Is the Observable Universe Consistent with the Cosmological Principle?

TL;DR

This review examines observational evidence questioning the Cosmological Principle, discussing potential deviations from isotropy and homogeneity, and considers whether current tensions in cosmological measurements challenge the foundational assumptions of standard cosmology.

Contribution

It synthesizes current observational hints and debates about potential violations of the Cosmological Principle, highlighting the need for further investigation beyond the standard FLRW framework.

Findings

Hints of anisotropies and inhomogeneities in cosmological data

Discrepancies in cosmic dipoles and quasar alignments

Possible implications for the validity of the FLRW paradigm

Abstract

The Cosmological Principle (CP) -- the notion that the Universe is spatially isotropic and homogeneous on large scales -- underlies a century of progress in cosmology. It is conventionally formulated through the Friedmann-Lema\^itre-Robertson-Walker (FLRW) cosmologies as the spacetime metric, and culminates in the successful and highly predictive $\Lambda$-Cold-Dark-Matter ($\Lambda$CDM) model. Yet, tensions have emerged within the $\Lambda$CDM model, most notably a statistically significant discrepancy in the value of the Hubble constant, $H_0$. Since the notion of cosmic expansion determined by a single parameter is intimately tied to the CP, implications of the $H_0$ tension may extend beyond $\Lambda$CDM to the CP itself. This review surveys current observational hints for deviations from the expectations of the CP, highlighting synergies and disagreements that warrant further study. Setting aside the debate about individual large structures, potential deviations from the CP include variations of cosmological parameters on the sky, discrepancies in the cosmic dipoles, and mysterious alignments in quasar polarizations and galaxy spins. While it is possible that a host of observational systematics are impacting results, it is equally plausible that precision cosmology may have outgrown the FLRW paradigm, an extremely pragmatic but non-fundamental symmetry assumption.