Author Topic: Black Holes eating our universe  (Read 7851 times)

Offline AcesHigh291

  • Little Puffin
  • **
  • Posts: 3
  • Karma: +0/-0
    • View Profile
Black Holes eating our universe
« on: January 27, 2013, 10:25:31 PM »
A question I asked Ben on twitter, but he said I could follow up on the forum here for more a long form response.

So. Assuming that the idea that we are in one of many big bang cycles is correct, wouldn't the universe eventually run out of material due to black hole consumption?

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Black Holes eating our universe
« Reply #1 on: January 31, 2013, 06:38:31 AM »
Hiiii!

okay, so the model of the universe being cyclic (periodically expanding and collapsing) is not correct. It's expanding and it probably always will expand. it *might* have started out with an era of contraction followed by a bounce.

BUT. you are right that if it WERE cyclic, everything would probably end up stuck in a black hole.

when the universe expands, it can kind of pulls stuff away from black holes. so the black holes won't eat as much as they could have if it weren't expanding.... like a dog eating a pile of kibble on a mountaintop.

when the universe contracts, the opposite it true, it as the universe gets denser, MORE stuff ends up inside a black hole than it otherwise would. ... like a dog eating kibble in a bathtub. heh.

what's more, when the universe is really dense, black holes get to FEAST. I'm *pretty sure* that the supermassive black holes at the center of galaxies all formed in a time when the universe was much denser than it is now. that's how they got so fat. so if the universe periodically got really really dense (as it would in a cyclic model), the black holes would get REALLY REALLY REALLY fat.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline stephako

  • emo emu
  • ******
  • Posts: 92
  • Karma: +0/-0
    • View Profile
Re: Black Holes eating our universe
« Reply #2 on: January 31, 2013, 10:37:47 AM »
But in the case of cyclic models wouldn't the space itself contract to areas that are comparable to the Schwarzschild radius of a black hole before it starts expanding again?

In that case do we even know what would happen to the black hole?
Null results, open questions and a bit of my writing: JUnQ

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Black Holes eating our universe
« Reply #3 on: January 31, 2013, 04:15:38 PM »
in all cosmological models worth talking about, space is infinitely big. so no matter how dense the universe is when the bounce occurs, there will still be places that don't get contracted to the inside of the black hole.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline bobmath

  • Miocene Terror Bird
  • *******
  • Posts: 100
  • Karma: +0/-0
    • View Profile
Re: Black Holes eating our universe
« Reply #4 on: February 01, 2013, 08:42:57 AM »
... but is there stuff in those places?

And what's the difference between the initial (pre-Bang) state of the universe and a black hole? They are both sometimes described as "singularities", but I guess they must be very different.

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Black Holes eating our universe
« Reply #5 on: February 01, 2013, 09:49:54 AM »
oh singularities.

so yeah, is there a difference between the big bang singularity and black hole singularities?

yes and no.
the same theorem is responsible for both.

but a big bang singularity is the type of singularity that stuff comes out of,
and black hole singularities are the types that stuff goes into.

a white hole singularity is kind of the same type as the big bang.
and black hole singularities are kind of the same type as big crunches.

well.

ask more questions. it's a great topic, but i'm having trouble grabbing it.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline stephako

  • emo emu
  • ******
  • Posts: 92
  • Karma: +0/-0
    • View Profile
Re: Black Holes eating our universe
« Reply #6 on: February 01, 2013, 10:23:13 AM »
OK if you want more questions:

Isn't the big bang a state where you initially have very little space? I kind of understood it the way that before the big bang there wasn't even space time. Whereas a white hole just spits out matter (again as far as I understand it).
Null results, open questions and a bit of my writing: JUnQ

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Black Holes eating our universe
« Reply #7 on: February 01, 2013, 10:41:24 AM »
yeah, great question.
so the way to imagine a big bang is by imagining a big crunch backwards.
somehow that's easier.

so imagine the universe, infinitely large. full of.. gas.
imagine the universe collapsing. so. ..
it doesn't collapse down to a point. instead
every second, a little imp goes and steals one centimeter out of every meter.

so the stars aren't getting sucked towards a point, but the distance between them is decreasing.
and gradually, the universe gets denser and denser.

and gravitational collapse accelerates. the denser the universe gets, the faster imps steal centimeters.

so that in a finite amount of time, every point in the infinite universe gets INFINITELY HOT AND DENSE.

So. That's what happens in the big bang.
the universe kind of starts out infinitely large and infinitely hot.


okay, so. there's a problem with out theories of physics and gravity. they are inconsistent when things get too dense. so technically we don't know what happened at t=0 seconds, even though we do know what happens from that point on :p
yep.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline Grawk1

  • talky crow
  • ****
  • Posts: 27
  • Karma: +0/-0
    • View Profile
Re: Black Holes eating our universe
« Reply #8 on: June 27, 2013, 09:15:28 AM »
So there is something I do not understand about white holes:

With a black hole, its matter intake is entirely dependent on the matter composition of the space around it. Asteroids, dust, stars, etc. fall in not as a uniform flow of matter converging in from all sides, but as a chunky, asymmetrical mass flow. However, as you have said, the only pieces of information a black hole contains are the mass and the angular momentum of the black hole.

If this the case, when we run time backwards to see a white hole, wouldn't we be observing a non-deterministic system? There are many possible ways the mass could be ejected from the hole which are consistent with the initial state of the system.

As I understand it, when time is reversed all laws of physics and causality are maintained except the 2nd law of thermodynamics. If the universe is deterministic, isn't it anathema to have a state which is equally compatible with a whole array of potential pasts of futures?

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Black Holes eating our universe
« Reply #9 on: June 27, 2013, 10:56:23 AM »
correct.
correct.

so white holes have a thing called a "naked singularity". that is, a singularity which is accessible to the outside universe.

singularities in general relativity are kind of bad, because, as you have correctly imagined, they can eat information... and thus they can also generate information.

so there is a conjecture (called "cosmological censorship", a pun) which says that "naked singularities" cannot exist. any time  you have a singularity, it must be enrobed in an event horizon.

cosmological censorship is a neat idea, because it is consistent with observed physics (we've never observed an astronomical singularity), and because it resolves the problem of naked singularities generating information and matter and dogs and cats and stuff.

that said. it's a controversial idea, because it does violate the time symmetry we believe the physical laws of the universe obey (at small scales).

it has not been proven, but it's fun to talk about.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline Grawk1

  • talky crow
  • ****
  • Posts: 27
  • Karma: +0/-0
    • View Profile
Re: Black Holes eating our universe
« Reply #10 on: June 27, 2013, 06:39:01 PM »
Awesome!!

I'm trying to get my head around what it would mean to destroy or generate information. I can understand the idea of creating a point of indeterminacy which could have many possible pasts leading to it and many possible futures leading from it, but what else would this imply?

By the way, is this what Hawking famously lost his bet over?

Offline bn

  • Titanium Physicist
  • Administrator
  • Godzilla!!!
  • *****
  • Posts: 1065
  • Karma: +1/-0
  • Compressing Hearts, Super Robo Style
    • View Profile
    • The Titanium Physicists Podcast
Re: Black Holes eating our universe
« Reply #11 on: June 27, 2013, 07:47:30 PM »
yeah.
this is what hawking lost his bet over.
I think he conceded.
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.