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

Universe began in a hot big bang

  1. Jan 17, 2004 #1
    This is my first post and I’ve got some questions in regards to our universe.
    From what I’ve been reading it is known that the universe began in a hot big bang, or evolved from a similar condition. Either way the information we have points to some kind of initial condition, and also a beginning of time.
    Is it reasonable to think that there was a preexisting universe, or a cyclic model? And if so would the features of preexistence have the same fundemental laws that govern our universe? I don’t think I’m trying to suggest transmission through a singularity, but merely understand if the dimensions we have in our universe would be the same, and if time had a beginning it should surely have an end, meaning the whole process repeats itself.
  2. jcsd
  3. Jan 17, 2004 #2


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Re: Preexistence

    the "beginning of time" idea is becoming obsolete
    the model of the universe that cosmologists have used for many decades, called the Friedmann equations or the Friedman model, had a glitch.

    it broke down and failed to compute at the beginning of the observed expansion----if you pushed the model back in time then at a certain point it would blow up and develop infinities (which means it was no longer working)

    the Friedmann model has been fixed (by Martin Bojowald and others) as of 2001 and the glitch or singularity has been removed, by quantizing the classical version.

    this was expected--quantizing a classical model not uncommonly removes singularities or other bad behavior.

    Bojowald's quantizing the classical model is straightforward without exotic extra baggage like "extra dimensions" or "supersymmetry" or "colliding brane worlds". He just goes ahead in the conventional and simplest possible way and quantizes the model
    with as few extra assumptions as possible
    and gets a model which contracts
    and then reaches a very high but finite density and then begins expanding
    and the model also drives an inflationary scenario

    Since Bojowald's landmark 2001 paper
    "Absence of Singularity in Loop Quantum Cosmology"
    a number of other authors have redone the calculation
    in various ways with various assumptions
    there has been a miniature "bandwagon"

    wavefunctions for the prior contracting phase have been calculated and the computer drawings of them are in some of Bojowald's other papers

    Here are some links
    scroll down to where it says "Loop Quantum Cosmology"
    Last edited: Jan 17, 2004
  4. Jan 17, 2004 #3
    Re: Preexistence

    I think you may be having trouble with the idea of conservation of energy with a creation from nothing model. But conservation laws are only relevant to a universe that exist and does not apply to nonexistence. So there is no violation of such conservation laws to have a universe develop from nothing. We only require that its quantities be conserved once it does start.
  5. Jan 17, 2004 #4

    Thank you Marcus, I appreciate your explanation.

    I did a little research into Bojowald’s paper, and as I understand it, the model pushes everything back to a point where the volume becomes zero. So quantum geometry extends space-time to a branch preceding the classical singularity, therefore removing any such beginning, or taking time out of the geometry. Can this be viewed as being some form of history? Does this still involve a “bounce”?, as you put it, “and gets a model which contracts, and then reaches a very high but finite density and then begins expanding”
  6. Jan 17, 2004 #5
    Re: Re: Preexistence

    Makes sense...
  7. Jan 17, 2004 #6


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Re: preexistence

    There are computer-generated picture of the wavefunction
    shrinking down (to very high but still finite density) and then
    reexpanding. For example on page 16 of Bojowald's
    "Homogeneous Loop Quantum Cosmology"

    Quantizing space prevents the volume from quite becoming zero, or if you imagine the classical glitch as one of infinite density, quantizing prevents the density from exceeding a very high but finite limit. So the classical glitch is eliminated and the universe just
    collapses and re-expands.
    But whether it will eventually start collapsing again, and repeat the process, is unknown and depends on other things like how the (so-far not understood) dark energy behaves.

    About all that can be said is there is no reason to suppose that time began at the same instant as expansion. The universe seems to have existed before. Maybe time has no beginning. Or maybe it has some other earlier beginning.

    The idea that time began with the BB was an artifact of using an imperfect model that had a glitch. Because they couldnt push the model back further than BB they said "time begins here"
    but now the model is improved and they can push it back further, so the point at which time begins eludes our grasp. Like grabbing a slippery fish that gets away. If there is such a beginning it must be somewhere else.
  8. Jan 17, 2004 #7
    Re: Re: preexistence

    Could it be that it depends on HOW it initially expanded. If there is a linear regression of things to t=0, then one might suppose that momenum will continue it to negative time. But if things proceed from a state of zero velocity and accelerated from there, then there is no mechanism to suppose there was negative time. Besides, if things do regress to absolute zero spacetime, then all information about any momentum is lost, and again we loose any mechnism to suppose negative time. So if it did proceed from zero, then what may have been prior to that is lost and become irrelavant and not provable.
  9. Jan 18, 2004 #8
    Under the first link you gave me, Bojowald’s Loop Quantum Cosmology papers indicates that as the universe proceeds inside a quantum geometry, there is no uniform instants of time. Or as you put it “disappearance of continuous time coordinates at the quantum level”.

    This only states that when applied to quantum conditions continuous time instants no longer apply. We don’t live at the quantum level, so time coordinates can be used, and given to calculate the age of our universe.

    However as continuous time coordinates do not apply at the quantum level, evolution would behave quite differently, but I think also behave in accordance to quantum laws.
  10. Jan 18, 2004 #9


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Vast, this is a technical point and I can provide links to
    papers if you want (Ashtekhar, Lewandowski, Bojowald and others)
    but it is, as I say, technical. Loop Quantum Cosmology
    as Bojowald and the others have developed it has a good
    semi-classical and classical limit. That is, even tho
    time-evolution is discrete after about as hundred steps it
    blends into the same evolution as the old model.

    The bounce appears to be qualitatively symmetric in nearly all the papers
    (there was one paper by Seth Majors and Franz Hinterleitner, IIRC,
    where things didnt turn out that way, but mostly it seems to)
    so what we are looking at is an evolution (contraction-bounce-expansion) where ordinary time appears as the limit of little steps, in the limit or as you get far enough away from where the singularity used to be,
    and the contraction is pretty much the same as the expansion run backwards.
    So time (as a smooth largescale limit of something discrete at small scale) extends back.
    Now what I am telling you is only my impression from reading a bunch of articles---and it is just my opinion! After mulling it over, I personally dont see any singularity, or any beginning to time, at that moment. Neither do the authors of the papers, of course.
    All that happens at that moment is that the quantized Hamiltonian operator that governs the evolution of the system (by means of a difference-equation) makes a contraction reach a high-density limit and turn around and become an expansion. The behavior appears to me to be not "put-in-by-hand" but a kind of business-as-usual continuation that comes from quantizing the system. Time, in as much as it exists as a continuous process at large scale, just continues.
  11. Jan 18, 2004 #10
    As I understand it, quantum mechanics can be formulated by the Fyneman path integral which assigns an amplitude to each path and adds up every possible path from initial to final states. It is easy to understand alternative paths inside a given space or manifold. But how can you have alternatives paths for the universe as a whole. There simply is no alternative to the given universe. There is no space outside the universe to plot alternative path of this universe. So it would seem that overall spacetime cannot be quantized because there is no higher dimensional space to form the required alternative paths of a quantization formulization of Feynmen. This would seem relevant since you are talking about jumps in time, which could only mean quantum jumps in the expansion of overall space.
  12. Jan 18, 2004 #11


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I dont understand why you think it necessary to assume the existence of a space outside the universe in which to plot alternative paths.

    Try reading Rovelli's discussion on page 52 and 53 of Quantum Gravity, the section 2.3.2 called "The Disappearance of Spacetime" where he quotes Einstein's words

    "...the requirement of general covariance takes away from space and time the last remnant of physical objectivity..."

    this is from the 1916 "Grundlage der allgemeinen Relativitaetstheorie"

    by general covariance what he means is "Diffeomorphism Invariance" the DI property that LQG preserved from General Relativity. For a good accessible discussion read Smolin's SciAm article in the January issue.

    There is no manifold. There is no continuum. The quantum states of space are relational and a quantum superposition of enough of them looks and acts like a continuum. It is not necessary to assume the existence of a differentiable manifold in which to define quantum states, or alternative paths, or Feynmann "sum over histories".

    Rovelli page 52: "...Therefore localization on [a manifold] M is just gauge: it is physically irrelevant. In fact, M itself has no physical interpretation, it is merely a mathematical device...M cannot be interpreted as a set of physical 'events', or physical spacetime points 'where' the fields take value...Contrary to Newton and Minkowski, there are no spacetime points where particles and fields live..."

    He is talking here about 1915 General Relativity, which we humans have been using as our fundamental model of space time and gravity for over 80 years. Apparently we have not gotten used to it yet. We have not yet heard what Einstein said:

    the spacetime continuum, the Riemannian manifold, does not have a physical meaning, is not real.

    "...the requirement of general covariance takes away from space and time the last remnant of physical objectivity..."

    Einstein agonized and waffled for 3 years (1912-1915) before taking this step. It was the only way he could get a workable theory. He was racing David Hilbert to get a modern theory of gravity and he still worried for 3 years. This step was not taken lightly.
    There is an historical account on pages 35-45 of Quantum Gravity, recounting Einstein's journey to reach GR, lots of quotes and firsthand source stuff.

    Its great. Read the book online if it is still available, or buy it when it comes out.

    This PF post has a bunch of LQG resource links
    The first link is to Rovelli's book.

    Or you can get it by just saying "Rovelli" to google. That gets you
    Rovelli's homepage and down the page from his photo is the link to the final prepublication draft of the book (if it is still available).
    Last edited: Jan 18, 2004
  13. Jan 18, 2004 #12
    I don't know what you mean by relational. What do you mean, what is it contrast to?

    All the quantum stuff I can remember is derived from diff eqs or integral formulations which assume the existence of some continuous variable being differentiated with respect to them or being integrated with respect to them.

    How can you say that something that does not exist can then be quantized?
  14. Jan 18, 2004 #13


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Mike, I think you are making a mistake not to read directly the non-mathematical parts of Rovelli's book that apply. I pointed to a few sections.

    We wont get anywhere if you take my sketchy summary of his points and argue with them. I cant reproduce the whole 5 or 10 page discussion.

    Best way to understand is read Smolin SciAm article (have you already?) and then look at a few non-mathematical parts of Chapter 2 of Rovelli.

    1915 GR substitutes the gravitational field for the old (diff. manif) idea of space.

    The old spacetime (a manifold, a continuum, Newtonian or Minkowskian absolute space, or whatever) has no more physical existence.

    You say, "what is to quantize then?"

    The gravitational field is what is to quantize.

    You will have to want to understand this---how the field can exist independently of a particular differentiable manifold---or you wont grasp the essential and we will find it impossible to converse.

    (the manifold, in those cases where it is used for purposes of definition, is a conventional mathematical convenience)

    The gravitational field is the arena where things happen where matter fields live and it the vehicle for the geometry----it takes the place of the spacetime continuum----and the gravitational field is what is quantized by the Loop theory.
    That is why it is called Quantum Gravity.

    Imagine a universe with only one star in it.
    I tell you that the star is turning.
    You ask, with respect to what is it turning?
    It is turning with respect to the gravitational field.

    See page 40 of Quantum Gravity for more examples, section
    2.2.3 "The Key Idea"

    BTW you are welcome to "pass" on quantum gravity. It is what is happening in our era in the history of science, I believe, and it is hardly necessary for everybody to know about it! Scientific revolutions occur largely without the knowledge of contemporaries. You can, if you chose, say "I dont need to know this" (as per Nibles) and just drop the conversation. I'm interested in talking with people who want to understand and dont have time to waste in argument.
  15. Jan 19, 2004 #14
    Thanks Marcus. Anything too technical is over my head, but I still have some of the other papers to read for now.

    The impression I’m getting is that the computer-generated universes that are created extending space-time back, use the cosmological origin as a point in which the density it can acquire is finite.
    The cosmological origin seems to be a point where contraction reaches it’s highest density and expands again, where as expansion of our universe reaches it’s lowest density and begins to contract again. (Supposedly)
    As the universe doesn’t have a static state, and is either expanding or contracting, our universe simply goes through such a process at the cosmological origin.

    Now when I read about the Feynman path integral, I take this to mean histories which contribute to our universe. (Correct me if I’m wrong) Histories which contribute to the inflation of our universe. The Feynman path integral allows every possible history. In a prior universe, it seems logical to me that something would contribute to the observable universe we have now.

    There’s only ever been our own universe to refer to, and this gives me the impression that the cosmological origin 13.7 billion years ago, is an initial state. On the other hand in such Ekpyrotic or Cyclic models the initial state is in the infinite past.
    I think the real question is whether or not anything can be conserved.
  16. Jan 19, 2004 #15
    Please provide the link to the document, and give the page numbers. Thanks.

    As I understand it, the quantized gravitational field exists in the form of a "graviton" which travels through some background space assumed to exist.

    In any event, doesn't the process of quantization require the mathematical tools of differentiation and integration which again assumes some sort of continuous variable to integrate and differentiate with respect to?

    I certainly understand that a manifold can take different coordinate systems. But I'm not aware of any mathematics that can produce quantized states from quantized space? How do the gravitons communicate or interact if not through some continuous space? Are you suggesting a communication through some discontinuity from one graviton boarder to the next? You seem to be suggesting that no distance function can be assigned between one graviton (or quantized bit of space) and the next. But you seem to suppose that they are individualized. Do they have a volume, or are they point particles? If no distance can be assigned between them, then they cannot interact, for you cannot say at which distance they interact.

    If everything is a graviton, then where do the photons, quarks, and leptons come from?

    I'm interested in the orgin of the universe. I'm not trying to be argumentative. But I find some objection to the premises being asserted. I've heard this quantized space argument before, and it seems to contradict the notion of information traveling from one point to the next through some none existent medium. If it travels, then there is a medium. Seems obvious enough to me.
  17. Jan 19, 2004 #16
    As far as logic is concerned, this is just begging the question as to where this prior universe came from. Also, logic allows for a conclusion to be true when the premise is false. You can logically get something from nothing. You are just supposing that conservation laws apply to a universe that does not exist yet. But it is only after it starts that conservation laws apply. The transition from nonexistence to existence is not something to which conservation law apply.
  18. Jan 19, 2004 #17


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Hi Mike, you asked for links and page numbers. In this thread I gave pointers for getting Rovelli's book in an earlier post, which I will quote here, and some page references. I have given rather a lot of page references to Rovelli's book scattered throughout several posts.

    I would say the two essential pieces of reading material for Loop Gravity are Smolin's SciAm article and the non-mathematical parts of Chapter 2 of Rovelli's book.

    If you want to learn about the subject these are good places to begin. The SciAm article is not free-online---it means a trip to the public library. If you have any trouble downloading Rovelli's book from his website please let me know. I am worried that he may take the draft version down, now that the book is at the publisher.
  19. Jan 19, 2004 #18


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    You've stimulated so many thoughts in my head that I dont know where to begin.

    where the universe came from is a Great High Question---the question of Origin.

    what happened 13.7 billion years ago is a practical mundane kind of question. The work of Bojo and others "demystifies" it. We have a model now that works better so we can work back in time past the 13.7 mark.

    So that mark does not have to be confused, any longer, with the mysterious Origin.

    However there is a huge amount that is not mundane and is still mysterious---huge questions to which Loop Cosmology and other quantumgravitybased theories provide little by way of clues.

    You seem to like the oscillating picture. But as far as I am aware, there is no hard evidence that the current expansion phase will ever end. Particularly because of the positive Lambda term in the model, associated with accelerating expansion, one cannot on the basis of present data predict that the expansion will ever turn around and give way to contraction. I think we need to be extremely patient, adopt a kind of "agnostic" open mind and honestly admit to how little of the whole story we really know.

    All right 13.7 ago there was no grand Origin. But what about 13.7 or 50 before that? And what about 13.7 billion years or 50 billion years in our future? Our minds do not like infinite expanses of time and tend to try to put bounds on them, or to put cycles on them (which is another way to achieve intellectual control of the infinite). For aesthetic or psychological reasons we have this urge to impose Termini---great beginnings and ends----on time, or to impose Cycles. The Ancient Hindu picture of time with its 50(?) billion year Kalpa or "Day of Brahma" was a nice cyclic picture. Perhaps it is even correct :wink: but I feel certain that we dont have nearly enough data to assess that!

    You ask challenging questions but all I have by way of anser is extremely limited. Loop Gravity is, as far as I know, the simplest quantization of the model of the universe that cosmologists have used for 80 some years. It just conservatively quantizes the model that has worked for us, with as little extra baggage and extra assumptions as humanly possible. So compared with other things it is barely different from the tried-and-true Gen. Rel. that has worked for us for 80 years and passed many a predictive test.
    And all the application of Loop Gravity to cosmology (i.e. Loop Cosmology) has done so far is get rid of the singularity 13.7 billion years ago and provide an explanation for how inflation may have been triggered. That is, Loop sheds a little light on one brief moment---and dispels the Aura of Mysterious Origin only from that one moment.
    All that happens is that the Aura of Mystery drifts off and collects somewhere else. It is a foggy nimbus which is always in some corner of the picture. I hope that my speculations about this in some sense parallel or paraphrase your own, so that they connect. Its a vague area of speculation for me, but interesting to think about.
  20. Jan 20, 2004 #19
    Mike, you seem to have misunderstood what I was saying. The prior universe that contributes to the universe we're in now, is our own. Our universe is basically Everything that exists right? So if we extend space-time back we should get a bounce which expands the universe into a prior existence.
  21. Jan 20, 2004 #20
    I accept that things are incomplete, and the mysteries of quantum mechanics won’t be solved any time soon.

    But can everything really come from nothing?
    Because for everything to come from nothing, it would seem that there really is no limit to how much can come into existence. The impression I get from LQC is that the density is finite, (At point of origin) which implies that it is not a state of non-existence, but simply an initial state for our universe.

    It even seems to suggest that the process of expansion and contraction is an infinite cycle.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Universe began in a hot big bang
  1. Big Bang pattern? (Replies: 12)

  2. Before the Big Bang (Replies: 12)

  3. Pre-Big Bang Scenario (Replies: 3)

  4. Holographic big bang (Replies: 1)