Spectral Representation and the Averaging Problem in Cosmology1

We investigate the averaging problem in cosmology as the problem of introducing a distance between spaces. We first introduce the spectral distance, which is a measure of closeness between spaces defined in terms of the spectra of the Laplacian. Then we define S _N, a space of all spaces equipped with the spectral distance. We argue that S _N can be regarded as a metric space and that it also possesses other desirable properties. These facts make S _N a fundamental arena for spacetime physics. Then, we apply the spectral framework to the averaging problem: We describe the model-fitting procedure in terms of the spectral representation, and also discuss how to analyze the dynamical aspects of the averaging procedure with this scheme. In these analyses, we are naturally led to the concept of the apparatus-dependent and the scale-dependent effective evolution of the universe. These observations suggest that the spectral scheme seems to be suitable for the quantitative analysis of the averaging problem in cosmology.