Black hole inspirals in an expanding universe
Radiation reaction is the small cumulative effect of radiation on the motion of its source. Celestial binaries, as steady emitters of gravitational radiation, eventually merge as a consequence of this. In this paper, we show how the expansion of the universe affects the decay and eccentricity evolution of the orbit of an extreme-mass ratio binary. Much previous work has quantified the effect of the cosmological constant on the gravitational wave fluxes of a binary, but here we study its impact on the orbit of the binary itself. We show that the cosmological constant has both long and short timescale effects on the orbital motion, which includes the accumulation of the pericenter advance in elliptical orbits and a decrease in plunge and circularization times.