Consequences of using a smooth cosmic distance in a lumpy universe: I

TL;DR

This paper investigates how fitting idealized cosmological models to observations in a lumpy universe affects parameter estimates, revealing environmental contamination and discrepancies in Hubble rate measurements from different methods.

Contribution

It introduces a method to analyze the impact of inhomogeneities on cosmological parameter inference using BAO and supernova data, highlighting environmental effects on the Hubble rate.

Findings

Hubble rate from Alcock-Paczyński monopole is in tension with the distance-redshift Hubble rate.

Environmental contamination affects the Hubble rate inferred from the Alcock-Paczyński parameters.

Discrepancy of about 9-12% in Hubble rate when environmental modes are restricted to the Hubble flow.

Abstract

How do we appropriately fit a model based on an idealised Friedmann-Lema\^{}tre Robertson-Walker spacetime to observations made from a single location in a lumpy Universe? We address this question for surveys that measure the imprints of the baryon acoustic oscillation in galaxy distribution and the peak apparent magnitude of the Type Ia supernova. These observables are related to the cosmological model through the Alcock-Paczy\'{n}ski parameters and the distance-redshift relation. Using the corresponding inhomogeneous spacetime expressions of these as observed data, we perform a parameter inference assuming that the background Friedmann-Lema\^{i}tre Robertson-Walker model is the correct model of the Universe. This process allows us to estimate the best fit Hubble rate and the deceleration parameter. We find that the inferred Hubble rate from the monopole of the Alcock-Paczy\'{n}ski parameters is in tension with the Hubble rate determined using the distance-redshift relation. The latter gives the best fit Hubble rate for the cosmological expansion. The constraint on the Hubble rate from the Alcock-Paczy\'{n}ski parameters is contaminated by the environment. When the environmental contribution is restricted to modes in the Hubble flow, we find about (9-12)\% discrepancy in the Hubble rate. Finally, we comment on the insufficiency of the method of cosmography in constraining the deceleration parameter.