The accelerated expansion of the Universe was discovered
in the late 1990s by two groups measuring accurate
distances to distant Type Ia supernovae [1, 2].
Even though the term Dark Energy is routinely used to
describe this acceleration, the cause of this phenomenon
is still unknown.
Contrary to regular matter, Dark Energy has negative
pressure. Regular matter has positive pressure
and when compressed the energy in a given volume
increases, which in turn increases the gravitational attraction.
If the mass is large enough this will give rise
to the runaway process that causes gravitational collapse
and gives birth to black holes. Positive pressure
then will cause the Universe to decelerate, Dark Energy
therefore has to have negative pressure. Equation
1 shows Einsteins equations with a cosmological constant
Λ, and the energy-momentum tensor Tμ
ν for a perfect