<p>Consider the limit of the observable universe.</p>
<p>Assume we received and analysed a photon coming from a distant region close to the edge of the observable universe.</p>
<p>If we believed  - based on energy measurement - that the photon we encountered was a result of annihilation of a particle with known mass, Would we not be right in assuming that another photon had entered space outside our observable universe, thereby giving us a measure of knowledge about this unobservable volume of space?</p>

CC BY-SA 4.0
2024-02-15T20:20:50.707Z
You're right that the light would spread in all directions, but the event still occurred within the observable universe - we didn't actually see anything play out beyond that limit, so nothing that was measured was past that boundary.

But the universe is thought to be isotropic, it is expected to be the same anywhere - the "observable universe" is relative to position. For example, from the distant galaxy GN-Z11, our own Milky Way would be an extremely high-redshift object, nearly out of its own "observable universe". I hope I understood your question correctly, please tell me if I didn't.
4NT4R3S
CC BY-SA 4.0
2024-02-19T17:18:30.390Z
You rightly point out that we don't measure anything outside the boundary. However we do know something, although it is a  modest knowledge. What we know is that a photon with known polarity is on its way through space outside the border.
Mikael Jensen
CC BY-SA 4.0
2024-02-19T18:45:34.097Z
That's true that such particles would have reached that area of space, but we don't know what happened to the photon or how it behaved after that point - there's not any data. Though as mentioned, the unobservable universe is thought to be similar to what we are familiar with, there would not be any reason for the light to act differently than from what we would gather from our angle.
4NT4R3S
<cosmological-horizon><quantum-mechanics>
Entanglement in- and outside the observable universe
Mikael Jensen
# Entanglement in- and outside the observable universe

(Asked by: username_0 on 2024-02-15)

Consider the limit of the observable universe.

Assume we received and analysed a photon coming from a distant region close to the edge of the observable universe.

If we believed - based on energy measurement - that the photon we encountered was a result of annihilation of a particle with known mass, Would we not be right in assuming that another photon had entered space outside our observable universe, thereby giving us a measure of knowledge about this unobservable volume of space?

**Tags:** cosmological-horizon, quantum-mechanics
> **Comments:**
> **username_1**: You're right that the light would spread in all directions, but the event still occurred within the observable universe - we didn't actually see anything play out beyond that limit, so nothing that was measured was past that boundary.  But the universe is thought to be isotropic, it is expected to be the same anywhere - the "observable universe" is relative to position. For example, from the distant galaxy GN-Z11, our own Milky Way would be an extremely high-redshift object, nearly out of its own "observable universe". I hope I understood your question correctly, please tell me if I didn't.  
> **username_0**: You rightly point out that we don't measure anything outside the boundary. However we do know something, although it is a  modest knowledge. What we know is that a photon with known polarity is on its way through space outside the border.  
> **username_1**: That's true that such particles would have reached that area of space, but we don't know what happened to the photon or how it behaved after that point - there's not any data. Though as mentioned, the unobservable universe is thought to be similar to what we are familiar with, there would not be any reason for the light to act differently than from what we would gather from our angle.  

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