Dark Energy should not to be confused with Dark Matter or Dark Fluid.
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% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount. The density of dark energy is very low much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the mass–energy of the universe because it is uniform across space.
Two proposed forms for dark energy are the cosmological constant, representing 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 the zero-point radiation of space i.e. the vacuum energy.Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow. Read more
Dark Energy Gets Weirder: Mysterious Force May Vary Over Time Space.com - January 30, 2019
Dark energy is apparently even more mysterious than astronomers had thought. Scientists first proposed the existence of this invisible force two decades ago, to explain the surprising discovery that the universe's expansion is accelerating. The most-used astrophysical model of the universe's structure and evolution regards dark energy as a constant. Indeed, many astronomers believe it to be the cosmological constant, which Einstein posited in 1917 as part of his theory of general relativity. But a new study of enormous, superbright black holes known as quasars suggests that dark energy could be miscast as the cosmological constant, or any kind of constant; the force may have varied since the universe's birth 13.8 billion years ago, research team members said.
Researcher finds hint of dark energy discussion in letters between Einstein and Schrodinger PhysOrg - December 11, 2012
Alex Harvey, a physics professor at the City University of New York has uploaded a paper in which he claims Albert Einstein and Erwin Schrodinger were writing letters suggesting the two men were on the precipice of discussing the possibility of the existence of dark energy. The letter exchange came in the years after Einstein had published his theories on general relativity, and revolved around the matter of the cosmological constant.
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