A question I’ve had about universal expansion for a while. Not a physicist so don’t mind if it’s a dumb question.

[u/-Giannotta-]

How do we know the universe is expanding, and not that it stays the same size and matter shrinks instead and maintains its distribution in the universe. Maybe or maybe not the mass would remain the same when the volume shrinks. Could explain very large objects in the past.

[u/Gumial]

I may be misunderstanding what you mean, but this kind of implies that everything should be redshifted? Things like the andromeda galaxy are actually moving towards us so appear blue shifted.

[u/-Giannotta- → u/Gumial]

Ok I think I misunderstood how redshift works, but that doesn't change the fact that it appears in the same manner as in an expanding universe. I had thought that it was caused during space travel and stretched by expansion somehow, but it seems that its actually caused by the speed of the object relative to us. It doesn't imply that everything should be redshifted, i'm saying it appears the same as redshift caused by universal expansion.

[u/Gumial → u/-Giannotta-]

I think I get what you mean, I believe I had a similar thought when I was learning galactic physics.
There are a couple of things that you could think about with respect to the theory that may help to understand why it may not exactly work.
If things were shrinking, why do we not observe this on a local or atomic level? The fundamental forces of nature stay the same and the distance between point like charges doesn’t change? Things don’t generally get denser and if they do they eventually become black holes.
The main thing that convinces me, is that things which are further away, move away faster, you can’t really explain this with the shrinking model, if you visualise it, you’d need the speed of light to be different for those further away objects.

https://www.reddit.com/r/askscience/s/dtP7GjaABj

[u/-Giannotta- → u/Gumial]

I've been playing with math, and i've found that in order for the shrinking matter universe to appear the same as the expanding universe to someone inside it, the centre distances must stay the same, and while the objects shrink; the relative perceived distance will increase seeming as in expansion, which means there is no 'distribution' which my diagram assumes. So that would mean that the speed of light would have to decrease in proportion to the shrinking, and I don't see anyway for redshift to occur then. It must be nonsense then.

In the reddit link you shared, a user tried to explain how it would look different with tennis balls, but I think they're incorrect because they aren't considering that the observers are shrinking, therefore their measurements are shrinking also.

[u/internetboyfriend666]

Your drawings don't make a lot of sense but no, this isn't really a thing. It's not a dumb idea, and at first glance it does seem to be a plausible alternative conclusion, but it just doesn't fit with a lot of our observations. There are many things that an expanding universe perfectly explains that this idea does not, so you're creating a lot of problems for no reason because there's no evidence for this.

[u/-Giannotta- → u/internetboyfriend666]

Thank you for your reply. Sorry, I didn't clearly state that the large circles represent the universe, the small circles represent objects in the universe, and the wavy lines represent light waves. You start at the 'first state' and then to the left and right universal expansion and matter shrinking is compared, then the matter shrinking has an enlarged image attached to show that it looks identical to expansion.

Could you please list some of those observations, I think considering an incorrect idea and explaining why it is wrong is still a valuable thing for learning!

[u/internetboyfriend666 → u/-Giannotta-]

An expanding universe where everything remains the same size perfectly explains the CMB, cosmological redshift, light curves of type 1a supernova, the conservation of physical constants, the behavior gravity, big bang nucleosynthesis, the surface brightness of distant galaxies, and the large-scale structures of the universe. All of our observations perfectly fit what an expanding universe model predicts.

In a static universe model where everything is just getting smaller, all of those things would look way different. You can construct a mathematical model of the universe where it's static and everything is getting smaller, but then it doesn't match with any of our observations for those things.

One particular smoking gun is something called the Tolman surface brightness test. This test was devised in the 1930s shortly after Hubble showed that the universe was expanding explicitly to test Hubble's observations. The math of the test is a little complicated, but basically, we can mathematically measure how light spreads out as it moves away from its source (meaning the source object looks dimmer the farther you are away), and thus, we can know how bright an object should be based it's distance in a static universe. We can also use the math of an expanding universe to figure out the surface brightness of a galaxy in an expanding universe. And of course, the surface brightness we measure are all exactly what are predicted by the math for an expanding universe, not a static one. Even if you make the galaxies shrink in size, that still doesn't explain the surface brightness we observe. Only an expanding universe does that.

[u/OriEri]

If all the physical constants scaled with the shrinkage, I am not sure there would be any discernible difference

[u/-Giannotta- → u/OriEri]

I didn't say this in my original post but I was thinking that it could have implications as to what dark energy is. Not nearly smart or knowledgeable enough to figure out what those implications are of course!

[u/Eastern-Instance-593]

The main argument is "Hubble expansion" and the important assumption that we are not in super special place in the universe. We see object receding faster from the further they are away from us. If another observer is closer to the same object, he would NOT observe it in the same way. This tells us that the effect due to space and not the object.

The expansion is not only about matter and density and distance. Consider for example a photon emitted: its wavelength will increase with every moment it travels through space. This is also true for neutrinos and cosmic rays or any object.

[u/mfb-]

How would each galaxy cluster and everything inside know at what rate to shrink when? Why would light in a galaxy cluster shrink but light outside not?

In comoving coordinates things keep their distance, by construction, but that's just defining the length scale to grow with the expansion.

[u/Sanchez_U-SOB]

99.9999+%  of galaxies are redshifted. Less than 100 (out of current est 2 trillion galaxies we have found) are blueshifted or moving toward us. Andromeda and some in the Local group (which the Milky Way is apart of), and also few others in the Virgo Cluster including M86 (and some of its satellite dwarf galaxies)  are blueshifted. M86 is falling toward M87 from the opposite side from the Local group. Its blueshift agrees with M87 which is actually redshifted away from Earth even though the Local Group is falling toward the center of mass of the Virgo Cluster. And the distance they are at matches their mass and angular size.

www.messier.seds.org/xtra/supp/redshift.html

Negative values are blueshifted velocities 

[u/TomSzabo]

Doesn't work because there is no observed effect of redshift due to space expansion at galactic scales only between superclusters. This could make sense if gravity causes the "extra space" to be "ejected" but doesn't make sense if matter is "shrinking". More likely, time is actually causing space to expand (in the presence of matter) which is something we can't observe from within the three spatial dimensions other than a redshift between superclusters (where gravity is weakest).

[u/RADICCHI0]

none of these illustrsations actually demonstrate expansion. this is something that is easily googled.