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In the case of the flatness problem, the parameter which appears fine-tuned is the density of matter and energy in the universe. This value affects the curvature of space-time, with a very specific critical value being required for a flat universe. The current density of the universe is observed to be very close to this critical value.
The density parameter is the average density of the universe divided by the critical energy density, that is, the mass energy needed for a universe to be flat. Put another way, If Ω = 1, the universe is flat. If Ω > 1, there is positive curvature. If Ω < 1, there is negative curvature.
Heliocentrism [a] (also known as the heliocentric model) is a superseded astronomical model in which the Earth and planets orbit around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism , which placed the Earth at the center.
The heliocentric model from Nicolaus Copernicus' De revolutionibus orbium coelestium. Heliocentrism, or heliocentricism, [9] [note 1] is the astronomical model in which the Earth and planets revolve around a relatively stationary Sun at the center of the Solar System.
In physical cosmology, the Copernican principle states that humans are not privileged observers of the universe, [1] that observations from the Earth are representative of observations from the average position in the universe. Named for Copernican heliocentrism, it is a working assumption that arises from a modified cosmological extension of ...
Evaluating the Hubble parameter at the present time yields Hubble's constant which is the proportionality constant of Hubble's law. Applied to a fluid with a given equation of state, the Friedmann equations yield the time evolution and geometry of the universe as a function of the fluid density.
According to Einstein's theory of general relativity, particles of negligible mass travel along geodesics in the space-time. In uncurved space-time, far from a source of gravity, these geodesics correspond to straight lines; however, they may deviate from straight lines when the space-time is curved.
In situations where either dimensionless parameter is large, then general relativity must be used to describe the system. General relativity reduces to Newtonian gravity in the limit of small potential and low velocities, so Newton's law of gravitation is often said to be the low-gravity limit of general relativity.