Timothy Clifton (London):

Does Dark Energy Exist?

Abstract:

Cosmological observations, when interpreted within the framework of a homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker (FLRW) model, suggest that the Universe is entering a period of accelerating expansion. This is usually taken to mean that the expansion of space itself is accelerating. In a general spacetime, however, this is not necessarily true.

In this talk I will attempt to clarify this point by considering a handful of local and non-local measures of acceleration in three different inhomogeneous cosmological models. Each of these measures reduce to the same quantity in the limit of exact spatial homogeneity and isotropy, but in spacetimes that are only statistically homogeneous and isotropic it can be shown that the acceleration inferred from observations made over large scales does not necessarily imply that the expansion of space itself is accelerating. This demonstrates that observations made in an inhomogeneous universe can imply acceleration without the existence of dark energy.

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