What force is needed to overcome expansion?

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Discussion Overview

The discussion revolves around the forces required to counteract cosmic expansion, particularly focusing on gravitationally bound systems and the implications of the Hubble constant. Participants explore theoretical models, assumptions about gravitational forces, and the effects of cosmic expansion on different scales.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that gravitationally bound objects do not move apart with cosmic expansion, questioning the minimum gravitational force needed to prevent separation under constant acceleration of expansion.
  • There is mention of the static universe model and its relation to Einstein's cosmological constant, which some participants believe counteracts gravity's attractive effects.
  • One participant suggests that while gravitationally bound systems generally remain intact, there could be edge cases where loosely bound objects might detach due to expansion.
  • Another participant distinguishes between unaccelerated and accelerated expansion, noting that the latter behaves like a repulsive force that becomes significant at cosmological distances.
  • It is noted that the gravitational force required to prevent separation depends on the mass of the objects and their distance apart.

Areas of Agreement / Disagreement

Participants express varying views on the effects of cosmic expansion on gravitationally bound systems, with no consensus reached on the specifics of the forces involved or the implications of different models.

Contextual Notes

Participants acknowledge the complexity of the topic, with some suggesting that advanced mathematical concepts, such as Jeans Instability, may provide further insights into the dynamics of gravitational binding in the context of cosmic expansion.

rede96
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As I understand it gravitationally bound objects don’t move apart with expansion. So I was wondering, assuming constant acceleration of expansion, what is the minimum gravitational force required to keep objects from moving apart?
 
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Are you talking about static universe model.
 
RyanH42 said:
Are you talking about static universe model.

I'm not really well up on current models as I am just an interested layman. So was wondering if we assume the current Hubble constant of 70 m/s per mega parsec
 
Albert Einstein added a positive cosmological constant to his equations of general relativity to counteract the attractive effects of gravity on ordinary matter, which would otherwise cause a spatially finite universe to either collapse or expand forever.This is the property of static universe.If you need numbers I can't help you.
 
RyanH42 said:
Albert Einstein added a positive cosmological constant to his equations of general relativity to counteract the attractive effects of gravity on ordinary matter, which would otherwise cause a spatially finite universe to either collapse or expand forever.This is the property of static universe.If you need numbers I can't help you.

Ok, thanks for that. I was probably thinking more practically than that. I read that expansion does not apply any forces to objects. So if this was the case then it seemed that any force at all attracting / binding two objects would be enough for it to stop them receding with expansion. But that assumes a constant rate of acceleration.
 
In a system of objects which are gravitationally bound they will in general remain gravitationally bound and not significantly affected by cosmic scale expansion.
I can imagine however that there could be edge cases where a very loosely bound object can become detached.
After all, the extent of an objects gravitational field is infinite, (is it not?), - although it becomes insignificantly weak far from the object.
So there is no place in the universe where gravity is totally absent, just extremely weak, and weak enough so that expansion could dissociate for example a tiny outlying galaxy from a loosely bound cluster.
This is not a theory, it just seems to me as if it could be possible, but probably someone can present me with impenitrable math which refutes the possibility, and I will happily take their word for it.
 
There's a distinction between unaccelerated expansion, which as I understand it has no effect whatsoever, and a cosmological constant (leading to accelerated expansion) which is similar to a tiny repulsive force proportional to distance - the latter doesn't prevent the formation of gravitationally bound systems up to the scale of say galaxy superclusters, but it becomes significant at cosmological distances. I am not sure of this but if my recollection is correct, the distance at which this repulsion balances gravity is roughly of the same order as the Hubble radius.
 
rede96 said:
As I understand it gravitationally bound objects don’t move apart with expansion. So I was wondering, assuming constant acceleration of expansion, what is the minimum gravitational force required to keep objects from moving apart?
Depends upon how massive the objects are and how far apart they are.

Read up on Jeans Instability if you want an in-depth analysis on how this works:
https://en.wikipedia.org/wiki/Jeans_instability
 

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