LARGE-ANGLE ANISOTROPY OF THE COSMIC MICROWAVE BACKGROUND IN AN OPEN UNIVERSE

We formulate a method to calculate systematically any multipole moment of the cosmic microwave background (CMB) anisotropy in an open universe. The formulation is very general in the sense that it takes care of all the terms which contribute to the anisotropy completely and rigorously. Using the method, we evaluate the effect of negative curvature to the quadrupole moment in the three representative cosmological models (pure baryonic, cold dark matter and hot dark matter models) for adiabatic and isocurvature initial density fluctuations with power-law spectra. It is found that the generalized Sachs-Wolfe effect, especially the part involving a time derivative of the potential, dominates the quadrupole moment in almost all of the low density models, while the intrinsic photon fluctuation at decoupling contributes to the quadrupole moment a great deal for some of isocurvature low density models. Implications of the results are discussed, including constraints on cosmological models from the observed upper limit of the CMB quadrupole anisotropy.