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The Mass of the Universe

  1. Nov 11, 2014 #1
    Forgive me if some of my question around here sound trite or have been covered before, I'm not a physicist, but I do have a great layerperson's interest and I feel like I need to move to the next level (beyond TV programmes I mean I mean)

    So I have an opening question. Of course the "singularity" before the Big Bang was not infinitely dense or the Universe itself would still be infinitely dense; so am I to presume that consistent with the first law of thermodynamics, the Universe has always had exactly the same mass, and always will?

    Unless of course, it is not a closed system and mass can "leak" out into other realms?

    Thanks
     
  2. jcsd
  3. Nov 11, 2014 #2
    The universe has always had the same TOTAL energy. The temperatures just after the big bang were too high for matter to form. But yes you are right.
    Oh, and what other 'realms' could there be?
    And why couldn't the universe have been infinitely dense at the big bang. Infinite density doesn't necessarily mean infinite mass( in which case you would be right) but it might also mean finite mass in an infinitesimal volume. So the universe could have been infinitely dense and the expansion of the universe means that you have regions that aren't infinitely dense.
     
  4. Nov 11, 2014 #3
    Cheers

    Other Universes. (Speculating)

    Sure, OK. But infinitesimal volume makes no more sense to me than infinite mass. It gives us the spectre of the physical singularity.
     
    Last edited: Nov 11, 2014
  5. Nov 11, 2014 #4

    Bandersnatch

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    I disagree with UncertaintyAjay's post. As indicated in the other thread the OP started, the total energy of the universe can't be easily defined. From what I understand there are some (most vocally Lawrence Krauss) who like to make statements about the total energy of the universe being zero, with the energy lost in the expansion "siphoned" into gravitational potential energy, but it's hardly the standard approach, and you are likely to find more people regarding that as muddling the issue - as indicated in that link I gave in the other thread.


    Similarly, singularities and infinities tend to suggest limitations to the theory, rather than actual physical reality. If you model the evolution of the universe backwards in time using General Relativity you end up with an infinitely dense singularity - but this is just an indication that you shouldn't try and use GR to model that region. And indeed, the Big Bang theory doesn't do that. It focuses on the expansion from a hot, dense state to what we see now, but claims no predictive power as to the beginning or what happened before (assuming it even makes sense to ask).
     
  6. Nov 11, 2014 #5
    But why doesn't infinitesimal volume not make sense? That's how black holes are formed. They shrink to incredibly small singularities. And I'm speculating too, but if there were other universes, then wouldnt the total energy in all the universes( combined) be the same, which I reckon amounts to the same thing.
     
  7. Nov 11, 2014 #6

    Bandersnatch

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    The singularities are pretty much what you get when you try to divide by zero. It's where the function you used to describe something stops being defined. It's a stretch to claim that it's has got any physical meaning.

    Both BB and black hole singularities are the extrapolations of GR to a region where it is no longer applicable. We'd need a quantum theory of gravity, that is, a theory that takes into account quantum interactions, to model those regions.
     
  8. Nov 11, 2014 #7

    phinds

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    No, that is NOT how they are formed, it is how the math says they END UP, which is not the same thing at all.
    Pointless speculation as far as anything practical is concerned. If there are other universes they are not in causal contact with ours so they are irrelevant.
     
  9. Nov 11, 2014 #8

    phinds

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    Me too.
     
  10. Nov 11, 2014 #9
    Enlighten me then phinds, how are they formed?
     
  11. Nov 11, 2014 #10

    Bandersnatch

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    I'm no phinds, but I can try to enlighten.

    All you need to do to get a BH is concentrate enough mass in a sphere of a radius lower than its Schwartzschild radius. That's when the event horizon forms and you've got a black hole. Whatever happens to the mass trapped inside is irrelevant.

    In fact, there were some recent papers suggesting the mass inside may bounce back in a finite amount of time, and emerge from the event horizon. Make a forum search for "planck stars" if you're interested.
     
  12. Nov 11, 2014 #11
    Thanks bandersnatch
     
  13. Nov 11, 2014 #12
    Oh and phinds, you seem to turn up in a lot of threads in which I post( nothing wrong with that) What I do take issue with is the fact that not one of your replies towards me has been politely phrased, or , in fact , told me anything of use.While both you and Bandersnatch corrected me( and I accept that I was wrong) only Bandersnatch had anything of value to add . You seem to rejoice in criticizing others and don't seem to understand that making mistakes is a part of a learning process. So please, be a little less brusque in the future and offer something constructive.
     
  14. Nov 11, 2014 #13

    Bandersnatch

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    Hey, don't be too harsh on phinds. His grumpy streak is legendary among PF users. It's just a something we have to live with. ;)
     
  15. Nov 11, 2014 #14
    Yeah, no, I get that he knows a lot of stuff, and understands things really well, but its time he became more polite. And seriously, I don't mind the criticism, its just that I don't learn much from his comments apart front the fact that I'm wrong.
     
  16. Nov 11, 2014 #15
    [Because E="UncertaintyAjay, post: 4910030, member: 529142"]But why doesn't infinitesimal volume not make sense? That's how black holes are formed. They shrink to incredibly small singularities. And I'm speculating too, but if there were other universes, then wouldnt the total energy in all the universes( combined) be the same, which I reckon amounts to the same thing.[/QUOTE]

    Because a finite matter in a practically infinitely small space sounds like it breaks every principle imaginable
     
  17. Nov 11, 2014 #16
    Yeah, forget the stuff I said I was wrong. But I'll take another go anyway( corrcet me if I'm wrong) but yes, it dies violate every principle, which is why relativity can't model singularities.
     
  18. Nov 11, 2014 #17
    I can't even imagine one in practice.
     
  19. Nov 11, 2014 #18

    phinds

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    I do apologize for the briskness of my replies. I don't mean to be rude in any way, I just try to get things across briefly and I don't expand much, if at all, many of the times, which is an unfortunate tendency that I fight, mostly as a losing battle. I also agree w/ your observation that I tend to be more negative than is sometimes necessary. My inclination is along the lines of the advice I once heard about being minimalist if on the witness stand and a lawyer asks you "do know what time it is?" You look at your watch and see that it's 3pm so your inclination is to answer "3pm". My inclination is to follow the advice and answer "yes". It comes across as rude when I don't mean for it to be.
     
  20. Nov 11, 2014 #19
    Oh, well, in that case, I'm sorry too phinds.( Not a very minimalistic post ,that one, eh?)
     
  21. Nov 11, 2014 #20

    phinds

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    One what? One singularity? It would be amazing if you COULD imagine one since it is not a "thing" it's just a word that represents "the place where the model breaks down". Someday, I hope, there will be a theory of Quantum Gravity that will give us a more meaningful insight into what is really going on at the center of a black hole. The word "singularity" has quite possibly caused more wasted keystrokes on this forum than any other single word/phrase.
     
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