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Apologetic value of the acceleration of the expansion of the universe

Discussion in 'Physical & Life Sciences' started by Johannes.Ar, Oct 13, 2019.

  1. Johannes.Ar

    Johannes.Ar New Member

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    Most of you probably know that astrophysical observations of vastly different kinds have consistently shown since 1998 that the expansion of the universe is accelerating, actually that it has been accelerating since a time roughly midway between now and "big bang". I want to point out that this already established fact has apologetic value for monotheistic faiths holding creation ex nihilo. This is so because there are several plausible mechanisms whereby a contracting universe would not contract all the way into a singularity, i.e. a state of truly infinite density, but at some maximum density would "bounce" and start to expand. Therefore, in principle it is possible that the universe would undergo a cycle of indefinite duration (both from an infinite past and into an infinite future) consisting of the phases:

    ... big bounce - decelerated expansion - stop at max size - accelerated contraction - big bounce ...

    Since such a cyclic universe would fit neatly the cyclic cosmologies of hinduism and buddhism, it is very good that the expansion of the universe is accelerating, as it invalidates this case. The reason of this invalidation is quite simple: let us assume that the universe indeed undergoes cycles. On the other hand, we know that the present cycle will not stop its expansion and then turn into contraction, but rather will expand forever and moreover in an accelerated way. Now, what could have changed between the previous cycles, in which the universe expanded to a maximum size/minimum density and then started to contract, and the present cycle? The only logical answer is that nothing could have changed, there were just no previous cycles.
     
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  2. SkyWriting

    SkyWriting The Librarian Supporter

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    Our conclusions are wrong, or there were no previous cycles.
    Either one.
     
  3. FrumiousBandersnatch

    FrumiousBandersnatch Well-Known Member

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    But we don't know that.

    It's tautologically obvious that if the expansion continues indefinitely, there will be no contraction, but since cyclic cosmologies are predicated on the expansion not continuing indefinitely, the fact that expansion is currently occurring doesn't, of itself, invalidate them.
     
  4. Occams Barber

    Occams Barber Newbie Supporter

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    An acceleration phase is a necessary part of an expansion/contraction cycle. To go from rest (i.e., maximum contraction) to a given speed (i.e., a given rate of expansion) is impossible without an acceleration phase.

    How do you know that the Universe will 'expand forever' in an 'accelerated way'?

    If the rate of Universal expansion is accelerating then logically it is expanding faster today than it was yesterday. For this to happen something must have changed. If a change causing expansion to go faster is possible then why is it not possible to have change moving the Universe towards contraction?
    OB
     
  5. Johannes.Ar

    Johannes.Ar New Member

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    General Relativity allow for three regimes of expansion of space in an universe with matter (an empty universe will expand linearly):

    a. one which at some point starts to accelerate towards an exponential regime (the actual case);
    b. one which keeps decelerating indefinitely but never stops, a(t) = t^(2/3);
    c. one which decelerates to a stop and turns into contraction.

    According to General Relativity, it is not physically possible for a universe to switch from one expansion regime to another. Since our universe is in regime a, it will always be in that regime.
     
  6. Johannes.Ar

    Johannes.Ar New Member

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    I am not referring to an initial stage of accelerated expansion (which an inflating non-cyclic universe also has), but to a stage of accelerated expansion that starts after a stage of decelarated expansion, which is the actual case, and accurs when the expansive effect of dark energy, whose energy density is constant over time, overwhelms the contractive effect of matter, whose energy density decreases with expansion.

    From General Relativity, which is a physical theory amply confirmed by observations. See my previous response to FrumiousBandersnatch.

    The ongoing change that starts the accelerated phase is that which I explained in the first paragraph of this post: the change in the balance of the energy densities of dark energy (which accordings to observations is just the Einstein-foreseen cosmological constant) and matter, as the expansion of space decreases the latter. General Relativity predicts this change and at the same time rules out any other change that will turn the acceleration into contraction.
     
  7. Michael

    Michael Contributor Supporter

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    Um, well, that's not actually "established fact". It's certainly a fact that the "majority (consensus) cosmology model" today would presume that cosmological redshift is caused by "space expansion" and "space expansion acceleration", whereas Edwin Hubble himself actually rejected that basic premise in favor of a "tired light" cause of cosmological redshift later in his life.

    Until Hubble found evidence of cosmological redshift, the (island galaxy) universe was assumed to be static, and potentially eternal.

    Hubble Eventually Did Not Believe in Big Bang: Associated Press

    Expansion/acceleration (forever) interpretations of cosmological redshift are probably more "attractive" to monotheistic faiths which teach "creation ex nihilo", which might in fact be part of the reason why that particular interpretation of the cause of cosmological redshift is more "popular" than other potential explanations for cosmological redshift which wouldn't necessarily imply or require a "creation" event, let alone a creation ex-nihilo scenario, like a big bounce concept or a static universe.

    Even if you presume that the universe is expanding now, and accelerating now, it's actually not a given that acceleration will continue indefinitely either.

    In short, you're making a number of various assumptions about the observation of cosmological redshift that "just so happen" to agree with your specific brand of "faith", which another individual may not automatically agree with depending on their faith (like Buddhism) or lack thereof (atheism).
     
  8. Michael

    Michael Contributor Supporter

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    With introduction of a non zero constant (not necessarily space expansion however), GR also allows for a static universe. That was Einstein's initial impetus for adding a non zero constant in fact.

    Well, even that premise is not actually 100 certain in GR. It depends on what you think "dark energy" and/or the cause of acceleration might be, and whether you believe that dark energy will continue to remain constant throughout expansion. If you don't make the assumption that dark energy will remain constant indefinitely, it's conceivably possible that the universe could expand and accelerate for awhile, but eventually slow down over time and even conceivably contract again.

    You can't entirely rule out the potential for later contraction simply based on what's happened in the past.

    And of course all of these ideas are predicated on one's interpretation of the exact cause of cosmological redshift. :)
     
  9. FrumiousBandersnatch

    FrumiousBandersnatch Well-Known Member

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    That assumes that dark energy is the 'cosmological constant' (vacuum energy) . If it is a scalar 'quintessence' field, there are a variety of possible scenarios, including becoming attractive rather than repulsive, as it is time-evolving, dynamic, and spatially dependent. Last I heard, we still don't know.
     
  10. Michael

    Michael Contributor Supporter

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    It's frankly a little weird to actually be in agreement. :)
     
  11. FrumiousBandersnatch

    FrumiousBandersnatch Well-Known Member

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    I'll alert the media ;)
     
  12. Ophiolite

    Ophiolite Recalcitrant Procrastinating Ape

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    I had to double my intake of blood pressure pills!
     
  13. FrumiousBandersnatch

    FrumiousBandersnatch Well-Known Member

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    The way I see it, it's not agreement with me, but with the accumulated wealth of human knowledge as I understand it ;)
     
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