A Tilt Instability in the Cosmological Principle

TL;DR

This paper reveals an instability in the FLRW cosmological model where fluid flow anisotropies can grow over time, challenging assumptions about late-time isotropy and providing new insights into cosmic dipoles and anisotropies.

Contribution

It demonstrates a tilt instability in the FLRW framework that can lead to growing anisotropies, even with a positive cosmological constant, which was previously overlooked.

Findings

Fluid flow anisotropies can grow despite acceleration.

Tilt instability is generic when w(t) approaches -1.

Implications for interpreting cosmic dipoles and anisotropies.

Abstract

We show that the Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) framework has an instability towards the growth of fluid flow anisotropies, even if the Universe is accelerating. This flow (tilt) instability in the matter sector is invisible to Cosmic No-Hair Theorem-like arguments, which typically only flag shear anisotropies in the metric. We illustrate our claims in the setting of ``dipole cosmology'', the maximally Copernican generalization of FLRW that can accommodate a flow. Simple models are sufficient to show that the cosmic flow need not track the shear, even in the presence of a positive cosmological constant. We also emphasize that the growth of the tilt hair is fairly generic if the total equation of state $w(t) \rightarrow -1$ at late times (as it does in standard cosmology), irrespective of the precise model of dark energy. The generality of our theoretical result puts various recent observational claims about late time anisotropies and cosmic dipoles in a new light.