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What does time mean in the context of BB theory?

  1. Sep 4, 2011 #1
    I would like to understand more about time. I am studying amongst other things, reality. If BB theory is the standard explanation for the start of everything then I would like to know how it explains the start of time.

    We link time to space and to events (entropy) and we measure time in defined units related to changes in the energy level of the caesium atom. Seems clear to me what time is. I don’t understand what is meant by claims that time is an illusion.

    BB theory says that time started with the existence of the singularity. I think it has to, otherwise we don’t get any BB sequence flow.
    Space also has to exist, otherwise there isn’t any expansion.

    If the singularity was a one-time event, how did information flow in the first fraction of a second in order to feed the expansion? In one place I read, it is ascribed to negative gravity and in another place it is ascribed to a vacuum effect i.e. the expansion was a sucking process rather than a pushing process. In this version, what did the sucking? Or if it was a pushing process using dark energy, how does dark energy push?

    Taking the negative gravity version, if in a high density, high mass environment gravity slows time down and presumably negative gravity speeds it up, was this factored into the BB timeline calculations? Am I correct in understanding that if the universe was initially highly compressed, time would hardly flow due to high gravity. Then with the reversal of gravity, time would suddenly speed up tremendously, hence the very high rate of initial expansion? Is this what is meant?

    BB theory uses such tiny fractions of a second after the BB, I start to wonder what meaning do such time calculations have? Alternatively, is it necessary that the BB proceeded so quickly? If it was neither Big nor a Bang, is it also possible that it proceeded extremely slowly? Have we got the wrong title here?

    So my question is, at what point in the history of the universe did time as we collectively perceive it on earth start? Did it start with the singularity or did it start with the decoupling? Does the inflationary time period (photon epoch) of 380,000 earth years mean anything in terms of time elapsed by today’s time flow standard? Until the photons were free, were they nevertheless travelling at c and how do we calculate the timing of BB events? What am I missing or misunderstanding, which would support the timeline of the BB, especially its very high rate of expansion from a standing start. Could it have been orders of magnitude slower?

    Please note, I only want to better understand BB theory, I don’t want to criticise it, although I am interested to know if there is more than one accepted BB version concerning the start of time.
  2. jcsd
  3. Sep 4, 2011 #2


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    Then maybe before you start making guesses, you should start at the very beginning. You might want to read these:


    Please note that the PF Rules that you had agreed to prohibit speculation based on something you still don't understand.

  4. Sep 4, 2011 #3

    Not a good start with my first posting! In fact I have been reading this forum for many months, so I should not have made this mistake. Sorry about that.

    As far as I can see, my OP contained only 1 speculation, that the BB timeline could have been considerably longer. I admit that I have no basis for saying this. I am just gagging at the accuracy of the timing of events in the first second or so. I would like to get some idea of how they are calculated. I would also like to get an idea of the margin of error of the BB timing of the events of the first 500,000 years. I have noticed that most of these timings are given as absolute numbers without a range.

    So far as the reversal of gravity is concerned, that's not my speculation, I took it from Alan Guth.

    Everything else were questions, not speculations, as far as I can see.

    Thank you for the links, which I have looked thru but found few clues to the answer to my questions about the development of time itself during the BB and how the time dimension was affected by the other events.
  5. Sep 4, 2011 #4
    There are no stupid questions. If they are asked honestly and without intent to advance a speculation then I don't see the harm in the OP.

    That said at t=0 or the BB when there existed a singularity there was no time or space as such. Time started at t>0 no matter how infinitesimal the increment in t.
  6. Sep 4, 2011 #5


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    Well, that second one certainly can't be correct since there was no vacuum in the very early universe.

    BB theory is just a specific case of general relativity, so time in BB theory has the same meaning as time in GR.
  7. Sep 4, 2011 #6
  8. Sep 4, 2011 #7

    Ken G

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    I think the problem here is that this is not actually a very good way to think of the BB theory, even though you see it in a lot of places. The BB theory is really just a theory that allows you to run time backward, using GR, and you get a universe that is much (much!) hotter and denser. Any universe like that is a BB universe. It's also true that if you run it back far enough, GR predicts a singularity, but few people take that prediction very seriously. Instead, it seems more appropriate to expect that the theory just breaks down at some point.

    Framed like that, we never have to talk about singularities, but instead we have a new version of your question: can we understand the expansion as a consequence of some physics that occurs within the realm where our theories still work? I think the answer to that would have to be no, though there are some proto-theories like inflation and branes that might shed some insight. Inflation has evidence to favor it as a model, but it's not really a theory yet, because it invokes a scalar inflaton field that really doesn't fit anywhere in our current physics. It's more like a wish list for a theory than a real theory, but many see it as promising. In any event, I suppose it depends on who you ask, but I would tend to see expansion as an initial condition on any BB model, rather than something that modern physics theories explain. That might change, and people like Hawking think it already has an explanation. But I'd say they are invoking these proto-theories to do it, not relying strictly on what we might call "modern physics."
  9. Sep 5, 2011 #8
    Thanks, I didn't realize that BB Theory doesn't include the singularity itself.

    I also found the following quote: "there is no well-supported model describing the action prior to 10^−15 seconds or so."

    Further, I read that the age of the universe estimated at 13.75 bn years has a margin of error of plus minus 0.11 bn, which means about 1% of the total age. But it didn't say whether the error would be mostly before decoupling. Still, if the singularity to decoupling is 0.11bn years off the estimate of 380,000 years, it could have been 500,000 years, which is the same order of magnitude for me.

    Can someone tell me, what are the conditions necessary for time to exist, so that I can see where it fits in the BB timeline sequence of events?
  10. Sep 5, 2011 #9

    Ken G

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    Presumably that 1% uncertainty applies globally, so it's only 1% uncertain for 380,000 years, so it could only be 380,000+3,800 = 383,300, not 500,000. But that uncertainty is a mixture of random and systematic uncertainties, it's hard to say just exactly what that means because systematic uncertainties have a way of changing when the paradigm shifts.
    Nobody knows what conditions are necessary for time to exist, though some have informed opinions.
  11. Sep 7, 2011 #10
    Thank you all for your help so far. Unfortunately I don’t feel that I am getting close enough to be able to say that my questions have been fully answered. My questions only require a clarification of BB theory in the time dimension.

    May I recap as follows, based on what I have learnt so far:

    - The BB started with the universe in a very dense and hot state. T=0 would refer to the Singularity, even if it’s technically not included in the Theory itself.

    - The description of the BB events and their sequence is very detailed with precise timings during the first fraction of a second. These events involve the total mass and energy of the universe, which I suppose was not smaller than what exists today.

    - The calculated timing of the total BB has a range of error of the order of 1-2%. Presumably this accuracy also applies to the first seconds.

    I would like to present my main questions in another way, within the scope of my OP:

    1. Do I correctly assume that, although there was neither space nor time at the Singularity, they both existed immediately afterwards. At the moment I don’t see any alternative, because I am assuming that anything which happens to or in the universe needs both space and time.

    2. Do the timings which I find in the Wikipedia Timeline of the Big Bang equal the equivalent measures of time which we refer to now using a caesium clock?

    3. Referring to the Photon Epoch (10s to 380,000 years) when the photons were interacting with other particles and only became free at the end of this period, thus making the universe transparent and producing the CMBR, I ask without speculating:

    a) Is it assumed that the velocity of photons before the decoupling was the same as afterwards i.e. the same as now? If so, why were the photons range bound? And why was the velocity of photons not influenced (reduced) by the presumably tremendous gravity?

    b) If the velocity of the photons before the decoupling was slower than afterwards, does this mean that a second (or year) lasted longer than a second as measured today by a caesium clock? Also, during the Photon Epoch, did any slower velocity of photons and/or stronger gravity dilate time?

    Thank you for your patience and I look forward to your answers.
  12. Sep 7, 2011 #11

    Ken G

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    "Immediately" is a little too quick. We have no idea where time came from-- we need the concept of time to even analyze how long it took the universe to support a meaningful concept of time! Some would point to a very short time called the "Planck time" (google it) as the beginning of a meaningful concept of time. I'm not sure it's important to specify how "long that took", it's just the start of even the more speculative versions.

    That is what I would call reverse logic. We develop concepts like space and time in order to talk about what the universe does, but the universe is never beholden to our concepts. It can do whatever it likes.
    That is presumably the meaning of the time concept.
    It is so assumed, yes, as it is standard to continue to apply out theories until we have evidence that they don't work. It seems the best approach when in doubt. The speed of photons would not be influenced by gravity, when you are in an inertial frame with photons whizzing by you (there are other ways to coordinatize spacetime such that the speed of light does appear to change with gravity, but never when you are on the scene, only when you are contemplating photon paths that are far away. The issue of coordinates in general relativity is a dicey area that will take you a long time to "grok.") The photons were range bound by scattering off, and being absorbed by, free electrons. What released them was when those electrons got bound in atoms.
    This is getting into coordinates used, rather than the physical truth of the situation. In a nutshell, physical truth is established by an observer on the scene, who never perceives time or light as going slow. But when we contemplate these distant and long ago epochs, we can use coordinates that spawn language like that. A coordinate is not a stance about what is really happening, it's just a mathematical contrivance for predicting what you see. You often hear that strong gravity "dilates time", but that is only when you contrast intervals happening at two different places-- and in that situation, there is no absolute way to match up the endpoints of the intervals such that you could really say they occur "at the same time" or not. As I said, the use of coordinates in GR will take you some time to get.
  13. Sep 8, 2011 #12
    Thank you for your excellent answers! I need just one further clarification concerning my central theme of time.

    You seem to be referring to another meaning of time than the one I am using. I am referring to the 4th dimension of spacetime.

    Time along with length, width and height are dimensions which we perceive and use, in order to come to conclusions about the size and age of the universe and changes which take place in it. I don’t know what you mean by a concept of time. Do we need a concept of length too? The measurement of space and time may produce different results in different circumstances, but that doesn’t have any impact on what the dimension length or the dimension time mean.

    When I say that the universe needs space and time to expand, I am thinking about expansion as a series of events. BB theory says that the universe changed over time and got bigger, so that’s a series of events in space and time.

    You call it reverse logic, I would call it simple deduction. I can't see any possibility of expansion without both space and time. Each event has its own coordinates in ST, which are usually different. Mutliple events may occur at the same time, but that is not the essence of BB Theory, otherwise the first 380,000 years would have been much shorter.

    You say immediately may be a little too quick and it’s also rather vague. I meant, as soon as anything wants to happen, it takes or makes time, even if it shares its time allocation with another event. In the first instant or Planck Epoch from zero to 10^-43 as far as I understand it, nothing changed, no events. But from 10^-43 we had the Grand Unification Epoch. I’m not sure if it needed any definable space, but it did take or make time, otherwise it could not have happened in the stated period.

    I think I have explained what I mean by time. Could you explain what you meant by time needing a concept?
  14. Sep 8, 2011 #13

    Ken G

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    Spacetime is built from the concept of time, so it wouldn't really be useful to then turn around and define time as a piece of spacetime! Time is what a clock measures, and then we build spacetime around that to agree with experiment.
    What I mean by the "concept of time" amounts to applying time when there are no clocks around, for example in the very early universe. That makes time a concept, we are generalizing what clocks do into a larger milieu of processes that are unified by the time concept. The time concept is along the lines of counting repetitions of periodic phenomena and noticing the consistency of the ratios between them. But when did there start to be periodic phenomena in the universe? How many periods does it take before something is periodic? The time concept emerges somewhat gradually over many periods, and more quickly for faster phenomena than for slower ones. There isn't going to be a line you can draw and say "time starts here," that's just not how our own concept of time works. Fortunately, most processes are very fast, so we are talking about very early in the grand scheme here.

    What those things means is the consistencies in the measurements, that's all they mean. So when the measurements produce different results, we have an immediate need to discover a way to restore that consistency. That's just exactly what the theory of relativity did. But there won't be any way to restore that consistency in, say, the first 10-35 of a second in the Big Bang. So the time concept must emerge along with the consistency.
    Beware the reverse logic again. Space, time, and expansion are all physics concepts. The universe does not need our concepts to do what it does, we build concepts to describe the universe. When our concepts break down, it is not the universe's problem.

    That is what is known as the logical fallacy of "argument by incredulity." It must be right because I can't get my mind around anything else the universe might do. But again, the universe does not need to care what you can and cannot get your mind around. Perhaps someone else will get their mind around what you cannot, or perhaps no human ever will. It's all the same to the universe.
    Things just happen. They don't need us to be able to fit our concept of time into the picture-- they don't need permission from our brains to happen.
    Those timescales are pure speculation, we have no theory of them. All we know is what we don't know, and some take that and say a little more than they really do know!
  15. Sep 8, 2011 #14
    :rofl: Do you really think you are going to get answers to the dozens of questions you have posed, many of which would require "god-like" knowledge of the nature of the creation of the universe as we know it, or that you could understand the answers even if they existed in anything but the most rudimentary form? If you think you can understand all that based on questions and answers posted on a forum, in words even, you are bound to be either disappointed or misled.
  16. Sep 8, 2011 #15
    Thanks, very interesting point about the necessity for fast multiple event flows in order that we can apply time measurement.

    OK, I see what you mean about reverse logic, but what other logic do you want?
    “We build concepts to describe the universe”, i.e. we make theories to fit what we observe. I guess that’s working in reverse.
    I didn’t mean the universe has a problem with us – rather that our physics concepts have to make sense and not break down. So far you did a great job!

    It’s correct that I shouldn’t argue by incredulity and I didn’t either. I was looking for an explanation of how it’s logically possible to have expansion without both space and time.
    However, I withdraw this point because you have by now given me enough background to understand time aspects of the BB Theory.

    As far as I am concerned, I have gotten as far I need to in this thread and I thank all contributors.
  17. Sep 8, 2011 #16

    Ken G

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    I would say it is forward to say we build models that fit what we see, and backward to say that what we haven't seen yet has to fit our models, unless we have very good reason to expect they will. I would call the early universe, on scales we have never observed and never built a model to fit, a place where we do not have very good reason to expect our models, built from very different regimes, to work. The way time works is one such model. It's fine to say "here's what we predict if our concept of time applies", but wrong to say "the universe needs our concept of time in order for anything to happen."

    We could imagine different types of space or time, that still support a concept of expansion (of course we'd need to see it to believe it, but that's always true in physics). Or, we could imagine that even the concept of expansion might no longer apply-- yet the universe could still exist and still do whatever it is doing. We cannot say that the universe "needs" time, we must say that we need time to work like we expect in order for our other concepts to work also.
    You are more than welcome.
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