Observational constraints from SNe Ia and Gamma-Ray Bursts on a clumpy universe

Abstract

The luminosity distance describing the effect of local inhomogeneities in the propagation of light proposed by Zeldovich-Kantowski-Dyer-Roeder (ZKDR) is tested with two probes for two distinct ranges of redshifts: supernovae Ia (SNe Ia) in 0.015 < z < 1.414 and gamma-ray bursts (GRBs) in 1.547 < z < 3.57. Our analysis is performed by a Markov Chain Monte Carlo (MCMC) code that allows us to constrain the matter density parameter Ω_m as well as the smoothness parameter $\alpha$ that measures the inhomogeneous-homogeneous rate of the cosmic fluid in a flat \LambdaCDM model. The obtained best fits are (Ω_m=0.285^{+0.019}_{-0.018}, α= 0.856^{+0.106}_{-0.176}) from SNe Ia and (Ω_m=0.259^{+0.028}_{-0.028}, α=0.587^{+0.201}_{-0.202}) from GRBs, while from the joint analysis the best fits are (Ω_m=0.284^{+0.021}_{-0.020}, α= 0.685^{+0.164}_{-0.171}) with a χ^2_{\rm red}=0.975. The value of the smoothness parameter $\alpha$ indicates a clumped universe however it does not have an impact on the amount of dark energy (cosmological constant) needed to fit observations. This result may be an indication that the Dyer-Roeder approximation does not describe in a precise form the effects of clumpiness in the expansion of the universe.

Accepted for publication in PRD, 8 pages, 2 figures

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