# B Gravitationally bound system in an expanding space

Tags:
1. Oct 24, 2016

### RMV

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 cant resist to think that there would be a certain amount of energy or time that is used up to win over expansion. If so cant we use this as one method to work out the expansion of space?
(Sorry, I am a little new here!)

2. Oct 24, 2016

### hsdrop

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

3. Oct 24, 2016

### Jorrie

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.

4. Oct 24, 2016

### hsdrop

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

5. Oct 24, 2016

### RMV

Thank you !
But will there be a measurable time lag between expected events due to expansion of space?

6. Oct 24, 2016

### Fervent Freyja

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.

7. Oct 24, 2016

### Jorrie

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.

Last edited: Oct 24, 2016
8. Oct 24, 2016

### Jorrie

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.

9. Oct 24, 2016

### Jonathan Scott

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.

10. Oct 24, 2016

### newjerseyrunner

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. Oct 24, 2016

### RMV

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?

Last edited: Oct 24, 2016
12. Oct 24, 2016

### Jorrie

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:
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.