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

TL;DR

This paper investigates how phantom scalar fields with a hidden sector influence cosmological observations, constraining their quantum cutoff scale and exploring their role as a dark energy source that may alleviate current cosmological tensions.

Contribution

It introduces updated constraints on the cutoff scale of phantom ghosts and explores their coupling to a hidden sector, proposing a novel phantom fluid model affecting dark energy and cosmological tensions.

Findings

Cutoff scale for ghosts constrained to be below ~19 MeV.

Ghost coupling to hidden sector can increase dark matter density at late times.

Model shows mild preference over standard cosmology and reduces Hubble and S8 tensions.

Abstract

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. Vacuum decays can then cause the dark matter density of the universe to grow at late times. The combined phantom plus dark matter fluid has an effective equation of state $w < -1$, and functions as a new source of dark energy. We derive constraints from cosmological observables on the rate of vacuum decay into such a phantom fluid. We find a mild preference for the ghost model over the standard cosmological one, and a modest amelioration of the Hubble and $S_8$ tensions.