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Accelerating Universe Expansion’s Implications? (layman’s questions)

  1. Jun 17, 2011 #1
    I’m new to these forums and as a matter of personal curiosity, and as someone who can’t follow the math behind the basic concepts, I have a few questions. The first set have to do with the accelerating expansion of our universe.

    A) Does this acceleration affect the inflationary model of the universe, and is the universe younger than previously thought?

    B) Has the rate of acceleration been determined? If so, how long before the speed of expansion greatly approaches the speed of light?

    C) What happens when the speed of light is reached? Will all matter (even black holes?) convert to pure energy?

    D) Who was the scientist that theorized that the universe’s accelerating expansion is fueled by an inverse property of gravity (a few years ago, I think). Where is she now and how has this theory faired? – Or has my memory really bungled this one?

    Thanks for any response; hopefully these questions aren’t too embarrassingly stupid/ignorant, and I’m deeply sorry if I’m not bright enough to realize these questions were answered elsewhere. . .
     
  2. jcsd
  3. Jun 17, 2011 #2
    Welcome to PF shawn-m; these are all excellent questions! I'll answer what I can

    I don't know the details to this question. There is definitely some effect; I think its actually fairly subtle however, as the current accelerated expansion, and inflation soon after the big-bang are fairly separate concepts.

    Yes and no. There are measurements for the rate of expansion; their uncertainties are very large (only recently is the community convinced that the expansion is accelerating at all. There are other measurements which directly relate (i.e. the matter/energy content of the universe, properties of the cosmic microwave background, etc), I'm not sure how they can be used to determine the value of the acceleration however.

    This is the most important question---and it shows that you're a little confused (understandably!). Every point in space is expanding (moving away) from every other point in space. Like the surface of a balloon as its being filled with air (note this is not "inflation"). The farther away two points are from eachother, the faster they are moving relative to each-other (related by http://en.wikipedia.org/wiki/Hubble%27s_law" [Broken]). Right now, there are parts of the universe which are moving away from us at speeds larger than the speed of light. The distance at which the average relative velocity is the same as the speed of light, is called the 'horizon' of the universe. There's nothing special about it, except that we'll never know-about, or interact-with, what's outside of it.

    We're all taught that 'nothing can exceed the speed of light'. This is true---but it only applies to local regions of the universe.

    I'm not sure who you're referring to, but I believe the first person to come up with the idea was Einstein, in his famous 'http://en.wikipedia.org/wiki/Cosmological_constant" [Broken]'. But I might be confused about what you're asking.
     
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  4. Jun 17, 2011 #3
    Generally I follow what you're saying, but the line above sounds too assertive. An inherently infinite and unbounded universe might be mathematically consistent, but it is just as fair to say that anything moving away from us faster than the speed of light cannot exist (as far as we are concerned, of course).

    Would it be correct to say that this horizon is 300,000 lyr beyond the CMB?
     
  5. Jun 17, 2011 #4

    marcus

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    Bob, Shawn, Hermes. What Hermes says here is right:
    ==quote Hermes==
    ...Every point in space is expanding (moving away) from every other point in space. Like the surface of a balloon as its being filled with air (note this is not "inflation"). The farther away two points are from eachother, the faster they are moving relative to each-other (related by http://en.wikipedia.org/wiki/Hubble%27s_law" [Broken]). Right now, there are parts of the universe which are moving away from us at speeds larger than the speed of light. ...
    We're all taught that 'nothing can exceed the speed of light'. This is true---but it only applies to local regions of the universe.
    ==endquote==

    In uniform expansion of distances nobody gets anywhere.
    It's not like ordinary motion. In ordinary motion there can in principle be some object you depart from and some object you approach. At least in principle you can be getting farther from something and closer to something else.

    There is no law of physics that says that distances cannot expand at rates exceeding the speed of light. That is not ordinary motion and it doesn't violate special relativity. Special Rel does not even apply to the changing largescale geometry of General Rel. Special Rel (where you are taught the speed limit for ordinary motion) uses fixed static geometry which applies only locally, in patches of the real universe which are small enough so expansion effects can be ignored.
     
    Last edited by a moderator: May 5, 2017
  6. Jun 17, 2011 #5

    marcus

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    Hermes, here is something for you to try. Google "cosmos calculator" which will get you
    http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

    The link is also in my signature.

    The standard cosmology model is built into this calculator. You first need to type in three numbers (the basic parameters of the model).

    .27 and .73 and 71, for "matter density" "cosmological constant" and "Hubble parameter".

    Then put in 1.7 for the redshift and see what you get.

    Most of the galaxies we are observing today have redshifts which are equal or greater than 1.7. Lots have redshifts like 5, or 6, or even 7....
    The CMB has redshift about 1090 or approximately 1100.

    So most stuff that we can see and are currently seeing, the distance from here to there was already expanding faster than c when the light was emitted. And is expanding faster than c to this day. Notice that I did not say "moving".
    I said the distances are changing. Dynamic geometry. Curved spacetime means dynamic spatial geometry. There is something important to realize when one is ready, that this calculator example with redshift 1.7 can help realize.

    You should have questions, like "how does the light get here" if the recession rates then and now were > c.

    The "charley" link in my signature can help with that. The first page is blank so scroll down. It is a SciAm reprint by Charley Lineweaver and Tamara Davis. But you can ask questions here too. But I suggest you try various redshifts in the calculator, first, and see what recession rates you get. A rate of recession is not necessarily to be thought of as a speed of ordinary motion (though people will say "recession speed" blurring the distinction).

    I want to thank you for the good answers you have been giving people. I realize you know a lot of this stuff and may have played around with cosmology calculators before. I differ with you on minor points about the various "horizons" (of which there are several in cosmology) but this does not matter.
     
  7. Jun 17, 2011 #6
    Wow, folks! Thanks for the answers!

    This nicely went to the heart of the matter, and was explained in such a way that I finally get it.
     
  8. Jun 18, 2011 #7
    I haven't seen one of these before, really cool--thanks!

    I never realized that before; also, very cool. And I indeed had exactly that question, and the article answered it quite nicely (in addition to suggesting what you meant by 'various horizons'). Thanks again!

    Really appreciate that, its my pleasure to give an occasionally-correct answer.
     
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