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This serves as a working definition even though there is no single agreed-upon definition of what constitutes a void. The matter density value used for describing the cosmic mean density is usually based on a ratio of the number of galaxies per unit volume rather than the total mass of the matter contained in a unit volume. [9]
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).
For up to 10 −35 seconds after the Big Bang, the energy density of the universe was so high that the four forces of nature – strong, weak, electromagnetic, and gravitational – are thought to have been unified into one single force. The state of matter at this time is unknown.
Since observations indicate the universe is almost flat, [73] [74] [75] it is expected the total energy density of everything in the universe should sum to 1 (Ω tot ≈ 1). 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 ...
A type of naturally occurring physical entity, association, or structure that exists within the observable universe but is a more complex, less cohesively bound structure than an astronomical body, consisting perhaps of multiple bodies or even other objects with substructures, such as a planetary system, star cluster, nebula, or galaxy. Though ...
Before 1998, it was expected that the expansion rate would be decreasing as time went on due to the influence of gravitational interactions in the universe; and thus there is an additional observable quantity in the universe called the deceleration parameter, which most cosmologists expected to be positive and related to the matter density of ...
In the Big Bang, the expanding Universe causes matter to dilute over time, while in the Steady-State Theory, continued matter creation ensures that the density remains constant over time. In cosmology , the steady-state model or steady-state theory is an alternative to the Big Bang theory.
The diffuse photoionized gas contains filaments of higher density, about one atom per cubic meter, [145] which is 5–200 times the average density of the universe [146]. The IGM is inferred to be mostly primordial in composition, with 76% hydrogen by mass, and enriched with higher mass elements from high-velocity galactic outflows. [147]