It's a *positive* pressure, graham, despite what the equations say. A negative pressure would make the universe contract, not expand. Gravity is a pressure *gradient*, and if the pressure is uniform there is no gravity, but the universe still expands like a ball of compressed gas. Let's face it, the early universe was very dense, but it didn't collapse. Look up stress-energy along with stress and pressure, and think *stress ball*. Squeeze it down in your fist, and let it go. It expands.

This may be good pseudoscience, but it has nothing to do with the relevant science.

To properly assign an energy density in General Relativity, one uses a 4X4 tensor, T

^{μν}, the 00 component of which is the energy density proper. Under certain general conditions allowable in cosmology, we can remove the off-diagonal elements of the tensor, but we cannot get rid of the diagonal elements. These are all equivalent, T

^{11} = T

^{22} = T

^{33} = p and they are a function of density and the behaviour of the particles or field in phase space. This function agrees with the usual definition of pressure in its use with ideal gases, but its role is as a source of gravity, not a pressure as classically used. This is especially important because the expansion of the universe is a gravitational effect, not one of force imparted through some other mechanism.

If we assign an energy density ρ

_{Λ} to the vacuum, then we must assign it in such a way that the energy density is associated with Lorentz invariant tensor. This is so because the properties of the vacuum itself don't transform along with a change in coordinates. The only Lorentz invariant tensor is η

^{μν}, that tensor with 1 in the 00 position, -1 on the other diagonal positions, and 0 on all off-diagonal positions. So we assign the vacuum energy tensor as T

_{Λ}^{μν} = ρ

_{Λ} η

^{μν}.

Combine these two requirements, and we see that the "pressure", p, associated with the energy density of the vacuum must be -ρ

_{Λ}. Thus if the energy density of the vacuum is positive, its pressure term, such as it is, is negative.

(The above adapted from pp. 130-132 of James Rich,

*Fundamentals of Cosmology*, Springer 2001.)