‘Acceleration’ from large-scale inhomogeneity?

As discussed in the previous chapter, the explanation of dark energy is a central preoccupation of present-day cosmology. Its presence is indicated by the apparent recent speeding up of the expansion of the universe indicated by SNIa observations, which is usually taken to be caused by quintessence or a cosmological constant, and is consistent with other observations such as those of anisotropies and large-scale structure studies. Like dark matter, its existence was discovered, not predicted. The astronomical observations are being refined in many sophisticated ways and used to confirm the acceleration data and test the equation of state of the hypothetical dark energy. Whether its density is constant or varying, its existence is a major problem for theoretical physics. It is therefore crucial to pursue the possibility of other theoretical explanations. The deduction of the existence of dark energy assumes that the universe has a RW geometry on large scales. However, the interpretation of the observations is ambiguous. They can at least in principle be accounted for without the presence of dark energy, if we allow inhomogeneity. This can contribute in one or both of two ways: locally via backreaction and associated observational effects, as discussed in the next chapter, and globally via large-scale inhomogeneity, considered in this chapter. Here we consider the possibility that what appears to be the acceleration of an FLRW universe due to dark energy, is in fact rather a manifestation of Hubble-scale inhomogeneity in a universe such as that described by the Lemaître–Tolman–Bondi (LTB) models discussed in the next section, where we are near the centre of a void.