Local Voids as the Origin of Large-angle Cosmic Microwave Background Anomalies: The Effect of a Cosmological Constant

We explore the large angular scale temperature anisotropies in the cosmic microwave background (CMB) due to homogeneous local dust-filled voids in a flat Friedmann-Robertson-Walker universe with a cosmological constant. In comparison with the equivalent dust-filled void model in the Einstein-de Sitter background, we find that the anisotropy for compensated asymptotically expanding local voids can be larger because second-order effects enhance the linear integrated Sachs-Wolfe (ISW) effect. However, for local voids that expand sufficiently faster than the asymptotic velocity of the wall, the second-order effect can suppress the fluctuation due to the linear ISW effect. A pair of quasi-linear compensated asymptotic local voids with radius (2 − 3)×102 h−1 Mpc and a matter density contrast δm ∼ −0.3 can be observed as cold spots with a temperature anisotropy ∆T/T ∼ O(10−5) that might help explain the observed large-angle CMB anomalies. We predict that the associated anisotropy in the local Hubble constant in the direction of the voids could be as large as a few percent. Subject headings: cosmic microwave background – cosmology – large scale structure 1. Introductio Recently, there has been mounting evidence that the statistical isotropy in the large-angle cosmic microwave background (CMB) anisotropy may be broken (Tegmark et al. 2003; Copi et al. 2004; Chiang et al. 2004; de Oliveira-Costa et al. 2004; Eriksen et al. 2004; Hansen et al. 2004; Larson & Wandelt 2004; Park 2004; Schwarz et al. 2004; Vielva et al. 2004; Cruz et al. 2005, 2006 ). Although the significance of any one of the findings is at most 3σ, the accumulation of anomalies hints at the possibility of the need for new physics to be added to the standard scenario (e.g., Luminet et al. 2003; Jaffe et al. 2005). 1Department of Science and Engineering, Kinki University, Higashi-Osaka, 577-8502, Japan 2University of Oxford, Department of Physics, Oxford, OX1 3RH, United Kingdom