  It was recently suggested that "cosmologically coupled" black holes with
masses that increase in proportion to the volume of the Universe might
constitute the physical basis of dark energy. We take this claim at face value
and discuss its potential astrophysical implications. We show that the
gravitational wave emission in binary systems would be significantly enhanced
so that the number of black hole mergers would exceed the observed rate by
orders of magnitude, with typical masses much larger than those seen by the
LIGO-Virgo-KAGRA network. Separately, if the mass growth happens at fixed
angular momentum, the supermassive black holes in matter-deficient elliptical
galaxies should be slowly rotating. Finally, cosmological coupling would
stabilize small black holes against Hawking radiation-induced evaporation.
