Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A couple of Big Bang questions

  1. Apr 2, 2007 #1
    Hello everyone, can someone clear up these Big Bang problems for me?

      • As far as I know quantum physics is a set of physical laws and rules describing our universe. If so, how can we say the Big Bang sprang from a quantum fluctuation if there was no Universe (and hence no laws of quantum physics) "before" then?
      • Also, the quantum theories about the Big Bang and inflation all rely on the fact that the Universe was very small (pointlike) at the Big Bang. Well this is true for the observable universe and a closed universe, but if the universe is flat (which it seems to be) or open then it will be infinite in extent, even at the Bang itself? So how can we use quantum physics to describe it? The Universe wouldn't make any special distinction between any small part of it and the whole, so how can we use quantum physics to describe an infinite universe?
      • Can someone give me a simple explanation of inflation and the mechanisms that are believed to have triggered and stopped it (particularly scalar fields?) I can't seem to find anything I can understand on the web. Can someone clear this up for me?
  2. jcsd
  3. Apr 2, 2007 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    practically speaking our universe extends back as far as we can crank the models. the models we use are always subject to doubt, challenge, and testing. right now LQC models are probing back a little ways before the big bang. they run deterministically (same physics)
    an introduction to read would be this recent paper of Ashtekar

    this is work in progress, they are considering more and more cases, in some cases they get a clean bounce and a prior semiclassical spacetime region, in other cases they don't get a recognizable prior region.

    people are working on ways to TEST the LQC models that are used for this work (by comparing to CMB data and galaxy-survey structure-formation data. A reference for that would be a recent Bojowald paper on LQC and structure formation, and a recent Magueijo Singh paper.
    That is not quite up to date. The LQC models do NOT rely on universe being spatially closed. They actually modeled the flat case first. You can cure the singularity and get a quantum bounce in the flat infinite universe cas and also in the spatially closed case. they have run a bunch of computer models on variations of these cases, and also solved various cases analytically. Ashtekar pointed out in a recent paper that in the closed case the singularity is not always replaced by something very small. the universe can be of macroscopic size when the bounce occurs, what matters is that the DENSITY reach a certain value which always seems to turn out to be about 80 percent of the Planck density (you can calculate the Planck density yourself it equals the planck mass divided by the cube of planck length, I forget, something more than 1090 times the density of water.

    LQC is also probing the black hole singularity. It is basically beginning to get a handle on the equation of state of the universe. kind of cool.
    there are a number of inflation "scenarios" and some involve rather far-fetched assumptions. I don't think one can generalize and speak for all the different scenarios which people have made up.

    In versions studied by Ganashyam Dateh and in some papers by Martin Bojowald, inflation happens naturally in LQC because of quantum corrections to gravity and the equation of state of matter at very high density.
    One does not need to assume an "inflaton". Inflation is triggered naturally by big-bounce conditions.
    Ganashyam Date has a paper called "Genericity of inflation in Loop Quantum Cosmology". Basically what he shows is that inflation is GENERIC in LQC---it is the natural thing to expect.
    A "graceful exit" also occurs naturally further down the line.
    (BTW in LQC very high density quantum corrections become important which cause gravity to be repulsive--thispartly explains but read the papers for a fuller story.)

    Interestingly, a guy who usually does string theory got interested in this and checked it out and published a paper about it. Tsujikawa IIRC.

    I think one still needs to check to see to what extent one NEEDS inflation in LQC, since with a quantum bounce there is no horizon problem. there was plenty of time in the prior contracting phase of the universe for all parts of the sky to come into thermal equilibrium---so the original problem that inflation scenarios were invented to address does not exist in LQC.

    Other lines of research besides LQC have tried various ways to resolve the big bang singularity. There would be different answers to your questions from those different perspectives. So the answers I have given are not the only answers and other people may see this thread and offer other responses.

    However LQC has become very prominent recently. The main researchers were invited to the Kavli Institute of Theoretical Physics to lecture at a 3 week workshop on using quantum gravity to resolve singularities. You can watch their presentations on video online if you want. Ask if you want links. The main LQC people lecturing were Ashtekar, Bojowald, and Thiemann. (Thiemann only does some LQC, but his work on the full QG theory supports what the other two are doing.)
    Last edited: Apr 2, 2007
  4. Apr 2, 2007 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    A problem with your "couple of questions" is that they necessarily involve quantum gravity----that is a quantum version or quantum extension of General Relativity that applies at very high density, very high curvature, energy etc.

    You can't really address things you are asking within the framework of ordinary mainstream cosmology (which only uses General Relativity as foundation model of spacetime)

    So the everyday mainstream cosmologists just make certain assumptions about what the quantum gravity models will give them when they are finally all officially worked out and tested.

    But as you may have found looking on web, most of what is out there is speculative scenarios---even if it is by prestige people like Alan Guth and Andrei Lindei.

    All I can do is give you pointers to what I think is the most advanced and detailed work---it recovers the classical limit and agrees with Gen Rel outside of the high-density regime, but beyond that it has not been tested so far!

    Here are some recent Ashtekar papers. You can get any of these free just by clicking on "pdf"

    1. arXiv:gr-qc/0702030 [ps, pdf, other] :
    Title: An Introduction to Loop Quantum Gravity Through Cosmology

    2. arXiv:gr-qc/0612104 [ps, pdf, other] :
    Title: Loop quantum cosmology of k=1 FRW models
    To appear in Physical Review D

    4. arXiv:gr-qc/0607039 [ps, pdf, other] :
    Title: Quantum Nature of the Big Bang: Improved dynamics
    To appear in Physical Review D

    5. arXiv:physics/0605078 [ps, pdf, other] :
    Title: The Issue of the Beginning in Quantum Gravity
    Based on an invited talk at the 7th International Conference on the History of General Relativity (HGR7), "Einstein and the Changing World View of Physics, 1905-2005", held at Tenerife, Canary Islands in 2005

    If you want links to the Bojowald papers I mentioned, or to Tsujikawa, or to Magueijo and Singh, just ask. If you want some attempts at non-technical explanation (which is harder than just citing research references:smile: ) then just keep asking and maybe someone will volunteer.


    Some people have the misconception that you can't test quantum gravity, but that's not true and work is in progress towards testing LQC.

    If the theory stands up under testing (using data like CMB and structure formation) then it may come to be accepted as a replacement to classical GR-based.

    If it is accepted, then what it says about the past has the same degree of validity as what Gen Rel and other accepted theories say about the past.
    Gen Rel makes many statements about the past which we cannot go back and observe because we can't go back in the past. But we accept them at least tentatively as part of the picture because we can test Gen Rel in the present with the data we have

    And that is what will happen with LQC. It will be tested in the present with the data we have, and it may or may not pass. If it passes, then we may choose to give some credit to what it says about the past (just as with other models that allow you to crank back in time.)
    Last edited: Apr 2, 2007
  5. Apr 3, 2007 #4


    User Avatar
    Science Advisor
    Gold Member

    Fey, do you have a proposal in mind that would falsify existing models that is not inconsistent with current observations?
  6. Apr 13, 2007 #5
    I have another question. What does the big bang theory predict to have been created first, light or matter? Or were they both created simultaneously and somehow coupled to one another?
  7. Apr 13, 2007 #6


    Kind regards
  8. Apr 13, 2007 #7


    User Avatar
    Science Advisor

    In the standard cosmological paradigm the matter of the universe is created in a process called reheating. This process takes place after inflation (a tiny time interval after big-bang) and consists basically in an energy transfer between the inflaton and the matter fields. The inflaton is the scalar field that drives the de-Sitter phase (exponential expansion of space) during inflation and accumulates energy density during this process. After inflation the inflaton decays and this energy is transferred to matter/antimatter fields (fermions as well as bosons, i.e. also light).

    However, most of the photons (light) of the universe have another origin and can be found in the cosmic microwave background. The origins of these photons can be traced back to the event of matter/antimatter annihilation. It is possible that this event took place at the same time as the decay of the inflaton, but this is by no means clear and it might have taken place later. Matter and antimatter annihilated in lots of gamma photons, and a small amount of matter remained (due to some unclear asymmetry). These photons we measure today redshifted in the microwave as part of the cosmic microwave background.
    Last edited: Apr 13, 2007
  9. Apr 13, 2007 #8
    ^thanks. :approve:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: A couple of Big Bang questions
  1. Big bang questions? (Replies: 3)