These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout the universe. As more space comes into existence, more of this energy-of-space would appear, thereby causing accelerated expansion. Because this energy is a property of space itself, it would not be diluted as space expands. According to Einstein, 'empty space' can possess its own energy.

Likewise, a universe which contracts slightly will continue contracting. The equilibrium is unstable: if the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. The mechanism was an example of fine-tuning, and it was later realized that Einstein's static universe would not be stable: local inhomogeneities would ultimately lead to either the runaway expansion or contraction of the universe. Einstein stated that the cosmological constant required that 'empty space takes the role of gravitating negative masses which are distributed all over the interstellar space'. Einstein gave the cosmological constant the symbol Λ (capital lambda). The cosmological constant was first proposed by Einstein as a mechanism to obtain a solution to the gravitational field equation that would lead to a static universe, effectively using dark energy to balance gravity. If considered as a 'source term' in the field equation, it can be viewed as equivalent to the mass of empty space (which conceptually could be either positive or negative), or ' vacuum energy'. The ' cosmological constant' is a constant term that can be added to Einstein field equations of general relativity. History of discovery and previous speculation Einstein's cosmological constant 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. However, scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be prolonged.ĭue to the toy model nature of concordance cosmology, some experts believe that a more accurate general relativistic treatment of the structures on all scales in the real universe may do away with the need to invoke dark energy. The cosmological constant can be formulated to be equivalent to the zero-point radiation of space, i.e., the vacuum energy. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. Two proposed forms of dark energy are the cosmological constant (representing a constant energy density filling space homogeneously) and scalar fields (dynamic quantities having energy densities that vary in time and space) such as quintessence or moduli. However, it dominates the universe's mass–energy content because it is uniform across space. Dark energy's density is very low: 6×10 −10 J/m 3 (~7×10 −30 g/cm 3), much less than the density of ordinary matter or dark matter within galaxies. The mass–energy of dark matter and ordinary (baryonic) matter contributes 26% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount. Assuming that the lambda-CDM model of cosmology is correct, as of 2013, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. As of 2021, there are active areas of cosmology research to understand the fundamental nature of dark energy. Since the 1990s, dark energy has been the most accepted premise to account for the accelerated expansion.

Without introducing a new form of energy, there was no way to explain an accelerating expansion of the universe.

Measurements of the cosmic microwave background (CMB) suggest the universe began in a hot Big Bang, from which general relativity explains its evolution and the subsequent large-scale motion. Before these observations, scientists thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time. Understanding the universe's evolution requires knowledge of its starting conditions and composition. 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. In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales.

Cosmological redshift equation examples