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The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually the dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved, but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant).
The measured dark energy density is Ω Λ ≈ 0.690; the observed ordinary (baryonic) matter energy density is Ω b ≈ 0.0482 and the energy density of radiation is negligible. This leaves a missing Ω dm ≈ 0.258 which nonetheless behaves like matter (see technical definition section above) – dark matter.
Fully 70% of the matter density in the universe appears to be in the form of dark energy. Twenty-six percent is dark matter. Only 4% is ordinary matter. So less than 1 part in 20 is made out of matter we have observed experimentally or described in the standard model of particle physics. Of the other 96%, apart from the properties just ...
In astronomy and cosmology, the dark fluid theory attempts to explain dark matter and dark energy in a single framework, as suggested by cosmologist Alexandre Arbey in 2005. [1] [2] The theory proposes that dark matter and dark energy are not separate physical phenomena, nor do they have separate origins, but that they are strongly linked together and can be considered as two facets of a ...
Dark energy is one of the greatest mysteries in science today. One of the simplest explanations is that it is a “cosmological constant” – a result of the energy of empty space itself – an ...
It is due to dark matter that galaxies are able to keep their shape, with the mass of dark matter creating enough gravitational force to hold the stars that make up a galaxy together. Dark energy, however, is a substance or force responsible for the accelerating expansion of the universe over time. [2]
where is the matter energy density, is the matter pressure, and is a constant. Then the strong energy condition requires w ≥ − 1 / 3 {\displaystyle w\geq -1/3} ; but for the state known as a false vacuum, we have w = − 1 {\displaystyle w=-1} .
Since the 1990s, studies have shown that, assuming the cosmological principle, around 68% of the mass–energy density of the universe can be attributed to dark energy. [6] [7] [8] The cosmological constant Λ is the simplest possible explanation for dark energy, and is used in the standard model of cosmology known as the ΛCDM model.