Ghost Condensation and a Consistent IR Modification of Gravity

We propose a theoretically consistent modification of gravity in theinfrared, which is compatible with all current experimental observations. Thisis an analog of Higgs mechanism in general relativity, and can be thought of asarising from ghost condensation--a background where a scalar field \phi has aconstant velocity, <\dot\phi> = M^2. The ghost condensate is a new kind offluid that can fill the universe, which has the same equation of state, \rho =-p, as a cosmological constant, and can hence drive de Sitter expansion of theuniverse. However, unlike a cosmological constant, it is a physical fluid witha physical scalar excitation, which can be described by a systematic effectivefield theory at low energies. The excitation has an unusual low-energydispersion relation \omega^2 \sim k^4 / M^2. If coupled to matter directly, itgives rise to small Lorentz-violating effects and a new long-range 1/r^2 spindependent force. In the ghost condensate, the energy that gravitates is not thesame as the particle physics energy, leading to the possibility of both sourcesthat can gravitate and antigravitate. The Newtonian potential is modified withan oscillatory behavior starting at the distance scale M_{Pl}/M^2 and the timescale M_{Pl}^2/M^3. This theory opens up a number of new avenues for attackingcosmological problems, including inflation, dark matter and dark energy.Comment: 42 pages, LaTeX 2