  Observations of gravitational waves from neutron star mergers open up novel
directions for exploring fundamental physics: they offer the first access to
the structure of objects with a non-negligible contribution from vacuum energy
to their total mass. The presence of such vacuum energy in the inner cores of
neutron stars occurs in new QCD phases at large densities, with the vacuum
energy appearing in the equation of state for a new phase. This in turn leads
to a change in the internal structure of neutron stars and influences their
tidal deformabilities which are measurable in the chirp signals of merging
neutron stars. By considering three commonly used neutron star models we show
that for large chirp masses the effect of vacuum energy on the tidal
deformabilities can be sizable. Measurements of this sort have the potential to
provide a first test of the gravitational properties of vacuum energy
independent from the acceleration of the Universe, and to determine the size of
QCD contributions to the vacuum energy.
