Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about the expansion of the universe:

  1. Jan 14, 2013 #1
    Well an interesting idea came across my mind the other day relating to the expansion of the universe, and I have yet to come across an answer to this little pondering through my own insight or digging:
    (I am sure everyone knows the following statement, but anyway): the theory of relativity states that matter cannot move faster than the speed of light, as the closer you get to it the more energy has to be exerted in order to continue to accelerate that object and too exceed the speed of light, you would need an infinate amount of energy (which not even the universe as a whole contains).
    The big bang theory states that the universe rapidly expanded from a single point roughly 14.7 billion years ago and has been expanding since.

    Combining these two points is how my mind came across my current thought: The universe is made of matter and energy, and as such cannot exceed the speed of light. If everything originated from a single point 14.7 billion years ago, logic would dictate that the maximum expansion of the universe cannot exceed a radius of 14.7 billion lightyears in any given direction for a max diameter of roughly 29.4 billion light years. At least from a stand point of matter and light.
    Yet it is known for a fact that the universe is larger than this, and objects have indeed been found that exceed the diameter of 40 billion light years at its widest point (the recent quasar cluster that also defied the commonly accepted scale of the universe rule).

    So my question is:
    How is it that the universe is larger than 14.7 billion light years in radius if it all theoretically started from a single point, and relativity dictates matter cannot move faster than the speed of light?
  2. jcsd
  3. Jan 14, 2013 #2
    This question gets asked alot. Recent age of the universe places it at 13.78 billion years old. Now light from that period travels to us at the speed of light. However spacetime is expanding. That light having already travelled part of the distance is essentally red shifted. In essence its frequency is stretched. Now spacetime itself can and does stretch faster than the speed of light.
    Expansion is not limitted by the speed barrier as expansion is not inertia

    For more detail and better accuracy of the above there is several posts of this nature including tha FAG section. Those posts explain it far better than I just did as Im sure others will point out lol

    In the cosmology forum there is currently a good explaination with tons of info in the bubble analogy thread. I highly recommend perusing it.
    Last edited: Jan 14, 2013
  4. Jan 15, 2013 #3
    The big bang doenst state the universe expanded froma single point, rather the bang happened everywhere. The unvierse may be infinite now in which case it was infinite at the bang . The bang simply marks the start of the expansion. As has been pointed out here read the FAq, it will answer your questions.
    Last edited: Jan 15, 2013
  5. Jan 18, 2013 #4
    I thought it started as a singularity which was an infinitely small point?

    EDIT: Ah! I read the FAQ. It seems that is not a popular theory anymore.
    Last edited: Jan 18, 2013
  6. Jan 18, 2013 #5
  7. Jan 18, 2013 #6
    Thanks for that. Do you not feel that it is counter to your above post however?

    "Thus, while some cosmologists do not have a problem with assuming that our universe began in a singular state, most are convinced that the big bang singularity is an artefact - to be replaced by a more accurate description once quantum gravity research has made suitable progress. To be replaced with what? Nobody knows for sure. In some models, we can go infinitely far into the past (one example is presented in the spotlight text Avoiding the big bang). In others, the big bang is replaced by a beginning of the universe which avoids all infinities, but in which time itself is rather different from what we are used to (some more information about this can be found in the spotlight text Searching for the quantum beginning of the universe).

    On the other hand, whenever cosmologists talk about something happening a second, a minute, or 400,000 years after the big bang, their reference point is indeed the big bang singularity. The reason is that, in the cosmological models based on general relativity, the formulae for the expansion of the universe become particularly simple if you define t=0, cosmic time zero, to coincide with the big bang singularity. This is a great advantage for physics calculations dealing with the early universe, so defining cosmic time in this way makes good sense."
  8. Jan 18, 2013 #7
  9. Jan 18, 2013 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Not true. Cosmological models based on general relativity do not describe the universe as expanding from a single point.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook