STELLAR STRUCTURE AND EVOLUTION
Cosmology and astrophysics
... Ordinary matter dilutes as it expands. According to the second law of thermodynamics, its pressure on
the walls should be positive, which exerts a force, and energy is lost in the expansion. On the other hand, vacuum energy is always the same, independent of the volume of the region, and thus, according to the second law, its pressure must be negative and of the same magnitude as the energy density. This negative pressure means that the volume tends to increase more and more rapidly, which explains the exponential expansion of the universe dominated by a cosmological constant.
Cosmic microwave background
Astronomy 162: Professor Barbara Ryden