  Some astrophysical sources of gravitational waves can produce a "memory
effect," which causes a permanent displacement of the test masses in a freely
falling gravitational-wave detector. The Christodoulou memory is a particularly
interesting nonlinear form of memory that arises from the gravitational-wave
stress-energy tensor's contribution to the distant gravitational-wave field.
This nonlinear memory contributes a nonoscillatory component to the
gravitational-wave signal at leading (Newtonian-quadrupole) order in the
waveform amplitude. Previous computations of the memory and its detectability
considered only the inspiral phase of binary black hole coalescence. Using an
"effective-one-body" (EOB) approach calibrated to numerical relativity
simulations, as well as a simple fully analytic model, the Christodoulou memory
is computed for the inspiral, merger, and ringdown. The memory will be very
difficult to detect with ground-based interferometers, but is likely to be
observable in supermassive black hole mergers with LISA out to a redshift of
two. Detection of the nonlinear memory could serve as an experimental test of
the ability of gravity to "gravitate."
