The cosmological constant and dark energy in braneworlds

Abstract

We review recent attempts to address the cosmological constant problem and the late-time acceleration of the Universe based on braneworld models. In braneworld models, the way in which the vacuum energy gravitates in the 4D spacetime is radically different from conventional 4D physics. It is possible that the vacuum energy on a brane does not curve the 4D spacetime and only affects the geometry of the extra-dimensions, offering a solution to the cosmological constant problem. We review the idea of supersymmetric large extra dimensions that could achieve this and also provide a natural candidate for a quintessence field. We also review the attempts to explain the late-time accelerated expansion of the universe from the large-distance modification of gravity based on the braneworld. We use the Dvali-Gabadadze-Porrati model to demonstrate how one can distinguish this model from dark energy models in 4D general relativity. Theoretical difficulties in this approach are also addressed.

Figures (13)

• Figure 1: A schematic picture of the braneworld. From Mar1.
• Figure 2: Constraints on Yukawa violations of the gravitational $1/r$ potential , $V(r) \propto (1/r) (1 + \alpha \exp (-r/\lambda))$. The shaded region is excluded at the 95% confidence level. From Kap.
• Figure 3: Removing a wedge from a sphere and identifying opposite sides to obtain a football geometry. Two equal-tension branes with conical deficit angles are located at either pole; outside the branes there is constant spherical curvature. From Car.
• Figure 4: Summary of the behaviour of gravity in the DGP model. At large scales $r>r_c$, the theory is 5D. On small scales $r<r_c$, gravity becomes 4D but the linearized theory is described by a Brans-Dicke theory. This affects the large scale structure (LSS) and the Integrated Sachs-Wolfe (ISW) effect and its cross-correlation to LSS. Below the Vainstein radius $r < r_*$, the theory approaches GR. This transition can be probed by weak lensing and cluster abundance as the non-linear dynamics is important for these measures. The solar system tests also provide constraints on the model in the 4D Einstein phase.
• Figure 5: Joint constraints [solid thick] on DGP models from the SNe data [solid thin], the BO measure $A$ [dotted] and the CMB shift parameter $S$ [dot-dashed]. The left plot uses SNe Gold data, the right plot uses SNLS data. The thick dashed line represents the flat models, $\Omega_K=0$. From Mar2.