[ajross]

> It isn't really a stance on the "beginning of time," which may have started long before

Well... yes it is, in the rigorous sense of "time" defined by general relativity. There's no "before" for a singularity. It may not be the whole story, but whatever metaphysical notion defines the "before/beyond/outside/why" that drives the big bang, it's not a place on the "time" axis of spacetime.

[mathematicaster]

IMHO this conflates model with reality. GR is a model.

[lvncelot]

Specifically, GR is a model that breaks down at singularities. That time "begins" at the Big Bang is a prediction of GR, but until we have a model of quantum gravity there's no telling whether that's actually true or whether the conditions at the big bang are something GR can't fully describe.

Similar to the singularities in black holes - everything up to a stone's throw of the event horizon is pretty well explained by GR, but as far as the horizon itself or the region beyond are concerned, there might be dragons as far as we know.

[skissane]

> That time "begins" at the Big Bang is a prediction of GR

I don’t think that’s right. If we interpret Big Bang theory as claiming that there is a singularity at a finite distance into the past history of every present event, then GR can’t predict what happened at or prior to that singularity. Whether time “began” then or whether there was “more time on the other side” is a question GR alone cannot answer, not a prediction of GR

[Beldin]

Black hole singularities do not start right after their event horizon. The event horizon only demarcates where the black hole singularity becomes an inevitable (inescapable) point in all possible futures.

[lvncelot]

That's not what I was trying to imply, sorry. It's the singularity at the center where GR entirely breaks down, but there's also weird stuff going on below the event horizon (space becoming time-like and vice versa), that aren't present in, e.g. String Theory's Fuzzballs [1] (which, of course, bring their own set of thorough weirdness). So what I was trying to say was that while GR predicts some behavior below the event horizon, a full model of quantum gravity could predict something entirely different, and not only for the area just around the singularity itself but (maybe) up to the event horizon.

[1] https://en.m.wikipedia.org/wiki/Fuzzball_(string_theory)

[Natsu]

> There's no "before" for a singularity.

How does that work for black holes? It seems like there would be a 'before' they formed in the time dimension of our universe, if not within the singularity itself.

[jjk166]

For blackholes it's the reverse, all paths lead to the singularity, and there is no 'after' as opposed to the big bang where all paths lead away from the singularity and there is no 'before.' If you hit rewind on a video of matter falling into a black hole's singularity, it would look like a big bang where everything was created from nothing at an infinitely dense point and starts flying outwards.

[yencabulator]

Think of singularities as unidirectional. We don't understand what if anything was before the big bang, there is no return from inside a black hole event horizon, we don't understand what would follow after an AI singularity. That doesn't mean that they don't have a threshold in time/space/spacetime, just that crossing that threshold breaks the rules we know.

[uoaei]

Singularities are mathematical constructs and are used to model different kinds of phenomena. Black holes and the big bang are only roughly comparable (but by no means similar) if you are considering a black hole from "inside" of one.