Unknown property in cosmology that causes the expansion of the universe to accelerate

<span class="mw-page-title-main">Dark energy</span>

Dark energy

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the universe does not expand at a constant rate; rather, the universe's expansion is accelerating. Two proposed forms of dark energy are the cosmological constant However, scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be prolonged. Due to the toy model nature of concordance cosmology, some experts believe in the real universe may do away with the need to invoke dark energy. Inhomogeneous cosmologies, which attempt to account for the back-reaction of structure formation on the metric, generally do not acknowledge any dark energy contribution to the universe's energy density. 