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Age of universe and time dilation.

  1. Mar 21, 2009 #1
    Something that always got me thinking about is time is relative to the speed of the object (time dilation) how can we determine age of universe.

    Here is an example of what I'm thinking.

    When the big bang happened, imagine if certain parts of the big bang it got flung at different speeds (clumping?), lets group them into two different speeds 1, and 2. Group 2 equals somewhat near the speed of light, and 1 is moving at slow speeds (hardly moving.)

    Now group 1 is the slowest moving and will develop first into galaxies first at normal speeds, while group 2 will develop the slowest (last.) Now imagine a scenario where in group 2 finally gets to a stage to develop galaxies, and due to some large stars and/or black holes with gravitational slingshots (or some other event) that ends up slowing a set of star systems down to group 1 speeds, i.e. normal speeds.

    Now imagine life sprouting up on these set of star(s), from their perspective they will see galaxies, and stars systems that range from very old from group 1, to relative young ones in group 2.

    Now in reality there is just no group 1 or 2, as there will be groups in all ranges between 1 and 2.

    We can calculate rate of expansion of galaxies all we want, but how do we calculate the age of the galaxies when we don't have any previous speed history of what were looking at?

    I daydream a lot, so feel free to pick at all my logic holes on this.
  2. jcsd
  3. Mar 21, 2009 #2


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    The main flaw is you are picturing the big bang as an explosion.
    This is a common misconception which has been pushed by journalists who don't know what they are talking about, and the pop-sci literature.

    Have a look at this Scientific American article by a prominent cosmologist
    http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf [Broken]
    It tries to counter some of the most widespread misconceptions.

    Expansion rates are not speeds that anything is moving. They are rates that distances are increasing. Picture dots on the surface of a balloon, so they can't move. Think of all existence concentrated on the 2D surface of the balloon, no surrounding 3D space. No space inside or outside, only the surface. A toy 2D universe that only 2D creatures can live in, it's just an analogy to the 3D situation.

    The dots don't go anywhere, yet the distances between them increase as the balloon inflates. By contrast, photons can travel across the surface of the balloon from dot to dot at some fixed speed like 1 millimeter per second. They travel with real motion, subject to special relativity (with it's speed limit).

    The dots do not travel and distances between widely separated pairs can be increasing at a much greater rate than 1 millimeter per second!

    In fact in the universe we observe, distances between galaxies are typically increasing at rates which are several times faster than the speed of light.
    This is not only allowed by General Relativity (the 1915 theory) but is required by it.
    If that were real motion, then it would be against the speed limit law of Special Relativity (the 1905 theory). The stretching of distance while light is in transit is what causes redshift. The expansion rate continually varies and it is the cumulative stretch over the whole journey that determines the redshift. So there is no welldefined speed to plug into a Doppler formula. Cosmic redshift is best not treated as a Doppler shift (what speed do you measure and at what point during the light's journey do you measure it?).

    Galaxies also have small individual random motions---a few tens or hundreds of km/s. These produce conventional Doppler shift. But the individual motions are negligible compared with the rate of distance increase between widely separated pairs. GR is a theory of dynamic geometry---it says you have no right to expect distances between stationary things to remain the same. Distances can increase without anything resembling an explosion.

    So it doesn't work to picture expansion of distances as an explosion. Also in mainstream cosmology there is no surrounding empty space to expand out into. All space is filled with matter and always has been. Approximately evenly distributed except for gradual clumping together as stars and galaxies condense.
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  4. Mar 21, 2009 #3
    Great information, thanks!
  5. Apr 2, 2009 #4
    Hi to all, my first post here.

    In my "daydreaming" usually think of this expansion as if we all (matter) are getting smaller and smaller. We don't notice that of course due to our references are also getting smaller.
    Meaning, things (matter) didn't got much out of their place, except for the re-combinations - overall meaningless movements.

    Cheers to all.
  6. Apr 2, 2009 #5


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    Welcome to PF, flyway. I hope you find some interesting discussions here and get some questions answered and generally enjoy PF.

    I'm open to imagining expansion cosmology as progressive miniaturization/shrinking. Clusters of galaxies that are not participating in the expansion would then be shrinking. Our Milky Way galaxy, with its hundreds of billions of stars, would be shrinking. And of course the solar system and everything on earth. To make the picture work you would have to do something about light. I guess light would have to be gradually slowing down.

    Mathematically it might be hard to make it work, awkward. But it is one way to visualize.
  7. Apr 3, 2009 #6
    Thanks for the welcoming.

    He!... I dream alot about this final explanation..lol.
    Learned "alot" Astronomy and Physics by my self, and enjoy very much the thinking of the final comprehension. But my math skills... are killing me.

    This "shrinking explanation" thought, comes naturally and easily from the idea of "me" inside (any position) a volume - Universe, and seeing everything getting away from me in all directions. Just the same as 2d explanation above (expanding balloon) but now in 3d thinking. Anyway, right now am struggling with time expansion, thinking of the same idea but in 4d space-time.
    Light would be faced the same way, remember; if am over surface of a shrinking sphere (my self also shrinking) would see light coming to me with same red-shifting. It's all about this shrinking rate matching Universe expanson rate.
    Or... maybe it would have to be slowing down as you say, can also agree with that, matching this idea of time expansion.

    BTW, have some more "day-dreaming" theories, but don't know if they're too much ridiculous to talk about'em over here... lol.

    Last edited: Apr 3, 2009
  8. Apr 3, 2009 #7
    Actually the main flaw is calling the start of the universe 'big bang'. How could one not picture that as an explosion?
  9. Apr 3, 2009 #8


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    Yes, that is a serious flaw. The name "Big Bang" was given to what is now standard cosmology in a spirit of hostility and contempt, by someone who had an alternative non-standard model and who didn't believe in what he called a "Big Bang".

    Fred Hoyle was the main proponent of the rival Steady State picture. The name he gave to the standard model is misleading, but it caught the attention of journalists/pubic, and it stuck.
  10. Apr 3, 2009 #9


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    Note that it is only erronous to consider the Big Bang as an explosion that happened as some particular point. The key thing to understand is the the Universe is (as far as we can tell) roughly the same everywhere. If you imagine the early universe as an infinitely large lump of dynamite that explodes, then this should give you a reasonable idea of 'the Big Bang' and the subsuquent expansion of the Universe.

    There certainly was some great impetus in the very early universe that caused the Universe to begin expanding. The fallacy is not thinking that this impetus is like an explosion, but thinking that it was explosion that occured in some small region contained in a larger empty region. That is certainly wrong. The explosion (or whatever you want to call it) happened everywhere at the same time.

    It's a subtle but very important difference.
  11. Apr 3, 2009 #10


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    In the usual picture, Hubble law (the expansion of largescale distances) does not apply to time. There is no "time expansion" corresponding in any simple way to spatial distance expansion. There is, however, something we could call disagreement about clocks, among different observers. But that's different.

    An example of disagreement about clocks is when two different observers in relative motion each see the other's clock running slow. That's called "dilation" and it depends on details about particular observers. As a rule, any physical measure of duration will depend strongly on the individual observer and his history. Except in the special case where you designate a universal criterion of rest (as is done in cosmology) there can be no overall measure of duration. So proposing an absolute time coordinate involves a certain arbitrariness---time is a bit tricky in General Relativity.

    I guess I'd urge not struggling too hard with "time expansion".

    A big problem with a daydream that tries to transpose expansion of largescale distances into shrinking of smallscale matter is that the actual expansion of distance isn't exactly uniform.

    A galaxy like Milky is 100 thousand lightyears diameter and it stays that size (barring collision and merger with other galaxies).
    Clusters of galaxies a few million lightyears across are not affected either.

    Then there is kind of grey area, vagueness. Some large collections of galaxies don't expand because they are gravitationally bound---but others aren't bound and do gradually drift apart. The threshold is hard to specify precisely. Whether a collection is bound together depends on their masses as well as their separations.

    Then when you get to much larger scale like 100 million lightyears the general pattern is definitely kicking in.

    If you try to transfer all those relationships to a shrinking model, then how do you decide which collections of galaxies to shrink?
    Individual galaxies like Milky you of course have to shrink (because in reality they stay the same size). But also some collections of galaxies in reality do stay the same size, and therefore in your dream-vision you must shrink them----while other collections of galaxies are not staying the same size but are drifting apart and spreading out, so in your vision you must keep them the same size. How do you know which to do?

    And light always takes 100 thousand years to cross Milky (that is Milky's diameter) but you are shrinking Milky in your dream-vision. So light cannot continue to travel the same speed, other wise it will be taking less than 100 thousand years to travel the diameter.

    So in the shrink-picture, light must have to be slowing down at different rates at different parts of the universe. Yet we know of no physical mechanism that would cause this to happen. So it gets quite confusing:surprised.
    Anyway, just my opinion: I get confused when I try to picture things that way. Other people may have better luck.
  12. Apr 3, 2009 #11
    Well, my knowledge of all the facts is limited.
    Always thought that every "thing" is getting away from every "thing".
    Maybe this shrink model have to adopt some non-linearity/ non-uniformity in its rate, maybe (as you said) based on local densities.
    About light problem, you're right, big chances of solving it, is thru a slow "it" down.
    But, can only relate it to time expansion concept (BTW, I know what Time Dilation is, thanks anyway).
    Now, of course there's no physical mechanism that explains this happening... only in my other DDtheories, where some actual concepts are viewed reversingly...:bugeye::redface:
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