Observational Constraints on Cosmology from the Modified Friedmann Equation

Abstract

Recent measurements of Type Ia supernovae, as well as other concordant observations, suggest that the expansion of our universe is accelerating. A dark energy component has usually been invoked as the most feasible mechanism for the acceleration. However, effects arising from possible extra dimensions can mimic dark energy through a modified Friedmann equation. In this work, we investigate some observational constraints on a scenario in which this modification is given by H 2 = (8π G /3)(ρ + C ρ n ). We mainly focus our attention on the constraints from recent measurements of the dimensionless coordinate distances to Type Ia supernovae and Fanaroff-Riley Type IIb radio galaxies compiled by Daly & Djorgovski and the X-ray gas mass fractions in clusters of galaxies published by Allen et al. We obtain the confidence region on the power index n and the density parameter Ω m of the universe from a combined analysis of these databases. We find that n = 0.06 ##IMG## [http://ej.iop.org/images/0004-637X/603/2/365/img1.gif] {img1.gif} and Ω m = 0.30 ##IMG## [http://ej.iop.org/images/0004-637X/603/2/365/img2.gif] {img2.gif} , at the 95.4% confidence level, which is consistent within the errors with the standard ΛCDM model. These parameter ranges give a universe whose expansion switches from deceleration to acceleration at a redshift between 0.52 to 0.73.