A peek outside our Universe
Enrique Gaztanaga, Pablo Fosalba

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 , like ours, is trapped inside an event horizon, . 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.
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