<p>We tend to take some things for granted, for example the light of speed is constant, but what if it isn't? How can we know that light is not slowing down at great distances for example, or that light speed is constant during time and not oscillating in time?</p>
<p>We see redshift observations on galaxies at the edge of the universe and our explanation is that the galaxies are moving away from us and accelerating, thus the universe must be expanding. How do we know that the light doesn't lose velocity at those distances and that's what creates the redshift-ness.</p>
<p>As light slows down, it takes more time to come to us, which appears as being further away. If light stops completely because it loses all momentum that's the observable universe horizon, instead of space expanding faster than light.</p>
<p>It's actually the same thing, but reading it differently, keeping constant the space and making variable the light speed, instead of the other way.</p>
<p>Can you convince me somehow that's not the case in layman's terms?</p>
<p>I'm noob so please be gentle.</p>

CC BY-SA 4.0
2022-09-24T10:37:13.893Z
See: [Does Universe really expand? What if the speed of light is decreasing instead?](https://physics.stackexchange.com/questions/315038/does-universe-really-expand-what-if-the-speed-of-light-is-decreasing-instead)
GrapefruitIsAwesome
CC BY-SA 4.0
2022-09-24T10:42:45.760Z
See on Astronomy: [(Are there other proofs of the expanding universe apart from the redshift?)](https://astronomy.stackexchange.com/questions/29061/)
James K
CC BY-SA 4.0
2022-09-25T00:17:28.140Z
Does this answer your question? [Are there other proofs of the expanding universe apart from the redshift?](https://astronomy.stackexchange.com/questions/29061/are-there-other-proofs-of-the-expanding-universe-apart-from-the-redshift)
GrapefruitIsAwesome
CC BY-SA 4.0
2022-09-25T00:54:47.390Z
@Grapefruit Your proposed duplicate target does not discuss variable speed of light theories.
PM 2Ring
CC BY-SA 4.0
2022-09-25T11:22:20.867Z
@PM2Ring That seems to be true for the possible dup on Astronomy Stack Exchange.  The dup on Physics Stack Exchange seems to cover it, but it's not a "true dup" as its on a different Stack Exchange.  There's not accepted answer there, so I don't feel comfortable just grabbing from there and posting here either.
GrapefruitIsAwesome
CC BY-SA 4.0
2022-09-26T13:47:34.190Z
@GrapefruitIsAwesome [Should we avoid calling questions that have answers in other SE sites as "duplicate", which already has a specific and different meaning in SE?](https://astronomy.meta.stackexchange.com/q/867/7982) It's pretty common in these cases to do something like that; leave a short answer here and a link to a more extensive answer in a different SE site, especially common for Astronomy linking to Physics. I think most agree this is better than closing the question because for some all that's needed is the short answer, and for others they appreciate the links to the other site.
uhoh
<expansion><redshift><observable-universe><speed>
How do we know universe is expanding and not light slowing down?
Dimitris
# How do we know universe is expanding and not light slowing down?

(Asked by: username_0 on 2022-09-24)

We tend to take some things for granted, for example the light of speed is constant, but what if it isn't? How can we know that light is not slowing down at great distances for example, or that light speed is constant during time and not oscillating in time?

We see redshift observations on galaxies at the edge of the universe and our explanation is that the galaxies are moving away from us and accelerating, thus the universe must be expanding. How do we know that the light doesn't lose velocity at those distances and that's what creates the redshift-ness.

As light slows down, it takes more time to come to us, which appears as being further away. If light stops completely because it loses all momentum that's the observable universe horizon, instead of space expanding faster than light.

It's actually the same thing, but reading it differently, keeping constant the space and making variable the light speed, instead of the other way.

Can you convince me somehow that's not the case in layman's terms?

I'm noob so please be gentle.

**Tags:** expansion, redshift, observable-universe, speed
> **Comments:**
> **username_1**: See: [Does Universe really expand? What if the speed of light is decreasing instead?](https://physics.stackexchange.com/questions/315038/does-universe-really-expand-what-if-the-speed-of-light-is-decreasing-instead)  
> **username_2**: See on Astronomy: [(Are there other proofs of the expanding universe apart from the redshift?)](https://astronomy.stackexchange.com/questions/29061/)  
> **username_1**: Does this answer your question? [Are there other proofs of the expanding universe apart from the redshift?](https://astronomy.stackexchange.com/questions/29061/are-there-other-proofs-of-the-expanding-universe-apart-from-the-redshift)  
> **username_3**: @Grapefruit Your proposed duplicate target does not discuss variable speed of light theories.  
> **username_1**: @PM2Ring That seems to be true for the possible dup on Astronomy Stack Exchange.  The dup on Physics Stack Exchange seems to cover it, but it's not a "true dup" as its on a different Stack Exchange.  There's not accepted answer there, so I don't feel comfortable just grabbing from there and posting here either.  
> **username_4**: @GrapefruitIsAwesome [Should we avoid calling questions that have answers in other SE sites as "duplicate", which already has a specific and different meaning in SE?](https://astronomy.meta.stackexchange.com/q/867/7982) It's pretty common in these cases to do something like that; leave a short answer here and a link to a more extensive answer in a different SE site, especially common for Astronomy linking to Physics. I think most agree this is better than closing the question because for some all that's needed is the short answer, and for others they appreciate the links to the other site.  

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