Bimetric gravity

Bimetric gravity or bigravity refers to two different classes of theories. The first class of theories relies on modified mathematical theories of <a href="/facts/Gravity/bqAN7DdQ">gravity</a> (or gravitation) in which two <a href="/facts/Metric_tensor/tbqek1gJ">metric tensors</a> are used instead of one. The second metric may be introduced at high energies, with the implication that the <a href="/facts/Speed_of_light/sKSQwdNx">speed of light</a> could be energy-dependent, enabling models with a <a href="/facts/Variable_speed_of_light/5QjmJTfw">variable speed of light</a>.
If the two metrics are dynamical and interact, a first possibility implies two <a href="/facts/Graviton/MVJUBjek">graviton</a> modes, one massive and one massless; such bimetric theories are then closely related to <a href="/facts/Massive_gravity/oK96Mhez">massive gravity</a>. Several bimetric theories with massive gravitons exist, such as those attributed to <a href="/facts/Nathan_Rosen/0ks08uEe">Nathan Rosen</a> (1909–1995) or <a href="/facts/Mordehai_Milgrom/cyqvKDY1">Mordehai Milgrom</a> with relativistic extensions of <a href="/facts/Modified_Newtonian_Dynamics/3kenEm7h">Modified Newtonian Dynamics</a> (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 <a href="/facts/General_relativity/jVsoIg8X">general relativity</a>, Rosen's theory has been shown to be inconsistent with observations of the <a href="/facts/PSR_B1913%2B16/GMTAbnBj">Hulse–Taylor binary pulsar</a>. Some of these theories lead to <a href="/facts/Cosmic_acceleration/F7X1PAMq">cosmic acceleration</a> at late times and are therefore alternatives to <a href="/facts/Dark_energy/6bWIYewA">dark energy</a>. Bimetric gravity is also at odds with measurements of gravitational waves emitted by the neutron-star merger <a href="/facts/GW170817/q78LSs5w">GW170817</a>.
On the contrary, the second class of bimetric gravity theories does not rely on massive gravitons and does not modify <a href="/facts/Newton%27s_laws_of_motion/XKzGTXK1">Newton's law</a>, but instead describes the universe as a <a href="/facts/Manifold/rXPERJtu">manifold</a> having two coupled <a href="/facts/Riemannian_manifold/5KYnmEgF">Riemannian metrics</a>, where matter populating the two sectors interact through gravitation (and antigravitation if the <a href="/facts/Topology/2EqW47cU">topology</a> and the <a href="/facts/Post-Newtonian_expansion/QxktcY1c">Newtonian approximation</a> considered introduce <a href="/facts/Negative_mass/dqGf2T4m">negative mass</a> and <a href="/facts/Negative_energy/2TgPAeto">negative energy</a> states in <a href="/facts/Cosmology/yM19zM7Y">cosmology</a> as an alternative to <a href="/facts/Dark_matter/y5HgX55N">dark matter</a> and dark energy). Some of these <a href="/facts/Physical_cosmology/ww4ChbEq">cosmological models</a> also use a variable speed of light in the high <a href="/facts/Energy_density/mwBXA7bC">energy density</a> state of the <a href="/facts/Scale_factor_(cosmology)/hN9JlB3v">radiation-dominated era</a> of the universe, challenging the <a href="/facts/Inflation_(cosmology)/SujXx3aD">inflation</a> hypothesis.

Bimetric gravity open-in-new

Bimetric gravity