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What's wrong with my logic?

  1. Jan 7, 2006 #1
    Is it reasonable to assume that if there are areas of space that are causally disconnected from us because they are receding at superluminal velocities then there must be galaxies that we cannot observe? And if this can be assumed then how do cosmologists estimate that the matter in the universe only comprises 5% of the total amount of matter that is required to explain the currect rate of expansion and attribute the remaining balance to Dark Matter and Dark Energy? Is it not possible that there are areas of space that we cannot see that also have an effect on this expansion?
     
  2. jcsd
  3. Jan 7, 2006 #2

    Garth

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    Cosmologists work with the density, not the total mass of the universe. And yes, if you break the cosmological principle and the requirement of homogeneity, then it is possible to imagine that we are in the middle of an immense relative void in the middle of a much denser universe beyond our particle horizon that may emulate some of the features of the accelerating LCDM model; but I doubt it reflects the real universe.

    Garth
     
  4. Jan 7, 2006 #3
    Thanks for the quick reply Garth!

    Is it then safe to assume that the current popular post bang model (that shows the rate of expansion decreasing until a few billion years after the bang and then progressively increasing after that point) is based on the current notions wrt the cosmological constant? i.e. this much dark matter vs. that much dark energy?
     
  5. Jan 7, 2006 #4

    Garth

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    That is the standard model called the [itex]\Lambda[/itex]CDM model, [itex]\Lambda[/itex] stands for the cosmological constant or Dark Energy and CDM for Cold Dark Matter. The epoch of cosmic acceleration is a fairly limited one and reverts back to normal deceleration at some point depending on the exact model. There are lots of threads and posts about it on this Forum, SpaceTiger's posts in his thread Review of Mainstream Cosmology are very good.
    Garth

    Edit: Link fixed!
     
    Last edited: Jan 8, 2006
  6. Jan 7, 2006 #5
    Thank you very much Garth....I will start there!
     
  7. Jan 8, 2006 #6

    SpaceTiger

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  8. Jan 9, 2006 #7

    Chronos

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    Causally disconnected parts of our universe [since the BB], will remain forever causally disconnected from our hubble bubble by definition, IMO. They may exist, but, will never have any observable consequences in our universe; for the reasons you already stated. There is some disagreement on this point, but, I think you have a very powerful argument.
     
  9. Jan 10, 2006 #8
    I had a similar question when I recently saw a rough estimate of the BB's energy level, which is estimated to be about 10^68J. Contemplating about the inflationary theory, I thought, wait a second, knowing the energy level of the original bang, it's a simple calculation to deduce the current mass, given the energy/mass ratio. Then compare it to the current space density, and voila, you know exactly how much is visible and how much is not. But obviously if it was that simple, we wouldn't be having this discussion. So, I'm guessing the 10^68 estimate is taken from a non-inflationary theory. Otherwise, I'm hopelessly confused.......
    Pavel.
     
  10. Jan 11, 2006 #9
    Hi Garth, thanks again for the link, its going to take me a bit of time to go through the entire thread, but Im going to have fun trying :)

    ....I was thinking about what you said earlier:


    And this made me wonder how it is possible that the cosmic acceleration will eventually revert back to a deceleration if the attractive gravitational effects of matter will weaken as space expands whereas the repulsive gravitational effect of the cosmological constant will stay the same? By this logic wouldn't spatial expansion continute to accelerare indefinitely over time? And if that is indeed the case, isnt the implication here that as spatial expansion accelerates, it will drive more and more galaxies beyond the particle horizon eventually leaving every galaxy causally disconnected from every other making the observable universe, from the perspective of our own milky way, an otherwise dark and empty place many hundreds if not thousands of billions of years from now?
     
    Last edited: Jan 11, 2006
  11. Jan 11, 2006 #10

    Garth

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    As I said, it depends on your model of DE.

    Before the discovery of 'fainter than expected' Type Ia Super Novae at about z = 1, which is normally interpreted as cosmic acceleration, it was believed that there were three possible destinies for the Universe linked to the shape of space: a spherically spatially curved, high density, Universe would eventually recollapse; a hyperbolically spatially-curved low density Universe would expand forever; and the flat spatially uncurved, critical density Universe would continue to slow down but would never recollapse.

    While it is now generally thought (the mainstream model) that the universe is flat, because of the WMAP data, the discovery of cosmic acceleration and dark energy has severed the original link between the universe's geometry and destiny.

    There are several suggestions as to the nature of the dark energy, and therefore the nature of cosmic acceleration, and so at present the destiny of the Universe in the mainstream model is unknown.

    The possibilities in that mainstream model are wide open.

    If the dark energy remains constant, as is consistent with the data presented today, and is in a form which mimics the cosmological constant, then the expansion will continue to accelerate and in a hundred billion years or so we will only be able to see a few hundred galaxies, compared to the hundreds of billions we can see today.

    On the other hand, it is also possible that dark energy will decrease with time so that eventually the normal gravitational fields overwhelm it and lead to a deceleration and recollapse, the 'Big Crunch'. (A nice thought!)

    Finally, the most radical possibility has been called the 'Big Rip', where the dark energy increases with time and within a 100 billion years or so rips apart every galaxy, star and atom in the Universe. (Another nice thought!)

    However, there are several caveats to remember.

    1. Cosmic acceleration is based on the observation of fainter than predicted distant Type Ia S/N, and there could be other explanations for their lack of apparent magnitude.

    2. The mainstream model depends on there being a period of intense inflation in the earliest moments of the BB, this itself depends on there being some fundamental particle, for example the Higgs Boson or Inflaton, which cannot be found even after many decades of intense investigation.

    3. The mainstream model depends on there being a non-baryonic Dark Matter particle which cannot be identified and discovered in a laboratory also after much intense investigation.

    4. Although the 'raw' version of GR has been well tested in solar system and laboratory experiments, the cosmological constant version, as required by Dark Energy, cannot be so verified. Attempts to identify it with the QM Zero Point Energy field fail by a factor of about 10120!

    5. Inflation was invoked to solve some major coincidences in the horizon, density and smoothness problems of GR cosmology. However we now find that it not only necessarily invokes DM and DE, so far undiscovered by laboratory physics, but does so in such a way that they at least at present are roughly equal in density. Is this not also rather a coincidence? A theory that explains one set of coincidences by introducing another one is somewhat unsatisfactory.

    Garth
     
    Last edited: Jan 11, 2006
  12. Jan 11, 2006 #11

    selfAdjoint

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    Garth just to discuss the three possibilities you laid out for dark energy (remain constant, decrese, or increase), it occurs to me there is a fourth. Dark energy could vary within bounds, not necesarily cycling but having a central attractor. Has anybody speculated along these lines?
     
  13. Jan 11, 2006 #12

    Garth

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    selfAdjoint: It's possible of course; anything goes that can be made to fit the present observations, that is until we identify what DE actually is and can measure its properties.

    Garth
     
  14. Jan 12, 2006 #13
    I was not aware of this and to your point it would be disheartening if the inflationary model were to depend on a set of precise conincidences much like the classical model was dependent on the precise value of the ATB critical density...ughh!
     
  15. Jan 16, 2006 #14

    Garth

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    It may indeed be varying:Is dark energy changing?.

    Garth
     
  16. Jan 16, 2006 #15

    SpaceTiger

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    If it's right, it's of course a very important result. However, there is a considerable amount of skepticism from the community, not just because of the questionable use of GRBs as standard candles, but also because of the way in which they did their analysis. The rumor is that they made a major (and result-changing) mistake.
     
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