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Expanding universe - some thoughts

  1. Oct 24, 2014 #1
    I had some interesting ideas concerning the mechanics behind the space expansion and its acceleration. Here they are. I know you expect blunders, but I hope you will find something interesting. Starting with obvious:


    1. In the presence of strong gravitational field, time flows slower than in its absence. We know it as time dilation. To my best knowledge, it has been confirmed experimentally.

    2. Universe had a beginning. Most evidence points to the age of about 13.8 billion years. I would call it only an assumption, but it is pretty solid and widely accepted. You know that.

    3. Galaxies have their own gravitational fields. You know that too :)

    4. It is only logical that there exists a time dilation between galaxies and the void inbetween them.

    5. If points 1 and 2 are both true, then the age of the universe can be relative. It could be said that a speck od dust found in the void between galaxies is actually older(exists longer) than a very similar speck of dust found within a galaxy. Any galaxy. In a way, the void is older then the galaxies. Actually, since this difference has existed since the beginning of time, it must have grown to be quite substantial, even if it is not very large. What unknown results can it possibly have? (And, taking time dilation to its extremes, can it be that only void actually ages, while matter ages only as a result of void's influence? Wow, getting side-tracked, sorry)

    6. What does it mean for the light travelling between galaxies? More specifically, how does it influence the light we see as emitted by distant galaxies? After all, light has to actually, physically travel through the void on its way to another galaxy.

    7. After much thought, I can't see how light could be influenced by travelling through "relative future". It's counterintuitive for me, and maybe it should be influenced, but I do not see how. However, travelling through "faster-time-zone" (no matter whether we call it relative future or present) should change something.

    8. Speed of light is constant in void. Since speed is measured as distance/time, then if time goes faster, and c must remain constant, it stands to reason that the distance should get stretched to compensate. Grow. Expand.

    What we see.

    Expanding universe.

    Einstein deformed time when he thought about the speed of light. He then moved to deforming space, believing that space and time are the same. So if time can change because of light and distance, why should space not change because of light and time?

    9. And why is it accelerating? As the void grows, so does the amount of time that light travels through relative 'faster-time'. Hence, the difference grows. Space has to stretch faster.


    All right guys. I'm posting it, before I waste more time thinking... Where have I erred :)
     
  2. jcsd
  3. Oct 24, 2014 #2

    PeterDonis

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    Yes, it has. But there is a significant qualifier to the statement quoted above: gravitational time dilation, as you describe it, only makes sense in a stationary system. So, for example, it makes sense to compare "rates of time flow" between someone at rest on the Earth's surface and someone at rest far out in space above the Earth, or someone at rest near the Sun in the solar system compared to someone at rest far out beyond the orbit of Pluto, and to attribute the difference in rates to "gravitational time dilation". (All of the experimental confirmations, which do exist, as you say, are in this kind of scenario.)

    But this sort of comparison won't work between, for example, someone at rest in the solar system and someone in the middle of a cosmic void a billion light-years away, because the expansion of the universe will be significant, and so the system as a whole is not stationary.

    More precisely, the evidence points to an age of 13.8 billion years as seen by a "comoving" observer, i.e., an observer (a hypothetical one, since no such actual observers exist) who has always seen the universe as (on average) homogeneous and isotropic. Observers who are moving relative to such "comoving" observers will attribute a different age to the universe (where "age" here means "proper time elapsed along a particular worldline").

    Yes.

    Only on distance scales small enough that the expansion of the universe is not significant, so that the galaxies surrounded by void can be treated as a stationary system. For example, our "Local Group" of galaxies (including our Milky Way, the Andromeda galaxy, and some other smaller ones) can be treated (approximately) as a stationary system in this way, so we can meaningfully talk about time dilation of someone in the Milky Way's gravity well compared to someone in the void between the Milky Way and the Andromeda galaxy.

    But, again, this kind of reasoning breaks down on larger distance scales (hundreds of millions to billions of light years) because the expansion of the universe can no longer be ignored.

    If there is such an effect in our universe (I'm not sure whether there is or not--see below), it isn't for the reason you give. The problem is that you are using an incorrect model. You are modeling the galaxy as an isolated gravity well, and the void as the empty space around it. This model is wrong because it is stationary, as I said above, and also because it is isolated. The model of the universe that is actually used in cosmology is a continous fluid (galaxies and galaxy clusters are the "particles" that make up this fluid, but the actual model averages over them rather than treating them as individual objects) which is expanding and decreasing in density as a result of the expansion.

    In the simplest model, the density of the fluid is the same everywhere in space (it decreases with time, as above). For this case, all "comoving" observers (observers who are "moving with the fluid flow", and who, as above, see the fluid as homogeneous and isotropic) experience the same proper time since the Big Bang (approximately 13.8 billion years).

    One could complicate this model somewhat by allowing fluctuations in the density, so that galaxy clusters would have higher average density and voids would have lower average density. I don't know exactly what the effect of this would be on the proper time experienced since the Big Bang by comoving observers in regions of different density. However, I do know that the answer can't be obtained using the model of "gravitational time dilation" that you are using.

    I'm going to refrain from commenting on the rest of your post because I think you need to re-think it in the light of what I've said above. (Also, please bear in mind the PF rules on personal speculations.)
     
  4. Oct 24, 2014 #3

    mfb

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    Just to add a number to PeterDonis' explanation: The difference in passage of time between "where we are in our galaxy" and "somewhere outside, but close to our galaxy" is roughly 1 part in a million. Over 13.8 billion years (ignoring that our galaxy and its environment did not always exist like that), this would just be ~10,000 years. This has to be compared with the experimental uncertainty on that number of 13.8 billion, which is roughly 50 million years (depending on which data sources are used).
     
  5. Oct 26, 2014 #4
    I would have to say that the light photon would remain at the constant of c because a body in motion within a gravitational field will remain in motion at the speed of light regardless of whether passing through a void or not.

    The real question is this: If a void between galaxies does not have a gravitational field because there is nothing producing a gravitational field what velocity can expect to be achieved by an object placed in the void where there is not much gravity? We know that a light photon's velocity is governed by gravity that if an object that is able to emit a light photon is placed within the void the light photon would retain the constant of c because of the gravity of the parent object. But if an object that is not a photon is placed within the same void be able to reach a faster velocity compared to the same object placed within a galaxy with normal gravitational fields while using less energy because of the gravitational field that causes work and energy to be created would not be present or very limited in the gravitational field strength?
     
  6. Oct 26, 2014 #5
    About forum rules. It is not my intention to break them in any way. I understand that if my ideas were at any point inconsistent with, for example, theories of relativity, then they are wrong. In case I still break the rules somehow, just tell me, and I will stop posting.


    Why do I get an impression that you could say that about every theory which assumes "the beginning"? :) In the first post I have simply shown my entire thought process. I know it would be much preferable to publish a theory with solid experimental and mathematical backup--but I can't, because it is not a theory yet, at least not in the scientific sense. It is just an idea.

    That is why it was slightly messy. Sorry. I left the part about the 'intergallactic' speck of dust being older than the 'galactic' one, simply because it seemed interesting. Let's agree, however, that you are right. It would mean that the universe is isolated. And for this reason let's disregard this part, please, and focus on the rest (although I am curios how Mfb came up with "~10,000 years in the future").

    The only really relevant thing is the rate of time flow. In the argumentation I used, that would be the reason why we percieve the space expansion. And so the model is not isolated.

    However, Mfb's "roughly 1 part in a million" difference in the time flow is enough. In fact, I would guess this is even too generous. Why? Because "1 part in a million" starts to matter when you consider the vast distances between galaxies. Light has to travel in this manner for millions, billions of years. That builds up to make the difference. If that light has to travel, say, 1 million parsecs, then an average difference of 2^-18 in the value of 'one meter' would be enough to explain the expansion.

    In Lorentz transformation, the length is maximum in the frame in which the object is at rest.

    If we apply the same transformation to gravity, wouldn't it be right if the maximum length was in the frame in which object is in "rest gravitation" field (meaning, gravitational acceleration as close to zero as possible)?


    Now this seems to be the major counterargument. I believe this objection is inadequate. To explain, let me rephrase our argument (if I take it too far, please point it out).

    What I am saying, essentially, is that universe expands because there is time dilation.
    You are answering, essentially, that there can't be time dilation, because universe expands.

    In yet another words:

    I say that the universe is not stationary, because there is time dilation.
    You answer that the universe is not stationary, so there isn't time dilation (in this case, to be precise).

    Perhaps now you can see that in my idea, the observable result is not a stationary model. That's why it is good--it is consistant with reality. Yes, it raises a question of what is the cause, and what is the effect. But it does not invalidate my assumptions.

    Also, I think that in a non-inertial reference frame time dilation still occurs. It's just that there are other effects at play, too, adding their own physics.

    (maybe this thread should go in relativity forum...)
     
  7. Oct 26, 2014 #6

    PeterDonis

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    In other words, you are saying a photon will move at ##c## because...it will move at ##c##. This is not a good argument.

    Who said it didn't? Gravity is everywhere in the universe. But just saying "there is a gravitational field" doesn't tell you anything useful.
     
  8. Oct 26, 2014 #7

    PeterDonis

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    If that's your impression, it's wrong (and I should have been clearer about what "isolated" means). By "isolated" I meant something very precise: that there is a single gravitating system with finite extent, surrounded by nothing but empty space. The technical term in GR is that the spacetime is "asymptotically flat". The spacetime of the universe is not asymptotically flat. This makes a big difference.

    PF is not really meant as a discussion forum about people's personal ideas (or theories). It's meant as a discussion forum about the physical models that are actually used by scientists, as shown in peer-reviewed literature or the equivalent. It's fine to describe your personal understanding of how those models work and ask for input or corrections. But if you're starting from your own personal model, constructed without reference to the models that are actually used by scientists, that's probably going to be ruled out of bounds for PF.

    Your OP appears to be a mixture of ideas that are part of mainstream scientific models (but it looks like you're misunderstanding some of them), and ideas that are your own. That's why I advised you to bear in mind the PF rules about personal speculations. If you're in doubt, a good rule of thumb is to rephrase your question in the general form of "mainstream model M appears to me to say X; does this seem correct? If so, I don't understand how that's consistent with A, B, C; it seems to me that..."

    No, it wouldn't. See above.

    I don't understand what you're saying here.

    No, it doesn't. 1 part in a million is still 1 part in a million. If the light travels for a million light-years, a 1 part in a million difference is a difference of 1 year. We can't measure intergalactic distances or travel times to anywhere near that accuracy anyway.

    A 1 part in a million difference in a million parsecs would be a difference of one parsec. Again, that's way smaller than measurement error. Our models of universe expansion are not based on effects that small; not even close.

    Yes.

    This doesn't make sense. First of all, "apply the same transformation to gravity" is meaningless; gravity is not a "transformation" and you don't model the effects of gravity by making a "transformation", and anyway a Lorentz transformation won't work because it only works globally in flat spacetime, and spacetime in the presence of gravity is curved. Also, what does "maximum length" mean in this connection? Finally, what does "gravitational acceleration as close to zero as possible" mean? Does it mean the object is in free fall?

    A general pattern I'm seeing here (which I alluded to above) is that you don't appear to have a very good understanding of the models that are used in GR for things like the universe, or the effects of gravity around a single massive body like a planet or star. If you get more familiar with those models, you might find that some of your questions get answered automatically, while others turn out not to matter because they were based on an incorrect understanding.

    No, I'm saying that "time dilation", in the sense you are using it (there are multiple senses of the term; you appear to be using it to mean "gravitational time dilation"), only makes sense in a stationary spacetime, and the spacetime of the universe is not stationary. It's true that a key line of evidence for the spacetime of the universe not being stationary is the observations that show it to be expanding. But the expansion, in itself, is not what creates problems in defining "gravitational time dilation"; it's the fact that the universe is not stationary. I gave the technical definition of what that means in a post in another recent thread; I'll see if I can find it.

    No, I can't. I can't even make enough sense of your idea to be able to deduce any consequences from it. Sorry to be blunt, but as I said at the end of my last post, I think you need to re-think your ideas; and per the general comment I made above, I think that before doing so you should get more familiar with the existing models that are used by scientists.

    This is another area in which I think you need to get more familiar with existing models.

    If you want to find out more about how existing models handle general issues like "time dilation" (both due to relative motion and due to gravity), yes, that forum is a good place to do so. But again, please bear in mind what I've said about PF rules. Also, you might want to consult a good relativity textbook. Taylor & Wheeler's Spacetime Physics is a reasonably good one on special relativity. For GR, a good starting point might be Sean Carroll's online lecture notes:

    http://preposterousuniverse.com/grnotes/
     
    Last edited: Oct 26, 2014
  9. Oct 27, 2014 #8
    I have decided not to publish my further arguments in this topic.

    Anyway, I can see that it does not precisely follow the forum rules. Nonetheless, thank you, Peter, for any and all input you have given. I will come back to this topic to see if you (or someone) had anything else interesting to say, but I will (try to) refrain from posting again.
     
    Last edited: Oct 27, 2014
  10. Oct 27, 2014 #9
    Regarding explanations of time dilation without gory equations (which in and of itself is a pretty bad idea,) I remember how someone once explained that an object approaching a black hole seemed to "slow down" because the photons bouncing off it were taking much longer to get to the distant observer, due to the curvature of spacetime (i.e: they had to travel an increasingly longer distance, thus taking more time, thus "slowing it down.") Then, time dilation does not sound like a magical, weird effect, but a quite logical consequence of geometry; but even then, the explanation is derived from actual equations, and not viceversa. Further deducing things from examples and analogies is as wrong as it can be. I probably got my own example pretty wrong.
     
  11. Oct 27, 2014 #10

    PeterDonis

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    This isn't quite right; the photons do take longer to get to the distant observer, because of spacetime curvature, but the main effect of the curvature is to reduce the apparent speed of the light according to the distant observer. (Note that I said "apparent"; this is not a violation of the law that light always travels at ##c## in a vacuum, because that law only applies to local measurements of the speed of light, which will indeed be always ##c##.) There is a "stretching" of the distance as well (compared to the expected distance if space were Euclidean), but it's not enough by itself to account for the greatly increased travel time of the light.

    This is true, and it's (IMO) the most important thing to remember about these types of scenarios. So you got the important part right.
     
  12. May 15, 2015 #11
    Whitefire, I absolutely agree with you and as a consequence to your theory you don't need the dark matter to explain same rotational speed of the stars in galaxy. Stars further away are moving slower but they are in faster time-frame (space-time is less stretched) so if you look to a galaxy as a whole it looks like they all move at same speed. So what it looks like dark matter and dark energy, it is all due to space-time distortions and gravitational time dilatation.
     
  13. May 15, 2015 #12

    PeterDonis

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    Have you done the calculations to confirm that this accounts for the observed rotation curves quantitatively? Or can you give a reference to a paper that shows these calculations? It's not enough to just say "stars further away are in a faster time-frame"; you have to show that the "time-frame" changes, quantitatively, by the right amount.
     
  14. May 19, 2015 #13

    berkeman

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    Thread closed for Moderation...
     
  15. May 19, 2015 #14

    PeterDonis

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    The OP's question has been answered. This thread will remain closed.
     
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