Circularities in the empirical grounding of the cosmological principle

Abstract

The standard model of cosmology relies on the cosmological principle. This thesis critically examines the principle's status, understanding, and justifications for its implementation and evaluates its potential as a testable hypothesis. I focus on data from the Planck mission, studying the CMB, revealing circularities that prevent independent verification of the isotropy of the signal. Even assuming the isotropy of the CMB, independently demonstrating homogeneity is challenging. The issues of fitting, averaging, and idealising observations add further circularity concerns. Their impact, however, diminishes with the realisation that cosmological models describe large-scale structures and as a historical science, cosmology prioritises explanatory value over testing individual hypotheses. Despite this, validating the cosmological principle independently remains an important way of making progress, which in the case of cosmology, is best understood as happening iteratively. This suggests being optimistic about cosmology, regardless of its weaker epistemic basis compared to the experimental sciences.