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 spontaneously. Around the new vacuum state, all excitations have a positive norm, and therefore the probabilities are positive definite.
We have a real scalar field φ with the following action
where a and b are positive constants and
in the +--- sign convention.
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.
Unfortunately, this theory allows for superluminal propagation of information in some cases and has no lower bound on its energy. This model doesn't admit a Hamiltonian formulation (the Legendre transform is multi-valued because the momentum function isn't convex) because it is acausal. Quantizing this theory leads to problems.
Famous quotes containing the word ghost:
“‘Dear Captain Smith,’ the ghost replied, ‘you’ve used me ungenteelly.
The crowner’s quest goes hard with me because I’ve acted frailly,
And Parson Biggs won’t bury me, though I am dead Miss Bailey.’”
—George Colman (1762–1836)