Redshift and blueshift in expanding universe

[michelcolman]

At first, people thought the universe was stationary. This seemed logical, why would any object tend to move in one direction or the other if there was the same amount of mass in all directions? Then they noticed the redshift of distant galaxies and concluded that the universe must be expanding. Then there were different schools of thought, some saying that the expansion in a homogenous universe should continue at the same pace (all objects experiencing the same gravity from all sides) and others saying that an infinite universe should tend to slow down and maybe even reverse its expansion due to the gravity of the matter in it. For some reason (I never understood why), the second school of thought won the argument and it was concluded that the expansion should tend to slow down. Until, that is, it was noticed that very distant supernovae of known brightness and therefore known distance were showing less redshift than what was calculated, which was interpreted as "expansion used to be slower, so the expansion must be speeding up". So now everyone is looking for dark energy to explain why the expansion is speeding up. Did I get everything correct so far?

Now, just a thought that popped into my head: suppose that gravity IS trying to slow down the expansion of the universe (I still don't understand why that should be the case, but apparently a lot of smart physicists seem to be certain of this). In that case, light from very distant places would be gaining energy while coming towards us, right? This energy would be proportional to the square of the distance (the supposed gravitational field being proportional to the distance in a homogenous universe). That would cause a blueshift that decreases the redshift, especially at large distances! Couldn't that be an easier explanation than dark energy? Or am i just being completely naive and ignorant? I assume the latter, obviously, but just thought I would ask.

Thanks for any insights!

Edit: I just realized that kinetic energy would also be proportional to the square of the distance. I just solved my own question, right?

 

[phinds]

At first, people thought the universe was stationary. This seemed logical, why would any object tend to move in one direction or the other if there was the same amount of mass in all directions? Then they noticed the redshift of distant galaxies and concluded that the universe must be expanding. Then there were different schools of thought, some saying that the expansion in a homogenous universe should continue at the same pace (all objects experiencing the same gravity from all sides) and others saying that an infinite universe should tend to slow down and maybe even reverse its expansion due to the gravity of the matter in it. For some reason (I never understood why), the second school of thought won the argument and it was concluded that the expansion should tend to slow down. Until, that is, it was noticed that very distant supernovae of known brightness and therefore known distance were showing less redshift than what was calculated, which was interpreted as "expansion used to be slower, so the expansion must be speeding up". So now everyone is looking for dark energy to explain why the expansion is speeding up. Did I get everything correct so far?

far as I know, yes.

Now, just a thought that popped into my head: suppose that gravity IS trying to slow down the expansion of the universe (I still don't understand why that should be the case, but apparently a lot of smart physicists seem to be certain of this)

Of course it is. It always has been and always will.

In that case, light from very distant places would be gaining energy while coming towards us, right?

No, because although gravity is TRYING to slow down the expansion, it is not succeeding.

 

[PeterDonis]

In that case, light from very distant places would be gaining energy while coming towards us, right?

No. You can't view us as at the "bottom" of a gravity well that the light from distant objects is falling into (which is where your intuition about light "gaining energy" is coming from). The universe is homogeneous; it's the same everywhere. Your intuitions are implicitly violating that assumption (because your intuitions come from things like the gravity well of a planet or star, which is not homogeneous).