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Infinite universe and energy/matter boundaries

  1. Jan 18, 2014 #1
    I know that although there are alternatives to this model (such as the torus model) most observations fit with the classic flat infinite universe, which is what I'd like to inquire about.
    If I understood properly, it means that the Universe has no boundaries whatsoever and, were it possible, one could travel in any direction indefinitely without ever coming back to the starting point. Now, if there is a finite amount of energy and matter in the Universe, would that mean we could reach a region in spacetime (please ignore faster-than-light travel limitations) where there is no matter and energy? Would this inevitably contradict homogeneity?
    I'm assuming there's a finite amount of energy and matter because I can't visualize how infinite amounts would make the law of conservation of energy any meaningful.

    Am I missing something very important and/or making a terrible reasoning mistake? By now it should be pretty clear that I'm by no means trained in physics, so I'd really appreciate it if you could keep it in layman's terms. Thank you very much in advance!
     
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  3. Jan 18, 2014 #2

    phinds

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    You are using contradictory suppositions and then getting surprised that there is a contradiction. If you assume an infinite universe, you cannot then assume a finite amount of mass/energy.

    The laws of conservation of energy do not apply on cosmological scales.
     
  4. Jan 18, 2014 #3
    Please note that no-data from CMB or other Cosmological data at least now ( and may be never) can decide that our universe is open or closed! The inflation theory says : the fraction of curvature density to density of the universe is negligible , but this does not state that the curvature of the universe is positive or negative or even zero.
     
  5. Jan 18, 2014 #4

    Chronos

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    The idea that energy conservation does not apply in general relativity is widespread, but, not universally agreed upon among physicists. Phil Gibbs, in http://arxiv.org/abs/1304.6728, states "Energy conservation in general relativity is real, exact, non-trivial and important." See also his blog entry dated 6 August 2010 at http://blog.vixra.org/category/energy-conservation/. John Baez, in http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html, responds to the question [Is energy conserved in general relativity?] by saying "In special cases, yes. In general — it depends on what you mean by "energy", and what you mean by "conserved"." pointing that merely defining energy under GR is non-trivial. IMO it is, at best, misleading to unequivocally claim energy is not conserved in GR. I agree with phinds that you cannot assume matter or energy is finite if you assume the universe is infinite.
     
  6. Jan 18, 2014 #5

    phinds

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    Chronos, thank you for that. I did not realize it was a contentious subject. I was merely parroting (with some limited understanding) what I thought others had agreed on here on this forum.
     
  7. Jan 18, 2014 #6

    Chronos

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    There is no compelling reason to assume energy conservation in GR any more than there is to the contrary. The easy part comes when you argue things like redshift and dark energy. If you assume energy is not conserved in GR, discussion ends prejudicially with "next question". I've never found that entirely satisfactory.
     
  8. Jan 19, 2014 #7
    Thanks for your answers guys, it's of great help. I wasn't aware conservation of energy may not apply on cosmological scales; trying to find information about this subject I came across that very article by Phil Gibbs but it's too technical for me. I've also found other articles debating Phil Gibbs' view but they're far beyond what I could possibly understand.
    It remains unclear to me how the size of the universe at its different stages of development could be calculated if infinity is assumed. Does it mean the observable universe?
     
  9. Jan 19, 2014 #8

    phinds

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    Any statement about the exact (more or less) size of the universe has to mean the observable universe since there is no known way at present (and may never be) to determine the size of the entire universe at ANY point in time.
     
  10. Jan 19, 2014 #9
    So far it is darn 99%(BOSS eye) flat still... it’s “more(cross finger)likely” the universe is infinite, extending forever into space and time.

    http://arxiv.org/pdf/1312.4877v1.pdf

    pg.36
     
  11. Jan 19, 2014 #10

    bapowell

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    In what sense is it "more likely"? We've examined an unknown amount of universe: the extrapolation you wish to perform might be incredibly unfounded.
     
  12. Jan 19, 2014 #11

    Chalnoth

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    I rather strongly disagree. Energy simply is not a conserved quantity in GR. You can recover energy conservation under specific conditions by defining things carefully. But that's not the same as energy conservation.

    What is conserved in General Relativity is the stress-energy tensor, which includes energy but also momentum, pressure, and twisting forces. The combination of all of these together is conserved (in a very specific way), but there's no universal way of defining energy such that it is always conserved.
     
  13. Jan 19, 2014 #12
    What amount. Are we talking observable universe? Their are data's coming from Baryon Oscillation Spectroscopic Survey measuring baryon acoustic oscillations/baryon waves with comparison to PLANCK+BAO+SN. (according to them).

    A strong case/consistent towards flat universe/infinite with uncertainty(that's me saying "more (x) likely"). WMAP ((2013)excluding the latest data) favors a flat universe 0.4% margin of error.Pointing towards Universe that is infinite in extent; however, since the Universe has a finite age, we can only observe a finite volume of the Universe. They can only conclude that the Universe is much larger than the volume they can directly observe.

    http://arxiv.org/abs/1312.4877

    "The acoustic features are detected at a significance of over 7σ in both the correlation function and power spectrum. Fitting for the position of the acoustic features measures the distance relative to the sound horizon at the drag epoch, rd, which has a value of rd,fid=149.28Mpc in our fiducial cosmology. We find DV=(1264±25Mpc)(rd/rd,fid) at z=0.32 and DV=(2056±20Mpc)(rd/rd,fid) at z=0.57. At 1.0 per cent, this latter measure is the most precise distance constraint ever obtained from a galaxy survey. Separating the clustering along and transverse to the line-of-sight yields measurements at z=0.57 of DA=(1421±20Mpc)(rd/rd,fid) and H=(96.8±3.4km/s/Mpc)(rd,fid/rd). Our measurements of the distance scale are in good agreement with previous BAO measurements and with the predictions from cosmic microwave background data for a spatially flat cold dark matter model with a cosmological constant."

    http://phys.org/news/2014-01-baryon-oscillation-spectroscopic-survey-universe.html
     
  14. Jan 19, 2014 #13

    bapowell

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    Yes, but all of these observations pertain to the observable universe! We have no grounds for inferring the geometry of those regions of the universe that lie outside the Hubble sphere. Saying that CMB/LSS data favor a flat, infinite universe (or in some vague sense make it "more likely") is just wrong.
     
  15. Jan 19, 2014 #14

    Chronos

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    The issue I have with an infinite universe is if it is infinite now, it has always been infinite, which implies the universe is also infinitely age. I just cant wrap my head around the idea we reside in a region which happens to be unnaturally young. On the other hand a finite universe is also unsettling with all the edge and what lies beyond questions you get peppered with. Steven Hawking likes the finite, but, unbounded approach. That works for me - even if I don't completely understand what it actually means. I believe the general consensus is the universe [not just the observable part] is really, really big. We don't know how big, and probably never will.
     
  16. Jan 19, 2014 #15

    jcsd

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    An infinite Universe doesn't imply it is infinite in age: take for example a flat FLRW Universe, its spatial slices are infinite in extent, but it has a definite starting point.

    "Finite, but unbounded" is a term that is usually applied to the closed FLRW Universe whose spatial slices are topologically 3-spheres and thus it has an always-finite (but changing) spatial volume.
     
  17. Jan 19, 2014 #16

    phinds

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    Why? What's your reasoning on this? Why should infinite size imply infinite time?
     
  18. Jan 20, 2014 #17

    Chronos

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    Good question, it just seems reasonable due to the whole spacetime thing. It is believed there is no spacetime without gravity and I assumed, perhaps in error, it would take an infinite amount of time for gravity to span an infinite universe. As usual, the multiverse comes to the rescue. I'm not convinced I'm satisfied with that alternative.
     
  19. Jan 20, 2014 #18
    Before anything else. I'm not an expert. Just a lurker in this forum trying to understand as much as i can.^^

    Here's the thing. We have a positive data showing a 'glimpse' of the geometry(Ob-Universe)with figures of uncertainty(open, closed, flat etc). When i say 'morelikey'(which seems to be the issue here lol). I'm referring to the 'constraint' of the data (PLANCK+BAO(boss)+SN) which is basically showing flatness to be dominant and favorable to the result. BTW What is outside the hubble sphere?




    "Among other cosmic parameters, says White, the BOSS analysis "also provides one of the best-ever determinations of the curvature of space. The answer is, it's not curved much."
    Calling a three-dimensional universe "flat" means its shape is well described by the Euclidean i.e straight lines are parallel and triangles add up to 180 degrees. Extraordinary flatness means the universe experienced relatively prolonged inflation, up to a decillionth of a second or more, immediately after the big bang."

    "One of the reasons we care is that a flat universe has implications for whether the universe is infinite," says Schlegel. "That means – while we can't say with certainty that it will never come to an end – it's likely the universe extends forever in space and will go on forever in time. Our results are consistent with an infinite universe."
     
  20. Jan 20, 2014 #19

    Chalnoth

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    This is taking the conclusions of the study too far. We have a detection of the universe being flat at a level of about 0.005 (that is, the spatial curvature contribution to total space-time curvature is less than roughly half a percent or so). In order to say that the universe is exactly flat, we would need to improve experimental accuracy enough to add an infinite number of zeroes between the decimal and the 5. Obviously, that cannot ever be done.

    That's the first point. The second point is that even if we dramatically improved our measurement of spatial curvature and that measurement remained flat, it would not say that our universe was infinite. It is perfectly possible to have a spatially-flat universe that wraps back on itself, for example (the shape would be a toroid, which is rather like the surface of a donut).
     
  21. Jan 20, 2014 #20
    I totally understand your point. As for the shape of the Universe, this depends on the overall curvature. The curvature depends on the amount of matter and energy in the Universe (Einstein's equations). The curvature and energy content can thus be estimated using a cosmological model.We can calculate the likelihood of the parameters in the model. Doing this one gets a curvature equal to, or very close to, zero. The analogue of which is a flat, smooth and infinitely extended bedsheet is dependent on shape. The shape (spacetime in GR), is in mathematical terms analogous to the shape of other geometrical objects. However, not necessarily the case for more general cosmological theories, some of which propose so-called topological defects and other geometrical objects which are not so easily visualized.

    Much of the confusion is contributed by the extent of unknowns. Beyond these horizons we can never hope to reach, with our current understanding of things. The reason for bringing up this matter of horizons is that making statements about the Universe beyond the horizons (and out to infinity beyond) is on the verge of philosophy. Many exotic ideas exist about the nature of the un-observable Universe. These include multiverse, eternal inflation, local void, and many others.

    Again going back. The claim is mostly bounded by positive observation/real datas. When they mentioned infinity it's not meant to be absolute(reachable?). As far as i understand. It could have been uncertain but definitely not unknown since we somehow have real values.^^
     
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