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

Will the universe shrink back?

  1. May 23, 2014 #1
    I know the universe in expanding and it's accelerating , so after some trillion of years all the stars will be burn out and the universe will become cooler , the universe will be left with black holes ....now my question is then if suppose it's starts shrinking back then again stars galaxies will be created and
    is there any possibility of life on any planet ?
     
  2. jcsd
  3. May 23, 2014 #2
    Greetings
    Although I find it particularly fascinating that many human beings care about the "Ultimate Fate of The Universe" when we will be dead for magnitudes of order longer than the time since it's "birth" till now, I think your question is so far out in "speculation land" that no answer or all "answers" are possible. IIRC, even given Dark Matter, there is presently not enough mass in our Universe to reverse expansion. This doesn't infer that it cannot happen, just that it doesn't seem likely with what we know so far.

    If we wish to assume just for sake of speculation, that some yet unknown action will come into play that could and would reverse expansion it would matter greatly what that action would be in that it does not necessarily follow that everything would essentially reverse order to the manner in which it expanded. So whether planetary systems would form and whether they would exist long enough for life to develop is another order(s) of magnitude into the realm of fantasy at this point.... fun to imagine, but no basis in evidence.
     
  4. May 23, 2014 #3

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    As noted by enorbet, current observational evidence suggests the universe is expanding, and, over the past 5 or so billion years the rate of expansion has been increasing [aka accelerated expansion]. Based on our current knowledge of nature only gravity can slow expansion and evidence again suggests there is not enough matter in the universe to ever allow it to recollapse. Stangely enough, to the best of our measurement ability, it appears the total energy content of the universe [under GR matter is just a form of energy and it all gravitates] is exactly what is required [aka critical density] to halt expansion in the unimaginably distant future. Under this scenario, expansion approaches zero as the age of the universe approaches infinity. This is one of the great mysteries of cosmology - why is the energy density of the universe immeasurably close to its critical density? A multiversalist would just shrug and say it is what it is. I think we can find a better answer than that.
     
  5. May 26, 2014 #4
    In the interest of accuracy and due to the fact that I don't see an "edit" button on my above post, I hope it is OK to make a correction here. As a friendly member brought to my attention, I used the term "infer" in an incorrect manner. It should have read "We can't infer" or "This doesn't imply". Hopefully this will be clearer now, especially for those who must translate into different languages. Cheers.
     
  6. May 30, 2014 #5
    No one knows, and by the time we know if, at all, we'll all be dead. There is nothing to suggest the present acceleration of expansion will not continue, resulting in the Big Rip. But new forces could be discovered changing all that, but again we won't know about it.
    An astronomer was giving a tour of a telescope and told the tour that in 5 billion years the sun would expand and destroy the earth. A sound of relief was heard from one of the parties, who said," whew, for a minute there I thought you said 5 million."
     
  7. May 31, 2014 #6
    you are right , but my question is different ...pls read it again . let's assume that some force will come in to action and the universe starts shrinking back then will the stars and planet be born again ?
    will the laws of physics imply this possibility ?
     
    Last edited: May 31, 2014
  8. May 31, 2014 #7

    You can always imagine some new force, but what would it be, how would it work, would it be strong enough to stop the acceleration?
    Same answer, we don't know, and by the time we did(if) we will be long gone. If there is a new force then the laws of physics are not the same as today, so the question is not well-formed.
     
  9. May 31, 2014 #8

    timmdeeg

    User Avatar
    Gold Member

    Well, the observations are consistent with the assumption that the dark energy acts like a cosmological constant with constant energy density over time. In this case the universe will continue to expand accelerated however without ending up in Big Rip scenarios. The latter depend on the rather exotic case that the density of the dark energy increases with the expansion.
     
  10. May 31, 2014 #9
    Your original question was about the possibility of the universe shrinking due to some new force. There is no reason at all to assume such a force, and to simply invent one is contrary to science. There has to be evidence first.
    If the universe does keep expanding at an accelerating rate forever, eventually our galaxy will be the only one we know of, and that will be our "universe". But after unknown trillions of years, even it will be torn apart when the expansion overcomes gravity. Then suns, then planets, then molecules, atoms, etc. But that assumes the accelerations keeps on going. Maybe it will level off sometime.
    We will not know. And we don't know now.
     
  11. May 31, 2014 #10

    phinds

    User Avatar
    Gold Member
    2016 Award

    vrmuth, basically, your question is equivalent to this: "if the laws of physics don't apply, what would the laws of physics say about <fill in any nonsense you like>". You see how this doesn't work?
     
  12. Jun 1, 2014 #11

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    In science, we are stuck with the laws of physics as currently known, not as they may vary due to unknown processes over time. In that sense, the laws of physics, as currently known, fail to support your premise.
     
  13. Jun 2, 2014 #12
    Nice !! that makes more sense even when i fill my nonsense :smile: , but please see my next reply .
    again you are missing what i am saying , I have read that the acceleration of the universe expansion was slowing down , but suddenly the amount of dark energy became more and the acceleration increased rapidly and it still continues, right ? , what i am saying , while the acceleration can suddenly increase it can also slow down ( some thing can come in to play , who knows ? ) , you cant rule out that possibility, am i right ? so assume all the star in the universe died and now some new force come to play and it's strong enough to shrink the universe back so the universe shrink back .....then read my question
     
  14. Jun 2, 2014 #13

    phinds

    User Avatar
    Gold Member
    2016 Award

    No, you have this (bolded) wrong. The amount of dark energy per unit of volume is not believed to have changed and it has been around since very early on. It's just that about 6 billion years ago it reached the tipping point relative to gravity and began to accelerate the expansion and has been doing so at an increasing rate, smoothly, ever since. There has not been the kind of discontinuity that you seem to envision.
    .
     
  15. Jun 11, 2014 #14
    so you mean with all the known laws of physics we know (for sure) the acceleration of expansion will be increasing for ever , it's impossible to assume the shrinking of universe ?
     
  16. Jun 11, 2014 #15

    phinds

    User Avatar
    Gold Member
    2016 Award

    I'd don't think we know enough to say with absolute certainty that's the case, but given the current state of our knowledge, the strong consensus among cosmologists, based on what I've read, is that the likelihood of the universe ever shrinking back is infinitesimally small.
     
  17. Jun 12, 2014 #16
    so there is also a small possibility for the shrinking of universe to happen ?
     
  18. Jun 12, 2014 #17

    phinds

    User Avatar
    Gold Member
    2016 Award

    Only in the sense that until we fully understand dark energy, we cannot say for sure that it will not have a state change at some point. This is considered extraordinarily unlikely, but cannot be absolutely ruled out because we just don't know enough.

    I encourage you to study ACTUAL physics instead of worrying about fairly useless speculation on stuff that may or may not (and almost certainly not) happen.

    EDIT: OK, I've got another minute now, so thought I'd give you what I think it an excellent example of what I'm talking about in the "study actual physics" comment. Instead of looking a what I consider to be pointless speculation on what might or might not happen at some point in the future, let's look at something that we strongly believe DID happen but which we don't understand.

    In a tiny fraction of a second, starting a tiny fraction of a second after the singularity, the universe apparently underwent a MASSIVE state change in what is referred to as "inflation" and then dropped back with ANOTHER state change. This is still a bit of an open question (whether or not inflation actually happened) but it explains the current universe much better than any other theory that leaves out inflation.

    SO ... what was that all about? Why did it happen? Why did it stop? THOSE are questions worth looking into.
     
    Last edited: Jun 12, 2014
  19. Jun 12, 2014 #18
    :devil:I guess my use of "speculation" wasn't strong enough in the first response in this thread. Consider this -

    If Cleopatra was immortal
    and
    If The Grand Slam Super Mega Ginormous Lottery operated throughout her life...

    is it possible to conclude she would be a multi-billionaire by 2020?

    As silly as that is, it is at least possible to calculate odds since we know the odds, the mechanism, of a lottery. To imagine some yet unknown force hundreds of billions of years from now, to counter what we actually have evidence for now, and further that this force could somehow recreate solar/planetary systems and further that life might form on one or more of these planets, is exponentially sillier than the above Cleopatra question.*

    * that is, unless you're using this at parties as a ploy to seduce Freshman Art Students and then it is perfectly reasonable, provide you're able to suspend moral judgment.
     
    Last edited: Jun 12, 2014
  20. Jun 25, 2014 #19

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    Hmm, do you have any idea where this 'center' might be located? According to the concordance model, the universe has no 'center'. Your 'reasons' are irrelevant if they conflict with observational evidence.
     
  21. Jun 26, 2014 #20

    phinds

    User Avatar
    Gold Member
    2016 Award

    Chronos, I got an email notification for the post that you are responding to but the post (by mxu193) isn't here. It would be helpful if you would use the quote button so we can see what you are responding too.

    This fruitcake (mxu193) has a blog where he uses the "center of the universe" as the basis for a discussion that is, as you say, in conflict with observational evidence.

    This isn't the first time that I've found a post not in the thread immediately after getting an email notification that it WAS in the thread.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Will the universe shrink back?
  1. The universe (Replies: 4)

Loading...