Inside_Olive5504 t1_japjy77 wrote on March 3, 2023 at 3:04 AM

Reply to comment by Any-Broccoli-3911 in Why do cosmologists say that gravity should "slow down" the expansion of the universe? by crazunggoy47

This is a nice Newtonian explanation, but I've always felt that there is more subtly than it acknowledges. It relies on Gauss' law applied to a frame centered on us, but why is that the correct frame? An argument can be made that gravity exerts no net force, just using Newton's law and symmetry.

Any-Broccoli-3911 t1_japkex4 wrote on March 3, 2023 at 3:08 AM

All inertial frames are correct in special relativity. Special relativity works well in this case.

Though we aren't perfectly inertial due to the acceleration of the sun and the galaxy, it's still close enough. Those accelerations are relatively small.

Inside_Olive5504 t1_japr7ao wrote on March 3, 2023 at 4:05 AM

You make an interesting point. I believe (possibly wrongly) that you are saying that one should only consider mass that is within the local Hubble volume to compute the force on test galaxies, in which case we are at the gravitational center of the frame (because it truly is a finite sphere centered on us). One should not compute the force from the test galaxy's frame, since it is non-inertial to us. I think I can buy that argument. To me, it hinges on the finiteness of the Hubble volume and the speed of gravity. If the universe is infinite and if Newtonian gravity acted instantly, I think one could still argue that the test galaxy would feel no gravitational force, even in our inertial frame.

Aseyhe t1_jaqbsge wrote on March 3, 2023 at 7:44 AM

> An argument can be made that gravity exerts no net force, just using Newton's law and symmetry.

That's what Newton believed, but a more careful look reveals that the integral over all space that determines the gravitational force does not converge to a well defined value. See for example the dynamics of Newtonian cosmology.

Inside_Olive5504 t1_jaqjuco wrote on March 3, 2023 at 9:35 AM

Thanks, I appreciate the response. I like the discussion of convergence and the care to consider the shape of the volume in the limit of infinite size.