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There also remain small numbers of astrophysicists, including Hans Alfven , Y.P. Varshni and Halton Arp, who argue that redshifts in galaxies do not correlate with distance and/or are not due to the Doppler effect, and that this invalidates the need for the Big Bang.
During the 1970s, observations were made that - assuming that all of the matter within the universe could be seen - created problems for the Big Bang theory, as it seemed to underestimate the amount of deuterium in the universe and lead to a universe that was much more "lumpy" than observed. These problems are resolved if one assumes that most of the matter in the universe is not visible, and this assumption seems to be consistent with observations that suggest that much of the universe consists of dark matter.
The effects that dark matter has on Big Bang calculations generally do not depend on the detailed properties of the dark matter. The main property of dark matter which influences cosmology is whether the dark matter consists of particles that are heavy and hence are assumed to move more slowly, thereby creating cold dark matter, or whether it consists of particles that are light and hence are assumed to move more quickly, thereby creating hot dark matter, or whether the dark matter consists of ordinary matter which is baryonic matter.
All the matter in the universe is gravitationally attracted to other matter which is within the observable horizon (defined by the age of the universe). This should cause the expansion rate of the universe to slow down over time. Exactly how much matter exists in any given volume, relative to how large the horizon is and how fast the universe is currently expanding can lead to one of three scenarios:
If the gravitational attraction of all the matter in the observable horizon is high enough, then it could stop the expansion of the universe, and then reverse it. The universe would then contract, in about the same time as the expansion took. Eventually, all matter and energy would be compressed back into a gravitational singularity. There are theories about what happens after this, but these remain uncertain as the physics of singularities remains in question. Also, the omega point theory suggests that an infinite amount of computational capacity might be available in the finite time before the Crunch.
If the gravitational attraction of all the matter in the observable horizon is low enough, then the expansion will never stop. As the matter disperses into ever greater and greater volumes, new star formation would drop off. The average temperature of the Universe would asymptotically approach absolute zero, and the Universe would become very still and quiet. Eventually, all the protons would decay, the black holes would evaporate, and the Universe would consist of dispersed subatomic particles. The Big Freeze is also known as the heat death of the universe.
If the gravitational attraction of all the matter in the observable horizon is just right, then the expansion of the universe will asymptotically approach zero. The temperature of the universe would asymptotically approach a stable value slightly above absolute zero. Entropy would increase, and the end result (with protons decaying) would be similar to the Big Freeze.
One extremely puzzling recent discovery comes from observations of type I supernovae which allow one to better calculate the distance to galaxies, from observations of the cosmic microwave background, from gravitational lensing, and from the use of large length scale statistics of the distributions of galaxies and quasars as standard rulers for measuring distances. It appears that the expansion of the universe is accelerating, an observation which astrophysicists are currently trying to understand (see accelerating universe). The currently favored approach is to reintroduce a non-zero cosmological constant into Einstein's equations of General Relativity, and adjust the numerical value of that constant to match the observed acceleration. This is akin to postulating a repelling "dark energy", also called quintessence.
See also the ultimate fate of the Universe.
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