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Physical cosmology

Key topics

Universe · Big Bang Age of the universe Timeline of the Big Bang Ultimate fate of the universe

Early universe

Inflation · Nucleosynthesis GWB · Neutrino Background Cosmic microwave background Expanding universe Redshift · Hubble's law Metric expansion of space Friedmann equations FLRW metric Structure formation Shape of the universe Structure formation Galaxy formation Large-scale structure Components Lambda-CDM model Dark energy · Dark matter History Timeline of cosmology... Cosmology experiments Observational cosmology 2dF · SDSS CoBE · BOOMERanG · WMAP Scientists Einstein · Hawking . Friedman · Lemaître · Hubble · Penzias · Wilson · Gamow · Dicke · Zel'dovich · Mather · Smoot · others

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The idea of a static universe is one which demands that space is not expanding nor contracting but rather is dynamically stable. Albert Einstein proposed such a model as his preferred cosmology by adding a cosmological constant to his equations of general relativity to counteract the dynamical effects of gravity which in a universe of matter would cause the universe to collapse. After the discovery by Edwin Hubble that there was a relationship between redshift and distance, Einstein declared this formulation to be his "biggest blunder".^[1]

Even after Hubble's observations, Fritz Zwicky proposed that a static universe could still be viable if there was an alternative explanation of redshift due to a mechanism that would cause light to lose energy as it traveled through space, a concept that would come to be known as "tired light". Subsequent cosmological observations have shown such a model to be an unviable alternative, leading most astrophysicists to conclude that the static universe is essentially falsified.

Einstein's proposal was a universe that followed the dust solution of Einstein's field equation for a value of the cosmological constant equal to Λ_E = 4πGρ / c², where G is Newtonian gravitational constant, ρ is the energy density of the matter in the universe and c is the speed of light. Value Λ_E is called Einstein's cosmological constant or, alternatively, the cosmological constant of Einstein's universe.

The radius of curvature of space of Einstein's universe, called "Einstein's radius", is equal to

"Einstein's universe" is one of Friedmann's solutions of Einstein's field equation, for the value of cosmological constant Λ_E. This is the only stationary solution of all Friedmann's solutions and because it is stationary it is thought to be non physical by majority of astronomers. Those astronomers think that the universe is expanding because there is observed a phenomenon of Hubble redshift and it is interpreted by those astronomers as a Doppler shift caused by galaxies moving away from our own Galaxy. Therefore, it is thought that the real solution of Einstein's field equation cannot be stationary.

Einstein came to consider this modification of his theory as his "biggest blunder." ^[2] Most physicists of the twentieth century assumed it to have the value zero; this assumption predicts a decelerating expansion of the universe. However, with the discovery of the accelerating universe, the cosmological constant has been revived as a possible explanation for dark energy.

Besides from Hubble's law, the cosmic microwave background radiation is used as empirical evidence of the Big Bang model. A static universe model has to explain this radiation in some other way. Also there has to be some process of "recreation of Hydrogen" since if the universe was not created some finite time ago, as in the Big Bang model, the stars would otherwise all have run out of fuel (Hydrogen) by now.

A very small number of proponents of the static universe continue to promote nonstandard cosmologies in defiance of scientific consensus, though the term "static universe" has fallen somewhat out of favor since it is argued by advocates of these theories that the universe is "dynamically evolving".

• Steady state theory
• Milne model

1. ^  In George Gamow's autobiography, My World Line (1970) he quotes Einstein: "Much later, when I was discussing cosmological problems with Einstein, he remarked that the introduction of the cosmological term was the biggest blunder of his life."