In particle physics, a ghost condensate is a speculative proposal in which a ghost, an excitation of a field with a wrong sign of the kinetic term, acquires a vacuum expectation value. This phenomenon breaks Lorentz invariance. Around the new vacuum state, all excitations have a positive norm, and therefore the probabilities are positively definite.

The theories of ghost condensate predict specific non-gaussianities of the cosmic microwave background. These theories have been proposed by Nima Arkani-Hamed, Markus Luty, and others.