  Circumbinary discs are generally thought to take up angular momentum and
energy from the binary orbit over time through gravitational torques mediated
by orbital resonances. This process leads to the shrinkage of the binary orbit
over time, and is important in a variety of astrophysical contexts including
the orbital evolution of stellar binaries, the migration of planets in
protoplanetary discs, and the evolution of black hole binaries (stellar and
supermassive). The merger of compact object binaries provides a source of
gravitational waves in the Universe. Recently, several groups have reported
numerical simulations of circumbinary discs that yield the opposite result,
finding that the binary expands with time. Here we argue that this result is
primarily due to the choice of simulation parameters, made for numerical
reasons, which differ from realistic disc parameters in many cases. We provide
physical arguments, and then demonstrate with 3D hydrodynamical simulations,
that thick (high pressure, high viscosity) discs drive sufficient accretion of
high angular momentum material to force binary expansion, while in the more
realistic case of thin (low pressure, low viscosity) discs there is less
accretion and the binary shrinks. In the latter case, tides, which generally
transfer angular momentum and energy from the more rapidly rotating object (the
binary) to the less rapidly rotating object (the disc), are the dominant driver
of disc-binary evolution. This causes the binary to shrink. We therefore
conclude that for common circumbinary disc parameters, binaries with
non-extreme mass ratios are expected to shrink over time. Expansion of the
binary can occur if the disc viscosity is unusually high, which may occur in
the very thick discs encountered in e.g. circumplanetary discs, super-Eddington
AGN, or the outer regions of passive protostellar discs that are heated by the
... (abridged).
