  We show that the vacuum (zero-point) energy of a low-temperature quantum
liquid is a variable property which changes with the state of the system, in
notable contrast to the static vacuum energy in solids commonly considered. We
further show that this energy is inherently anomalous: it decreases with
temperature and gives negative contribution to system's heat capacity. This
effect operates in an equilibrium and macroscopic system, in marked contrast to
small or out-of-equilibrium configurations discussed previously. We find that
the negative contribution is over-compensated by the positive term from the
excitation of longitudinal fluctuations and demonstrate how the overall
positive heat capacity is related to the stability of a condensed phase at the
microscopic level.
