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Big Bang as an extrapolation

  1. Oct 31, 2007 #1

    OOO

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    Today I have been travelling across the night sky with Google Earth a bit. As always I'm surprised about how inspiring the cosmos is for thinking about physics. But I've always been quite skeptical about the big bang theory. Nowadays we're unable to simulate even a single hadron. And the observation of the sun seems to be a constant source of surprise about it's inner workings.

    Yet people claim in detail about how the big bang took place and what happened when. I don't say the big bang is wrong because I'm not an expert in cosmology. But isnt't it a rather foolhardy extrapolation ? I have no philosophical (or whatever psychologically or religiously motivated) problem with a big bang apart from finding it a little strange that there should have been a singular event where such a huge amount of energy has been accidentally created from the vacuum. But if it was that way, well, okay.

    What I do have a problem with is the assumption that the laws of physics have been the same for 13 billion years and/or over a distance of 13 billion lightyears. Who can say that ? Probably masses and coupling constants are just a matter of boundary conditions.

    If I were to bet my son's life on the big bang, I would never ever do so.

    So what makes you cosmologists so sure about that thing ?
     
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  3. Oct 31, 2007 #2

    EL

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    This is a common misunderstanding of the Big Bang theory. We cannot extrapolate all the way back to the "beginning" since we do not know what physical laws to use in that extrapolation. We can "only" go about 13,7 billion years back until the temperature of the Universe corresponds to energies we have measured in accelerator experiments. Before that we just have some fair speculations of what happened. What the Big Bang theory says is that the Universe ones was in a much hotter and much denser state than it is today.

    Measurements indicates so. Maybe someone's got a good reference?

    Please don't bet your son's life om anything.
     
  4. Oct 31, 2007 #3

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    Of course I won't. :smile: That was just a metaphor for how carelessly we take some things for granted.
     
  5. Oct 31, 2007 #4

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    So big bang cosmology is all about thermodynamics in a gravitational background ?
     
  6. Oct 31, 2007 #5

    EL

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    Well, you need to add atomic, nuclear and particle physics as well.
     
  7. Oct 31, 2007 #6

    EL

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    In know.:smile:
    But seriously, the Big Bang theory is very well founded. See e.g. http://en.wikipedia.org/wiki/Big_bang
     
  8. Oct 31, 2007 #7

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    I bet there has gone a lot of energy into it, but that doesn't necessarily mean it is well founded. Take this from http://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis as an example:

    "During the 1970s, there was a major puzzle in that the density of baryons as calculated by Big Bang nucleosynthesis was much less than the observed mass of the universe based on calculations of the expansion rate. This puzzle was resolved in large part by postulating the existence of dark matter."

    Know what ? There is less money on my bank account than I expected. This puzzle was resolved by postulating a giant white money-eating rabbit.
     
  9. Oct 31, 2007 #8
    That same rabbit has been munching in my bank account
     
  10. Oct 31, 2007 #9

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    Probably we should try to convince this silly animal of becoming a macrobiotic. I mean, who does he think he is ?
     
  11. Oct 31, 2007 #10

    marcus

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    What about $50? Would you be willing to bet fifty dollars that it's wrong?

    I'm curious how you imagine the bet would work. How would we decide who wins?

    Or if you wouldn't be willing to bet against, would you bet fifty bucks that something like that has happened? And how would we decide?
     
  12. Oct 31, 2007 #11

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    That's precisely the point.
     
  13. Oct 31, 2007 #12

    marcus

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    then maybe we are in at least partial agreement. I still wish you would spell out what you think is precisely the point of what. But I imagine that what you mean is something professional cosmologists know extremely well. They are the toughest skeptics of their own models, and by far the best informed doubters. Professionals know very well that one cannot verify some particular version of early universe cosmology.

    So it would be senseless to propose a bet like that.

    All you can do is use the best most accurate theory of gravity we have, so far, and fit the data to it as closely as you can, making the fewest assumptions you can to get a close fit to observation, and see what that says.

    You don't BELIEVE theories, you test them and use them (always provisionally in case a better theory comes along, and always skeptically.) Conclusions should always be qualified: "if suchandsuch model is correct, then..."

    I am curious where you are getting your ideas about who these people are and what they claim. I hope you are not talking about popular journalism or pop-sci books. I'd like to have a link to somewhere a professional cosmologist makes some unqualified claim about events and conditions.

    Assertions really ought to be qualified by reservations like "according to the usual LCDM model" or "according to GR"

    GR is the most precise theory of gravity we have, it fits observations to many decimal places, but I think any working cosmologist would tell you that it can't be relied on in extreme situations. You dont believe GR, you apply it where it works and keep mental reservations about where it doesn't.

    Conventional cosmology is, of course, entirely based on GR. So it is subject to the same reservations.
    ==================
     
    Last edited: Oct 31, 2007
  14. Oct 31, 2007 #13

    Wallace

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    The issue, as marcus suggests, lies not with Big Bang theory but in understand the nature of scientific 'knowledge' in general. Modern Cosmology has a pretty detailed theory of how the Universe has evolved that is referred to commonly as 'the Big Bang', although the theory is much more complex than the simple name implies.

    The point though is that your initial problem is a straw man argument, professional cosmologists do not hold the view that you are arguing against. Based on the best evidence we have we attempt to construct what appears to be the most likely explanation for what we observe, and that is about all we can ever say about a scientific theory, that it is supported by the evidence.

    The evidence for the Big Bang is overwhelming, but that doesn't mean that it is an unassailable truth in scientific cannon.

    Pop-sci writers such as Richard Dawkins, or more relevant to cosmology, Simon Singh, sometimes, in my view, go a bit over the top in terms of the certainty of scientific knowledge. It's probably a forgivable sin, but it does lead to misunderstanding and confusing, such as that expressed by the OP.
     
  15. Oct 31, 2007 #14
    Would it be a stupid question to ask the size of the universe at this early stage?
     
  16. Oct 31, 2007 #15

    russ_watters

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    The big bang is not taken for granted.

    But evidence shows clearly that everything in the universe is flying apart. So what does that imply about the past...?
     
  17. Oct 31, 2007 #16

    russ_watters

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    That's a very difficult question. The best that can be said is it must have been very hot and very dense.
     
  18. Oct 31, 2007 #17

    pervect

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    We are pretty sure from the CMB readings that the entire observable universe originally occupied a very small size, because the temperature of the CMB appears to be the same in all directions.

    However, the entire universe may be and probably is much bigger than the observable universe. For more on this, see for instance

    http://map.gsfc.nasa.gov/m_uni/uni_101bb2.html

     
  19. Nov 1, 2007 #18

    EL

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    I see you have already got some anwers to your question, but I'll just add some words.
    No, it is not a stupid question, but mearly slightly unprecise. It depends on what you mean by "the universe". Usually in the scientific community "the universe" is a synonym to "the observable unvierse", that is the part of the universe from where light has had time to reach us. In that case "the universe" was very tiny at the moment the temperature corresponded to the energy scales we have been able to study in labs. In principle it should be possible to calculate the size of "the universe" at that moment. At least a rough estimate shouldn't be a problem, but I do not have the numbers in my head.
    If by "the universe" you mean also the parts which are unobservable it gets more complicated, and more philosophical. See pervect's reply.
     
  20. Nov 1, 2007 #19

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    I was comparing cosmology with the ability to make long-term predictions in other branches of physics. Take the weather forecast as an example and compare it's time scale (days) with cosmology (13 billion years). Add to this the assumption that we don't even understand the standard model very well, let alone theories beyond the standard model.

    That's fine, so maybe I just got the wrong impression of the whole thing.

    Star Trek certainly is not one of my sources, although sometimes I think that it could be helpful... :smile:

    I admit that to a large extent I'm referring to the view of popular science journalism. That's because I always avoided cosmology while learning GR (shouldn't have done this, I guess).

    Maybe I just haven't found an adequate attitude towards pop journalism yet. It's the same with my own field, theoretical particle physics (in which I consider myself a novice): Everywhere the public is informed about how fantastic the standard model is and how cool physicists are. But if you look more closely it boils down to some highly accurate predictions, some quite reasonable ones, and the bloody rest.
     
  21. Nov 1, 2007 #20

    EL

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    What do you mean? The standard model is extremely well tested!
     
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