> Is this understanding wrong by generally accepted science?
Yes. It's also not relevant to the discussion in this thread. The properties of gravity under discussion here have nothing whatever to do with expansion of the universe.
> The properties of gravity under discussion here have nothing whatever to do with expansion of the universe.
At least part of the discussion is about gravity being (caused by? understood as?) the curvature of spacetime. Why would matter curve spacetime, if not for the accelerated expansion of spacetime + matter resisting acceleration due to inertia?
I'm also OOTL on the boson part (remember, high school physics). I'm asking to hope to understand the subject better.
The idea behind that is that, for all other interactions (electromagnetic, weak, strong), our Standard Model of particle physics tells us that there are "gauge bosons" (for electromagnetism it's the photon, for weak it's the W+, W-, and Z bosons, and for strong it's the eight gluons) that mediate the interaction. The full details of how this works involve quantum field theory and are way too long to go into here; all we need for this discussion is the basic idea that any interaction in quantum field theory has one or more "gauge bosons".
So if we are going to try to quantize gravity, the obvious way to do it--at least if you are a Standard Model particle physicist--is to treat it the same way and have a "gauge boson" for it. This has actually been done: the quantum field theory of a massless spin-2 field, which is what a "gauge boson" for gravity would look like, was developed in the 1960s and early 1970s. Furthermore, the classical limit of this quantum field theory is known to be General Relativity, i.e., the classical theory of gravity that we already have and that we already know works. So if we want to treat gravity as if it were a Standard Model interaction and give it a "gauge boson", this would be one obvious way to do it.
The reddit questioner was asking the more basic question of why we would want to do that at all--why do we need to quantize gravity. If we don't need to, the fact that we could do it using this known mathematical model (there are other issues that come into play with that, which one of the reddit responses mentions, but leave that aside) is irrelevant. Not all mathematical models have physical realizations.
Probably a dumb question, but... why massless? If I understand GR correctly, the gravitational field itself (especially in the form of a gravitational wave) has energy, and therefore contributes to the mass-energy tensor, further bending spacetime. Would that correspond to a gauge particle with mass?
Meaning, why is the spin-2 field massless? The easiest short answer is, because gravity is a long range interaction, and only massless gauge bosons can give you a long range interaction. That's why electromagnetism is also long range (the photon is massless) but the weak interaction is not (W+, W-, and Z are massive).
The strong interaction, unfortunately, breaks this simple heuristic (the gluon is massless but the strong interaction is short range), because it has other factors involved which aren't as easy to explain. But we know that those other factors don't come into play with the spin-2 field model of gravity, so we can still safely say that the graviton should be massless.
> If I understand GR correctly, the gravitational field itself (especially in the form of a gravitational wave) has energy
Only in a certain sense, which is...
> and therefore contributes to the mass-energy tensor, further bending spacetime.
...not this sense. The stress-energy tensor does not contain any "gravity" contribution. In the Einstein Field Equation, "gravity" is on the LHS, represented by the Einstein tensor, and "mass-energy" is on the RHS, represented by the stress-energy tensor. That has to be the case in order for local conservation laws to hold.
What all that means is that there is no valid local concept of "energy stored in the gravitational field" (because there is no tensor quantity that corresponds to this). But globally, you can still look at a system that emits gravitational waves and come up with a meaningful concept of "total energy" that decreases with time as gravitational waves are emitted. (This concept is called the "Bondi energy".) So we can say that "gravitational waves carry energy" in this sense. We just can't localize where that energy is.
One of the things I love about HN: You don't just get "you're stupid not to know that" (though you sometimes get that too). You also get real explanations from people that actually know things and take the time to explain it well.
If you mean elaboration on how the expansion of the universe works, I can give that, but as I've said, it's irrelevant to this discussion.
> At least part of the discussion is about gravity being (caused by? understood as?) the curvature of spacetime.
More precisely, the curvature of spacetime around an isolated gravitating body. Which is a very different spacetime geometry from the spacetime geometry of the universe as a whole, which is what cosmologists use to account for the observations that lead us to say that the universe is expanding.
> Why would matter curve spacetime, if not for the accelerated expansion of spacetime
You have it backwards. What you are calling "the accelerated expansion of spacetime" is an effect of spacetime curvature, not a cause of it. Plus, as I said above, that spacetime curvature is the curvature of the universe as a whole, which is very different from the curvature around an isolated gravitating body, that we are discussing here.
> matter resisting acceleration due to inertia?
This has nothing to do with spacetime curvature at all. It would be present even if spacetime were flat.
You lead with saying you have no background in physics, proceed to give an incorrect explanation, and then asked if it was wrong. Someone simply told you it was in fact wrong -- which apparently didn't meet your expectations. If you wanted someone to explain it to you, why not ask for an explanation or for critique rather than to _expect_ a total stranger to correct all of your understanding out of the goodness of their heart?
Why not just come out and state "Someone please offer me their valuable time to explain physics exactly at my level and at my pace. Thanks!"
Perhaps because that's just how matter and spacetime work?
I mean, you could ask the same question just about anything in physics. Why is there such a thing inertia at all, for example? You can try explaining things in terms of other things, but at some point you inevitably have to arrive at "it's just how the universe works".
Yes. It's also not relevant to the discussion in this thread. The properties of gravity under discussion here have nothing whatever to do with expansion of the universe.