Cosmology Seminar: Bridging the μHz gap in the gravitational-wave landscape with lunar laser ranging

Thanks to LIGO/Virgo, we now have direct observational evidence for gravitational waves emitted by binary systems. But binaries can also absorb gravitational waves, leaving potentially detectable imprints on their orbital parameters. We expect every binary in the Universe to be immersed in a persistent background of gravitational waves, causing their orbits to execute a random walk through parameter space on long timescales. In this talk, I will describe how this effect can be used to search for gravitational waves at frequencies that are inaccessible to all other current and future experiments — in particular, how laser-ranging measurements of the Moon’s orbit around the Earth will allow us to explore the "μHz gap” between the frequencies probed by pulsar timing arrays and future space-based interferometers such as LISA. As examples of the discovery potential of this approach, I will show how it is able to place unique constraints on gravitational waves from cosmological phase transitions, and will discuss how it will shed light on the possible gravitational-wave background recently detected by various pulsar timing arrays.