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Speed of light and big bang

  1. Sep 29, 2010 #1
    When I think of what happened in the big bang theory and in today's world when we look through telescopes and look at distant objects it raises the question of how far back in time can we actually see with more and more powerful telescopes?

    One thing that I find confusing is that at the time of the big bang I assume many things were expanding at or close to the speed of light and then slowed down as they changed to particles with more mass. In which case, everything on the boundary of expansion would have its 'history' moving at the same speed as everything else. How is it possible to see light that has already passed us and is heading outwards from the centre of expansion?

    Sure we can see distant stars and we see them as they were 100 or so light years away.....but the light that was present when the universe was close would surely have passed the earth with nothing to reflect it back..... is this right?
    If this is true then the light or information that we can view that shows the very early universe is way passed the earth with nothing in the beyond that is able to reflect it back.
  2. jcsd
  3. Sep 29, 2010 #2


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    Expansion is the short answer. Immediately following the big bang, the universe expanded faster than light, then slowed down.
  4. Sep 29, 2010 #3
    So in effect are we still awaiting the light from the big bang to catch up to us here on earth?

    The particles that formed earth must have been travelling faster than light for a LONG time before they slowed down, in order that 13.75billion years have passed and the light from the big bang has still not passed us. ??? Is this a correct statement?
  5. Sep 30, 2010 #4
    Anybody have a reply for this?

    I used to be good at physics at 'A' level but its been a while since I took a questioning interest on the huge subject that it is.
    I'm hoping to get back in to building my knowledge on physics but first I need to clear up some good old fundamental questions that are running around in my head.
  6. Sep 30, 2010 #5


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    See #3 and #5 in this thread. Make sure you click the links in #5.
  7. Sep 30, 2010 #6
    Thanks Fredrik.
  8. Sep 30, 2010 #7
    well the idea is that after the big bang there was a period of "inflation" where space was actually expanding faster than light there fore thee universe before this time is opaque. Then expansion settled down about 100,000 years after the big bang and we can see this as the cmb or cosmic microwave background. And we can see that with telescopes today, but we cannot see any further back. Thats why gravity waves have become a hot topic as gravity waves travel with space,

    As for distant stars that would be "in front " of us, thats a bad way to think of it. There is no real center of the universe there fore they are not in front or behind us they are simply traveling with respect to us. We can see them because stars emit radiation atall wavelengths in every direction.
  9. Oct 15, 2010 #8
    If an object travels the speed of light it becomes infainitly heavy, so it would effectivly implode hence not exsist so no light.

    Einstein’s theories
    No physical object can travel at or faster than the speed of light. The speed of light is generally considered to be a physical speed barrier.
    As objects approach the speed of light, their "rate of time" approaches zero, and distances (travelled by the object) approach zero, and their mass increases.
  10. Oct 26, 2010 #9
    If the speed of light limit has been breached before then we can breach it again. It is only a matter of time.

    If only we can create conditions that were similar to the infant Universe....

    Conversely if there is no upper limit for a particle then it can exist in multiple locations at the same time which puts a sort of dampner on our hopes.

    The only reason I want us to breach the speed of light is because I want to see us visit other stars and galaxies in a short space of time.
  11. Oct 26, 2010 #10

    When we say that in the early universe, space was expanding faster than the speed of light, we mean just that, space -- not mass. The concept of "inflation" does not suggest that any object with mass ever moved at C (velocity of light). And even though some have suggested that in the early universe, the speed of light may have differered from its current value, no one has ever suggested that any object containing mass can reach C (whatever C's velocity is or was). The postulates of the special theory of relativity would still hold, regardless of the value of C.
  12. Oct 26, 2010 #11


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    This is false.

    Objects moving relativistically will gain mass as observed from an external frame of reference. They do not not gain in hteir own frame of reference, and thus will not "implode".
  13. Oct 26, 2010 #12


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    Well, one of the conditions during this phase is that the universe was smaller than an atom.

    Which means...
    ...no stars or galaxies.

    And, well, come to think of it, no atoms either...
  14. Oct 27, 2010 #13
    LBrandt -- So what exactly is 'space' if there is no mass to occupy it, or to define its boundary?

    And next question; is this saying that a universe can be defined as a small vacuum?
  15. Oct 27, 2010 #14

    George Jones

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    In special relativity (flat spacetime), there is a standard definition of (spatial) distance that can be applied both locally and globally. In other words, this definition of distance applies to nearby objects, and to objects that are far away. Speed is change in distance divided by elapsed time, so this standard definition of distance can be used to calculated speeds of objects that are near and far. Speeds of objects, near and far, calculated in this way always have the speed of light as their speed limit.

    The situation in general relativity (curved spacetime) is far different. Because of spacetime curvature, the definition of (spatial) distance used in the flat spacetime of special relativity can only be applied locally, just as the Earth looks flat only locally. This leads to speeds of nearby objects that limited by the the speed of light, but it say nothing about the behaviour of objects that are far away.

    Even though the special relativity definition of distance cannot be applied globally in curved spacetime, there is a standard cosmological definition of distance that is used in the Hubble relationships. Strangely, this cosmological definition of distance can be applied to the flat spacetime of special relativity (Milne universe), and when this is done, it produces a definition of distance (for special relativity) that is different than the standard definition of distance in special relativity!

    A different definition of distance gives a different concept of speed, since speed is distance over time. This alternative definition of speed, even within the context of special relativity, produces speeds of material objects that are greater than the standard speed of light! In other words, this definition of speed produces, in both cosmology and in special relativity, speeds that are greater than the standard speed of light.

    If v is standard speed in special relativity, and V is cosmological "speed" applied to special relativity, then some corresponding values (as fractions of the numerical value of the standard speed of light) are:

    Code (Text):

      v                   V
    0.200                0.203
    0.400                0.424
    0.600                0.693
    0.800                1.10
    0.990                2.65
    Even though there can be different definitions of spatial distance, there is no ambiguity with respect to the prediction of experimental measurements. One just has to keep in mind what definition is being used.
    Last edited: Nov 18, 2010
  16. Oct 27, 2010 #15


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    Is this tantamount to saying that the superluminal expansion of the universe cannot be adequately explained without general relativity? i.e. that it is the curvature of space that allows objects to recede superluminally?
  17. Oct 27, 2010 #16
    Okay. Thank you for clearing that up for me.

    So for reaching large distances in a short space of time we can use the same principle right?

    If we can create an inflation drive that can expand/contract the space around an object then reaching far away distances could be made easier. I think our understanding of space is rudimentary at best so maybe if we can figure out ways to manipulate it then this seemingly large distances might just become short.

    There is the other paradox that I mentioned in my post - i.e if objects can say move really fast(say using inflation drives/ warp drives or whatever) then they can be at two places at the same time.
  18. Oct 27, 2010 #17


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    http://en.wikipedia.org/wiki/Alcubierre_drive :biggrin:
    How does this follow? Being in two places almost at once is not the same as actually being in two places a once.
  19. Oct 28, 2010 #18
    Regarding the Alcubierre drive.. His explanation seems plausible. I just thought that we need to contract space to move toward an object and expand space to move away from an object. Like objects stuck on a flat rubber sheet.

    We need to do exactly what space did to expand when the universe was born. I always believe that if something has occured in nature then we can replicate it too.

    About the second point, you are right. :)
  20. Oct 28, 2010 #19
    Blimey...a lot of this is over my head but I still find it fascinating.
  21. Oct 28, 2010 #20


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    This might take you far away from home, but it won't take you closer to anything else.
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