Is matter not expanding together with the rest of the universe?

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Discussion Overview

The discussion revolves around the nature of cosmic expansion and its effects on matter within the universe. Participants explore analogies, such as the balloon analogy, to understand whether matter expands along with the universe or remains unaffected by this expansion. The conversation touches on concepts of gravity, local versus cosmic scales, and the dynamics of galaxy interactions.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants compare the universe's expansion to blowing up a balloon, questioning whether matter (represented by ink dots or pennies) expands with the universe or remains unchanged.
  • There is a suggestion that gravity might be extraneous to the expansion, raising questions about the role of gravity in local versus cosmic scales.
  • One participant proposes that to halt local expansion in a galaxy, gravity must counteract the expansion force, leading to inquiries about the conditions under which galaxies can collide despite the overall expansion of the universe.
  • Another participant emphasizes that the Friedmann metric governs expansion and may not apply effectively at smaller scales, such as within galaxies.
  • Concerns are raised about the relative strength of gravitational forces compared to the expansion of the universe, with some arguing that the expansion force is significantly weaker than gravity.
  • Clarifications are sought regarding the definition of "local" in the context of gravitationally bound clusters of galaxies and how this relates to objects in intergalactic space.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between cosmic expansion and local gravitational effects. There is no consensus on whether matter expands with the universe or remains static, and the discussion includes multiple competing perspectives on the role of gravity and the nature of local versus cosmic scales.

Contextual Notes

Participants highlight limitations in applying cosmic expansion models to local scales, indicating that assumptions about gravitational binding and the nature of forces at play may vary significantly across different contexts.

TVI_1405
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I was wondering, if the inflation of the universe can be compared to blowing up a balloon with, say, some ink dots on it representing matter, then, as the balloon grows in size, the ink dots should be getting bigger too since they are part of the same fabric. It does not seem to be the case in the real universe...or is it? Is it only the empty space that is expanding and what would we consider empty in this case?
 
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TVI_1405 said:
I was wondering, if the inflation of the universe can be compared to blowing up a balloon with, say, some ink dots on it representing matter, then, as the balloon grows in size, the ink dots should be getting bigger too since they are part of the same fabric. It does not seem to be the case in the real universe...or is it? Is it only the empty space that is expanding and what would we consider empty in this case?

See this:

www.phinds.com/balloonanalogy
 
Thanks phinds, that's a very good link. However, I was thinking about the pennies glued on the balloon analogy. Pennies are extraneous to the balloon objects, i.e. not part of the balloon fabric (hence, they don't change their size during the balloon inflation). Would it mean that gravity is also somehow extraneous to the expansion as it is so effective in keeping objects together? Or would it be matter and its properties that are extraneous?
 
I was also thinking, in order to stop the expansion of the universe locally (say, in one galaxy), it would require the force of gravity to be equal to that of the expansion... It also means that if hypothetically, someone was in an intergalactic space where gravity would have little effect, we would expect that person to expand rapidly in relation to the objects in the galaxy...?

Also, I am curious why would gallaxies collide if the universe is expanding? As far as I understand, it happens due to so-to-say local gravity that exists between the two gallaxies...which means the value of the local gravity should be higher than that of the universe expansion (going against the expanstion). Where could we get such a strong force from in a closed system of the universe?

Please, could anyone explain it to me? I am not a physicist but I am really curious about these things... :)
 
Expansion is governed by the Friedmann metric, it is inappropriate to apply it at small scales - like galaxies.
 
TVI_1405 said:
Thanks phinds, that's a very good link. However, I was thinking about the pennies glued on the balloon analogy. Pennies are extraneous to the balloon objects, i.e. not part of the balloon fabric (hence, they don't change their size during the balloon inflation). Would it mean that gravity is also somehow extraneous to the expansion as it is so effective in keeping objects together? Or would it be matter and its properties that are extraneous?

This is answered in the link.
 
TVI_1405 said:
I was also thinking, in order to stop the expansion of the universe locally (say, in one galaxy), it would require the force of gravity to be equal to that of the expansion... It also means that if hypothetically, someone was in an intergalactic space where gravity would have little effect, we would expect that person to expand rapidly in relation to the objects in the galaxy...?

Also, I am curious why would gallaxies collide if the universe is expanding? As far as I understand, it happens due to so-to-say local gravity that exists between the two gallaxies...which means the value of the local gravity should be higher than that of the universe expansion (going against the expanstion). Where could we get such a strong force from in a closed system of the universe?

Please, could anyone explain it to me? I am not a physicist but I am really curious about these things... :)

Local clusters are gravitationally bound and some galaxies are moving towards each other locally and will eventually collide/merge

You don't seem to "get" that the force of the expansionion of the universe is STAGGERINGLY weak on local scales. MANY MANY orders of magniture weaker than gravity, to say nothing of the strong nuclear force.
 
phinds said:
Local clusters are gravitationally bound and some galaxies are moving towards each other locally and will eventually collide/merge

You don't seem to "get" that the force of the expansionion of the universe is STAGGERINGLY weak on local scales. MANY MANY orders of magniture weaker than gravity, to say nothing of the strong nuclear force.

Could you define "local", please? So, the object in the intergalactic space will not expand due to the strong nuclear force?
 
TVI_1405 said:
Could you define "local", please? So, the object in the intergalactic space will not expand due to the strong nuclear force?

"Local" in this context means a gravitationally bound cluster of galaxies.

That is correct, a ball bearing in intergalactic space will be totally unaffected by the expansion of the universe, other than that everything will keep receeding from it.
 

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