Average and dispersion of the luminosity-redshift relation in the concordance model

TL;DR

This paper analyzes how inhomogeneities in the universe affect luminosity-distance measurements and their implications for dark energy parameter estimation, finding a fundamental scatter limit of about 10% due to cosmic perturbations.

Contribution

It provides a gauge-invariant averaging method to quantify the impact of stochastic perturbations on luminosity distances in the concordance model, including non-linear effects.

Findings

Inhomogeneities do not eliminate the need for dark energy.

They limit the precision of parameter determination to about 10^{-3} to 10^{-5}.

Predicted dispersion matches current observational estimates, challenging future measurements.

Abstract

Starting from the luminosity-redshift relation recently given up to second order in the Poisson gauge, we calculate the effects of the realistic stochastic background of perturbations of the so-called concordance model on the combined light-cone and ensemble average of various functions of the luminosity distance, and on their variance, as functions of redshift. We apply a gauge-invariant light-cone averaging prescription which is free from infrared and ultraviolet divergences, making our results robust with respect to changes of the corresponding cutoffs. Our main conclusions, in part already anticipated in a recent letter for the case of a perturbation spectrum computed in the linear regime, are that such inhomogeneities not only cannot avoid the need for dark energy, but also cannot prevent, in principle, the determination of its parameters down to an accuracy of order $10^{-3}-10^{-5}$, depending on the averaged observable and on the regime considered for the power spectrum. However, taking into account the appropriate corrections arising in the non-linear regime, we predict an irreducible scatter of the data approaching the 10% level which, for limited statistics, will necessarily limit the attainable precision. The predicted dispersion appears to be in good agreement with current observational estimates of the distance-modulus variance due to Doppler and lensing effects (at low and high redshifts, respectively), and represents a challenge for future precision measurements.