Constraints on an Anisotropic Universe
Physical Review D(2024)
Abstract
We analyze the possibility of global anisotropy of the universe. We consider
an altered FLRW metric in which there are different scale factors along the 3
different axes of space. We construct the corresponding altered Friedmann
equations. We show that any initial anisotropies decrease into the future. At
late times, the difference in Hubble parameters changes as 1/√(t) in a
radiation dominated era and as 1/t in a matter dominated era. We use
constraints from Big Bang Nucleosynthesis and the Cosmic Microwave Background
to constrain the level of anisotropies at early times. We also examine how the
approach back in time to the singularity is radically altered; happening much
more abruptly, as a function of density, in an anisotropic universe. We also
mention improved bounds that can arise from measurements of primordial
gravitons, WIMPs, and neutrinos.
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