Observing gravitational waves from the first generation of black holes
  The properties of the first generation of black-hole seeds trace and
distinguish different models of formation of cosmic structure in the
high-redshift universe. The observational challenge lies in identifying black
holes in the mass range ~100-1000 solar masses at redshift z~10. The typical
frequencies of gravitational waves produced by the coalescence of the first
generation of light seed black-hole binaries fall in the gap between the
spectral ranges of low-frequency space-borne detectors (e.g., LISA) and
high-frequency ground-based detectors (e.g., LIGO, Virgo and GEO 600). As such,
these sources are targets for proposed third-generation ground-based
instruments, such as the Einstein Telescope which is currently in design study.
Using galaxy merger trees and four different models of black hole accretion -
which are meant to illustrate the potential of this new type of source rather
than to yield precise event-rate predictions - we find that such detectors
could observe a few to a few tens of seed black-hole merger events in three
years and provide, possibly unique, information on the evolution of structure
in the corresponding era. We show further that a network of detectors may be
able to measure the luminosity distance to sources to a precision of ~40%,
allowing us to be confident of the high-redshift nature of the sources.
