Fredagskollokvium: Iain Brown: Cosmology's Hidden Secret

Cosmology is the theory of the universe on the largest scales. Based on general relativity and assumptions of average homogeneity and isotropy, the standard model of cosmology has become an impressive success, fitting almost all the observable data with very few parameters. However, this model contains "dark energy" and "dark matter", which make up 95% of the matter in the universe and are entirely mysterious. Examining the assumptions that lead to the standard model reveals repeated use of the word "average". Unfortunately for cosmology, averages cannot be defined in general relativity. We examine some aspects of this issue and some possibly consequences for cosmology.