Gravitational Wave Signals from the First Massive Black Hole Seeds
  Recent numerical simulations reveal that the isothermal collapse of pristine
gas in atomic cooling haloes may result in stellar binaries of supermassive
stars with $M_* \gtrsim 10^4\ \mathrm{M}_{\odot}$. For the first time, we
compute the in-situ merger rate for such massive black hole remnants by
combining their abundance and multiplicity estimates. For black holes with
initial masses in the range $10^{4-6} \ \mathrm{M}_{\odot}$ merging at
redshifts $z \gtrsim 15$ our optimistic model predicts that LISA should be able
to detect 0.6 mergers per year. This rate of detection can be attributed,
without confusion, to the in-situ mergers of seeds from the collapse of very
massive stars. Equally, in the case where LISA observes no mergers from heavy
seeds at $z \gtrsim 15$ we can constrain the combined number density,
multiplicity, and coalesence times of these high-redshift systems. This letter
proposes gravitational wave signatures as a means to constrain theoretical
models and processes that govern the abundance of massive black hole seeds in
the early Universe.
