system
Given the following post, respond as if you're commenting on Reddit:
human
Instead of the Universe expanding, could matter actually be shrinking?

Would the same characteristics of the universe not apply? Everything would be moving away from us rapidly, not because space is expanding, but because all matter is actually shrinking while the fabric of space stays the same?
gpt
You can build a model where it's true (technically, it's just choosing new spatial coordinates) but then the speed of light changes...

Imagine that you cut a stick at a length of 1 meter, using the definition of the meter (the distance light travels in roughly 1/30000000 second). You measure the distance to a distant galaxy and find that it's 10^23 times your stick (that's a long and impossible method, but that's not a problem with thought experiment). Then expansion happens. If you measure the distance to the same galaxy, you will find 2.10^23 times the stick.

The first interpretation is to say: the stick is 1 meter, so the distance increased from 10^23 m to 2.10^23. Cosmological distances change but the matter is staying the same.

Your interpretation would be: the distance used to be 10^23 sticks and now it is 2.10^23, if we assume space is not expanding, it means the stick is now half a meter long. Matter is shrinking!

Up to now, both interpretations are good and you can prefer yours if you want to keep cosmological distances constant. But...

We actually have a way to make a choice between these interpretations. We can get back to the definition of the meter, to see if the stick is now 1 or 0.5 m. So we measure the time light needs to go from one end to the other and we find it's still 1/30000000 s.

So in the first interpretation the stick is always 1 m, the speed of light is always around 30000000 m/s but cosmological distances increase with expansion.
In the second interpreation matter shrinks. The speed of light varied from 30000000 sticks/second to 60000000 sticks/second but cosmological distances stay the same. A varying speed of light is not very good but why not?

Actually, cosmologists can use both interpretations. The second one is called "comoving coordinates" (because it moves with the expansion) while the first is "physical coordinates" (because they are the ones that really matter for physics, like for the speed of light).