitational field of the earth. However, they are also inherently capable of detecting gravitational waves.
The plot below shows the strain noise of the GRACE Follow-On satellites along with a collection of prospective sources. At it’s current sensitivity GRACE Follow-On is sensitive to binary mergers of intermediate mass black holes within the Milky Way. This sensitivity will be swamped by LISA once it launches. However, the GRACE Follow-On satellites are already in orbit and taking data so they may allow insights into the local distribution of intermediate mass black holes in the near future.
“Limits on the Stochastic Gravitational Wave Background and Prospects for Single Source Detection with GRACE Follow-On,” M.P. Ross, C.A. Hagedorn, E.A. Shaw, A.L. Lockwood, B.M. Iritani, J.G. Lee, K. Venkateswara, J. H. Gundlach. Physical Review D 101, 102004 (2020).
“GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral“, B.P.Abbott et. al. (LIGO Scientific Collaboration and Virgo Collaboration), Physical Review Letters 119 (16), 161101 (2017).