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Speed Of Light In Relation To The Size Of The Universe

  1. Jul 18, 2012 #1
    Hello Everyone. This is my first post started on this site. Please, please, please excuse my ignorance but I couldn't seem to find the answer to my question online (although i'm sure it is). If the size of the know universe has a diameter of 93 billion lightyears with a radius in accordance to us being half of that, does that make it impossible for the light (suns/galaxies) of a great deal of the "observable universe" nonobservant? If this is true how do we observe what hasn't reached us yet? Thank you very much for reading and for any responses.

    Last edited: Jul 18, 2012
  2. jcsd
  3. Jul 18, 2012 #2
    The Big bang happened 13.7 billion years ago, and from that moment inflation expanded time and space a great deal in all direction along with energy flying out in all directions away from the 'center'. I don't know the radius of the universe so I can't answere one of your main questions. However the milky way galaxy, along with almost all objects in the universe, are traveling at speeds near the speed of light away from the starting point. The universe is also undergoing expantion, meaning not only is everything traveling as fast as it was a billions of years ago, its accelerating at a relatively even pase. Because of this, objects that emit light that are traveling in the opposite direction then our galaxy is headed light will not reach us. The combined speed of us and other universes traveling away end up being significantly faster then the speed of light.

    Picture 2 cars going the speed limit of 50 mph, and 'light speed' is the speed limit of 50. If those cars are driving perfectly away from one another the combined speed is 100mph and it is impossible for another car to travel this gap without going faster then 100.

    So yes, it is reasonable to say their are parts of the universe we will never be able to observe. We call the 'observable universe' just that because their may very well be a significant amount happening just past it's edges, we just can't see.

    Edit: You would also probably like to know that light emitted from a galaxy that is 10 billion light years away and currently visible may not be in the observable sky in the next 10 billion years due to the rapid pase of acceleration the universe is undergoing.
    Last edited: Jul 18, 2012
  4. Jul 18, 2012 #3


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    No! This is wrong, and it leads to many misconceptions.
    There is no center, and no object is moving with any significant velocity.
    See the balloon analogy for a discussion.

    The objects which are now ~46 billion light years away were much closer in the past, and space expanded between them and the travelling light, too.
  5. Jul 18, 2012 #4
    Is your question why, even though the universe is only 13.7 billion years old, we can see light from roughly 46.5 billion light years away?

    If so, the answer is that the universe has expanded considerably in the past 13 billion years. So, the light from the early universe not only travels the 13.7 billion light years (because the universe is 13.7 billion years old), but it must also traverse extra distance that is created by the expansion of the universe. Put yourself in the position of light - imagine trying to run across a large field that is, say, 1000 meters long. If the field was to start expanding while you were running, it would be much larger than 1000 meters when you reach the end. Say that it grows to 2000 meters. So, you've traveled much farther than just the normal 1000 meters, because the field has expanded.

    The situation is similar for light. The light is from 13.7 billion years ago, but it has had to travel 46.5 billion light years to reach us.
  6. Jul 18, 2012 #5
    When I say 'center' I'm referring to the point in which the big bang occurred, which we can roughly infer from the cosmic microwave background
  7. Jul 18, 2012 #6


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    Nonsense. There was no "point" where the BB happened, it happened everwhere. The universe has no center, just the observable universe.


  8. Jul 18, 2012 #7
    As Phinds said, there is no center, as the big bang was not a 'bang'. It happened everywhere. This is the essential story of what happens:

    1) The very early universe is hot, dense, and filled with radiation. This isn't concentrated to a single point, it's everywhere. It's also homogeneous, it's the same everywhere.

    2) The universe expands. By this, I mean that the metric of space itself is expanding at every point. The distance in between everything grows. It doesn't 'expand into' anything. See more about this on phinds balloon analogy page. This causes the radiation to cool, and the plasma to break up into gas. The slightly denser regions of gas pull the rest in, while the space around them expands, isolating them from each other. These develop into galaxies.

    That's the gist of the story. No explosion, no center.
  9. Jul 18, 2012 #8


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    Let's take the CMB, for example - which is now at a proper distance of about 46 billion light years. When the CMB photons we now receive were emitted, the source of those protons were only about 42 million light years away [again, proper distance]. The recession velocity of the source of those photons was about 65 times the speed of light at that time, so, it took a very long time [about 13.7 billion years] for those photons to span what was originally a distance of 42 million light years. The photons being emitted NOW by the CMB source will never reach us because those sources have long since left our cosmological event horizon. In the LCDM model this is true of any object with a current redshift exceeding about 1.8 [the CMB redshift is about 1100].
  10. Jul 19, 2012 #9
    Thanks guy's you have answered my question sufficiently about that.

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