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Aseyhe t1_jaka1l0 wrote

Cosmic expansion really does just mean that things are moving apart in a uniform way. There is nothing fundamentally physical about the idea that space itself is expanding; that's just a mathematical convention that is convenient in some contexts. (It's a coordinate choice.)

Thus, the gravitational attraction of the matter in the universe slows the expansion precisely as it would slow the expansion of a distribution of matter inside the universe. Indeed, Newtonian gravity predicts exactly the correct expansion dynamics for a matter-dominated universe. Similarly, the gravitational repulsion of the dark energy accelerates the expansion (although there is some subtlety to this).

Further reading on expanding space not being a physically real phenomenon:

Further reading on cosmological dynamics with Newtonian gravity:

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Resaren t1_jam94cw wrote

This is the answer I’ve been looking for after years of having this concept explained only with vague references to expanding balloons, or allusions to the expansion of space being some sort of intrinsic property. Thank you! This quagmire of a concept reminds me a lot of the confusion around ”relativistic mass”, which has thankfully fallen out of usage.

A follow up question to this, does this mean there is some unique Center Of Momentum frame, or is this precluded by SR? And how is this related to the CMBR rest frame?

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Aseyhe t1_jamb3p2 wrote

The CMB rest frame is the frame of a comoving observer, that is, one who is at rest with respect to their (cosmologically) immediate surroundings. At different locations, the CMB rest frames are different. There's no global "center of momentum frame" if the universe is homogeneous, only local ones.

(I should also note that in curved spacetimes, reference frames only make sense locally. However, this is more minor consideration, in a certain technical sense. While the impact of the difference in the velocities of different comoving observers scales linearly with their separation, the impact of curvature scales as the square of the separation. So the latter only becomes important at very large separations.)

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dphseven t1_jakrf04 wrote

This blows my mind a bit, thanks to the steady diet of documentaries I've had for years.

If I'm reading Diatribe correctly, there would be no "big rip" scenario because local gravity would overcome large scale expansion. And there are lots of similar implications... Am I getting that right?

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Aseyhe t1_jalbzp5 wrote

Not quite because as I noted, dark energy supplies gravitational repulsion. In the big rip, the energy density of dark energy increases over time, and so does the repulsive force. That is what rips everything apart.

(Observations currently do not support that the energy density of dark energy is increasing.)

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theotherquantumjim t1_jamo41v wrote

Recent study suggests otherwise doesn’t it? Yet to be confirmed independently I guess, but hasn’t it very recently been posited (maybe also evidenced) that black holes are driving expansion by returning energy to the quantum vacuum? Does this not mean expansion would indeed be physical?

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Aseyhe t1_jampj7l wrote

Indeed, that's one reason to be highly skeptical of that study. The "cosmological coupling" doesn't make sense in the context of general relativity. The global scale factor is not locally even a thing. In certain cosmological spacetimes, the scale factor isn't even uniquely defined globally.

The authors motivate the cosmological coupling by citing the behavior of black holes placed in otherwise homogeneous universes. Such black holes grow over time, but they grow by accreting the surrounding fluids (which are present due to the assumption of homogeneity), not by magically eating the scale factor, as the authors seem to suggest.

The only interpretation of the black-holes-as-dark energy idea that might make sense relativistically is that black holes have a negative-pressure coupling to other black holes. Then as the black holes separate from each other due to cosmic expansion, the negative pressure feeds them mass. This achieves the same outcome without positing a magical coupling to the global expansion factor.

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