A peek outside our Universe

Understand

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.