Accelerated expansion and the Goldstone ghost

TL;DR

The paper explores an infrared modification of gravity in which a massless Goldstone ghost arises from Lorentz symmetry breaking in a hidden sector. This ghost, produced via vacuum instability at a scale $\Lambda_L$, transfers energy to standard matter at rate $\varepsilon$, yielding an evolving effective dark-energy equation of state $w_{\mathrm{eff}}$ that transitions from $-\tfrac{3}{2}$ toward $-1$ as the ghost+normal fluids come to dominate. A de Sitter attractor is reached with $H_{\mathrm{ss}}=\left(\frac{\varepsilon\delta w}{9(1+w_n)(1+w_g)m_{\mathrm{Pl}}^2}\right)^{1/3}$, and the present cosmology is consistent with $w_{\mathrm{eff}}\approx -1.2$ when $\rho_n+\rho_g$ is ~0.73 of the critical density. Observational constraints require suppressing photon production relative to neutrinos, which points to a hidden-sector coupling scale $\Lambda_H \sim 10^3\,\mathrm{TeV}$ and a Lorentz-breaking scale $\Lambda_L \lesssim 10\,\mathrm{keV}$ (often near $10^2\,\mathrm{eV}$), with neutrinos dominantly produced alongside ghosts. The work connects Lorentz symmetry breaking and Goldstone ghost dynamics to an observable cosmology without a cosmological constant and hints at links between the Lorentz-violating scale, neutrino masses, and potential dark-matter states in a UV-complete theory.

Abstract

A vacuum instability due to a massless ghost in a hidden sector can lead to an effective equation of state for dark energy that changes smoothly from $w=-3/2$ at large redshifts, to $w\approx-1.2$ today, to $w=-1$ in the future. We discuss how this ghost can be the Goldstone boson of Lorentz symmetry breaking, and we find that this breaking in the hidden sector should occur at a scale below $\sim$10 KeV. The normal particles that are produced along with the ghosts are then predominantly neutrinos.