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The cosmological Big Rip and a shrinking hubble radius

  1. Jul 6, 2013 #1
    The cosmological "Big Rip" and a shrinking hubble radius

    Doesn't an expanding universe forecast such outcomes as a faded and unobservable cmb, galaxies moving away from each other greater than the speed of light, a gravitationally unbound solar system, and eventually an observable universe which is microscopic in size resulting in ultimately a "Big Rip" ? If so, does this necessarily imply a shrinking hubble radius?
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  3. Jul 6, 2013 #2
    The big rip was one proposal but its no longer considered as a fate of the universe. We are rather headed towards heat death if expansion stays the same. Galaxies are not moving anywhere near the speed of light and will never move at the speed of light. What pop media articles are referring to by the statement this galaxy is moving at 3c. what they are really referring to is the recessive velocity to distance relation.

    Hubbles law states the greater the distance the greater the recessive velocity. This is an observer dependent relation. the article on this thread will explain that for you

  4. Jul 6, 2013 #3
    All of this is model dependent on the cosmological model...reigned in [parameters constrained] where necessary by observational evidence:

  5. Jul 6, 2013 #4


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    Uh ... "universe which is microscopic in size" ??? Oh ... I guess you mean there will be nothing left bigger than microscopic particle. No ... entire galactic clusters may hang together. Certainly there is no evidence that anything as "small" (on cosmological scales) as a galaxy will be affected, but you are certainly right about the unobservable CMB and in fact the total lack of ANYTHING observable outside of our galaxy since all others (except perhaps the local group) will have eventually receded out of detectable range.
  6. Jul 6, 2013 #5
    A note on the Hubble parameter that is not always made clear....

    The Hubble parameter is currently, right now, a 'constant'. That means it is the same in all directions based on the assumptions of the favorite cosmological model...the FLRW or FRW model.

    But it varies over time: As noted in the explanations I posted, it is decreasing currently.....

    And you can calculate much of the sort of stuff....You can probably find such in the link Mordred posted.

    Marcus of these forums explained elsewhere:

    elsewhere, somebody posted the Hubble constant is asymptotically approaching a constant value....which leads approximately to the 1/160 increase.
  7. Jul 6, 2013 #6
    In one sense this is true, but I would explain it differently.....eventually if expansion continues long enough, all the galaxies matter will be absorbed by black holes. Those BH may all coalesce or may not, but as the universe cools [Mordred calls it 'heat death expansion' above those black holes will eventually evaporate.....in a disappointing 'poof' as Chalnos of these forums expressed it.
  8. Jul 6, 2013 #7
    Ok, so what am I missing here? . . . .
    1) The hubble radius is defined as the distance at which objects are moving away from us at the speed of light.
    2) The rate at which the universe is expanding is accelerating causing objects objects to move faster as time goes on.
    3) In my mind, as a consequence of points 1 and 2, I imagine the hubble radius contracting.
  9. Jul 6, 2013 #8


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    Ah ... I was overlooking that, possibly because I was not aware that it was established fact that the black holes WILL eat the galaxies. Has this really been confirmed as a probable outcome? I had thought that they wouldn't.
  10. Jul 6, 2013 #9


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    Don't let the words confuse you, the word "acceleration" can give people the wrong idea. When they say expansion is accelerating cosmologists are talking about the standard cosmic model in which the Hubble radius is NOT expected to ever get smaller.

    R is currently 14.4 Gly (billion light years) meaning large-scale distances expand 1/144 of a percent per million years
    R is expected to continue increasing towards a limit of 17.3 Gly meaning in far distant future distances will be growing at a rate of 1/173 % per million years. (no "big rip" stuff)

    The percentage growth rate is actually DECLINING. So why do cosmologists say that expansion is accelerating?

    they say that because the decline in percentage growth rate is so slow that if you watch a particular distance between two widely separated objects it will be showing nearly exponential growth at a nearly constant percentage rate. As the distance grows in size, the actual speed it is expanding grows.

    It's like you put money in a savings account at some fixed percentage interest. Your principal keeps increasing so the yearly DOLLAR gain keeps increasing. That's a kind of acceleration in terms of yearly dollar gain. Now imagine that the bank very very slowly reduces the percentage interest rate that it pays. Your account could still be "accelerating" because the principal is growing, and the percentage rate is almost constant.

    To see what the standard cosmic model says about change of Hubble radius over time, go here:

    and click on "set sample chart range" and then click "calculate"

    T is the year and R is the Hubble distance. The present is year 13.8 Gy
    The table will start at around 0.067 Gy, that is at year 67 million when R is quite small (distances are growing a full 1% per million years)
    The table will go up to the present T=13.8 Gy and then continue on a few billion years into the future and show R beginning to converge to its eventual limit of 17.3 Gly. (eventual growth rate 1/173 % per million years.)
    Last edited: Jul 6, 2013
  11. Jul 6, 2013 #10
    That must be the update to the prior [older] 1/60 % I posted earlier.....darn scientists keep changing the numbers!!

    and go 'poof'.....

    Absolutely, positively, without any doubt...............NOT confirmed....[LOL]

    I checked Wiki:
    Wiki says heat death is 'most commonly accepted'....here
    Heat death of the universe

    and here, Wiki implies black holes will be the last to go...


    How 'mainstream'??? I have no idea....I have a knack for infuriating a few 'Gods of the
    forums" so maybe one will appear here to pummel me....If Marcus did not comment adversely, likely such an idea is at least in the ballpark....[I have to 'cool it' until some infractions expire...hopefully that will be before the end of the universe!!!]
  12. Jul 9, 2013 #11


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    You are perhaps missing the consequences of Hubble's law, which others already mentioned. The recession velocity v of a co-moving object is proportional to it's proper distance r, v = Hr, with H the Hubble parameter. In the case of a photon the proper distance coincides with the radius R of the Hubble sphere, R = c/H. As H decreases in an expanding (and even in an accelerated expanding) universe, R increases. Should the universe expand exponentially, then H and R will asyptotically approach constant values. So, R will not shrink. This is true, if the the w-parameter > -1.

    With unlimited growth of antigravitating energy (w < -1, big rip case) the horizon becomes zero, as you said. That's unlikely, but to my knowledge not yet definitely excluded.
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