Phantom fluid cosmology: Impact of a phantom hidden sector on cosmological observables

Understand

Phantom scalar theories are widely considered in cosmology, but rarely at the quantum level, where they give rise to negative-energy ghost particles.

• These cause decay of the vacuum into gravitons and photons, violating observational gamma-ray limits unless the ghosts are effective degrees of freedom with a cutoff $\Lambda$ at the few-MeV scale.
• We update the constraints on this scale, finding that $\Lambda \lesssim 19$ MeV.
• We further explore the possible coupling of ghosts to a light, possibly massless, hidden sector particle, such as a sterile neutrino.