Being able to make precise measurements of distances and redshifts will help us understand how the universe is evolving. With the advent of gravitational wave observatories, we can make these measurements by using black holes in a very different way than before.
To measure how the universe is expanding, we need to simultaneously obtain the distances and redshifts to sources. When it comes to measuring large distances in space, astronomers have typically leaned on “standard candles” — objects whose intrinsic brightness is known. The dimmer a standard candle appears, the farther away it is.
Merging binary black holes (BBHs) can serve as standard candles, in a way. When compact objects like black holes merge, they produce gravitational waves, which can be picked up by observatories like the Laser Interferometer Gravitational-Wave Observatory (LIGO). The emitted gravitational waves have a characteristic energy, meaning that these mergers could be used to measure distances as “standard sirens”.
The trouble comes when trying to simultaneously measure the redshift of these sources. The gravitational wave detection on Earth gives us a mass measurement for the black holes that’s a combination of their redshift and their true masses in the source frame. If we know ...