Negative Pressure
Independently from its actual nature, dark energy would need to have a strong negative pressure in order to explain the observed acceleration in the expansion rate of the universe.
According
to General Relativity, the pressure within a substance contributes to
its gravitational attraction for other things just as its mass density
does. This happens because the physical quantity that causes matter to
generate gravitational effects is the Stress-energy tensor, which contains both the energy (or matter) density of a substance and its pressure and viscosity.
In the Friedmann-Lemaître-Robertson-Walker
metric, it can be shown that a strong constant negative pressure in all
the universe causes an acceleration in universe expansion if the
universe is already expanding, or a deceleration in universe contraction
if the universe is already contracting. More exactly, the second
derivative of the universe scale factor, 
, is positive if the equation of state of the universe is such that w < − 1 / 3.
, is positive if the equation of state of the universe is such that w < − 1 / 3.
This accelerating expansion
effect is sometimes labeled "gravitational repulsion", which is a
colorful but possibly confusing expression. In fact a negative pressure
does not influence the gravitational interaction between masses - which
remains attractive - but rather alters the overall evolution of the
universe at the cosmological scale, typically resulting in the
accelerating expansion of the universe despite the attraction among the
masses present in the universe.
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