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staedtler
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- TL;DR Summary
- Time Dilation
Shouldn't we question why cosmic voids are expanding but not galaxies?
Shouldn't the time dilation in cosmic voids be immense?
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In summary, galaxies are gravitationally bound while cosmic voids are not, and it's not clear why the universe is expanding but not the volumes inside galaxies. It's also worth noting that momentum is relative and galaxies that are moving apart in the universe are still gaining momentum relative to each other. Time dilation is not a meaningful concept in a spacetime that is not stationary, and Einstein was not wrong about spacetime.
- #2
berkeman
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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.
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.
- #3
staedtler
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It isn't common knowledge that the universe is expanding ..but not the volumes inside galaxies?
- #4
berkeman
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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. 
- #5
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PeterDonis
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staedtler said:
Sure, and the answer is that galaxies are gravitationally bound, while cosmic voids are not.
- #7
PeterDonis
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staedtler said:
Talking about "volumes" expanding is meaningless. "Expansion" means things are moving apart. That's all it means.
- #8
staedtler
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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.
- #9
PeterDonis
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staedtler said:
"Volumes" aren't things. A much better term would be "galaxies", or names for similar objects.
staedtler said:
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.
staedtler said:
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.
- #10
staedtler
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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.
The Local Group is not expanding.
- #11
PeterDonis
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staedtler said:
Not in the least. Why would you think that?
staedtler said:
Yes. For example, our galaxy and the Andromeda galaxy.
staedtler said:
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".
- #12
staedtler
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Fine with me, cosmic voids are expanding though.
- #13
PeterDonis
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staedtler said:
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.
- #14
staedtler
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huh? something has to be expanding ..and the cosmic voids are the only areas that make sense.
- #15
staedtler
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I suspect a certian amount of mass in necessary to enact spacetime. Cosmic voids don't have it, so they go bananas.
- #16
PeterDonis
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staedtler said:
I already told you what "expanding" means: it means galaxy clusters are moving apart. That's all it means.
staedtler said:
No, they're an area that doesn't make sense. See above.
staedtler said:
Please review the PF rules on personal speculation.
Also, since you have refused to provide any references despite repeated requests, this thread is closed.
1. What is time dilation in cosmic voids?
Time dilation is a phenomenon in which time appears to pass slower for an observer in a region with a strong gravitational field or at high speeds. This is due to the curvature of spacetime, as described by Einstein's theory of general relativity.
2. Why is time dilation expected to be immense in cosmic voids?
Cosmic voids are vast regions of space that contain very few galaxies and have a low density of matter. According to general relativity, the curvature of spacetime is directly related to the distribution of matter. In cosmic voids, where there is very little matter, the curvature of spacetime is expected to be extremely low, resulting in significant time dilation.
3. How does time dilation in cosmic voids affect our perception of time?
For an observer in a cosmic void, time would appear to pass slower compared to someone in a region with a higher density of matter. This means that events in the void would seem to take longer to occur from an outside perspective. For example, a clock in a cosmic void would appear to tick slower than a clock on Earth.
4. Is time dilation in cosmic voids measurable?
Yes, time dilation in cosmic voids can be measured through various methods, such as using precision clocks or observing the redshift of light from distant galaxies. These measurements have confirmed the existence of time dilation in cosmic voids and have provided valuable insights into the structure of the universe.
5. How does time dilation in cosmic voids impact our understanding of the universe?
The presence of significant time dilation in cosmic voids challenges our current understanding of the universe and its evolution. It suggests that the distribution of matter and the curvature of spacetime may be more complex than previously thought. Further research and observations in cosmic voids can help us gain a deeper understanding of the fundamental principles of the universe.
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