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I recently asked this question about whether there was a "distance" between two galaxies where both the gravitational force and the influence of dark energy would be balanced. The answers and comments seem to indicate that there is indeed such a "radius" around a galaxy.

I was very interested in this, so I contacted the authors of this paper about this phenomenon. I asked them if it could be possible to have a satellite galaxy orbiting a bigger one just in the point where there would be a balance between the gravitational attraction of the bigger galaxy and dark energy, so that the satellite galaxy orbit would not decay (through gravitational waves, tidal forces...) and avoiding its fall eventually towards the bigger galaxy.

They replied that the answer was basically yes, and that they could keep that orbit as long as there was no external perturbation modifying these orbits. But I had one more question about this scenario.

My question is:

If that balanced state would be possible, would there still be tidal effects between the two galaxies (So that some of the orbits of planets and stars inside the galaxies could be somewhat modified) but without making the orbits of the galaxies decay over time...?

I mean, imagine a satellite galaxy orbits a bigger galaxy just in the radius distance where the influence of gravity and dark energy are balanced out. Is it physically possible (at least theoretically) that the tidal forces between the galaxies may affect some of the planetary systems' orbits in these galaxies (for example changing the orbits of planets around their stars like for example making them orbit their stars further apart)?

And would these tidal forces disrupt the satellite galaxy from the distance where gravity and dark energy are balanced out? Or without any external perturbation, it should keep orbiting at that distance (even with these tidal forces between the galaxies or the gravitational waves emitted from the orbits around the bigger galaxy)?

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There would be no stable orbit at this balanced point, since there would be no centripetal force. There would be an orbit at a separation closer than this however.

I can't see that these deliberations have any influence over the tidal forces felt by one galaxy due to the other. These would be absolutely tiny, since the tidal forces diminish as the cube of the separation between the galaxies. As such the tidal force will have an utterly negligible effect on bound planetary orbits within a galaxy, and far, far smaller than the influence of the tidal field that is due to the galaxy within which they reside.

For example, the Hill radius of the Solar System - the radius at which tidal forces in our own Galaxy become equal to the gravitational pull of the Sun - is at around 2 light years from the Sun. The additional tidal field due to the Andromeda galaxy is totally negligible and this is far closer to us than the separation of the galaxies in your scenario.

Similarly, the great separation of the galaxies means that their tiny tidal influence on each other will have no significant bearing on their structures.

Would tidal effects change the overall orbit? Probably, on some ridiculously long timescale. This would be because galaxies contain dissipative gas and spin faster than the likely orbital periods of this galaxy pair. This is analogous to the situation of the Earth and the Moon and would act to increase the separation, but it can't act in such a way as to unbind the pair altogether.

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  • $\begingroup$ what do you mean with "There would be no stable orbit at this balanced point, since there would be no centripetal force. There would be an orbit at a separation closer than this however"? Wouldn't there be forces like tidal effects at that "zero gravity" radius distance? @ProfRob $\endgroup$
    – vengaq
    Commented Oct 21, 2023 at 21:05
  • $\begingroup$ @vengaq I mean you need an inward force $>0$ to have a stable orbit. $\endgroup$
    – ProfRob
    Commented Oct 21, 2023 at 21:31
  • $\begingroup$ And could a galaxy orbiting another one get further from it due to e.g. tidal effects (as the moon is receding from earth) reaching the distance where gravity is balanced by dark energy? what would happen then at this distance if the orbit would become unstable? @ProfRob $\endgroup$
    – vengaq
    Commented Oct 24, 2023 at 0:53

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