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What Caused the Big Bang?

  1. Jul 2, 2011 #1

    owr

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    I know that there is by no means a definitive answer to this question, but, could someone list all the major current theories for what caused the big bang please?

    Thank you
     
  2. jcsd
  3. Jul 2, 2011 #2

    Drakkith

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    I've actually never seen any theories on what caused the big bang, so I don't know. I didn't think there were any.
     
  4. Jul 2, 2011 #3
    1. A colision of pre-existing brane universes.
    2. God.

    That's all I've got...
     
  5. Jul 2, 2011 #4

    bcrowell

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    Hi, owr,

    Welcome to PF!

    First off, any real answer to your question probably requires quantum mechanics, but we don't have a theory of quantum gravity. The best we can really do based on well-tested theories is to answer this kind of question using general relativity, which is classical.

    So in terms of GR, the picture we have is that time and space exist only for t>0. For any t>0, there is an earlier time, so we can always give cause and effect explanations. Conditions at any given time were caused by conditions at an earlier time. GR can't answer questions like why there is something rather than nothing. It can, however, say that there was something rather than nothing at time t because there was something rather than nothing at an earlier time. In GR's model, there is never any creation of matter out of nothing (in technical terms, energy-momentum is locally conserved). Matter exists at all t>0, and there was never a universe full of empty space.

    GR's description of cosmology is expected to be wrong due to quantum-mechanical effects at times earlier than the Planck time, which was about t=10-43 s. There are speculative attempts to construct a quantum-mechanical theory of gravity that would fix these problems. In one such approach, called loop quantum cosmology, the result seems to be that the big bang is a bounce, so that the time axis extends on both sides of t=0. Marcus works in that field, and he will probably chime in with a better description of that than I can give. In another theory, called perpetual inflation, new universes spontaneously branch off from old ones. I think the idea mrspeedybob is referring to is called the ekpyrotic universe.
     
    Last edited: Jul 2, 2011
  6. Jul 2, 2011 #5
    Hi Owr, Unfortunately I havent seen a real ultimate cause for the BB in any theory.
    With the brane theory you can always ask what caused the branes to first exist?
    With any cyclic theory you can always ask what caused the first cycle?
    With any religious faith you can always ask what created God and the reality he exists in?

    I guess it is like that Russian matryoshka doll conundrum!
    http://images.search.yahoo.com/search/images?&p=russian+matryoshka+dolls&rs=0&fr2=


    So I guess it is a pretty hopeless task really, I suspect we have to accept that we cannot know of any top level reality or ultimate source of existance.

    We can however learn all that can be learnt about our reality during the short time we have and try to deduce some overall purpose from what we see and learn of our universe.
     
    Last edited: Jul 2, 2011
  7. Jul 2, 2011 #6

    bcrowell

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    Owr didn't ask about an ultimate cause, just about a cause for the big bang.
     
  8. Jul 2, 2011 #7

    marcus

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    It would make the job easier if we restricted to empirical (testable) theories. That's sort of the idea of science as envisaged e.g. by Francis Bacon around Shakespeare's time. Theorists should voluntarily restrain themselves to proposing only theories where you can in good faith see some way to test by making observations. Eventually at least.

    Ben Crowell made a good point.
    And he also mentioned something I'm interested in.
    Indeed there are several proposed models that do not fail at the classical singularity but extend causality and time-evolution on back before the t=0 point where the classical model breaks. My reason for being especially interested in the LQC model is that it is unusual in the research activity around it that has to do with testing.

    Many of the papers are by early universe phenomenologists (professionally concerned with testing theories) rather than by the LQC theorists themselves. Theorists might be considered to have a stake. Pheno people score points however the test comes out, pass or fail. If they can find a valid way to put a theory on trial, and it gets falsified, so much the better. I like this. So I pay especially close attention to what's going on in that field.

    But there are other criteria for preference, obviously. You can list the various proposals for what led up to the start of expansion and then pick one on aesthetic grounds, that agrees with your taste, or that seems intuitively reasonable to you, beautifully simple...
    That's not a bad way to proceed either! And subjective judgements based on taste can vary!

    I'll get some links about LQC, so anyone who wants can check out the pheno activity. And get an overview. A nice thing is they come up with a simple quantum modification of the Friedmann equation---the basic equation used in Cosmology to model the universe. It agrees with the classical version except at very high densities where quantum effects resist collapse and cause a bounce. So using this quantum-corrected Friedmann equation they can calculate numbers, run computer models of the bounce, make predictions and so forth. It gets quantitative, which I guess is part of the appeal.

    Here are some papers bearing on LQC phenomenology. It is not a perfect search, but it comes up with quite a few goodies:
    http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

    Here is a broad search for Quantum Cosmology in general (not just Loop). Both searches restrict to 2009 or later. (It is an actively changing field so good to concentrate on recent research.)
    http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=dk+quantum+cosmology+and+date+%3E+2008&FORMAT=WWW&SEQUENCE=citecount%28d%29 [Broken]
     
    Last edited by a moderator: May 5, 2017
  9. Jul 2, 2011 #8
    Ben true. I was reading between the lines a bit, thinking that what Owr was really asking was what the ultimate cause for the big bang might be, but I suspect it is unknowable. jmho.

    Marcus I agree, for me it is the most interesting part of Physics and I would be sad to have to leave the universe not knowing what it was all about. Mind you I think I would be sadder if I left it believing in a theory or religion which turned out to be a falsehood and never know that what I believed was false. I might very well end up believing in nothing just to be safe :)
     
    Last edited: Jul 2, 2011
  10. Jul 2, 2011 #9

    marcus

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    I appreciate your comment, Tanelorn. You might find inspiration in a cultural history/personal thoughts book due to come out in September called The First Scientist.
    https://www.physicsforums.com/showthread.php?t=483523
    http://www.westholmepublishing.com/thefirstscientist.html [Broken]
    Our models of the world HAVE ALWAYS BEEN WRONG and probably always will be. The great thing is that we criticize, test, improve them. And the other conscious species in the galaxy, if there are any, are probably doing the same with their concepts. We devise ways to measure the speed of light, and stuff like that, and we all try to think the world as clearly and perceptively as we can.

    And there is always room for improvement. It is not to feel sad about (you mentioned sad) but I think rather to rejoice in belonging to such a tradition or movement.
    ===============

    My response to OWR's good question is so far quite lop-sided. He would like a LIST OF ALL THE MODELS. Some I think are rather fanciful, with little or no idea of testability, or depending on some drastic assumptions like all the matter in the universe eventually decaying into pure low-temperature light. Or extra large dimensions we have no evidence to suggest existence of. And so on. I'm not an expert in the whole list of things that have been proposed.

    So other people's responses supply an essential ingredient of completeness that I cannot.
     
    Last edited by a moderator: May 5, 2017
  11. Jul 3, 2011 #10

    Chalnoth

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    The major ones?

    Well, here is a very rough breakdown of the big ones:

    1. Quantum vacuum fluctuation (this includes but is not limited to eternal inflation).
    2. Bounce from previous collapsing phase (esp. loop quantum cosmology).
    3. Collision of branes (ekpyrotic universe).
     
  12. Jul 3, 2011 #11
    Marcus, thanks very much for the book link, I will look out for it. Perhaps sad wasnt the right word, but there again maybe it was. I am thinking of all the people throughout all history that have searched and eventually, sadly expired in vain trying to understand what it was all about. All the Religions Philosophies and Science that people have used tried to try explain it all, so far without complete success. I do think that we are very lucky to have a view of so much in the last few decades that no one before has ever had, particularly the CMBR and the Hubble telescope views, it certainly reduces the number of cosmological possibilities.
    If everyone was offered one last wish after passing I think I can guess what a Physicist or Cosmologist would ask for!

    Chalnoth thanks for the list, those were also the ones that I recall reading about.

    Chris
     
    Last edited: Jul 3, 2011
  13. Jul 6, 2011 #12

    owr

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    Thanks everyone - that'a all really helpful
     
  14. Jul 9, 2011 #13
    I hope that the version of LQC in which it is the whole universe that "bounces" (the Ashtekar version) isn't seen as the ONLY LQC, because it would be a pity to abandon an interesting model on the basis of a subtle, but significant, error. IMO, it is reasonable to hypothesise that a black hole is the first stage of a real, observed bounce, but we can't see the second stage of it because of the event horizon. A combination of Smolin's LQG and his fecund universes (FU) looks much closer to the mark. A pity he, himself, rejected FU on spurious grounds, to do with a misconception of the relevance "natural selection" outside of biology.
     
  15. Jul 9, 2011 #14
    Has anybody ever thought about the way that the conception, development, and death of the universe somewhat parallels the way in which a living things are conceived, develop, and eventually die? Not sure if it means anything, but there we are. In this thread we have been talking about the conception of the universe and in the other thread we were talking about its heat death.
     
    Last edited: Jul 9, 2011
  16. Jul 12, 2011 #15
    Tanelorn, are you asking whether universes have sex? :)
    Actually, in Smolin's "fecund universes" cosmology, they at least get pregnant (by having black holes), but it's a virgin birth, I'm afraid - there was no daddy universe!
     
  17. Jul 12, 2011 #16

    bcrowell

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    He didn't hypothesize cosmological natural selection (CNS) because he wanted it to be analogous to biology. He constructed it as a well-motivated physical theory, which happened to be analogous to a process in biology. The natural selection part is the only thing that gives it predictive value. If you take out the natural selection part, then it no longer makes any testable predictions and doesn't even qualify as a scientific theory. It's actually quite remarkable that it was falsified by the observation of a high-mass neutron star. The fact that a theory was proposed and then falsified shows that he was doing real science.
     
  18. Jul 12, 2011 #17

    epenguin

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    I thought that one well-regarded idea was that nothingness is unstable and so collapses into something and then a little something is also unstable so it inflates into a big something or rather a massive something very fast, all something to do with a Mexican hat, I am sure someone knows what I am talking about even if I don't.

    As well as the what I am talking about there was also a why which I am even less clear about.
     
  19. Jul 12, 2011 #18

    Chalnoth

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    Well, the difficulty there is how do you define "nothingness" in the first place? Most of our understanding of the universe relies upon some sort of background space-time, and even General Relativity, which has a dynamical space-time, can't describe "nothing". So how would you do that? I don't think anybody has come up with a sensible answer.
     
  20. Jul 16, 2011 #19

    bcrowell

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    This thread degenerated into a dialog with a crackpot poster. I've deleted the dialog with the crackpot and closed the thread.
     
    Last edited: Jul 16, 2011
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