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S.o.L vs The Big Bang

  1. Nov 28, 2011 #1
    Hello,

    My question is regarding the speed of the Big Bang explosion.

    As I understand it, within the first 10 minutes of the Big Bang the universe had expanded hundreds of light years (this of course being faster than the Speed of Light). Bringing me to my question, if nothing can travel faster than the Speed of Light, then surely the Big Bang explosion would not have been able to exceed that speed at the suggested growth rate?
     
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  3. Nov 28, 2011 #2

    phinds

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    The creation of distance is NOT the same as things moving in the same frame of reference. For example, at present, the objects that we are currently getting the light from at the edge of our observable universe are receding from us at approximately 3 times the speed of light. That's because the geometry is changing, NOT because anything is moving faster than c. Nothing moves faster than c, but distances CAN change faster than c. UN-intuitive, but true.
     
  4. Nov 28, 2011 #3
    Thank you for the reply phinds. I'm not an expert so please forgive my stupidity, but I am still finding it difficult to contemplate.

    I can understand that the distance between Earth and a Star on the edge of the Galaxy are receding away at 3 times the speed of light and that is measured in distance, not speed, but that is the distance between two objects where as the Big Bang is a singular object (object may not be the best description here...).

    The Big Bang came from one central point and expanded outwards. Which is what I am failing to comprehend?
     
  5. Nov 28, 2011 #4

    phinds

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    No, there is no "point" from which the big bang expanded. If that were the case, then the universe would not exibit its two fundamental attributes of isotropy and homegeneity which are the foundations of modern cosmology.

    The expansion of the very early universe ("inflation") is not well understood [actually, I'm not sure it is understood AT ALL] but it was surely a creation of distance, NOT a case of things exceeding the speed of light.
     
  6. Nov 28, 2011 #5

    Drakkith

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    As Phinds stated the Big Bang is thought to have occured throughout the entire universe and is not an "explosion in space from a single point". The easiest way to understand inflation and expansion, although not technically correct, is that nothing can move THROUGH space at greater than the speed of light, however space itself has no such limit and can "carry" things with it as it expands.

    Imagine you are in an above ground swimming pool and can only swim up to 5 mph through the water. If the swimming pool suddenly bursts and empties all over your yard while you were swimming away from your friend, you are still swimming 5 mph THROUGH the water, but now the water is carrying you with it at a velocity greater than 5mph away from your friend. Again that isn't technically what's happening but is just a way of visualizing it.
     
  7. Nov 28, 2011 #6
    Does this mean gravity can "not accelerate" me faster than the speed of light towards an object then? I have trouble understand force-less gravity motion so I think of it as the space being consumed by mass, so to speak, and dragging the objects with it
     
  8. Nov 28, 2011 #7
    Thank you for your patience. That swimming pool metaphor really helps!

    However, Stephen hawking himself said that if you converge everything in the cosmos it returns to one central point in space. Which makes sense, for instance, if you take a nuclear bomb exploding, the explosion comes from one central point (specifically the point in space where the atom is split in two - which is in fact one central point) and expands outwards (or in the Big Bangs case, inflates).
     
  9. Nov 28, 2011 #8
    As I understand it, gravity cannot accelerate you faster than the speed of light. You can use the force of gravity to accelerate, as shown with quite a few spacecraft throughout history, but whether it is possible to 'reach' the speed of light using this method, I'm not sure.
     
  10. Nov 28, 2011 #9

    DaveC426913

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    It is not.
     
  11. Nov 28, 2011 #10

    Drakkith

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    I haven't read or seen whatever cosmos is, but I have to believe that he means the "observable universe", which is just the large sphere around us that we can see. Beyond a certain distance we cannot see anything however it is believed that there is still plenty more stuff out there and that the universe is either infinite or finite but without bounds. (Meaning that you go in one direction long enough and you eventually come back around to your starting point.)
     
  12. Nov 28, 2011 #11
    Is what Drakkith said:
    correct? and if so, how does gravity works in relativity without being a force if not that space "carries" test particles nearer the object on a geodesic without changing their velocity?

    and lastly, if those are both the case, how does the carrying differ in gravity and inflation?
     
  13. Nov 28, 2011 #12

    DaveC426913

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    Yes.

    If Milky Way and Andromeda were 2 million light years apart, and then an amount of space were inserted between them - say 2 million light years - they would now be 4 million light years apart. No force has acted on them and the galaxies themselves have not been accelerated at all, let alone faster than light.

    Space doesn't literally get inserted this way, I'm simply trying to demonstrate how the expansion of space can result in distances increasing without acting as forces or acting to move objects.
     
  14. Nov 28, 2011 #13
    Right, but during inflation space did something different than normal, here it is being called creation of distance, surely this creation of distance carried the mass suspended within it along, as per the previous description. So if it is the case that mass being carried by "moving space" does not accelerate the mass, but causes it's distance to change, then is that also a description of what is going on in relativity's gravity, whereby distance changes without acceleration?
     
    Last edited: Nov 28, 2011
  15. Nov 29, 2011 #14
    EDIT: I misunderstood your question, but I'll leave the post anyway... :-)

    Acceleration is defined with respect to some inertial frame. You must consider what happens to your particle as well as what happens to its inertial frame during the modification of spacetime through e.g. inflation. The particle is all the time at rest in its own inertial frame during this process, since the inertial frame itself is "carried along" with the inflation in the same way that the particle is "carried along".

    Therefore, the particle is not accelerating. Newton's laws are still valid at low speeds in this inertial frame even during such an inflationary cosmological phase. And there is no law that limits how much the physical distance between two particles can increase due to modification of spacetime through the evolution of Einstein's equation.

    At least that was my understanding of this.
     
  16. Nov 29, 2011 #15
    Yes but this still implies that there is a starting point for the Big Bang, which is my initial point. I think I understand the concept of what is being said here, though it is just the concept that the Big Bang did not have a central starting point that eludes me! Ohh my sleep will be disrupted this night!
     
  17. Nov 29, 2011 #16
    The inflating balloon analogy is pretty good. If you live on the surface of an inflating balloon. Also, you can only detect things that happen on the balloon surface, because e.g. light moves along the balloon surface in this analogy. There is no "ambient space" like we have when looking at a balloon, of course.

    Then there is no starting point for the inflation process. At one instant there is just a point, i.e. the big bang, and at any time after that there is a 2-sphere. It just gets bigger and bigger. That's why some say that the big bang happened "everywhere" at once.
     
  18. Nov 29, 2011 #17
    The concept of the Big Bang not having a starting point is not a problem if you redefine "Big Bang" to be in accordance with what the theory currently states (no starting point). The confusion arises because it's commonly taught as an explosion which we all know has a point of origin.

    Don't think "explosion". Just think "spacetime that suddenly expanded a long time ago, and cooled, and now we live in it". Nobody ever said it magically acquired some extra dimensions and went from point to volume. It was volume, then it got bigger.

    What happened before that? We can only guess. But putting any faith behind a guess based on everyday experience is probably not a good idea.
     
  19. Dec 9, 2011 #18
    Thank you for the replies everyone :)
     
  20. Dec 9, 2011 #19
    I've heard of a lot of analogies to help explain spaceinflation, but these are always combined with the clause that space doesn't literally inflates in that way. What is what literally happens? How well understood is what exactly happens and what are the differences in observable results with inserting space like in all the analogies?
     
  21. Dec 9, 2011 #20

    phinds

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    All we really know is that distances increase. Everything else is just speculation, far as I know.
     
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