  Scalar-tensor theories are a compelling alternative to general relativity and
one of the most accepted extensions of Einstein's theory. Black holes in these
theories have no hair, but could grow "wigs" supported by time-dependent
boundary conditions or spatial gradients. Time-dependent or spatially varying
fields lead in general to nontrivial black hole dynamics, with potentially
interesting experimental consequences. We carry out a numerical investigation
of the dynamics of single and binary black holes in the presence of scalar
fields. In particular we study gravitational and scalar radiation from
black-hole binaries in a constant scalar-field gradient, and we compare our
numerical findings to analytical models. In the single black hole case we find
that, after a short transient, the scalar field relaxes to static
configurations, in agreement with perturbative calculations. Furthermore we
predict analytically (and verify numerically) that accelerated black holes in a
scalar-field gradient emit scalar radiation. For a quasicircular black-hole
binary, our analytical and numerical calculations show that the dominant
component of the scalar radiation is emitted at twice the binary's orbital
frequency.
