# cosmological constant

The cosmological constant is a parameter introduced in 1917 by Albert Einstein into the description of the universe in terms of the general
theory of relativity, in order to ensure static (i.e., not expanding
or contracting) cosmological models.
At that time, the expansion of the universe was still unknown. Einstein
later described the invention of the cosmological constant as his greatest
blunder. Einstein's cosmological constant was later interpreted as the energy
density of the vacuum. The observational upper bound on the value of
the vacuum energy density is 40 to 120 orders of magnitude smaller than that predicted from quantum
field theories of elementary particles – the so-called **cosmological
constant problem**. The cosmological constant may be the cause of
the acceleration of the universe recently inferred from observations of
Type Ia supernovae, but again there is
as yet no theoretical understanding of why it would have the small, non-zero
value needed to explain these observations.