# Einstein's universe: Wikis

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Physical cosmology
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Age of the universe
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A static universe or "Einstein universe" is one in which space is neither expanding nor contracting. Albert Einstein proposed such a model as his preferred cosmology in 1917[citation needed]. He added a positive cosmological constant to his equations of general relativity to counteract the attractive effects of gravity on ordinary matter, which would otherwise cause the universe to either collapse or expand forever. This motivation evaporated after the discovery by Edwin Hubble that the universe is in fact not static, but expanding; in particular, Hubble discovered a relationship between redshift and distance, which forms the basis for the modern expansion paradigm. According to Gamow this led Einstein to declare this cosmological model, and especially the introduction of the cosmological constant, 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". However, subsequent cosmological observations have shown that this model is not a viable alternative either, leading most astrophysicists to conclude that the static universe is not the correct model of our universe.

## Einstein's universe

Einstein's static universe is closed (i.e. has hyperspherical topology and positive spatial curvature), and contains uniform dust and a positive cosmological constant with value precisely ΛE = 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. The radius of curvature of space of the Einstein universe is equal to

$R_E = \Lambda_E^{-1/2} = {c \over \sqrt{4\pi G\rho}}.$

The Einstein universe is one of Friedmann's solutions to Einstein's field equation for dust with density ρ, cosmological constant ΛE, and radius of curvature RE. It is the only non-trivial static solution to Friedmann's equations[citation needed].

Because the universe is now known to be expanding the Einstein universe is no longer regarded as a viable model for our universe. Moreover, it is unstable in the sense that any change in either the value of the cosmological constant, the matter density, or the spatial curvature will result in a universe that either expands and accelerates forever or re-collapses to a big crunch.[citation needed]

After observations indicated that the universe was expanding, most physicists of the twentieth century assumed the cosmological constant is zero. If so (absent some other form of dark energy), the expansion of the universe would be decelerating. However, with the discovery of the accelerating universe, a positive cosmological constant has been revived as a simple explanation for dark energy.

## Other problems with the model

Aside 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. Yes there are problems and I do mean big problems!

## Nonstandard cosmologies

A very small[citation needed] 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".

## References

1. ^  In George Gamow's autobiography, My World Line (1970), he says of 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."