Cosmological principle

In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is uniformly isotropic and homogeneous when viewed on a large enough scale, since the forces are expected to act equally throughout the universe on a large scale, and should, therefore, produce no observable inequalities in the large-scale structuring over the course of evolution of the matter field that was initially laid down by the Big Bang.

Astronomer William Keel explains:

The cosmological principle is usually stated formally as 'Viewed on a sufficiently large scale, the properties of the universe are the same for all observers.' This amounts to the strongly philosophical statement that the part of the universe which we can see is a fair sample, and that the same physical laws apply throughout. In essence, this in a sense says that the universe is knowable and is playing fair with scientists.

As Andrew Liddle puts it, "the cosmological principle [means that] the universe looks the same whoever and wherever you are."

The two testable structural consequences of the cosmological principle are homogeneity and isotropy. Homogeneity – constant density – means that the same observational evidence is available to observers at different locations in the universe. Isotropy – looking the same in all directions – means that the same observational evidence is available by looking in any direction in the universe. Isotropy implies homogeneity, but an homogeneous universe could be anisotropic.

The cosmological principle is first clearly asserted in the Philosophiæ Naturalis Principia Mathematica (1687) of Isaac Newton.^{[dubious – discuss]} In contrast to some earlier classical or medieval cosmologies, in which Earth rested at the center of universe, Newton conceptualized the Earth as a sphere in orbital motion around the Sun within an empty space that extended uniformly in all directions to immeasurably large distances. He then showed, through a series of mathematical proofs on detailed observational data of the motions of planets and comets, that their motions could be explained by a single principle of "universal gravitation" that applied as well to the orbits of the Galilean moons around Jupiter, the Moon around the Earth, the Earth around the Sun, and to falling bodies on Earth. That is, he asserted the equivalent material nature of all bodies within the Solar System, the identical nature of the Sun and distant stars, and thus the uniform extension of the physical laws of motion to a great distance beyond the observational location of Earth itself.

Since the 1990s, observations assuming the cosmological principle have concluded that around 68% of the mass–energy density of the universe can be attributed to dark energy, which led to the development of the ΛCDM model.

Observations show that more distant galaxies are closer together and have lower content of chemical elements heavier than lithium.^{[citation needed]} Applying the cosmological principle, this suggests that heavier elements were not created in the Big Bang but were produced by nucleosynthesis in giant stars and expelled via a series of supernovae and new star formation from the supernova remnants, which means heavier elements would accumulate over time. Another observation is that the farthest galaxies (earlier time) are often more fragmentary, interacting and unusually shaped than local galaxies (recent time), suggesting evolution in galaxy structure as well.