Ghost condensation and a consistent infrared modification of gravity

TLDR: In this article, the authors proposed a theoretically consistent modification of gravity in the infrared, which is compatible with all current experimental observations and opens up a number of new avenues for attacking cosmological problems, including inflation, dark matter and dark energy.

Abstract: We propose a theoretically consistent modification of gravity in the infrared, which is compatible with all current experimental observations. This is an analog of the Higgs mechanism in general relativity, and can be thought of as arising from ghost condensation — a background where a scalar field has a constant velocity, = M2. The ghost condensate is a new kind of fluid that can fill the universe, which has the same equation of state, ρ = −p, as a cosmological constant, and can hence drive de Sitter expansion of the universe. However, unlike a cosmological constant, it is a physical fluid with a physical scalar excitation, which can be described by a systematic effective field theory at low energies. The excitation has an unusual low-energy dispersion relation ω2 ~ 4/M2. If coupled to matter directly, it gives rise to small Lorentz-violating effects and a new long-range 1/r2 spin dependent force. In the ghost condensate, the energy that gravitates is not the same as the particle physics energy, leading to the possibility of both sources that can gravitate and anti-gravitate. The newtonian potential is modified with an oscillatory behavior starting at the distance scale MPl/M2 and the time scale MPl2/M3. This theory opens up a number of new avenues for attacking cosmological problems, including inflation, dark matter and dark energy.