  Vacuum energy changes during cosmological phase transitions and becomes
relatively important at epochs just before phase transitions. For a viable
cosmology the vacuum energy just after a phase transition must be set by the
critical temperature of the next phase transition, which exposes the
cosmological constant problem from a different angle. Here we propose to
experimentally test the properties of vacuum energy under circumstances
different from our current vacuum. One promising avenue is to consider the
effect of high density phases of QCD in neutron stars. Such phases have
different vacuum expectation values and a different vacuum energy from the
normal phase, which can contribute an order one fraction to the mass of neutron
stars. Precise observations of the mass of neutron stars can potentially yield
information about the gravitational properties of vacuum energy, which can
significantly affect their mass-radius relation. A more direct test of cosmic
evolution of vacuum energy could be inferred from a precise observation of the
primordial gravitational wave spectrum at frequencies corresponding to phase
transitions. While traditional cosmology predicts steps in the spectrum
determined by the number of degrees of freedom both for the QCD and electroweak
phase transitions, an adjustment mechanism for vacuum energy could
significantly change this. In addition, there might be other phase transitions
where the effect of vacuum energy could show up as a peak in the spectrum.
