2.3. Dark energy
While the standard cosmological model given above features a cosmological constant, in order to explain observations indicating that the Universe is presently accelerating, further possibilities exist under the general heading dark energy. 2 Unfortunately this is rather a misnomer, as it is the negative pressure of this material, rather than its energy, that is responsible for giving the acceleration. A particularly attractive possibility (usually called quintessence, though that word is used with various different meanings in the literature) is that a scalar field is responsible, with the mechanism mimicking that of early Universe inflation . As described by Olive and Peacock, a fairly model-independent description of dark energy can be given just using the equation of state parameter w, with w = - 1 corresponding to a cosmological constant. In general, the function w could itself vary with redshift, though practical experiments devised so far would be sensitive primarily to some average value weighted over recent epochs. For high-precision predictions of microwave background anisotropies, it is better to use a scalar-field description in order to have a self-consistent evolution of the `sound speed' associated with the dark energy perturbations.
Present observations are consistent with a cosmological constant, but it is quite common to see w kept as a free parameter to be added to the set described in the previous section. Most, but not all, researchers assume the weak energy condition w - 1. In the future it may be necessary to use a more sophisticated parametrization of the dark energy.
2 Unfortunately this is rather a misnomer, as it is the negative pressure of this material, rather than its energy, that is responsible for giving the acceleration. Back.