-3

If after a Big Crunch, the new singularity explodes in a Big Bang, would we get the same Universe all over again? Since black holes retain all the information they've stored, would we get an exact copy? And if not, why not? Where would the randomness come in? Is this the 9th time we're doing this . . . all this?

A Webb
  • 71
  • 1
  • 3
  • Even if the new singularity were identical to the old one, we don't know since we don't know whether the universe is deterministic. See: Is the universe fundamentally deterministic? – lemon Jun 28 '16 at 13:05
  • 2
    There are no signs that a big crunch will be happening, nor can we know what the mechanism of such a hypothetical event would look like. Black holes don't retain "information", physicists are simply (and unfortunately) using the word "information", when they mean "degrees of freedom", which leads to any number of misunderstandings. At this point it is not even known if black holes retain such simple standard model invariants as lepton number. General relativity says no, quantum mechanics demands a positive answer, so we really don't know what's going on. – CuriousOne Jun 28 '16 at 13:07
  • 2
    The Big Bang is NOT an explosion in space. One cannot simply explode a singularity (into Mordor?) and produce a universe – Jim Jun 28 '16 at 13:23
  • BTW, if you want to literally "see" how little actual "information" the big bang has left us with about whatever happened at its "beginning", just look at the cosmic microwave background map. That's a two dimensional projection of the three dimensional CMB and it is pretty boring. Of all the degrees of freedom in the entire universe, only a few hundred, maybe a few thousand really matter early in the game. One could, of course, argue that we have lost much of the third dimension, already (as we have not been measuring the time dependence), but it's still a very small number. – CuriousOne Jun 28 '16 at 13:35
  • CuriousOne: When I read this - http://phys.org/news/2015-04-black-holes-dont-erase-scientists.html - it sure sounds like information in the most prosaic sense is being talked about. Perhaps the article is faulty, I don't know. – A Webb Jun 28 '16 at 13:38
  • Jim: Perhaps this just my linear mind at work, but the Universe seems to have been produced by exactly that, an explosion of a singularity -- everything that happened afterwards happened not by adding anything, but by interactions of things (matter, energy, forces) that had come into existence. And my question is, what stops the next one (if there is a next one) from differing in any way? – A Webb Jun 28 '16 at 13:41
  • 2
    It's misleading if you think that what is meant is that if you throw a teapot into a black hole that somehow, after some time, a teapot will come out. Unfortunately these kinds of ideas are around, but they are completely false, of course. If you throw a teapot into a black hole a bunch of x-rays will come out, right away, and then nothing for a very long time and then, eventually, the rest of the teapot's degrees of freedoms will emerge as quanta. What quanta... that is the question. – CuriousOne Jun 28 '16 at 13:43
  • 1
    No, the Big Bang refers to a moment in time, like yesterday, not an event located somewhere in space. The singularity the big bang includes is a curvature singularity; a moment of time when the scale factor of the metric was zero (or extremely close to zero). There was no explosion, the scale factor simply became non-zero and increased continuously – Jim Jun 28 '16 at 13:45
  • 1
    A simple model of a black hole evaporating may be ice cubes. Imagine you have three ice cubes of the same volume, weight and temperature. One is a cube, the second a sphere, the third is heart shaped. I give you three cups of water, each with a molten ice cube in it... which one was the cube, which the sphere and which one was the heart? That's similar to what black hole evaporation and a hypothetical big crunch do. – CuriousOne Jun 28 '16 at 13:48
  • Also, the stuff that happened after the Big Bang was not exclusively without adding more stuff. At every moment, energy is added to the universe in the form of dark energy and removed through cosmological red-shifting of radiation. Because much of what has happened is in some way a result of expansion and dark energy drives expansion, we can say much of what has happened was in part because there was stuff added after the Big Bang – Jim Jun 28 '16 at 13:53
  • CuriousOne: Thank you -- the ice cube example helps. – A Webb Jun 28 '16 at 14:04
  • Jim: Without wanting to appear obtuse . . . I was always taught that matter and energy can't be created or destroyed, simply converted from one form to another. When you say that dark energy is added to the universe, that sounds at odds with what I thought was a law of physics. – A Webb Jun 28 '16 at 14:06
  • @AWebb The law of conservation of energy is only valid locally (at small distances and time scales). It is, in fact, an extension of the global (universe-sized scales) law of conservation of the stress-energy-momentum tensor (a mathematical object). On very large scales, energy and momentum are not independently conserved. However, on small scales, like the size of a galaxy, the stress-energy-momentum tensor can be separated into energy and momentum components, which allows them to be independently conserved – Jim Jun 29 '16 at 12:48
  • that means in your every day life, you will always see energy and momentum conserved. But on large scales that you will neither travel far enough nor live long enough to observe, neither energy nor momentum need to be conserved – Jim Jun 29 '16 at 12:49
  • No, because then the universe would have to be rotationally symmetric, assuming the laws of physics are rotationally symmetric. Since this is not the case, the outcome of a big bang must be probabilistic. – Abhimanyu Pallavi Sudhir Jun 15 '18 at 17:25

2 Answers2

2

First off, let's start with the more common misunderstanding. The Big Bang was not an explosion of any kind. Popular science likes to depict it as an explosion because of the name "Big Bang" and also because it's more visually appealing than what the Big Bang actually was. The actual definition of the Big Bang is a little complicated, but suffice to say it refers to a moment in time (similar to the word "yesterday") and not an event in space.

Second, the Big Crunch scenario is considered an extremely unlikely scenario in modern cosmology. It is widely accepted that the expansion of the universe is accelerating and will continue to do so for the foreseeable future.

Even assuming black holes retained all information they captured, a black hole cannot explode. It is theorized that they can evaporate, but that is a far cry from any process violent enough to be call an explosion.

Continuing down the hypothetical, if all of the energy in the universe were "recompressed" into a similar state as at the beginning of the universe and a new Big Bang occurred, there is no reason for everything to come out looking the same. Matter and radiation would have to be recreated and the inherent randomness of quantum physics (not to mention even the minor fluctuations in the primordial background energy of that "new universe", which are expanded to relevant scales during inflation) allows for literally almost anything to happen. It could be entirely different.

This is likely not the 9th time we are doing this. This is either going to be the first time or an extremely large number-th time (as with any Big Bounce model)

Community
  • 1
Jim
  • 24,458
  • Thank you. I can see how the word "explosion" is a problem. It's that whole "inherent randomness of quantum physics" part that gets me. But you answered my question. – A Webb Jun 28 '16 at 14:01
0

It may be early to say it is not a cyclic universe. Everything we see around us (in spite of increasing entropy), appears to be cyclic. See atoms, solar systems, galaxies, clusters etc. Even the rate of expansion of universe has gone through cycles of acceleration and slow down. The last switch from slowed down expansion to accelerated expansion is expected to have taken place 5 billion years ago. To me, a cyclic universe makes more sense otherwise, why it would even happen once. Whether it will be an identical repeat or not is much more complex question. Presence of Inherent quantum randomness does not mean it is not regulated at some level. If it was not, then it would violate laws, and would never reconcile with them at a later point. There is something that knows that laws have been broken momentarily and enforces the return of order within short period of time. It is possible that no laws were broken in the first place, it may be just appearing (calculating) to us for a moment or so.

kpv
  • 4,509
  • In the world of advanced physics, one should never approach a situation by first using one's intuition. Too often, reality is contrary to intuition. One should, instead, find the observations and data first and only apply intuition afterward to check if there is internal logical consistency and if everything makes sense in hindsight – Jim Jun 29 '16 at 12:55
  • Also, this seems more like a response to my post than an answer to the question. – Jim Jun 29 '16 at 12:56
  • @Jim: They have put it on hold anyway based upon non-mainstream. So, I could use my intuition here because of same reason. The quantum randomness part (which likely made you think it was response to your post), is response to OP's comment on your response "It's that whole "inherent randomness of quantum physics" part that gets me". – kpv Jun 29 '16 at 15:58