Abstract: Current cosmological observations, when interpreted within the framework of a homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker (FLRW) model, strongly suggest that the Universe is entering a period of accelerating expansion. This is often taken to mean that the expansion of space itself is accelerating, implying the existence of a non-zero cosmological constant, an exotic fluid with negative pressure, or even modifications to the theory of gravity. In a general, inhomogeneous spacetime, however, this is not necessarily the case.
In this talk, I define a number of possible measures ("types") of cosmic acceleration, each corresponding to a theoretical or observational procedure that has previously been used to study acceleration in cosmology. Some of the measures are purely local quantities, defined at a spacetime point, whereas others depend on null geodesics or spatial averages. While they all give identical results in the FLRW case, the measures can be very different from one another in an inhomogeneous spacetime. I demonstrate this by calculating the various measures in a handful of exact inhomogeneous relativistic solutions, and find in some cases that a subset of the measures can show acceleration while the rest appear to be decelerating! This leads to the conclusion that observations made in an inhomogeneous universe can imply acceleration without the existence of dark energy. I discuss the implications of this finding, and suggest observational tests that could in principle distinguish between the different types of acceleration.
Speaker's homepage: http://www.physics.ox.ac.uk/users/bullp/