Galaxies do not affect black hole spectroscopy
When a black hole is disturbed, it “rings” like a bell before settling down - a phenomenon known as black hole ringdown. The study of the vibration modes of black holes is called spectroscopy, in analogy with atomic physics inaugurated by Niels Bohr.
Ringdown can be used to extract the mass and spin of the black hole and even to test General Relativity: perhaps Einstein was wrong and black holes are different from the predictions of his theory, in which case they will “ring” differently. This potential has turned black hole spectroscopy into a vibrant field.
Data from current gravitational wave detectors has been used by members of the Strong group at the Center of Gravity with great success, and future space-based detectors will offer even more precise measurements. All this raises an important question: can these detectors also reveal details about the black hole’s surroundings?
Are tests of General Relativity in jeopardy? Our work, Ringdown of Black Holes in Galaxies, published in Physical Review Letters, investigates the ringdown of black holes surrounded by matter, specifically black holes sitting in the center of galaxies. Remarkably, the only detectable effect is a redshift of the gravitational wave spectrum as it climbs the gravitational potential: black holes in galaxies sound more like a baritone, whereas in vacuum they would be a tenor or a contralto.
This work ensures that the ringdown phase will provide reliable measurements of a black hole properties without significant interference from its surroundings.