Redshift Dipoles from Non-Geodesic Observer Congruences in Covariant Cosmology

Abstract

Recent analyses of large-scale structure and redshift surveys have reported significant dipolar anisotropies in the local Universe that are not straightforwardly attributable to a kinematical boost. When interpreted within standard frameworks, these signals may correspond to coherent bulk flows that have been reported to exhibit tension with $\Lambda$CDM expectations. On the other hand, signals inferred from different astrophysical probes are not always consistent with the Cosmic Microwave Background (CMB) dipole, challenging the assumption of dipoles that are pure kinematical in origin. In more general cosmological scenarios, the congruence associated with cosmological observers need not be geodesic. Within a fully covariant framework, we show that a non-geodesic observer congruence introduces an additional contribution to the propagation of redshift along the past light cone, proportional to the line-of-sight projection of the observer four-acceleration. This generates an anisotropic modulation in the redshift itself, which could propagate to any observable defined in redshift space. We discuss different physical mechanisms that may produce a non-geodesic observer congruence, including interactions within the dark sector, modified theories of gravity, and relative motion between cosmological frames in tilted cosmologies.