dark energy

Form of energy that is thought to fill the universe, causing it to expand at an accelerating rate. There are several theories of the nature of dark energy. The one generally regarded as best fitting the available evidence involves the cosmological constant, symbolized by Λ. The cosmological constant describes a force of universal repulsion and was first introduced by Albert Einstein as an addition to his general theory of relativity because he believed it was necessary to explain why the universe does not collapse on itself.

Supernova observations reported in 2005 by the multinational Supernova Legacy Survey indicate that the value of Λ (the amount of energy the vacuum possesses per unit volume) has remained constant since the Big Bang. This means that as space expands, the total repulsion energy in the universe increases, boosting the rate of expansion. Before about nine billion years ago, this repulsion was dominated by the gravitational self-attraction of all the matter in the universe - both ordinary matter and dark matter - and the expansion of the universe had been slowing.

Cosmologists were forced to postulate the existence of dark energy in the late 1990s when studies of supernovae revealed that the expansion of the universe, discovered by the US astronomer Edwin Hubble in the 1930s, is not slowing under the mutual gravitational attraction of the matter in the universe, but rather is accelerating.