Cosmological acceleration and gravitational collapse

The acceleration parameter defined through the local volume expansion is negative for a pressureless, irrotational fluid with positive energy density. In the presence of inhomogeneities or anisotropies the volume expansion rate results from averaging over various directions. On the other hand, the observation of light from a certain source in the sky provides information on the expansion along the direction to that source. If there are preferred directions in the underlying geometry one can define several expansion parameters. We provide such definitions for the case of the Tolman–Bondi metric. We then examine the effect of a localized inhomogeneity on the surrounding cosmological fluid. Our framework is similar in spirit to the model of spherical collapse. For an observer in the vicinity of a central overdensity, the perceived local evolution is consistent with acceleration in the direction towards the centre of the overdensity, and deceleration perpendicularly to it. A negative mass leads to deceleration along the radial direction, and acceleration perpendicularly to it. If the observer is located at the centre of an overdensity the null geodesics are radial. The form of the luminosity distance as a function of the redshift is consistent with acceleration for a certain range of redshifts.