Gravitationally bound system in an expanding space

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Discussion Overview

The discussion revolves around the effects of cosmic expansion on gravitationally bound systems, particularly focusing on galaxies and their potential mergers. Participants explore the relationship between gravitational forces and the expansion of space, questioning how these dynamics might influence measurements of cosmic expansion.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that while gravity dominates over cosmic expansion for gravitationally bound systems, there may still be energy or time considerations involved in overcoming expansion when galaxies are close enough to merge.
  • Others argue that the expansion of space occurs universally, not just between galaxies, and suggest rethinking the implications of this expansion on gravitational interactions.
  • It is noted that measuring the expansion of space is typically done through redshift observations of distant galaxies, and gravitationally bound systems do not provide information about cosmic expansion.
  • Some participants express curiosity about whether there is a measurable time lag in events due to the expansion of space, with responses indicating that this depends on the specific events being discussed.
  • One participant mentions that while mergers of gravitationally bound systems are unaffected by expansion, events at cosmological distances may exhibit time effects that are complex to define.
  • There is a discussion about the concept of a "tethered galaxy problem," which involves understanding the critical distance at which gravitational and expansion forces balance each other.

Areas of Agreement / Disagreement

Participants generally agree that gravitationally bound systems are not influenced by cosmic expansion in terms of their internal dynamics. However, multiple competing views remain regarding the implications of expansion on energy, time, and the measurement of cosmic phenomena, leaving the discussion unresolved.

Contextual Notes

Participants highlight that the relationship between gravitational forces and cosmic expansion is nuanced, with unresolved mathematical steps and dependencies on specific definitions and reference frames. The complexity of defining events at cosmological distances is also noted.

Who May Find This Useful

This discussion may be of interest to those exploring cosmology, gravitational physics, and the dynamics of galaxies, particularly in the context of cosmic expansion and its implications for measurements in astrophysics.

RMV
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I came to understand that the space between galaxies are expanding and not that they are speeding from each other and as space expands the gravitationally bound systems remain in their own relative position as the effect of gravity is more compared to the expansion of space.
But now consider two galaxies close enough that they are expected to merge over a appreciable period of time.
Even though gravity wins over expansion , I can't resist to think that there would be a certain amount of energy or time that is used up to win over expansion. If so can't we use this as one method to work out the expansion of space?
(Sorry, I am a little new here!)
 
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i had it explained to me that it not just the space between the galaxy that are expanding but space ever wair is expanding (like between adams and even sub atomic particles) so with that in mind you might want to rethink your question
 
RMV said:
Even though gravity wins over expansion , I can't resist to think that there would be a certain amount of energy or time that is used up to win over expansion. If so can't we use this as one method to work out the expansion of space?
We "work out the expansion" (measure the Hubble constant) by observing the redshift of distant galaxies and clusters. Gravitationally bound systems do not expand, so there is really no way to get expansion information from them.

#hsdrop: Similarly, spatial expansion does not work at molecular/atomic scales.
 
Jorrie said:
We "work out the expansion" (measure the Hubble constant) by observing the redshift of distant galaxies and clusters. Gravitationally bound systems do not expand, so there is really no way to get expansion information from them.

#hsdrop: Similarly, spatial expansion does not work at molecular/atomic scales.
sorry for the miss lead i was just sharing what i was told on the forum but i don't mind being set straight with the facts
 
Thank you !
But will there be a measurable time lag between expected events due to expansion of space?
 
RMV said:
I can't resist to think that there would be a certain amount of energy or time that is used up to win over expansion. If so can't we use this as one method to work out the expansion of space?
(Sorry, I am a little new here!)

The velocity of each galaxy and the distance between them probably determine whether or not they 'win over expansion' and merge. Someone say if I am wrong here.
 
RMV said:
Thank you !
But will there be a measurable time lag between expected events due to expansion of space?
It depends on what event you're talking about. Mergers are generally gravitational bound and there will be no influence from expansion. For events at cosmological distances from each other, there will be time effects, but one must specify the event coordinates exactly in some chosen reference frame. Such events may not be easy to define.
 
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Fervent Freyja said:
The velocity of each galaxy and the distance between them probably determine whether or not they 'win over expansion' and merge. Someone say if I am wrong here.
Yes, if you specify two non-gravitationally bound objects, the distance and dynamics will be the deciding factor.

PS: I'll be out for some hours; will check back later.
 
The universe is getting bigger but locally nothing interesting happens as a result of expansion (at least not on a time scale where the rate of change of expansion is negligible). If you think of the universe as being like a cone where time is the distance from the apex and space is around the cone, then there is more space the further you get from the apex. Objects which started off equally spaced remain equally spaced and hence diverge. However, locally the cone is flat, so parallel paths remain parallel and expansion does not have any physical effects.
 
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RMV said:
Thank you !
But will there be a measurable time lag between expected events due to expansion of space?
Sort of? Information traveling through a system (in this case space) lags if the system itself is expanding during the process. This is why the observable universe is now 90 billion light years, when in a non-expanding space, it'd be roughly 13.8 billion.
 
  • #11
Jorrie said:
It depends on what event you're talking about. Mergers are generally gravitational bound and there will be no influence from expansion. For events at cosmological distances from each other, there will be time effects, but one must specify the event coordinates exactly in some chosen reference frame. Such events may not be easy to define.

Thank you again!
So it is possible to see for time effects but not easy to define...has it ever been tried?
and can you please give examples of such events?
 
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  • #12
newjerseyrunner said:
Sort of? Information traveling through a system (in this case space) lags if the system itself is expanding during the process. This is why the observable universe is now 90 billion light years, when in a non-expanding space, it'd be roughly 13.8 billion.
Yes, but this is not a time lag. The light took only 13.8 billion years to get to us from the farthest parts of the observable universe.
I think we are trying to answer RMV's uncertainty:
RMV said:
Even though gravity wins over expansion , I can't resist to think that there would be a certain amount of energy or time that is used up to win over expansion.
For our accelerating expansion universe, if you consider just two galaxies in an otherwise empty region of space, and you would tether them together so that the physical (proper) distance between them do not increase, and then you cut the tether, there is a critical tether length where the accelerated expansion effect cancels the gravitational effect. Make the tether slightly shorter and they will fall towards each other; make it slightly longer and they will drift apart.

So yes, there is an effect, but in all cosmological effects, one should not think in terms of "a certain amount of energy or time that is used up to win over expansion". It is something that comes out of the expansion dynamics, as a solution of Einstein's gravitational field equations.

If you are not put off by some math, then the "Tethered galaxy problem" by Davis and Lineweaver may be of interest to you.
 
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