Dark energy

Dark energy is the name given to the unknown form of energy that is hypothesized to drive the accelerated expansion of the universe. Observations of distant Type Ia supernovae in 1998 revealed that the expansion of the universe is not slowing down under the influence of gravity, as had been assumed, but is speeding up. The entity responsible for this acceleration — approximately 68% of the total energy density of the universe — is dark energy.

The leading candidate is the cosmological constant Λ, first introduced by Einstein in 1917 and later abandoned as his 'greatest blunder.' In quantum field theory, the vacuum is not empty but teems with virtual particles whose collective energy should contribute to Λ. The problem: the predicted value is 10^120 times larger than the observed value. This is the cosmological constant problem, one of the most severe fine-tuning problems in physics.

Alternative proposals include quintessence — a dynamic scalar field whose energy density varies with time — and modifications to general relativity on cosmological scales. None has been observationally distinguished from Λ.

Dark energy connects to emergence in an unsettling way: the largest-scale dynamical feature of the universe may be the result of a microscopic quantum effect whose magnitude we cannot calculate. The universe, at the largest scale, may be shaped by physics at the smallest scale — or by something we have not yet imagined.