Galaxy cluster Abell 2029

Dark energy

In the 1990s, scientists studying exploding stars called supernovae in far-flung galaxies discovered that the Universe's expansion is accelerating, not slowing as theorists predicted. This discovery led them to the conclusion that some unknown process was causing the Universe to speed up, and they named it dark energy.

One of the biggest goals in science is to explain this mysterious energy, which is thought to make up about 70% of the energy density of the Universe.

Image: A Chandra X-ray Observatory image of the distant galaxy cluster Abell 2029. Astronomers use images like this to better understand dark energy's effects. (credit: NASA/CXC/IoA/S.Allen et al.)

Watch and listen to clips from past programmes TV clips [2] Radio Programmes [2]

Galaxy cluster Abell 2029


A mysterious process speeds up the Universe's expansion.

About Dark energy

In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe. Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate.

Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68.3% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, respectively, and other components such as neutrinos and photons contribute a very small amount. Again, on a mass–energy equivalence basis, the density of dark energy (~ 7 × 10−30 g/cm3) is very low, much less than the density of ordinary matter or dark matter within galaxies. However, it comes to dominate the mass–energy of the universe because it is uniform across space.

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy. Scalar fields that do change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.

High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time and space. In general relativity, the evolution of the expansion rate is parameterized by the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring the equation of state for dark energy is one of the biggest efforts in observational cosmology today. Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model of cosmology" because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.

Read more at Wikipedia

This entry is from Wikipedia, the user-contributed encyclopedia. If you find the content in the 'About' section factually incorrect, defamatory or highly offensive you can edit this article at Wikipedia.