Cosmological Surprises from Braneworld models of Dark Energy

Properties of Braneworld models of dark energy are reviewed. Braneworld models admit the following interesting possibilities: (i) The effective equation of state can be w ≤ −1 as well as w ≥ −1. In the former case the expansion of the universe is well behaved at all times and the universe does not run into a future ‘Big Rip’ singularity which is usually encountered by Phantom models. (ii) For a class of Braneworld models the acceleration of the universe can be a transient phenomenon. In this case the current acceleration of the universe is sandwiched between two matter dominated epochs. Such a braneworld does not have a horizon in contrast to LCDM and most Quintessence models. (iii) For a specific set of parameter values the universe can either originate from, or end its existence in a Quiescent singularity, at which the density, pressure and Hubble parameter remain finite, while the deceleration parameter and all invariants of the Riemann tensor diverge to infinity within a finite interval of cosmic time. (iv) Braneworld models of dark energy can loiter at high redshifts: 6 < ∼ z < ∼ 40. The Hubble parameter decreases during the loitering epoch relative to its value in LCDM. As a result the age of the universe at loitering dramatically increases and this is expected to boost the formation of high redshift gravitationally bound systems such as 109M⊙ black holes at z ∼ 6 and lower-mass black holes and/or Population III stars at z > 10, whose existence could be problematic within the LCDM scenario. (v) Braneworld models with a time-like extra dimension bounce at early times thereby avoiding the initial ‘Big Bang singularity’. (vi) Both Inflation and Dark Energy can be successfully unified within a single scheme (Quintessential Inflation).