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Thread: Inflation theory

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    Inflation theory

    Why is it necessary that inflation happens so early in the universes history?
    Last edited by Copernicus; 2019-Dec-12 at 10:30 PM.
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    Have a look at the motivations for inflation. One is the lack of evidence for monopoles that should have only formed in the very early universe. The horizon and flatness problems also suggest something happening in the early universe.

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    Quote Originally Posted by Reality Check View Post
    Have a look at the motivations for inflation. One is the lack of evidence for monopoles that should have only formed in the very early universe. The horizon and flatness problems also suggest something happening in the early universe.
    Interesting. Does the entropy of our universe correspond with 13.8 billion years?
    The moment an instant lasted forever, we were destined for the leading edge of eternity.

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    Quote Originally Posted by Copernicus View Post
    Interesting. Does the entropy of our universe correspond with 13.8 billion years?
    Nothing to do with the thread but I would guess no. The increase in entropy from the early universe to now would be consistent with an aging universe. Offhand, I do not recall a calculation of the age according to the entropy increase.

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    Quote Originally Posted by Reality Check View Post
    Nothing to do with the thread but I would guess no. The increase in entropy from the early universe to now would be consistent with an aging universe. Offhand, I do not recall a calculation of the age according to the entropy increase.
    Entropy was included in the article about inflation you referenced.
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    Quote Originally Posted by Copernicus View Post
    Entropy was included in the article about inflation you referenced.
    The word entropy was included 6 times and not in the context of the age of the universe.
    1. "Their models failed, however, because of the buildup of entropy over several cycles"
    2. "Gravitational interactions, in this case, circumvent (but do not violate) the first law of thermodynamics (energy conservation) and the second law of thermodynamics (entropy and the arrow of time problem)."
    3. "He stressed that the thermodynamic arrow of time necessitates low entropy initial conditions, which would be highly unlikely"
    4. "the reheating at the end of the inflation era increases entropy,"
    5. "Supporters admit that their model "does not solve the entropy and flatness problems of standard cosmology"
    6. "then it represents a definite increasing of the entropy."

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    Quote Originally Posted by Copernicus View Post
    Does the entropy of our universe correspond with 13.8 billion years?
    Sean Carroll addresses this in From Eternity to Here: The Quest for the Ultimate Theory of Time [2010]
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by Cougar View Post
    Sean Carroll addresses this in From Eternity to Here: The Quest for the Ultimate Theory of Time [2010]
    I'll check it out.
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    I'm not sure how to ask this. If the initial conditions of the universe were bigger than after inflation. Would inflation still be necessary?
    The moment an instant lasted forever, we were destined for the leading edge of eternity.

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    Quote Originally Posted by Copernicus View Post
    I'm not sure how to ask this. If the initial conditions of the universe were bigger than after inflation. Would inflation still be necessary?
    Yes it would be. Inflation is not about the size of the universe, it is about the size of the bit of universe that gave rise to to the universe that we can see (the horizon problem in the article Reality Check linked to). Changing the overall size of the universe doesn't change the volume of it that would have been in thermal equilibrium in the early universe.

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    Quote Originally Posted by Shaula View Post
    Yes it would be. Inflation is not about the size of the universe, it is about the size of the bit of universe that gave rise to to the universe that we can see (the horizon problem in the article Reality Check linked to). Changing the overall size of the universe doesn't change the volume of it that would have been in thermal equilibrium in the early universe.
    Why couldn't the universe always have been in thermal equilibrium?
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    Quote Originally Posted by Copernicus View Post
    Why couldn't the universe always have been in thermal equilibrium?
    Because there is no mechanism to maintain this. Because the universe is expanding we can observe regions which have never been causally connected. Hence in the absence of inflation we need to add in some additional non-local effect that somehow sets and maintains the temperature of the universe to be the same everywhere to a high degree of accuracy. But not quite perfectly the same.

    The Wiki article mentioned covers this under the horizon problem section. Without inflation you end up with a really sensitive fine tuning problem related to the cosmic microwave background radiation.

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    Quote Originally Posted by Shaula View Post
    Because there is no mechanism to maintain this.
    Could an alternative be some sort of mechanism that kept the universe in a (quasi)stable state where there was no (or very little) expansion allowing it to reach equilibrium before something triggered expansion (therefore not needing inflation).

    I realise that this introduces yet more unknown "somethings" (the latter one could be false vacuum decay, maybe?) and would be hard to find tests for, but possible in principle?

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    Quote Originally Posted by Strange View Post
    Could an alternative be some sort of mechanism that kept the universe in a (quasi)stable state where there was no (or very little) expansion allowing it to reach equilibrium before something triggered expansion (therefore not needing inflation).

    I realise that this introduces yet more unknown "somethings" (the latter one could be false vacuum decay, maybe?) and would be hard to find tests for, but possible in principle?
    I'm dosed on the winter cold medications so apologies if this is incomplete / garbled / suddenly starts talking about invisible pink unicorns.

    My understanding is that while this is possible from a basic thermodynamics point of view it breaks down when you look at the evolution of the anisotropy we do see. QGP has a very low viscosity and so tends to be a great playground for structure derived from turbulence. This leads to a scale dependent set of anisotropies (even if it is only something like Kolomgorov's power law). Quantum fluctuations, however, provide a scale independent set of fluctuations. Studies of the CMB show only scale independent anisotropies. So you need something else as well - an extra damping and mixing mechanism that constrains the evolution of the plasma.

    That said there is a lot of criticism of the inflationary model out there that focuses on how it only tangentially addresses these issues by pushing them back into an era we don't understand, effectively just assuming different initial conditions that seem to make more sense at the moment - although that might be because we have a much shakier grasp of the detailed physics of the era. Everything I've said should be taken with a caveat of "based on what we currently know, but we know that we don't know all that much"

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    Quote Originally Posted by Shaula View Post
    I'm dosed on the winter cold medications so apologies if this is incomplete / garbled / suddenly starts talking about invisible pink unicorns.

    My understanding is that while this is possible from a basic thermodynamics point of view it breaks down when you look at the evolution of the anisotropy we do see. QGP has a very low viscosity and so tends to be a great playground for structure derived from turbulence. This leads to a scale dependent set of anisotropies (even if it is only something like Kolomgorov's power law). Quantum fluctuations, however, provide a scale independent set of fluctuations. Studies of the CMB show only scale independent anisotropies. So you need something else as well - an extra damping and mixing mechanism that constrains the evolution of the plasma.

    That said there is a lot of criticism of the inflationary model out there that focuses on how it only tangentially addresses these issues by pushing them back into an era we don't understand, effectively just assuming different initial conditions that seem to make more sense at the moment - although that might be because we have a much shakier grasp of the detailed physics of the era. Everything I've said should be taken with a caveat of "based on what we currently know, but we know that we don't know all that much"
    what do you mean by constraining the evolution of the plasma?
    The moment an instant lasted forever, we were destined for the leading edge of eternity.

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    Quote Originally Posted by Copernicus View Post
    what do you mean by constraining the evolution of the plasma?
    Preventing the turbulence I mentioned, removing coherent structures and injecting/stabilising just the right kind of anisotropy so that when it does begin to expand we see what we do.

    As I said I don't think we understand this kind of state well enough to be certain, but I believe the current consensus is that the anisotropy we see in the CMBR is a poor fit to anything other than inflation. Inflation takes microscopic features which naturally have the right properties to match what we see and imprints them on the macroscopic.

    That said if someone can come up with a convincing inflation free model I think it would make a lot of physicists very happy.

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    Quote Originally Posted by Shaula View Post
    Preventing the turbulence I mentioned, removing coherent structures and injecting/stabilising just the right kind of anisotropy so that when it does begin to expand we see what we do.

    As I said I don't think we understand this kind of state well enough to be certain, but I believe the current consensus is that the anisotropy we see in the CMBR is a poor fit to anything other than inflation. Inflation takes microscopic features which naturally have the right properties to match what we see and imprints them on the macroscopic.

    That said if someone can come up with a convincing inflation free model I think it would make a lot of physicists very happy.
    Should be fun times.
    The moment an instant lasted forever, we were destined for the leading edge of eternity.

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