Why the Universe Didn't Become a Black Hole 109
StartsWithABang writes: With some 10^90 particles in the observable Universe, even stretched across 92 billion light-years today, the Universe is precariously close to recollapsing. How, then, is it possible that back in the early stages after the Big Bang, when all this matter-and-energy was concentrated within a region of space no bigger than our current Solar System, the Universe didn't collapse down to a black hole? Not only do we have the explanation, but we learn that even if the Universe did recollapse, we wouldn't get a black hole at all!
Because of the expansion (Score:5, Interesting)
You can't use the Schwarzschild radius calculation for expanding space. The only kind of new part was the bit about not becoming a black hole if it should re-collapse.
Business relationship (Score:5, Interesting)
So does ./ have some kind of promotional relationship with startswithabang? If so you should disclose it.
The blog does have interesting material, and its appropriate for /., so its not like its bad that every article on there is making the /. front page. But its kind of odd that every article on there is making the ./ front page.
Re:Early universe (Score:4, Interesting)
If you are going faster than light then you can escape from a black hole.
There is no part of physics that says speed has anything to do with escaping a black hole. If you could produce enough thrust to travel at just one meter per billion years, you could escape a black hole ... assuming you could keep that speed while inside the event horizon of the black hole. Unfortunately, from a mathematical perspective this appears to be impossible.
After a certain point (the event horizon) light simply bends two quickly back on itself to escape a black hole and stays inside the radius of the event horizon. It doesn't slow down, it changes directions, because space is all sorts of fubar inside the event horizon of a black hole.
What they've proved mathematically as that at the event horizon of a black hole the math fails. It falls apart and no longer makes any sense because the numbers get too large on one side of the equation.
In reality, this doesn't mean 'nothing can escape a black hole'. It means 'nothing we've observed can escape a black hole'. Well, except it can. Hawking radiation escapes a black hole as it evaporates, but all the explanations for why are just silly as they are pretty arbitrary compared to 'light' not escaping.
Another obvious but often overlooked theory is that our universe IS a black hole inside a larger universe. It explains a great many aspects that don't make sense ... but then it also introduces a whole bunch of aspects that don't make sense without making a bunch of assumptions about what is outside our universe, and these assumptions are so absurd from our view point that we just assume they are false.
The truth of the matter is ... science knows a lot less than they claim to about black holes, the big bang, and the nature of the universe. Many scientist treat theories with holes the size of the planet in them as obvious fact when they are no such thing. They have no fucking clue why the universe exists in the state it exists today, but many of them refuse to acknowledge that FACT to anyone. The good ones do. Einstein as an example, had no problem admitting his theories were nothing more than theories and that they were often wrong because they were simply based on the little bits of the universe we can observe.
Re:Early universe (Score:5, Interesting)
"What they've proved mathematically as that at the event horizon of a black hole the math fails. It falls apart and no longer makes any sense because the numbers get too large on one side of the equation."
Not so. The maths dies at the singularity at the centre of the hole, but it doesn't at the event horizon except in a badly-chosen coordinate system. Alas, the usual coordinate system we'd present the Schwarzschild solution in is indeed badly-chosen and has an apparent singularity at the horizon, but this is not an actual singularity, as can be seen quickly by calculating a scalar curvature invariant - the Ricci scalar is the immediate choice, it's basically a 4d generalisation of the more-familiar Gaussian curvature - and seeing that it's entirely well-behaved except at the centre of the hole. So we look for a coordinate system well-behaved at the horizon and quickly come across Painleve-Gullstrand coordinates, in which spacetime is locally flat and perfectly behaved at the horizon. The implication is the poor sod wouldn't be able to tell that he'd got to the horizon, except through tidal forces (which depend on the size of the hole), and then he'd struggle to navigate before slamming into a singularity.
Even more confusingly, for a *realistic* hole, the insides are rather different. A Schwarzschild hole has a singularity inevitably in the future - all future-directed paths one can travel on, or light can travel on, end at the singularity. That's a bit of a bummer if you happen to be in a Schwarzschild hole. But a Schwarzschild hole is not physical; it is a non-rotating, uncharged hole, and that's not a realistic setup. In a charged (Reisser-Noerdstrom) or a rotating (Kerr) or, come to that, a charged rotating (Kerr-Newman) hole the singularity is "spacelike" -- there exist paths on which we could, in principle, travel, that avoid the singularity. In the case of a Kerr(-Newman) hole it's even smeared out into the edge of a disc. In reality, good luck navigating in there, but the singularity is not inevitably in the future in there.
A bit closer to the point, you're right that speed doesn't really have anything to do with it. Instead it's the type of path you can travel on, and where *they* go. An event horizon can be defined as the surface on which "null" geodesics, on which light travels, remain equidistant from the hole. If you travel, as massive particles do, on a "timelike" geodesic then you're fucked; you're never going to be able to accelerate enough that you even travel on a null geodesic, let alone a "spacelike" geodesic along which you can basically access anywhere. On a spacelike geodesic you could get out of a hole no problem. You could also travel in time, and you could break causality fifteen times before breakfast. I'd like to travel on a spacelike geodesic - it would be fun. Though managing to get back to a timelike geodesic might be significantly less so.
"Another obvious but often overlooked theory is that our universe IS a black hole inside a larger universe."
That's an extraordinarily strong statement. Our universe might be indistinguishable from a black hole from the outside, yes, but there's a big "might" in there, and an "outside" that doesn't necessarily make much sense either. It all depends on the setup you're assuming. Sure, we could end up finding that the universe is "inside" a black hole for a given definition of "universe", "inside" and "black hole", or we might find that that statement does not make any extent. I wouldn't want to say anything stronger than that, frankly, not least as I'm aware of models of cosmology that are observationally indistinguishable from a standard, infinitely-extended, flat universe, which are also flat, but which have finite extent. One way to do so is to simply put the universe into a toroidal topology. Since GR is a local theory it says nothing about topology, and it would be hard to argue that a universe extended on a torus would look like a black hole from the "outside", since that would be the entire extent of spacetime.
Re:Summary (Score:4, Interesting)
So to extrapolate from the TFA: The laws of physics do not exist in a vacuum...
There's a difference between 'a vacuum' and 'nothing'.