Cosmological Principle

The Cosmological Principle is a principle invoked in cosmology that severely restricts the large variety of possible cosmological theories:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:On large scales, the Universe is homogeneous and isotropic.

Related Topics:
Homogeneous - Isotropic

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places. Or, in other words, on a large scale the Universe looks the same in all directions for an observer at any place. This principle is consistent with observations of the universe.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

If one assumes these two properties to be true about the Universe, then the only possible cosmic evolution is a global expansion or contraction.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In this case, at a time, the velocity between two points must be proportional to their separation; this is Hubble's law.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

An extension proposed by Fred Hoyle is the Steady-State Principle, also known as the Perfect Cosmological Principle, which is that the universe is homogeneous and isotropic both in space and in time. The original steady-state principle is inconsistent with the Big bang and is widely seen as having been disproved by the mid-1970s.

Related Topics:
Fred Hoyle - Steady-State - Big bang

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

This principle may be compared to the more metaphysical Anthropic principle.

Related Topics:
Metaphysical - Anthropic principle

~ ~ ~ ~ ~ ~ ~ ~ ~ ~