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The big bang model, time, and general relativity

  1. Mar 28, 2012 #1
    It had been my understanding that the big bang model of the universe is a theory of the origins of the structure of the universe in which the expansion of the universe is extrapolated back in time until we arrive at a near-singularity and general relativity breaks down. Accordingly, the big bang theory starts when general relativity begins to work, and before that anything could be the case and still be consistent with the big bang model. In my reading of the physics literature this seems to be the case.

    However, isn't this inconsistent with the interpretation of big bang cosmology in which spacetime is created? For instance, it may be asked what happened before the big bang. Which is the better answer?--That nothing happened because the big bang involves the creation of time which nevertheless general relativity can say nothing about? Or is it better to say that there was a singularity before the big bang and that what happened before is debatable.

    I am asking this because science writers like Paul Davies (1978, 'Spacetime singularities in cosmology') will say things like the following:

    "If we extrapolate this prediction to its extreme, we reach a point when all distances in the universe have shrunk to zero. An initial cosmological singularity therefore forms a past temporal extremity to the universe. We cannot continue physical reasoning, or even the concept of spacetime, through such an extremity. For this reason most cosmologists think of the initial singularity as the beginning of the universe. On this view the big bang represents the creation event; the creation not only of all the matter and energy in the universe, but also of spacetime itself."

    But to me, this is either untrue or at least a misleading overstep on the actual content of the the big bang model. So isn't it true that:

    1. The big bang model begins when general relativistic effects allow the evolution of the universe to be predicted.

    2. The big bang model would still remain if it was determined that time extends beyond the origin of the universe.

    Lastly, do theories which attempt to explore the earlier pre-universe, or mutliverse, or universe creation via brane collision, etc. assume that time extends further back, or was time still created before (or at) the big bang?
     
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  3. Mar 28, 2012 #2
    The terminology is very convoluted on what exactly the big-bang is, and there definitely isn't a single, unified definition of 'the big-bang'.

    The classical concept of the big-bang starts from a singularity---i.e. when GR doesn't quite apply---and includes the initial expansion there-from. The classical big-bang theory generally holds (as Paul Davies says) that space-time formed at/around that singularity.

    Because there is no quantum theory of gravity, we have no idea what would have been going on at the singularity (or more generally on any scales below about the planck-length, or above the planck temperature).

    More modern movements in cosmology are considering the possibility that the initial condition wasn't a true singularity, but instead a point of incredibly high density/temperature/etc. This is still often referred to colloquially as the 'big bang', but is perhaps better described as a modern addition/variant to the big bang theory. These ideas may permit space-time to exist before-hand, e.g. if there was a previous 'big crunch', and re-expansion.

    There are even suggestions of avoiding a singularity, while still being the origin of space-time (e.g. Hawking-Hartle model).

    In summary: 'big bang' often used loosely, no single definition exists. Generally, 'big bang' refers to an initial singularity and creation of space-time; modern variants may-or-may-not have a singularity or creation of space-time.
     
  4. Mar 28, 2012 #3

    marcus

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    I agree with the general spirit of your remarks. I think it is misleading to talk about time being created at the start of expansion.
    I don't think there is any scientific evidence to support the assumption that time began at that time. There are lots of people working on resolving the singularity failure of classic GR and developing models that go further back in time. The new models recover the good fit to data of GR and predict the general features of the U just as well as GR does.

    It's an active research area. Some of the new cosmological models giving just as good a fit to the observational data are also pretty much as simple: don't assume extra dimensions or unusually fancy exotica that hasn't been observed. The key thing now is to find ways to TEST the new nonsingular cosmic models.

    At present there is no scientific evidence that ordinary time-evolution does NOT extend back further. So it is unscientific and misleading to talk as if that were known to be the case. All we know is that a certain vintage 1915 theory, General Relativity, breaks down at that point.
     
  5. Mar 28, 2012 #4

    marcus

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    :zzz:

    What does it take for people to realize that there is an active QG field and a very active field of quantum cosmology.
    People are getting hired to faculty positions. Hundreds of research papers a year. Rising visibility at the most important international conferences.

    Check out the April meeting of the APS (American Physical Society) in Atlanta. Two sessions on QG including QC one session of invited talks, one of shorter contributed talks.
    Check out the triennial GR and Gravitation conferences. Marcel Grossmann Paris 2009 Marcel Grossmann Stockholm 2012.

    We have some definite ideas of what could have been going on slightly before the start of expansion. Computer models. Also analytical models. Both covering many different cases. And there are definite ideas about how to TEST some of the models. Phenomenologists working on this.

    It is not correct to say that we have no QC and no idea.
     
  6. Mar 28, 2012 #5

    marcus

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    Last edited: Mar 28, 2012
  7. Mar 28, 2012 #6
    There has been a lot of controversies among many scientists on this topic . Its the matter of view point which to accept . If you go on with the pre dimensional category i-e up to 4 dimensions , in that case you will have to make the big bang from singularity and nothing before it , but if you decide to involve the 4+7 dimensional category , it opens the gate for non existence of singularity and existence of membranes ...
     
  8. Mar 28, 2012 #7

    marcus

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    By far the greatest amount of quantum cosmology research that gets rid of the singularity and goes back further in time is in 4D spacetime.
    It is a misconception (probably spread by some popular mass market books) that you need to go to extra dimensions to "open up" the possibility of nonsingular cosmology.

    The problem now is to TEST nonsingular QC models by deriving their predictions about the CMB temperature and polarization map that stem from the way the singularity is resolved.
    (CMB=cosmic microwave background, the most ancient light we can see.)

    Here are some recent research papers relating to this:
    http://inspirehep.net/search?ln=en&...Search&sf=&so=d&rm=citation&rg=100&sc=0&of=hb
    These are all from 2008 or later and are mostly about testing QC by comparing its predictions with observations of the ancient light.
    The search finds some 49 papers.
    Just looking down the list of titles is interesting.

    Plus there are other QC testing papers that this keyword search does not find. The list is not complete. We'd have to put in more keywords to extend the search to get them. Let me know if you want more links. It's a fascinating field.
     
  9. Mar 28, 2012 #8
    what caused gravity to break away from the other forces to disintegrate the symmetry of the singularity? in doing so, space-time was created. I know my question is so elementary and perhaps even dumb. but, just in the simpliest terms, assuming there was a singularity, why did the symmetry break apart?
     
  10. Mar 28, 2012 #9
    There's an 'active field' for just about everything---including in a 'static universe'; that doesn't mean there's any accepted model for QG in the mainstream, and thus my point stands. Show me the slightest evidence that a quantum theory is correct (or verifiably better than GR or QFT by itself), and I'll revise my statement.
     
  11. Mar 28, 2012 #10

    bapowell

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    The unified symmetry of the early universe, if there was one, has nothing to do with the initial singularity (which is a mathematical artifact and not regarded as something physical.) Instead, the symmetry refers to the laws of physics -- the equations that govern the dynamics of the fundamental particles and forces. Conventional wisdom suggests that all four fundamental forces were unified in the earliest moments after the big bang; however, nobody has succeeded in fitting gravity into a grand unified theory yet. Assuming this can be done, then symmetries in the early universe break for the same reason that symmetries in the present day break: instability.

    Can you think of some examples of symmetry breaking in everyday life? A pencil standing on its tip has rotational symmetry, until it falls. The orientations of magnetic domains above the Currie temperature are random, and the system possesses symmetry, until it cools and the domains "pick" directions in which to point. In fact, most ordinary phase changes are associated with some kind of symmetry breaking. In many cases in everyday life, the energy of the system controls this degree of symmetry: generally, at high energies systems are stable and are able to retain whichever symmetries they possess. At low energies, this stability can be lost and the symmetry can break.

    Going back to the pencil analogy, we can increase the energy of the system by tying a string to the top of the pencil and using it to support the pencil in an upright position. It's pretty stable in this configuration. But, once I cut the string, even the tiniest perturbation will send the pencil toppling over. So, pretty generally, as we lower the energy of a system that possesses symmetry, it becomes possible that the system will "spontaneously" lose this symmetry, and a tiny nudge is sometimes all it takes.

    So what exactly possesses the grand unified symmetry in the early universe? As I said, the laws of physics do. More specifically, the symmetry is exhibited by the configurations of all the dynamic fields (particles) in the theory. It's a little difficult to explain, because the symmetries are rather abstract -- they are not spatial symmetries of the type I described above. The symmetries pertain to various "internal" characteristics of the fields, and these characteristics depend on the energy of the universe. But we can imagine these internal symmetries as abstract pencils, and we can imagine that each force has a pencil (this is sort of a simplification, but let's go on). As the universe cools, these pencils become more likely to tip over: if one tips over, then the corresponding force decouples from the others, and the grand unified symmetry breaks. The first pencil to tip over would gravity, but we don't understand how that happens. As the universe continues to cool, the pencil representing the strong force tips over, and so on.
     
  12. Mar 28, 2012 #11
    Wow, that was a very good response. Thank you for taking time to help me understand this issue. I was surprised to read that breaking of the symmetry did not cause the big bang, but the big bang gave rise to the symmetry that eventually broke. Thus, the original singularity can only be described to exist with complex mathematics. Thank you again for your help.
     
  13. Apr 2, 2012 #12
    Marcus, thank you very much for the later search which covers the last line of this quote. Could you give me some search words to help me find articles describing (some of) the models (I'm having difficulty framing the 'tests' without having some background on the models themselves)? I'm not looking to be spoonfeed (although if you can recommend a good starter pack that would be great), but seperating the general concepts from the detailed tests ain't so easy (for me).

    Much appreciated,

    Noel.
     
  14. Jun 3, 2012 #13

    marcus

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    Hi, I missed this post asking for some keywords or perhaps just links to articles themselves would do as well. I'll get some links.

    The gist is very simple. Standard cosmology is based on an equation called Friedmann equation that relates the "size of the universe" better called the SCALE FACTOR a(t) as it evolves over time to the density of matter and energy rho(t) written with the Greek letter ρ pronounced "rho" and to the Hubble expansion rate H(t) as they evolve over time.

    By putting in numbers for today's values of these things, one can then run the equation back in time and it gives an impressive fit to the observational data.

    But it breaks down right at the start of expansion.

    The Friedmann equation is "classical" (i.e. non-quantum) it was first derived from General Relativity in 1922. There is a QUANTUM version of the Friedmann equation which has been developed over the past 10 years by researchers in what's called "Loop Quantum Cosmology" LQC.
    The new equation has an extra term in it called a quantum correction term. It is ordinarily negligible and becomes large only at very very high energy density. Very high "rho".
    Near a critical value of rho called "rho-crit".
    So the new equation reproduces perfectly all the ordinary observed behavior at lower density because it is basically the same equation (the quantum correction term effectively vanishes). But at density near critical the quantum term gets large and dominates the other terms and makes gravity effectively REPEL rather than attract. It is as if nature resists being pinned down and compressed beyond a certain density.

    So when you run the new Friedmann equation back in time, you get a bounce instead of a breakdown. The universe re-expands as you go back in time. Another word for when a theory breaks down or blows up is "singularity". The old Friedmann eqn developed a singularity and would not evolve at a certain point. The new Friedmann eqn gives a non-singular evolution back past that point.

    So it sounds like you want a pedagogical paper that presents the LQC quantum corrected version of the Friedmann equation.
    Abhay Ashtekar writes that kind of paper from time to time, he is the main senior researcher in LQC. He is head of a group at Penn State, a central figure who has mentored most of the younger LQC people.
    For starters you could check out the titles of his papers over the past few years. There should be an introductory or pedagogical paper or two.

    Here is a search that goes 2005-2012
    http://arxiv.org/find/grp_physics/1/au:+Ashtekar/0/1/0/2005,2006,2007,2008,2009,2010,2011,2012/0/1

    I will look thru and see if I spot a title that suggests introductory LQC. But you could also scan down the list of titles to get an idea.

    Here we go! Look on page 7 of this 2010 paper of Ashtekar
    http://arxiv.org/pdf/1005.5491v1.pdf
    It is the most general audience non-specialist paper I could find in recent years, for LQC.

    On page 7 you see the simplest possible presentation of both the non-quantum (classical) Friedman and the quantum version. They are equations (3.1) and (3.2).

    On other pages you also see some figures plotting how the scalefactor behaves in bounce, in various cases.
    This is the model that the other papers are concerned with TESTING that were listed by that other search link that you already saw.
     
    Last edited: Jun 3, 2012
  15. Jun 3, 2012 #14
    Thanks very much Marcus. This is exactly the type of information that I was looking for. Very much appreciated.

    Regards,

    Noel.
     
  16. Jun 3, 2012 #15

    Ken G

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    I'd say it's hard to classify theories like cosmologies based on quantum gravity. There is a lot going on at the "fringes" these days, where we have things like string theory, modifications to Newtonian gravity, multiverses, and quantum gravity. Personally, I'm glad people are thinking about these things, because you just never know what new promising ideas might get generated, but the general tendency is for all these areas to get vastly oversold. We need to face facts: all these areas are highly speculative, and also highly unlikely to deliver when they get too full of themselves and make all kinds of grandiose promises-- promises they will likely not fulfill, but funding is funding. You can certainly get a conference together of like-minded people, and these subgroups can tend to kind of take on a life of their own, but most mainstream scientists outside these subgroups are pretty skeptical of their results-- until they really have a dramatic prediction to show for themselves.
     
  17. Jun 3, 2012 #16

    marcus

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    A good way to get perspective on what is fringe and what is more central, in mainstream cosmology, is to look at the representation in the main international conferences.

    Any one person's IMPRESSIONS can be outdated because things change, or they can have an individual bias. So it is helpful to have some objective indicators that one can watch change over the years.

    For example I watch two large triennial conferences related to general relativity, cosmology, and gravitation (both theoretical and experimental gravity waves etc.). The GR conference series (General Relativity and Gravitation) and the MG series (Marcel Grossmann meetings).

    MG-13 is this year in Stockholm (already 907 participants have registered!)
    http://www.icra.it/mg/mg13/parallel_sessions.htm
    Out of 19 parallel session categories how many are directly dealing with cosmology?
    Check out CM, GT2, HR, OC, QG4, TC
    No multiverse stuff AFAICS, it has largely gone out of fashion. Not of much scientific interest to the community of working cosmologists, it appears. Maybe in some other mainstream venue.
    But there are several sessions devoted to LOOP gravity and cosmology.

    GR-20 is next year in Warsaw. It doesn't have a website yet, but to get some idea here is the program announcement for GR-19, the 19th in this series of triennial conferences.
    http://hyperspace.aei.mpg.de/2010/03/08/gr19-scientific-program-update/ [Broken]
    It was held in 2010, which is why the next one is coming up next year in 2013.
    One can get an idea from the program just how fringe and how mainstream the various quantum theories of gravity are (in the view of major conference organizers). It will be interesting to see the lineup of GR-20 when it is posted. Certainly not a black-and-white issue, more a matter of degree (and obviously shifting.)
     
    Last edited by a moderator: May 6, 2017
  18. Jun 3, 2012 #17
    The reason that LQG is interesting is that

    1) it's a refreshing new approach. Rather than trying to come up with a "theory of everything" just worry about quantizing gravity

    2) even if it is wrong it's useful. A lot of the math involved in figuring out how to "quantize gravity in the real world" turns out to be the same as figuring out how to "quantize gravity in a computer program". So even if it turns out that LQG doesn't work, we can get some useful computer programs out of it.
     
  19. Jun 3, 2012 #18

    Ken G

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    Excellent point.
    Now there's some hard evidence for you, though I note there was no reaction. I think multiverse enthusiasts continue to pursue it, but I'm not aware that it was ever considered mainstream by anyone other than multiverse enthusiasts.
    I think it is natural to expect every new idea to generate some buzz for awhile, and a high level of speculation is tolerated at first, but if it doesn't produce any hard evidence, it falls out of favor before it ever picks up much steam. Even string theory is losing some of its momentum, though it is largely maintained by successes that are far outside of the original set of goals for the theory, and still attracts recruits by virtue of not having much in the way of competition for fundamental new theories.
     
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