In
cosmology, the
cosmological constant (usually denoted by the Greek capital letter
lambda: Λ) is the value of the energy density of the vacuum of space. It was originally introduced by
Albert Einstein in 1917 as an addition to his theory of
general relativity to "hold back gravity" and achieve a
static universe, which was the accepted view at the time. Einstein abandoned the concept after
Hubble's 1929 discovery that all galaxies outside the
Local Group (the group that contains the
Milky Way Galaxy) are moving away from each other, implying an overall expanding universe. From 1929 until the early 1990s, most cosmology researchers assumed the cosmological constant to be zero.