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:
““The work is done,” grown old he thought,
“According to my boyish plan;
Let the fools rage, I swerved in nought,
Something to perfection brought;”
But louder sang that ghost “What then?””
—William Butler Yeats (1865–1939)