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Pre-Big Bang Knowledge

  1. Sep 22, 2015 #1
    The Big Bang process (with or without inflation) followed known rules which had to exist prior to time zero (start of big bang). For example, some of the obvious rules of the game included: mathematics, conservation laws (symmetry), thermodynamic laws (energy conserved, entropy increases), quantum mechanics (uncertainty principle, locality, and probability waves), special relativity (c constant, E = mc2), and general relativity (equivalence principle). Thus, we know a lot about the environment which set the stage for our universe.

    I do not understand why no one addresses this issue; rather, all discussions on the big bang or multiverse assume this environment was created with the big bang.
     
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  3. Sep 22, 2015 #2

    marcus

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    Not so. I'll get you a paper. You know there is a lot of research into quantizing GR. and then when some method of QG (quantum GR, quantum geometry/gravity) is applied to cosmology it is called QC (quantum cosmology).
    One of the main goals of QC research is to get rid of the singularity (considered a symptom that the classical theory breaks down and is unreliable at extreme density) and to extend time evolution on back.

    In terms of research output, activity, number of papers the most active line of QC research is probably the types which find a rebound from a prior contracting phase of essentially the same universe---same laws of physics, constants, types of matter.

    As GR is quantized in these approaches, at extreme density quantum effects take over and in effect make gravity repel, so when you run the model back in time at some critical density there is a rebound and the universe expands. So that was the contracting phase. There are hundreds of papers exploring this that have appeared just in the last few years, but I'll just mention one or two.
     
    Last edited: Sep 22, 2015
  4. Sep 22, 2015 #3

    marcus

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    The standard cosmic model nearly everybody uses these days is called LambdaCDM. Lambda is the cosmological constant introduced by Einstein around 1917. CDM is cold dark matter. These people took the trouble to see what happens when you set up a bounce cosmology with the prior contracting phase like our standard universe! Same Lambda constant, same CDM, same physics. Not a toy model.

    They found they could dispense with inflation! The effects we normally appeal to inflation to produce, the motivating raisons d'être for making up exotic "inflaton" fields and inflation scenarios, were achieved more simply. So that by itself was interesting. Here's the paper, since you are interested in research about conditions before the start of expansion you might like to see if there is any of it you can understand---it has some relatively non-technical parts at the beginning and end (introduction and conclusions).

    You can get it by googling "LambdaCDM bounce", the PDF file with diagrams is freely downloadable. Here's the link and abstract summary.
    http://arxiv.org/abs/1412.2914
    A ΛCDM bounce scenario
    Yi-Fu Cai, Edward Wilson-Ewing
    (Submitted on 9 Dec 2014 (v1), last revised 28 Jan 2015 (this version, v2))
    We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Importantly, as this scenario predicts a positive running of the scalar index, observations can potentially differentiate between it and inflationary models. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
    14 pages, 8 figures
     
  5. Sep 22, 2015 #4

    marcus

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    Another paper might be of interest because it EXEMPLIFIES one of the main trends in this line of research---the desire to have PREDICTIONS derived from bounce cosmology models that we can test by examining the ancient light of the Cosmic Microwave Background. A physical theory should make predictions that can be tested against observation.
    There are quite a large number of papers along this line (someone in the forefront is Aurelien Barrau, but here is a very recent paper by someone else)

    http://arxiv.org/abs/1509.05693
    Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
    Ivan Agullo, Noah A. Morris
    (Submitted on 18 Sep 2015)
    We provide an exhaustive numerical exploration of the predictions of loop quantum cosmology (LQC) with a post-bounce phase of inflation for the primordial power spectrum of scalar and tensor perturbations. We extend previous analysis by characterizing the phenomenologically relevant parameter space and by constraining it using observations. Furthermore, we characterize the shape of LQC-corrections to observable quantities across this parameter space. Our analysis provides a framework to contrast more accurately the theory with forthcoming polarization data, and it also paves the road for the computation of other observables beyond the power spectra, such as non-Gaussianity.
    24 pages, 5 figures
     
  6. Sep 22, 2015 #5
    Thanks Marcus, I will read these references. In the bounce scenario, as you say the laws are the same; thus, to restate my basic question with a specific example: did the rules of quantum mechanics exist prior to the big bang (bounce) or did they self-create (just exist)? I was thinking if anything was missing, say uncertainty principle, then the big bang would evolve quite differently most likely in chaos. Thus, the statement we know a lot about the pre-bounce environment. Please comment.
     
  7. Sep 22, 2015 #6
    I am glad you mentioned that the two papers were quite technical. Although, I did understand that the bounce theory of LQC can replace inflation and achieve the same result. But I did not understand what “environment” was required (and what we know about it) for LQC. I am guessing the response is, “it (the environment) is what it is”. However intuitively, I visualize the environment features/principles/laws as changeable - analogous to the 30 dimensionless parameters (particle physics and cosmology) addressed by fine tuning.
     
  8. Sep 23, 2015 #7
    Here is something you might find interesting:


    There is discussion about how the laws of nature could create themselves, and how it's mathematically possible to create a causality loop where the universe creates itself. There is intense discussion about where the laws themselves came from.

    About 25 minutes in it gets into that, but I would recommend watching the whole thing.
     
    Last edited: Sep 23, 2015
  9. Sep 23, 2015 #8

    bcrowell

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    This is far from certain. In standard cosmological models based on classical general relativity, time only exists for t>0. It is possible that this is not really true, and that a theory of quantum gravity will show that time existed at t<=0. This is what is claimed by the the bounce models that Marcus is referring to, but keep in mind that these are tentative models with zero experimental evidence to back them up, and they are not even necessarily the most popular candidates for a quantum-mechanical theory. It is quite possible that we will one day have a theory of quantum gravity that is empirically verifiable, and that it will agree with classical GR that time exists only for t>0.

    As Marcus has outlined, people do try to address this issue. However, addressing it is very difficult, uncertain, and controversial, and we have almost no evidence that would help us to figure out how to proceed.
     
  10. Sep 23, 2015 #9
    Newjerseyrunner, thanks, the video directly answered my question about scientists not addressing the issue, they are. It also addressed opinions on the prior existence of laws (in my initial question, inherent features included laws, mathematics, symmetry…). Krauss says they came into existence with the universe. Holt says they are platonic timeless entities or do not exist - just patterns of nature. Gott says they would “just exist” in a 4 dimensional loop of universes.
    My uninhibited thoughts, visualize the platonic timeless entities in an abstract reality. Then, time symmetry is broken, initial conditions initiate the big bang, the big bang evolves. Initial conditions could be inflation or a bounce or colliding branes or quantum gravity or something else. One or multiple universes would result depending on a unique solution or many solutions to the process. To continue the thought process, are there timeless entities with different laws, would they produce a physical universe (as in the video, one without quantum mechanics)? I think discussions of multiverse should acknowledge variables at two stages; the first stage contains the inherent features, and the second stage addresses about 30 dimensionless parameters within our laws. The video touched on this but it was not emphasized.
    Appreciate any more references or comments. Thanks
     
  11. Sep 23, 2015 #10
    Ben, thanks for the clarification, I missed Marcus’s point. Also, see my previous post which explains that inherent features included time plus mathematics, symmetry…
     
  12. Sep 24, 2015 #11
    Just one thing to add the LWC bounce does not replace inflation
    Just one thing to add, there are bounce models that replace inflation but I think most papers in LQC assume inflation happens after the bounce. An interesting result in LQC is the claim that the supposed fine tuning of inflation which is said to present in GR is not present in LQC. This is discussed in our video here:
    https://www.youtube.com/playlist?list=PLANt-1sb3M3o82YWDDm3oiFwToHVj07Vo
     
  13. Sep 24, 2015 #12

    marcus

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    Skydive,

    LQC bounce is certainly COMPATIBLE with inflation. For something like 10 years it has been a common practice to include an "inflaton" field and an episode of inflation. Your video interviewing leaders in the field like Ashtekar and Agullo is very much to the point. It shows how well that has been worked out.
    The paper I just cited some three posts back, by Agullo and Morris does in fact follow that line.

    My point is that in LQC inflation does not seem to be NECESSARY to get agreement with observation.

    That was highlighted in December 2014 by the other paper I cited, by Cai and Wilson-Ewing. Essentially it gets a simple version of the standard cosmic model, LambdaCDM, just with the Loop cosmology bounce, and without inflation.

    You still get the nice features that inflation was invented to explain. This removes much of the motivation for imagining inflation, which involves making up a lot of stuff.

    http://arxiv.org/abs/1412.2914
    A ΛCDM bounce scenario
    Yi-Fu Cai, Edward Wilson-Ewing
    (Submitted on 9 December 2014)

    In that sense the Cai Wilson-Ewing paper was a "game changer".

    But there's no reason to expect an immediate flipflop in the field. There is still a lot of respectable research to be done, exploring the earlier idea of LQC+inflation. Like the Agullo Morris paper:
    http://arxiv.org/abs/1509.05693
    Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
    Ivan Agullo, Noah A. Morris
    (Submitted on 18 Sep 2015)
     
    Last edited: Sep 24, 2015
  14. Sep 25, 2015 #13
    Marcus, I have to apologize for not comprehending the articles you referenced (too technical for me). Your subsequent comments did clarify however.

    My focus is on the conceptual level but no one has commented on my observation - discussions of multiverse (or our universe) should acknowledge variables at two stages; the first stage contains the inherent features (mathematics, conservation laws, thermodynamic laws, quantum mechanics, special relativity, and general relativity); and, the second stage addresses about 30 dimensionless parameters within our laws (typically referred to as fine tuning).

    Thus, am I correct in inferring that this proposed framework adds no value? Please advise.
     
  15. Sep 26, 2015 #14

    Chronos

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    My guess is physical constants align as an inevitable consequence in an evolving universe. They self adjust in a way that produces the universe we observe after billions of years. While that suggests a higher order in the laws of nature, but, i see no way to avoid that conclusion without invoking some kind of grand design or deity argument.
     
  16. Sep 26, 2015 #15
    Thanks everyone, I am now aware of LQG/LQC (Marcus your brief summaries were most helpful) and CCC (plan to buy Penrose book).
    Since I was not familiar with the definition of the current term of pre-big bang, this was a source of misunderstanding on my part. After figuring this out, I clarified my thoughts which are really about the pre-pre-big bang and the multiverse (although they could apply to one universe). My observation is that first stage inherent features require a “fine tuning” (analogous to stage two) or a grand design (only one solution possible) or a multiverse theory.
    As Chronos restated, a two stage approach really addresses a higher order of laws. One speculative multiverse, the Ultimate (Tegmark), is a good example.
     
  17. Sep 28, 2015 #16
    I agree with the idea that there are probably higher laws of nature, I don't think that the universe's laws evolve in any way. Self-adjustment can happen outside of our universe, simple Darwinian evolution could fine tune laws that produce universes that live the longest and produce the most number of offspring. There could be universes branching off our ours as easily as virtual particles pop into existence, black holes may be spitting them out like factories, or some alien species is busy recursively building simulated universes.
     
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