Bimetric theory | Wikipedia audio article
Channel: wikipedia tts
Date: 2018-12-30

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Bimetric_gravity


00:02:13 1 Rosen's bigravity (1940)
00:24:06 2 Janus cosmological model
00:33:04 2.1 The bimetric universe according to Janus model
00:38:04 2.2 Cosmology with variation of constants
00:39:12 2.3 Geometrization of Janus model
00:44:00 2.4 The bridges between the sheets of the universe ("Black Holes")
00:48:03 2.5 Formation of large structures in the universe according to the Janus model
00:48:15 2.5.1 Density fluctuations in the universe
00:48:42 2.6 Formation and stability of galaxies and structures on a large scale
00:49:45 2.7 Future of the bi-sheeted universe according to the Janus model
00:50:08 3 Massive bigravity
01:01:32 4 See also



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SUMMARY
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Bimetric gravity or bigravity refers to two different class of theories.
The first class of theories relies on modified mathematical theories of gravity (or gravitation) in which two metric tensors are used instead of one. The second metric may be introduced at high energies, with the implication that the speed of light could be energy-dependent, enabling models with a variable speed of light.
If the two metrics are dynamical and interact, a first possibility implies two graviton modes, one massive and one massless; such bimetric theories are then closely related to massive gravity. Several bimetric theories with massive gravitons exist, such as those attributed to Nathan Rosen (1909–1995) or Mordehai Milgrom with Modified Newtonian Dynamics (MOND). More recently, developments in massive gravity have also led to new consistent theories of bimetric gravity. Though none has been shown to account for physical observations more accurately or more consistently than the theory of general relativity, Rosen's theory has been shown to be inconsistent with observations of the Hulse–Taylor binary pulsar.
 Some of these theories lead to cosmic acceleration at late times and are therefore alternatives to dark energy.On the contrary, the second class of bimetric gravity theories does not rely on massive gravitons and does not modify Newton's law, but instead describes the universe as a manifold having two coupled Riemannian metrics, where matter populating the two sectors interact through gravitation (and antigravitation if the topology and the Newtonian approximation considered introduce negative mass and negative energy states in cosmology as an alternative to dark matter and dark energy). Some of these cosmological models also use a variable speed of light in the high energy density state of the radiation-dominated era of the universe, challenging the inflation hypothesis. The Janus cosmological model introduced by Jean-Pierre Petit belongs to this class.