Dark Energy Dominance
– Dark energy contributes 68% of the total energy in the observable universe.
– Dark matter contributes 26% of the total energy.
– Ordinary matter (baryonic matter) contributes 5% of the total energy.
– Neutrinos and photons have negligible contributions.
– Dark energy has a low density compared to ordinary matter and dark matter.
Observational Evidence
– Type 1A supernovae are used as accurate distance measures.
– Comparing the distance to the redshift of supernovae shows the universe’s expansion is accelerating.
– Prior to this observation, scientists believed the expansion would slow over time.
– Multiple independent lines of evidence support the existence of dark energy.
– Dark energy’s exact nature remains a mystery with various explanations.
History of Discovery and Previous Speculation
– Einstein proposed the cosmological constant to obtain a static universe.
– The equilibrium of the cosmological constant was unstable.
– Edwin Hubble’s observations showed the universe is expanding.
– Einstein referred to his failure to predict a dynamic universe as his greatest blunder.
– Alan Guth and Alexei Starobinsky proposed inflationary dark energy in the early universe.
Inflationary Dark Energy
– Inflation postulates a repulsive force driving exponential expansion after the Big Bang.
– Inflation occurred at a higher energy density than observed dark energy.
– Most cosmological research in the 1980s focused on critical density models.
– Observations of large-scale galaxy clustering and cosmic microwave background led to modified CDM models.
– Direct evidence for dark energy came from supernova observations in 1998.
Nature of Dark Energy and its Contribution to Cosmological Constant
– Dark energy is more hypothetical than dark matter.
– It is not dense and does not interact with fundamental forces other than gravity.
– Dark energy is believed to uniformly fill empty space.
– It has a very low mass of approximately 10^-27 kg/m.
– The vacuum energy, generated by particle-antiparticle pairs, is often considered a main contribution to dark energy.
– The vacuum energy is expected to contribute to the cosmological constant.
– The cosmological constant problem arises from the disagreement between observed and theoretical values of vacuum energy density.
– The unresolved problem challenges the understanding of dark energy.
– Dark energy would need to have a strong negative pressure to explain the observed acceleration of the universe’s expansion.
– The pressure within a substance contributes to its gravitational attraction. Source: https://en.wikipedia.org/wiki/Dark_energy
In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. Its primary effect is to drive the accelerating expansion of the universe. Assuming that the lambda-CDM model of cosmology is correct, dark energy is the dominant component of the universe, contributing 68% of the total energy in the present-day observable universe while dark matter and ordinary (baryonic) matter contribute 26% and 5%, respectively, and other components such as neutrinos and photons are nearly negligible. Dark energy's density is very low: 6×10−10 J/m3 (≈7×10−30 g/cm3), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe's mass–energy content because it is uniform across space.
The first observational evidence for dark energy's existence came from measurements of supernovae. Type 1A supernovae have constant luminosity, which means they can be used as accurate distance measures. Comparing this distance to the redshift (which measures the speed at which the supernova is receding) shows that the universe's expansion is accelerating. Prior to this observation, scientists thought that the gravitational attraction of matter and energy in the universe would cause the universe's expansion to slow over time. Since the discovery of accelerating expansion, several independent lines of evidence have been discovered that support the existence of dark energy.
The exact nature of dark energy remains a mystery, and explanations abound. The main candidates are a 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. A cosmological constant would remain constant across time and space, while scalar fields can vary. Yet other possibilities are interacting dark energy, an observational effect, and cosmological coupling (see the Theories of Dark Energy section).
