Detectability of Cosmic Topology in Almost Flat Universes

TL;DR

This paper investigates how close to flat the universe can be while still allowing the detection of its possible non-trivial topologies, showing that as the universe approaches flatness, many topologies become undetectable or can be ruled out based on current cosmological bounds.

Contribution

The study provides a detailed analysis of the detectability of cosmic topologies in universes with $oxed{ ext{Ω}_0 ext{ close to 1}}$, highlighting how observational bounds influence which topologies can be identified or excluded.

Findings

As $oxed{ ext{Ω}_0 o 1}$, many topologies become undetectable.

Non-zero bounds on $|oxed{ ext{Ω}_0 - 1}|$ can exclude certain topologies.

Current observational bounds allow some hyperbolic and spherical topologies to be detectable or ruled out.

Abstract

Recent observations suggest that the ratio of the total density to the critical density of the universe, $\Omega_0$, is likely to be very close to one, with a significant proportion of this energy being in the form of a dark component with negative pressure. Motivated by this result, we study the question of observational detection of possible non-trivial topologies in universes with $\Omega_0 \sim 1$, which include a cosmological constant. Using a number of indicators we find that as $\Omega_0 \to 1$, increasing families of possible manifolds (topologies) become either undetectable or can be excluded observationally. Furthermore, given a non-zero lower bound on $|\Omega_0 - 1|$, we can rule out families of topologies (manifolds) as possible candidates for the shape of our universe. We demonstrate these findings concretely by considering families of topologies and employing bounds on cosmological parameters from recent observations. We find that given the present bounds on cosmological parameters, there are families of both hyperbolic and spherical manifolds that remain undetectable and families that can be excluded as the shape of our universe. These results are of importance in future search strategies for the detection of the shape of our universe, given that there are an infinite number of theoretically possible topologies and that the future observations are expected to put a non-zero lower bound on $|\Omega_0 - 1|$ which is more accurate and closer to zero.