I have seen references to gravity not being able to keep pace during the Inflationary epoch, as gravitational field is only able to propagate at the speed of light. This was an explanation for why the very early universe did not collapse into black hole, but I do not recall where I saw this.

I now question this response. If gravitational field had been established across the entirety of space-time, and then the space-time expanded, it seems to me that the gravitational field would remain intact across the entirety of space-time, although reduced in strength by increased distance, regardless of expansion rate. My reasoning is that all the forces exist every where / when in space-time regardless of expansion rate (i.e., there is no lag between superluminal expansion and arrival of force), or am I wrong about this?

  • $\begingroup$ Both this statement and your response are kind of vague things that don’t really make sense in the context of general relativity. In the real theory, there’s no such concept as “the gravitational field remaining intact”. All of these explanations are all just metaphors used to roughly but very imperfectly allude to what is actually going on. $\endgroup$
    – knzhou
    Dec 12 '19 at 21:19
  • $\begingroup$ Sorry, profoundly unsatisfying answer, but that’s the only productive possible answer to 90% of questions involving GR on this site... $\endgroup$
    – knzhou
    Dec 12 '19 at 21:20
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    $\begingroup$ The gravitational field doesn’t get established across spacetime. The geometry of spacetime (specifically, its metric) is the gravitational field. In GR, there is no way to have spacetime without a geometry and thus without a gravitational field. There is nothing disruptive about inflation, although it is certainly amazing. $\endgroup$
    – G. Smith
    Dec 12 '19 at 23:12
  • $\begingroup$ Given your background, I recommend that you stop reasoning about how GR might or should work and learning the math about how it does work. It’s just some differential equations, and you are familiar with those. You don’t even have to worry about things like why the curvature tensor is what it is. You just need to know that it depends on the second derivatives of the metric. $\endgroup$
    – G. Smith
    Dec 12 '19 at 23:18
  • $\begingroup$ In a previous question, I asked about your background, and you said you were a layperson. But in your user profile you say you have a BS in applied physics and a PhD in materials science. I don't know if you were just trying to be modest, or if you meant that you're not a specialist in this subfield -- but I agree with G. Smith. Given your background, you should just learn some of the relevant physics rather than asking questions in nonmathematical language. $\endgroup$
    – user4552
    Dec 13 '19 at 4:25

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