Extending the LCDM model through shear-free anisotropies

TL;DR

This paper explores shear-free anisotropic cosmological models that violate the cosmological principle but preserve CMB isotropy, showing they are viable alternatives for large-scale universe evolution with distinct CMB signatures.

Contribution

It introduces shear-free anisotropic models as attractors in cosmology, providing a new framework consistent with CMB isotropy and analyzing their perturbations and observational signatures.

Findings

Shear-free models are attractors in anisotropic cosmologies.

These models produce a Sachs-Wolfe effect similar to standard cosmology.

Distinct CMB temperature spectrum signatures are predicted.

Abstract

If the spacetime metric has anisotropic spatial curvature, one can afford to expand the universe isotropically, provided that the energy-momentum tensor satisfy a certain con- straint. This leads to the so-called shear-free metrics, which have the interesting property of violating the cosmological principle while still preserving the isotropy of the cosmic mi- crowave background (CMB) radiation. In this work we show that shear-free cosmologies correspond to an attractor solution in the space of models with anisotropic spatial curva- ture. Through a rigorous definition of linear perturbation theory in these spacetimes, we show that shear-free models represent a viable alternative to describe the large-scale evo- lution of the universe, leading, in particular, to a kinematically equivalent Sachs-Wolfe effect. Alternatively, we discuss some specific signatures that shear-free models would imprint on the temperature spectrum of CMB.