Speaker
Description
Precision cosmology suggests that late-time inhomogeneities may no longer be treated as small corrections to the FLRW paradigm. Moreover, recent observational hints of axisymmetric anisotropies in the local expansion rate further motivate analyses beyond linear perturbation theory. Motivated by these issues, the framework called "Covariant Cosmography'' is adopted which describes the anisotropies in the local cosmic expansion rate within a fully relativistic framework and in a model-independent manner, free from a priori symmetry constraints on the underlying gravitational field and without resorting to linear perturbation theory approximations.
In this work, we address the intriguing evidence of an axisymmetric pattern in the angular anisotropies of the local cosmic expansion rate within the Covariant Cosmographic (CC) framework by considering the cosmological set-up of an off-center observer in a spherically symmetric Lemaître-Tolman-Bondi (LTB) spacetime. By estimating the amplitudes of the covariant cosmographic parameters—including the Hubble, deceleration, curvature, and jerk parameters, we compare it with the observed value. Moreover, we test luminosity-distance reconstructions with the CC parameters within this model. Finally, we compare the LTB relativistic distance for small inhomogeneities with the corresponding result derived from linear perturbation theory (LPT) in the standard cosmological model.
For moderate central contrasts ($\delta \leq 1$), LPT reproduces exact distances within $10\%$ for observers inside the structure. However, Covariant Cosmography (CC) extends this regime of validity up to $\delta \leq 2.5$. At larger radii, the situation reverses: for observers at three times the characteristic size, LPT remains accurate up to $\delta \leq 3$, while CC already exceeds $10\%$ error for $\delta \geq 1.5$. At sufficiently large distances from the structure, both methods converge to the exact solution.
This analysis is instrumental in interpreting expansion-rate anisotropies, facilitating investigations of the local Universe beyond the FLRW framework with a fully non-perturbative metric approach.