Generalized LTB model with Inhomogeneous Isotropic Dark Energy: Observational Constraints

Abstract

We consider on-center and off-center observers in an inhomogeneous, spherically symmetric, isocurvature (flat) concentration of dark energy with typical size of a few Gpc. Such a concentration could be produced e.g. by a recently formed global monopole with core size that approaches the Hubble scale. In this case we would have what may be called `topological quintessence' in analogy with the well-known topological inflation. We show that the minimum comoving radius r_{0min} of such a dark energy inhomogeneity that is consistent with the Union2 Type Ia supernovae (SnIa) data at the 3σlevel is r_{0min}\simeq 1.8 Gpc. As expected, the best-fit fractional dark energy density at the center, Ω_X,in, approaches the corresponding LCDM value Ω_X,in =0.73 for large enough values of the inhomogeneity radius r_0 (r_0 > 4Gpc). Using the Union2 data, we show that the maximum allowed shift r_{obs-max} of the observer from the center of the inhomogeneity is about 0.7 r_0 which respects the Copernican principle. The model naturally predicts the existence of a preferred axis and alignment of the low CMB multipoles. However, the constraints on r_{obs-max} coming from the magnitude of the CMB dipole remain a severe challenge to the Copernican principle and lead to r_{obs-max}< 110 Mpc even for an inhomogeneity radius as large as r_0=7 Gpc.