  Galaxy mergers lead to the formation of massive black hole binaries which can
accelerate background stars close to the speed of light. We estimate the
comoving density of ejected stars with a peculiar velocity in excess of $0.1c$
or $0.5c$ to be $\sim 10^{10}$ and $10^5$ Gpc$^{-3}$ respectively, in the
present-day Universe. Semi-relativistic giant stars will be detectable with
forthcoming telescopes out to a distance of a few Mpc, where their proper
motion, radial velocity, and age, can be spectroscopically measured. In
difference from traditional cosmological messengers, such as photons,
neutrinos, or cosmic-rays, these stars shine and so their trajectories need not
be directed at the observer for them to be detected. Tracing the stars to their
parent galaxies as a function of speed and age will provide a novel test of the
equivalence principle and the standard cosmological parameters.
Semi-relativistic stars could also flag black hole binaries as gravitational
wave sources for the future eLISA observatory.
