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Time from big bang / size of universe

  1. Jul 30, 2011 #1
    How can the elapsed time from the big bang be known if it isn't known how far the universe extends past the ability to observe it? In other words, would a larger universe take longer to contract towards a singularity? Or, if the universe were infinitely large how could it have expanded from a big bang in less than an infinite time?
  2. jcsd
  3. Jul 30, 2011 #2


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    You seem to be imagining the big bang as an explosion at a certain point in a previously empty space. That's not what it was. We have a FAQ about that: https://www.physicsforums.com/showthread.php?t=506991 [Broken]

    If it's infinite now, then it was also infinite at all previous times. Cosmological models don't start out finite and then become infinite.
    Last edited by a moderator: May 5, 2017
  4. Jul 30, 2011 #3
    There is a type of cosmology called inflationary cosmology, where with a certain burst from a negative pressure energy field that pushes with a uniform energy out into the fabric of space, this energy field yields a vast amount of energy that could have multiplied the size of the universe millions of times in a fraction of a second, if this cosmic expansion was faster then the speed of light then light from the furthest reaches of space would not have reached us. therefore we can only see what is in our space, say lets call the radius of what we can see our "cosmic horizon", then that is all we can see
    we can measure the size of this energy field and also the speed at which is must be expanding and accelerating at, meaning we can measure the size and the amount of time it must have been expanding for ;)
  5. Jul 30, 2011 #4
    Here is an explanation I saved from a usually reputable source here...Crowell:

    Here are some other questions and answers about the age of the universe....they are consistent with Crowell's post above...but may offer some different perspective:


    You may be interested to know if you don't already that the most distant galaxies are receding from our vantage point (Earth) at faster than the speed of light...and always have been. And so are the most distant galaxies from all other similar vantage points throughout the universe! But nobody knows for sure if the universe is infinite or not...
  6. Jul 30, 2011 #5
    Another perspective:

    About 380,000 years after inflation concluded, what we now see as cosmic microwave background radiation (CMBR) could begin to get through early matter after all the initial high energy ionization subsided...we see it coming from everywhere..as Crowell implied...it all happened at once, everywhere....

    We only get the CMBR light from some particular batch of early matter once and it passes by. Tomorrow we will get light from matter that is farther away than that batch whose light we got yesterday because the universe continues to expand. The source of the CMBR is NOW about 45b light yrs away but the light we get from it was emittied 41 million light years ago. It has taken so long to get here because the universe has expanded....by a factor of about 1080 times...

    At the UCLA souce I posted previously, there is a "nedwright calculator"...you can plug in numbers and it will do calculations for you....

    In essence, it makes no real difference how much bigger the universe is than the observable universe.
  7. Aug 4, 2011 #6
    I think that the age of the universe was determined from the properties of its background radiation. Not sure though.
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