What do very nearly flat detectable cosmic topologies look like?

Abstract

Recent studies of the detectability of the cosmic topology of nearly flat universes have often concentrated on the range of values of Ω0 given by current observations. Here we study the consequences of taking a range of bounds satisfying |Ω0 − 1| ≪ 1, which include those expected from future observations such as the Planck mission, as well as those predicted by inflationary models. We show that in this limit, a generic detectable non-flat manifold is locally indistinguishable from either a cylindrical or toroidal manifold, irrespective of its global shape, with the former being more likely. Importantly, this is compatible with some recent indications of the alignment of the quadrupole and octupole moments, based on the analysis of the first year WMAP data. It also implies that in this limit an observer would not be able to distinguish topologically whether the universe is spherical, hyperbolic or flat. By severely restricting the expected topological signatures of detectable isometries, our results provide an effective theoretical framework for interpreting cosmological observations, and can be used to confine the parameter spaces which realistic search strategies, such as the ‘circles in the sky’ method, need to concentrate on.