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-LemaˆıtreRobertson-Walker (FLRW) cosmologies as the spacetime metric, and culminates in the successful and highly predictive Λ-Cold-Dark-Matter (ΛCDM)
model. Yet, tensions have emerged within the ΛCDM model, most notably a
statistically significant discrepancy in the value of the Hubble constant, H0.
Since the notion of cosmic expansion determined by a single parameter is intimately tied to the CP, implications of the H0 tension may extend beyond ΛCDM
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.
