A peek outside our Universe

TL;DR

This paper explores the possibility of a different universe outside our cosmological horizon, suggesting our universe may resemble a black hole for external observers and leaving observable imprints on the CMB sky.

Contribution

It introduces a novel model where our universe is akin to a black hole, using Israel junction conditions to analyze potential external regions and their observational signatures.

Findings

CMB maps may contain imprints from an external universe

Our universe could be a black hole for outside observers

Challenges traditional views on the primordial universe's origin

Abstract

According to general relativity (GR), a universe with a cosmological constant $\Lambda$, like ours, is trapped inside an event horizon, $r< \sqrt{3/\Lambda}$. What is outside? We show, using Israel (1967) junction conditions, that there could be a different universe outside. Our universe looks like a black hole for an outside observer. Outgoing radial null geodesics cannot escape our universe, but incoming photons can enter and leave an imprint on our CMB sky. We present a picture of such a fossil record from the analysis of CMB maps that agrees with the black hole universe predictions but challenges our understanding of the origin of the primordial universe.