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OK. So was the Big Bang the beginning or not?

  1. Apr 5, 2007 #1
    The biggest "debate" I've seen on these cosmology forums appears to be "did the Universe, time, space and the whole whatevery actually begin with the Big Bang?" or "there is no scientific evidence the Big Bang was the beginning of the Universe, why should we believe it?" I haven't seen a thread for it yet however.

    This issue is really confusing me; can people from both camps put forward evidence for us to weigh up?

    (Note: By "Big Bang" I mean the classical moment of creation itself rather than the burst of energy and matter that occurs a little while after creation in some inflationary scenarios.)
     
  2. jcsd
  3. Apr 5, 2007 #2

    hellfire

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    Current physics break down in the Planck era. This is earliest period of time in the history of the universe, going from the initial singularity at t = 0 to t ~ 10-43 sec. after the initial singularity. Especially, the classical notion of gravitational field as a geometric quality of spacetime, as given by general relativity, is not valid anymore during the Planck era. So the fact is that we don't know and that any claim about the initial singularity is very speculative: time might be a more fundamental quantity with extended validity at the sinitial singularity and even may be beyond it, or the notion of time may also break down at that stage, we simply do not know.

    However, there exist models about the origin of the universe. The most promising ones are based on the two most solid theories that try to extend standard physics beyond its validity, string theory and loop quantum gravity. These are two very different theories based on very different postulates and formalisms so one would expect very different models about the origins of the universe. Models are in fact very different, but suprisingly they have one common feature: both approaches describe an extension of the classical regime beyond the singularity which acts as a strange transition event. Nevertheless, one has to keep in mind that there is still no verification of any of the extensions of standard physics or cosmology.
     
    Last edited: Apr 5, 2007
  4. Apr 5, 2007 #3

    marcus

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    I agree in general with what hellfire says here, and have some extra detail to add.

    there is no scientific reason to believe it.
    you can believe it if it makes you feel good, or fits in nice with your religion or your aesthetic ideas of what ought to be

    I stick very close to mainstream cosmology---not attracted by fringe ideas.
    Mainstream cosmo has accumulated a magnificent bunch of data of many kinds. Two models fit equally well, classic GR and LQC---that is, the current consensus model LambdaCDM can be based either on classic GR or on LQC.

    Both basis theories provide you with the Friedmann equation that you need, the only difference is that one version is quantized and the other isnt.

    The unquantized (classic) model breaks down at the start of expansion. The quantiized model (LQC basis) does not break down and continues back into the past before the start of expansion.

    The LQC version is so far indistinguishable from classic GR in what it says about what we observe because to a close approximation it DUPLICATES classic GR as soon as a few planck units of time have passed after the start of expansion. The convergence to the classic theory as a limit is very fast.

    However there are some differences which may afford the opportunity to distinguish between the two in the future. There was a recent article by Magueijo and Singh about that. (The quantized version does not have a "horizon problem"---which was what motivated people to invent "inflation" scenarios---so it appears that inflation is simply an option. Differences in structure formation may show up. Currently, no available data can distinguish between the two, but future observations are expected to.)

    At present there is no scientific reason to prefer one version to the other. since only the classic version breaks down, there is no scientific reason to prefer a breakdown at the start of expansion.

    =====================
    BTW the success of LQC in achieving this has only been completed in the past 2 years. It has NOT been paralleled by developments in string theory (ekpyrotic model is more speculative and does not as yet give a deterministic evolution thru the big bang moment).
    However string-based cosmology COULD catch up---it simply has not so far. It would have to reproduce the classic Friedmann equation, which hasnt happened as yet in the string context.
    Since these developments (associated with new LQC dynamics, 2005 and 2006) are recent, not everybody knows about them.
    So you won't get exactly this story (some people will discuss it as if string cosmology and loop cosmology were about on par, which at present they are not).
     
    Last edited: Apr 5, 2007
  5. Apr 7, 2007 #4

    Chronos

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    The big bang theory only claims the universe did not always look exactly the way it does now [to us]. There is a mountain of very credible evidence that something very much like a big bang occured a finite time in the past. If, as is logical, you presume the big bang was preceded by something else [usually called initial conditions], the big bang was not the 'true' beginning, just the point at which we humans can coherently describe its properties.

    This is not unlike asking when a person begins. Humans are objectively recognizable as a distinct entity at birth. We can even push that 'beginning' back to the moment of conception. Prior to that, we can only say there existed the potential for any given person to come into being. So it is with the universe. Reverse engineering our observable universe past the big bang is like creating a picture of someones parents from a DNA sample - only harder.
     
  6. Apr 8, 2007 #5
    Is there any one physicist who can provide us with a scientific base of the notion "begin". Each existing notion of "begin" IMO is "not even wrong".
    As long as such a scientific based notion does not exist, it seems IMO useles to discus the beginning of the universe.

    Kind regards
    hurk4
     
  7. Apr 8, 2007 #6

    marcus

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    For a broad perspective based on the history and philosophy of science

    "The Issue of the Beginning"

    this identifies the common threads of SEVERAL contemporary development in theory. It was a talk at a conference of science historians and philosopy of science experts---where they asked this guy (a quantum cosmologist) to come and give a big-picture talk about modern ideas of Beginning.

    To get the whole article, go here and click on PDF
    http://arxiv.org/abs/physics/0605078
    The Issue of the Beginning in Quantum Gravity
    Abhay Ashtekar

    "Abstract: The goal of this report is to provide an up to date account of results on the quantum nature of the big bang, obtained in loop quantum cosmology. They suggest a radical modification of the paradigm provided by general relativity for the issue of the Beginning. The article is addressed primarily to historians and philosophers of science."

    He's probably the top world expert on this now. Hawking's activity and influence has been dwindling for a while.
     
    Last edited: Apr 8, 2007
  8. Apr 9, 2007 #7
    Hi Marcus
    Thank you.
    General relativity gave the wrong paradigm (See introduction of Ashtekar’s “the issue of the Beginning in Quantum Gravity”) “In the beginning there was the big bang”. The conclusion IMO must be: the notion of beginning does not exist, the search for a such a begin makes no sense, but what really makes sense is, to continue the search for the endless evolution which happened to the ever during existence from which our universe is a part of.

    Kind regards
    Hurk4
     
  9. Apr 10, 2007 #8
    Dear Chronos,
    Apparently you still agree with Douglas Adams’ statement about the notion of “begin” which shows up as a kind of one-egg twin-sister of “the” universe?
    On the other hand you gave a nice example of a non-begin which is the birth of a human.
    To me it seems that the notion of a creation moment resembles the characteristic impedance of a long line cable with the difference that indeed such an impedance is a physical reality where as I can not see physical reality in a creation moment.
    Kind regards
    hurk4
     
  10. Apr 12, 2007 #9

    hellfire

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    I am not sure whether this critique is fair. I think you may be right that string cosmology has not a description of the dynamics near to the singularity as loop cosmology has. It is made use of an effective action with limited validity at high energies. But, on the other hand, however, the Friedmann equations are reproduced in the same way than in loop cosmology: by working assumption on a minisuperspace model, since the effective action actually contains classical general relativity. Moreover, and unless loop cosmology, there are lots of papers that describe phenomenological results of string cosmology, especially related to the spectrum of gravitational waves that emerge from the pre-big-bang scenarios (see the work of Gasperini, Veneziano, etc.). Unfortunately I am not versed on this to give a detailed opinion about it, but it seams to me that lot of promising work was done.
     
  11. Apr 13, 2007 #10

    Chronos

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    The universe may very well be infinitely old, perhaps a landscape of prior incarnations preceded the one in which we currently reside. But this is not terribly interesting. At best, prior incarnations are observationally inaccessible to us. The universe is currently observationally accessible to us back to about z~1100 [the CMB]. CMB photons were emitted when the universe, as we know it, was about 300,000 years removed from the theoretical point at which the universe became observationally accessible by science as we know it. Its a bit like a crime scene. A defining event must transpire before any meaningful investigation is possible. Establishing that a crime occurred is simple. Defining the circumstances involved is not so simple.
     
    Last edited: Apr 13, 2007
  12. Apr 15, 2007 #11
    Seems to me that any cosmological notions of the 'beginning' need to be divorced from our biases (notions) of being embedded in space-time, which is apparently very difficult to do.

    When people ask about the cosmological 'beginning', I'd think what they're really asking is "why does the universe exist at all, rather than not?"

    If that makes sense...
     
  13. Apr 17, 2007 #12

    Chronos

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    Many people are uncomfortable with the notion of 'something' arising from 'nothing'. This is understandable given there is no compelling observational evidence that nothing has ever existed in our universe.
     
  14. Apr 17, 2007 #13
    In such a case "why" can only point in a 'direction', not to a (IMO) non existing 'begin' or 'end'. I think in researching an evolutionary path the question "why" is kind of useful motivation, to many of us, to look around.
    The question only makes sense if no final answer is expected.
    Kind regards
    hurk4
     
  15. Apr 18, 2007 #14
    Do the physical laws count as 'something'?
     
  16. Apr 18, 2007 #15

    jal

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    Good day everyone!
    Further reading.
    http://arxiv.org/PS_cache/gr-qc/pdf/0605/0605011v2.pdf
    Gravity, Geometry and the Quantum
    Abhay Ashtekar
    14 July 2006
    If you look at the bottom of p. 12
    When I use QMLS (Quantum Minimum Length Structure), I do not reach the Planck Scale. I do not make the interpretation/speculation of “a bounce”.
    This is where Abhay Ashtekar reached the 2d structure of the universe. The triads are in a dynamic 2d configuration as they would be for the horizon for a black hole.
    Continuing…
    As you can see, a new mechanism, “make gravity repulsive” is needed to get the “big bounce”.
    QMLS does not need to invent a new mechanism/speculation. To get 3d, all that is needed is for the triads to get into a dynamic 3d configuration.
    It is all speculation until the theories can come up with a model to do dynamic simulations. Read my blog for more info.

    There are still lots of questions to be answered.
    A black hole is the result bending the units of space into a sphere.
    There are two length scales in quantum gravity: the Planck constant lP = p¯hG/c3 and the quantum of area A0 = 8√3πιl2 P .

    If you have triads, what happens when a black hole gets bigger? Are you adding more triads? Are the new triads the same size as the old triads? If the new triads are not the same size as the ones already there, why not? If the number of new triads are not sufficient to close the sphere, What happens? Would the horizon be unstable? What are the numbers of triads that must be added to have a stable horizon? Photons come in quantas which are stable. Black hole grow to stabilized quantas. What are the stabilized quantas? Where did the triads come from? How did they merge with the other triads?

    If you are not adding new triads, but are making the existing triads expand, then what does it mean when all triads are not the same size?
    Does that mean that light would take longer to traverse a bigger black hole with expanded triads? How do you keep the speed of light a constant if you double the surface area of a unit of a triads?

    The questions do not need to be about triads. The question could be about strings or your favorite “articles” that make up your “curved space” and “black hole” and “big bang”.

    Stay tuned to CERN. They might be able to answer some of those questions when they open the “box”.
    jal
     
  17. Apr 19, 2007 #16

    Chronos

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    Permit me a bold prediction. The LHC will not produce new physics. The universe has already produced higher energy laboratory data for us to ponder. e.g., gravity waves - see inspiralling binary neutron stars.
     
  18. Apr 19, 2007 #17

    jal

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    Chronos: Something else to read (To balance/counter you prediction)
    http://www.arxiv.org/PS_cache/arxiv/pdf/0704/0704.2276v1.pdf
    Title: Physics Beyond the Standard Model and Dark Matter
    Authors: Hitoshi Murayama
    Comments: Lectures at Les Houches Summer School, Session 86, Particle Physics and Cosmology: the Fabric of Spacetime, July 31- August 25, 2006
    jal
     
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