Abstract

The Cosmological Principle (CP) was a major achievement of modern cosmology to turn qualitative statements into quantitative ones. It makes the strong assumption that space is globally homogeneous and isotropic. By doing so, it reduces Einstein’s Field Equations to provide highly symmetric cosmological models. The latter then enable us to interpret a very sparse amount of data and put them into a common cosmological context. Consequently, cosmologists could probe large-scale properties of the known universe starting in the 1920s. Observational techniques greatly advanced over the last century and provided myriads of data with increasing quality. So, what started as “all places in the universe are alike” – a vague but well-working formulation back then – now faces a semantic debate how to define “isotropy” and “homogeneity” in the era of statistical, data-driven inferences. Given the increased amount of observations, we may ask whether the model-driven approach of using the CP as an a priori assumption is still adequate or can be abandoned in favour of a data-based approach.

Within this setting, I will give a review on the current status of the usage and the validity of the CP in recent cosmic data evaluations based on our community white paper Aluri et al. Class.Quant.Grav. 40 (2023) 9, 094001, which jointly discusses several tensions that challenge not only our cosmological concordance model but the entire class of homogeneous and isotropic cosmologies.

Our paper (Aluri et al. 2023): https://doi.org/10.1088/1361-6382/acbefc

Our communities: https://cosmoprinciple.wordpress.com and https://cosmoversetensions.eu