<p>You're right, different images of an object will generally come from different cosmological times, and if the times are different enough it may not be recognizable as the same object.</p>
<p>But this problem doesn't exist for the CMB, because all of the CMB light was emitted at the same cosmological time (about 400,000 years after the big bang). You can think of each small region of the gas that emitted the CMB as an &quot;object&quot; with a very brief lifetime. If we see two images of it, they must come from the same era of its life because there is only one era.</p>

CC BY-SA 4.0
2024-12-19T05:54:15.850Z
2024-12-19T05:54:15.850Z
benrg
<p>I'm not into physics, so these thoughts may not make sense at all:</p>
<p>I wonder whether searches for repeating patterns in the sky (or CMB) to detect a &quot;pacman&quot; geometry of the universe wouldn't have to account for different light travel times of the observed instances of the repeated object (or whether this is actual practice).</p>
<p>When we see objects multiple times, the light of each instance followed a different path, each with a different distance. For example, in such a geometry we could reach the moon &quot;straight away&quot; or we could travel in some other direction and reach the moon &quot;from behind&quot;. As a consequence, the various instances of one and the same object would look different, each snapshot showing it at a different point of time. So how can we find repeated views of the same things when they do not appear to us in an identical way?</p>

<universe><space-geometry>
Wouldn't searches for repeating patterns in the sky to detect a "pacman" geometry of the universe have to account for different light travel times?
Shakesbeer
# Wouldn't searches for repeating patterns in the sky to detect a "pacman" geometry of the universe have to account for different light travel times?

(Asked by: username_0 on 2024-12-18)

I'm not into physics, so these thoughts may not make sense at all:

I wonder whether searches for repeating patterns in the sky (or CMB) to detect a "pacman" geometry of the universe wouldn't have to account for different light travel times of the observed instances of the repeated object (or whether this is actual practice).

When we see objects multiple times, the light of each instance followed a different path, each with a different distance. For example, in such a geometry we could reach the moon "straight away" or we could travel in some other direction and reach the moon "from behind". As a consequence, the various instances of one and the same object would look different, each snapshot showing it at a different point of time. So how can we find repeated views of the same things when they do not appear to us in an identical way?

**Tags:** universe, space-geometry

---

## 1 answers

### Answer by username_1 on 2024-12-19. Score: 3 | Accepted answer

You're right, different images of an object will generally come from different cosmological times, and if the times are different enough it may not be recognizable as the same object.

But this problem doesn't exist for the CMB, because all of the CMB light was emitted at the same cosmological time (about 400,000 years after the big bang). You can think of each small region of the gas that emitted the CMB as an "object" with a very brief lifetime. If we see two images of it, they must come from the same era of its life because there is only one era.


---

