Shouldn't the time dilation in cosmic voids be immense?

[staedtler]

TL;DR

Time Dilation

Shouldn't we question why cosmic voids are expanding but not galaxies?

 

[berkeman]

Welcome to the PF.

We require links for such questions. Can you post a couple of links to what you have been reading about this, so we can understand the context of your question better? Thank you very much.

 

[staedtler]

It isn't common knowledge that the universe is expanding ..but not the volumes inside galaxies?

 

[berkeman]

Sort of. You should be able to find some applicable links, no? Instead you are making us guess. You've marked the level of this thread at Basic (thank you), so if you are having trouble finding good links to start the discussion, I can help.

 

[berkeman]

Also paging @phinds who can likely help with good links...

 

[PeterDonis]

Shouldn't we question why cosmic voids are expanding but not galaxies?

Sure, and the answer is that galaxies are gravitationally bound, while cosmic voids are not.

 

[PeterDonis]

the universe is expanding ..but not the volumes inside galaxies?

Talking about "volumes" expanding is meaningless. "Expansion" means things are moving apart. That's all it means.

 

[staedtler]

The volumes are moving apart without gaining momentum from it. So it is not meaningless. Time dilation on the extreme end seems to add more nothingness.

 

[PeterDonis]

The volumes are moving apart

"Volumes" aren't things. A much better term would be "galaxies", or names for similar objects.

without gaining momentum from it

Momentum is relative. Galaxies that are moving apart in the universe are certainly gaining momentum relative to each other, since their relative velocities are increasing.

If you think there is a sense in which galaxies are not gaining momentum, you will need to explain in more detail what you mean and why you think that.

Time dilation

Is not even a meaningful concept in a spacetime that is not stationary, i.e., that does not have time translation symmetry. The spacetime that describes the universe as a whole is not stationary, so there is not a meaningful concept of "time dilation" for it, or for objects like galaxies that are modeled as moving in it.

 

[staedtler]

Are you saying Einstein was wrong about spacetime? Some galaxies are moving towards each other ..clearly the "expansion" is not influencing them.

The Local Group is not expanding.

 

[PeterDonis]

Are you saying Einstein was wrong about spacetime?

Not in the least. Why would you think that?

Some galaxies are moving towards each other

Yes. For example, our galaxy and the Andromeda galaxy.

clearly the "expansion" is not influencing them.

Yes, because those galaxies are part of a larger gravitationally bound system. In the case of our galaxy and the Andromeda galaxy, both are part of the Local Group, which is gravitationally bound, and is (AFAIK) also believed to be gravitationally bound to a larger cluster of galaxies.

So a better term for the things whose moving apart, on average, is what we call the "expansion of the universe" would be "galaxy clusters" (or perhaps "superclusters", or even "supercluster complexes" in some cases), not "galaxies".

 

[staedtler]

Fine with me, cosmic voids are expanding though.

 

[PeterDonis]

cosmic voids are expanding though

Please give a reference, as you have already been asked to do. As you state it, this makes no sense, since a void, consisting of nothing, can't "expand". The fact that you often see talk like this in pop science sources does not mean it's correct.

 

[staedtler]

huh? something has to be expanding ..and the cosmic voids are the only areas that make sense.

 

[staedtler]

I suspect a certian amount of mass in necessary to enact spacetime. Cosmic voids don't have it, so they go bananas.

 

[PeterDonis]

something has to be expanding

I already told you what "expanding" means: it means galaxy clusters are moving apart. That's all it means.

the cosmic voids are the only areas that make sense

No, they're an area that doesn't make sense. See above.

I suspect a certian amount of mass in necessary to enact spacetime. Cosmic voids don't have it, so they go bananas.

Please review the PF rules on personal speculation.

Also, since you have refused to provide any references despite repeated requests, this thread is closed.