Dr David Wiltshire (Department of Physics and Astronomy, University of Canterbury, Christchurch, NZ)

Averaging in Cosmology Without Dark Energy - Dr David Wiltshire Colloquium

AAO Lecture Theatre

Abstract

I will present a possible alternative to "dark energy": a model
universe with only clumped matter at the present epoch, consistent with general relativity and primordial inflation. It is based on a new solution to the problem of averaging lumpy geometries in cosmology.

The observed universe has an inhomogeneous structure of galaxy
clusters on bubble walls surrounding voids. Despite being inhomogeneous, the growth of initial perturbations from primordial inflation provides a particular self-similar fractal structure to this inhomogeneity. An observer at an average spatial position finds herself in a void, and I argue that the clocks of such a comoving void "observer" differ from those of the average galaxies in the gravitational wells of bubble walls. Nonetheless the scale-invariant nature of the perturbations generated by inflation means that there is a homogeneous cosmic time in average galaxies, defined by the epoch at which they broke from the Hubble flow.

This understanding would mean a recalibration of cosmic clocks and
rulers, which I quantify, with interesting consequences. The age of the Universe is 15 ± 1 billion years. The universe is older at high
redshifts than is conventionally assumed, consistent with the
existence of complex galaxies at such redshifts, which is otherwise
problematic. The fraction of baryonic matter predicted by primordial nucleosynthesis bounds is three times larger than is conventionally assumed, significantly changing the issues associated with dark matter.

A statistical analysis of type Ia supernova data shows that the model
fits comfortably. Although the standard "dark energy" model with a
cosmological constant wins on a Bayesian analysis, the present
model is the first approximation in averaging, and further anisotropic
corrections are expected. This makes the present "fractal bubble
model" a serious candidate to describe the Universe in which we live.