Gauss-Bonnet cosmologies: crossing the phantom divide and the transition from matter dominance to dark energy

Abstract

Dark energy cosmologies with an equation of state parameter $w$ less than -1 are often found to violate the null energy condition and show unstable behaviour. A solution to this problem may require the existence of a consistent effective theory that violates the null energy condition only momentarily and does not develop any instabilities or other pathological features for a late time cosmology. A model which incorporates a dynamical scalar field $\varphi$ coupled to the quadratic Riemann invariant of the Gauss-Bonnet form is a viable proposal. Such an effective theory is shown to admit nonsingular cosmological evolutions for a wide range of scalar-Gauss-Bonnet coupling. We discuss the conditions for which our model yields observationally supported spectra of scalar and tensor fluctuations, under cosmological perturbations. The model can provide a reasonable explanation for the transition from matter dominance to dark energy regime and the late time cosmic acceleration, offering an interesting testing ground for investigations of the cosmological modified gravity.

44 pages, several eps figures; section 2 expanded with discussions on cosmological/astrophysical constraints, final in JCAP

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