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The Big Bang And Question(s)

  1. Oct 26, 2011 #1
    I'm pretty ignorant in physics, so please help me out.

    OK... So I was watching the special "How The Universe Works", and the episode was about the origins of the universe. (Lame, I know, but Netflix had nothing else that was interesting). They keep talking about the speed at which the Big Bang created the universe, which was something like a millionth of a millionth of a millionth of a millionth of a second to expand from the size of a golf ball to the size of the Earth. They go on to describe Planck time and more exact figure and stuff, but my question is more general.

    The question is this: How are we even able to measure any form of time, when (at least as it has been explained to me), time was created with the Big Bang? I mean... they talk about expansion into not even space, just emptiness or whatever existed before the Big Bang... so how can we even give that a referenced scale? Is our initial point of reference just referring to the first observable time unit within the universe and its expansion from that? Because to me it seems like everyone talks about it as though the 'emptiness' that existed before the Big Bang can still be measured in time.

    This also goes along with the question about how the Big Bang expanded faster than the speed of light.

    If anyone could shed light on this (please avoid the obvious pun) then I would be grateful. If my question is being asked incorrectly, please logically explain to me what is wrong with it so I can understand.

  2. jcsd
  3. Oct 26, 2011 #2


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    Matter does not 'move' during inflation. The speed limit c only applies to matter with respect to inertial reference frames. It does not apply to the space between particles of matter.
  4. Oct 26, 2011 #3
    OK. I understand that. But what does it mean to say that the Big Bang expanded at such and such a speed/time when that speed/time doesn't exist outside it's own creation?
  5. Oct 26, 2011 #4
    BBT cosmology uses human scale or matter as the reference frame.
    So space is expanding relative to us.

    Personally, I think that choice of reference frame may be flawed, and a slight bending of the cosmological principle.
  6. Oct 26, 2011 #5
    It means that in few tiny fractions of a second it expanded exponentially. Once dimensionality was manifested, you can measure it. You can assign time coordinate to beginning of inflation, and you can do it for end of inflation, thus you know its duration.

    I don't understand what are you trying to say.
  7. Oct 26, 2011 #6
    The concept of expansion implies a reference frame for a coordinate system in which space is expanding against some unit of measurement.
    BBT choose to measure the universe against the locally experienced human reference frame.
    The metric expansion of space offers an infinit number of possible alternative reference frames all expanding or accelerating relative to each other.
    The basic laws of physics and motion can only be correct in one of the over long periods of time.
    You can get some very interesting alternative cosmolgies from alternative reference frames.
  8. Oct 26, 2011 #7
    No, concept of expansion implies comoving frame of reference. Nothing to do with locally experienced human reference frame. Comoving observers are the only ones that expirence universe as isotropic, which is a requirment for using FLRW metric, which in turn enables you to derive scale factor as a function of cosmological time.

    Edit: I should have said "concept of isotropic expansion", because there is really no reference frame in which universe is not expanding. Question is if expansion is perceived as isotropic, or not. That makes case of comoving coordinates as most natural ones even stronger.
    Last edited: Oct 26, 2011
  9. Oct 26, 2011 #8
    Indeed one of my favourite alternative cosmologies "Condensing universe" has something very close the comoving distance as the absolute distance. ie space not expanding.
    It does look a bit nasty because it assume a reference frame for physics where matter is shrinking at a bit more than 6% a billion years. (an atom on your metre ruler every 6 years or so) Also time dilation is an indirect property of shrinking matter.

    Anyway bringing the thread back on track ....:smile:
  10. Oct 26, 2011 #9
    Then you should ask yourself why is that your favorite cosmology. It utterly complicates things without one single gain in understanding. We need to have starting point. There is no questioning that distances between galaxies are increasing relative to the size of atoms. Can we describe same physics by saying that space is not expanding on large scales, but instead atoms are shrinking and space is contracting on small scales? Probably yes, but we are introducing two variables instead of one, which is bad. So, what's the point?
  11. Oct 27, 2011 #10
    Mostly just as a fun hobby.
    But this case is very interesting, most alternate cosmologies you can kill off almost instantly they just don't match observation. I have looked at loads of them.
    This one is very hard to break.
    It gives a very different view of a universe that matches raw observations, but not their interpretation.
    If space is not expanding then there was no big bang, etc.

    If conservation of momentium works in that reference frame, then in the BBT reference frame matter speeds up relative to local space as space expands.
    This sounds very odd till you combine it with gravity, it make gravity look stronger or masses bigger over large time scales.
    If I still can't break it, I will post a thread on it. It is on other forums, but I have not updated any of them for a while.
  12. Oct 27, 2011 #11


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    Nah, the "condensing universe" is just a change in coordinates. You can't change the physics with just a change in coordinates. The big bang singularity is physical (within the theory), meaning it is there no matter which coordinates you use. It looks different, in that at that singularity matter is infinitely-large, instead of space being infinitely-small. But the singularity is still there.

    Of course, the prediction of a physical singularity is the main reason why the big bang theory cannot be trusted that far back. And you can't get rid of it without replacing General Relativity with a quantum theory of gravity.
  13. Oct 27, 2011 #12
    OK. Assigning a time coordinate makes sense I guess. I'll just have to think about the whole thing a little longer.

    The conversation you guys are having is helping though! Keep going :)
  14. Oct 27, 2011 #13


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    Really!? Have I misunderstood your point?

    So if we were to look at an equation for the curvature of space-time around a mass, we'd not find a 'c' term in it?
  15. Oct 27, 2011 #14


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    "c" is a unit conversion factor, so you see it in all sorts of places depending upon what units you use.
  16. Oct 27, 2011 #15
    It does not change physics, but the two reference frames or coordinate systems are accelerating/expanding relative to each other.
    An object moving at constant velocity in one is accelerating in the other ref frame.
    If you assume the TCU reference frame is the correct one for the laws of physics then the correction factors for BBT look similar to dark matter and dark energy, at the very large scale.

    In the TCU version of things the universe also starts infinitely slowly as well as large (thousands of billions of years to get going)

    The maths for the conversion back BBT, does make space expand faster than the speed of light, but objects can never exceed the speed of light relative to there local reference frame.:wink:
  17. Oct 27, 2011 #16


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    I have no idea what you're trying to say here. There is no physical difference between the two.
  18. Oct 27, 2011 #17
    In the BBT ref frame galaxies move further apart as space expands.
    In the TCU ref frame galaxies stay the same distance apart but slowly shrink.

    If you put a fixed size box around the nearest 1000 galaxies (size fixed in coord system of that ref frame)
    In BBT the galaxies expand out of the box, but in TCU they all just stay still only perturbed by gravity.
    Viewed rom the TCU ref frame the BBT box shrinks leaving the galixies behind.

    In TCU an object traveling between two galaxies arrives in the same way as a person just crossing the street at constant velocity.
    But in BBT the two galaxies get further apart as the object travels increasing travel time.

    Viewed from TCU and object traveling at constant velocity in BBT is slowing as that reference frame shrinks.

    Conservation of momentum cannot work in both reference frames. One has to be wrong.
  19. Oct 27, 2011 #18


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    Well, momentum isn't conserved in an expanding universe anyway.

    However, in any space-time, you can always transform to a local coordinate system where momentum is conserved.
  20. Oct 27, 2011 #19
    Anyway, that reference frame does provide an interesting alternative cosmology, in which it is a bit easier to explain some observations that get very messy in BBT.
  21. Oct 27, 2011 #20


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    Again, it's not an alternative. And it's generally expected that some things are easier to understand in certain coordinate systems rather than others.
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