shows how the contents of the universe influence its expansion rate. Here, G is the gravitational constant, ρ ρ is the energy density within the universe, p is the pressure, c is the speed of light, and Λ Λ is the cosmological constant. A positive energy density leads to deceleration of the expansion, a ¨ ¨ < 0 , and a positive pressure further decelerates expansion. On the other hand, sufficiently negative pressure with p < − − ρ ρ c 2 / 3 leads to accelerated expansion, and the cosmological constant also accelerates expansion. Nonrelativistic matter is essentially pressureless, with | p | ≪ ≪ ρ ρ c 2 , while a gas of ultrarelativistic particles (such as a photon gas) has positive pressure p = ρ ρ c 2 / 3 . Negative-pressure fluids, like dark energy, are not experimentally confirmed, but the existence of dark energy is inferred from astronomical observations.