[pdonis]

I don't know what theory of gravity you are trying to use, but it isn't the correct one.

In our actual model of the universe, using the correct theory of gravity, the "total amount of gravity" affecting us (or any point) from the rest of the universe (i.e., once we factor out local influences like our solar system and our galaxy) is zero. That is because the average matter distribution in the universe is the same in all directions from us, so the "gravity" from it cancels out. The average matter distribution in the universe affects its overall rate of expansion over time, but this is not the same as the kind of "gravity" you are thinking of.

Also, while our universe does have a cosmological horizon (due to accelerating expansion), this horizon does not work the way your hypothetical "event horizon" does.

In short, your "theory" is not even wrong, because it doesn't even start from a correct underlying theory of gravity.

[015a]

> while our universe does have a cosmological horizon (due to accelerating expansion), this horizon does not work the way your hypothetical "event horizon" does.

I think this statement is really the crux of the counter-argument. Your statement that matter has average uniform density in all directions around us is obviously only correct in sufficiently large frames of reference; there are galactic voids, and galactic-super-strands, uniformity really only exists within the "mathematically and hypothetically infinite" frame.

You should expand on why the cosmological horizon does not function in the same way the GP's "event horizon" analogue does; and/or possibly, expand on how large the frame would have to be to achieve reasonable uniformity.

[pdonis]

> Your statement that matter has average uniform density in all directions around us is obviously only correct in sufficiently large frames of reference

Yes, agreed. But the distance to our cosmological horizon is large enough that the averaging assumption is fine on that scale.

> You should expand on why the cosmological horizon does not function in the same way the GP's "event horizon" analogue does

Fair enough. But first I'll reference an excellent paper by Davis & Lineweaver (2003) [1]. You can find much more detail there than I'm going to give here.

Some simple facts about our cosmological horizon are:

(1) It is currently receding from us very slowly (in terms of proper distance); and asymptotically it will be at a constant distance from us forever (in more technical language, our universe approaches de Sitter spacetime asymptotically, as all other matter and energy becomes negligible compared to dark energy; and in de Sitter spacetime the cosmological horizon is at a constant distance forever). It certainly is not receding from us at the speed of light.

(It is true that the horizon is a lightlike surface--but that does not mean it's receding from us at the speed of light. In fact, counterintuitively, to the extent it is "moving", locally, in any direction, that direction is towards us, not away from us! But due to the curvature of spacetime, its proper distance from us will asymptotically be constant.)

(2) It is not a boundary between things that can affect us gravitationally and things that can't. To the extent there is such a boundary in GR, it's our past light cone (which is shown in the diagrams in Figure 1 of the paper I referenced). But one also has to consider the other caveats I gave in my earlier post.

(3) The cosmological horizon is a boundary in spacetime, not space. It is a boundary between the region of spacetime that will be able to causally affect us into the infinite future, and the region that won't. Particular objects, as they recede from us, will move beyond the cosmological horizon and will then no longer be able to causally affect us. But events that occurred in those objects before they moved beyond the horizon will still be able to causally affect us--though of course it will take time for those effects to propagate to us. For example, the light we see from distant objects is a causal effect, but we see the objects not as they are "now" but as they were when the light was emitted. It's quite possible for a distant galaxy whose light we are seeing now to be beyond our event horizon "now", but they weren't when they emitted the light.

The spacetime diagrams in Figure 1 of the paper I referenced can be very helpful in making all this clearer.

> expand on how large the frame would have to be to achieve reasonable uniformity.

A few billion light years is certainly large enough. Our cosmological horizon is quite a bit further away than that (about 16 billion light years, according to the paper I referenced).

[1] https://arxiv.org/abs/astro-ph/0310808

[mxkopy]

> That is because the average matter distribution in the universe is the same in all directions from us, so the "gravity" from it cancels out.

Can't gravity have an infinitesimally small effect, which means this matter distribution has to be perfectly balanced for its gravity to cancel out to zero at some point?

[pdonis]

> this matter distribution has to be perfectly balanced for its gravity to cancel out to zero at some point?

In principle, there are of course effects due to the matter distribution not being perfectly spherically symmetric about us. But except for the obvious effects that are due to nearby obvious objects--like the Earth, the Sun, our galaxy--in practice the gravitational effects of the rest of the universe on us are negligible.

[unsupp0rted]

I don't think we know how small of an effect gravity can have over a large distance. Is there a lower Planck-like limit?

[pdonis]

> Is there a lower Planck-like limit?

Only in speculations that we have no way to test now or in the foreseeable future. We are many, many orders of magnitude away from being able to probe spacetime on such scales.

[mxkopy]

From my (complete lack of) understanding, it’s unlikely that spacetime is discrete, so maybe not.

[unsupp0rted]

Sure, and whenever Warren Buffet gives a speech to 1000 college students, the average wealth of every person in the auditorium briefly jumps to millionaire-level, until Buffet leaves the room.