Observing the Dimensionality of Our Parent Vacuum

TL;DR

This paper explores how a universe originating from a lower-dimensional parent vacuum could leave observable anisotropic signatures in the CMB, offering insights into the universe's dimensional history and the nature of vacua.

Contribution

It introduces the idea that anisotropic curvature from a lower-dimensional parent vacuum can produce detectable CMB signals, linking cosmological observations to vacuum dimensionality.

Findings

Anisotropic curvature affects the entire sky and can be detected through CMB correlations.

Different expansion rates of dimensions lead to observable quadrupolar anisotropy.

Isotropic curvature observations suggest a higher-dimensional parent vacuum.

Abstract

It seems generic to have vacua with lower dimensionality than ours. We consider the possibility that the observable universe originated in a transition from one of these vacua. Such a universe has anisotropic spatial curvature. This may be directly observable through its late-time effects on the CMB if the last period of slow-roll inflation was not too long. These affect the entire sky, leading to correlations which persist up to the highest CMB multipoles, thus allowing a conclusive detection above cosmic variance. Further, this anisotropic curvature causes different dimensions to expand at different rates. This leads to other potentially observable signals including a quadrupolar anisotropy in the CMB which limits the size of the curvature. Conversely, if isotropic curvature is observed it may be evidence that our parent vacuum was at least 3+1 dimensional. Such signals could reveal our history of decompactification, providing evidence for the existence of vastly different vacua.