A Matter Bounce By Means of Ghost Condensation

TL;DR

This work builds a non-singular matter-bounce cosmology using ghost condensation in GR, tuning a ghost sector potential to make the ghost energy density outpace regular matter and anisotropies before the bounce. It shows that curvature perturbations generated in a matter-dominated contracting phase remain scale-invariant after passing through the ghost-driven bounce, while ghost-induced fluctuations are blue and subdominant, leaving the observable spectrum intact. The bounce is argued to be robust against radiation and anisotropic stress, though a brief gradient instability exists during the bounce, which does not affect observable scales under the chosen parameter regime. Overall, the paper provides a concrete non-inflationary path to the observed primordial fluctuations via a stable ghost-condensation–driven matter bounce.

Abstract

Assuming the existence of a scalar field which undergoes "ghost condensation" and which has a suitably chosen potential, it is possible to obtain a non-singular bouncing cosmology in the presence of regular matter and radiation. The potential for the ghost condensate field can be chosen such that the cosmological bounce is stable against the presence of anisotropic stress. Cosmological fluctuations on long wavelengths relevant to current cosmological observations pass through the bounce unaffected by the new physics which yields the bounce. Thus, this model allows for the realization of the "matter bounce" scenario, an alternative to inflationary cosmology for the generation of the observed primordial fluctuations in which the inhomogeneities originate as quantum vacuum perturbations which exit the Hubble radius in the matter-dominated phase of contraction.

Figures (5)

• Figure 1: Sketch of the effective potential of the ghost condensate field.
• Figure 2: Evolution of the ghost field $\phi$ as a function of time in our numerical simulation.
• Figure 3: Evolution of the ghost field velocity $\dot{\phi}$ as a function of time in our numerical simulation.
• Figure 4: Evolution of the Hubble parameter $H$ as a function of time in our numerical simulation.
• Figure 5: A sketch of the evolution of perturbations with different comoving wave numbers $k$ in the matter bounce. The horizontal axis represents comoving coordinates, the vertical axis is time, with the bounce point being $t = t_B$. The two vertical lines correspond to the wavelengths of two different scales, the thick black curve gives the Hubble radius.