Why Not Consider Closed Universes?

We consider structure formation and CMB anisotropies in a closed universe, both with and without a cosmological constant. The CMB angular power spectrum and the matter transfer function are presented, along with a discussion of their relative normalization. This represents the first full numerical evolution of density perturbations and anisotropies in a spherical geometry. We extend the likelihood function vs. Omega from the COBE 2-year data to Omega>=1. For large Omega the presence of a very steep rise in the spectrum towards low ell allows us to put an upper limit of Omega<=1.5 (95%CL) for primordial spectra with n<=1. This compares favorably with existing limits on Omega. We show that there are a range of closed models which are consistent with observational constraints while being even older than the currently popular flat models with a cosmological constant. Future constraints from degree scale CMB data may soon probe this region of parameter space. A derivation of the perturbed Einstein, fluid and Boltzmann equations for open and closed geometries is presented in an appendix.