HET-Discovery seminar by David Wiltshire – University of Copenhagen

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HET-Discovery seminar by David Wiltshire

Title: Models of inhomogeneity and backreaction in cosmology: A status report

Abstract: The possibility of backreaction - that inhomogeneous structures on small
(< 100/h Mpc) scales change average cosmic evolution relative to a
homogeneous isotropic FLRW model - can be tested in different ways.

1. In our own < 100/h Mpc environment the assumption of a FLRW geometry can be tested model-independently. We present very strong Bayesian evidence that the standard CMB rest frame is not the frame in which the spherically averaged variation of the Hubble expansion is minimized. Using large ray tracing simulations in exact solutions of Einstein's equations we further show that observations are consistent with a 0.5% differential expansion of space on
< 70/h Mpc scales that is not included in the standard cosmology. This has
testable implications for large angle CMB anomalies.

2. On > 100/h Mpc scales detailed models of backreaction are required for cosmological tests. The Timescape Cosmology provides such a phenomenology.
We report on the status of observational tests of this model, including the fitting of the acoustic peaks in the CMB anisotropy spectrum. The Timescape Cosmology passes current tests, and can be distinguished from the FLRW
cosmology with future Euclid satellite data. In terms of the CMB we find that the effects of backreaction in the primordial plasma still need to accounted for to deal with systematic uncertainties of 8-13% in particular cosmological parameters.