A peek outside our Universe

Abstract

According to general relativity (GR), a universe with a cosmological constant $\hat{I}›$, like ours, is trapped inside an event horizon, $r<\sqrt{3/\hat{I}›}$. 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.

3 pages. Essay written for the Gravity Research Foundation 2021 Awards for Essays on Gravitation