Braneworld models of dark energy

Abstract

We explore a new class of braneworld models in which the scalar curvature of the (induced) brane metric contributes to the brane action. The scalar curvature term arises generically on account of one-loop effects induced by matter fields residing on the brane. Spatially flat braneworld models can enter into a regime of accelerated expansion at late times. This is true even if the brane tension and the bulk cosmological constant are tuned to satisfy the Randall-Sundrum constraint on the brane. Braneworld models admit a wider range of possibilities for dark energy than standard LCDM. In these models the luminosity distance can be both smaller and larger than the lu- Within the conventional framework, 'phantom energy' with w < −1 is beset with a host of undesirable properties, which makes this model of dark energy unattractive. Braneworld models, on the other hand, have the capacity to endow dark energy with exciting new possibilities (including w < −1) without suffering from the problems faced by phantom energy. For a sub-1 class of parameter values, braneworld dark energy and the acceleration of the universe are transient phenomena. In these models, the universe, after the current period of acceleration, re-enters the matter-dominated regime so that the deceleration parameter q(t) → 0.5 when t ≫ t 0 , where t 0 is the present epoch. Such models could help reconcile an accelerating universe with the requirements of string/M-theory.

The phantom divide in string gas cosmology

“…The study of NEC-violating cosmologies was initiated in [17]; they are often singular [see [18] [19] for more recent references discussing the singular case], but the fact that they can be non-singular and asymptotically locally de Sitter was pointed out in [20], and this observation has been developed in various ways in [21] [22][23] [24][25] [26]. "Phantom cosmologies" involving true violations of the NEC encounter well-known objections [27], but these are avoided by more sophisticated models in which the violation is merely effective 2 : for example, in brane-world theories [29] [30], or through quantum effects [31] [32], or by considering string-motivated Gauss-Bonnet corrections [33] to the Einstein-Hilbert Lagrangian. String theory apparently permits true NEC violation in the very early Universe; specifically, there are in fact explicit NECviolating cosmologies which have been constructed within string theory: Kachru and McAllister [34] have given such an example in the context of a warped deformed conifold in a Calabi-Yau compactification of II B string theory 3 .…”

Avoidance of big rip in phantom cosmology by gravitational back reaction

“…(10,15,17), we know how Φ evolves with time on the wavelength scale smaller and larger than the Hubble radius. Thus we can express Φ pk (t > t H ) in terms of t 0 , t H and Φ mk (t 0 )…”

“…The above conclusions are based upon a constant negative value of w. However, if the value of w could change during the evolution of the universe and then in principle the big rip can be avoided [12]. Attempts have also been made toward avoiding the big rip by modifying the original Caldwell's phantom model [16,17,18,19,20]. Note that it has been argued that a singularity can also develop at a finite future time even if ρ + 3p is positive [21].…”

Crossing the phantom divide without phantom matter

“…Instead, gravity itself produces phantom-like acceleration. Sahni and Shtanov [3] showed that a class of braneworld models exhibited effective phantom behaviour. In these models, the 4-dimensional brane universe contains only matter and a cosmological constant Λ.…”

“…At late times, gravity leakage screens the cosmological constant, leading to an effective dark energy [3,5] …”