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When was (or will be) inflation confirmed?

  1. Mar 26, 2014 #1
    I found different opinions regarding when inflation was confirmed or when could it be confirmed. Lets write down the facts I know (surely this isnt an exhaustive list):

    1) it was proposed in order to solve some problems (horizon, flatness -not sure if monopoles should be included because Im not sure if it is considered mainstream physics-). As it was the most simple Idea that solves these problems we may say that this put inflation in "mainstream physics"
    2) around the 90s it was found that the CMB temperature was not equal in 10^-5 scales. In order to account for these fluctuations, inflation or something similar is needed (or am I wrong? Can this fluctuations be explained without assuming inflation or they are too big to be explained by quantum fluctuations of the standard model + just gravity)
    3) with the discovery of E modes in CMB at the begining of the new milenium.
    4) last week with b modes

    So, they were several questions related to one principal doubt: when and why inflation became mainstream physics?

    Thanks in advance as always
  2. jcsd
  3. Mar 26, 2014 #2
    You mentioned flatness, horizon and can include monopole problems. Inflation also predicted the presence of the CMB. The correct mixture of elements found in the CMB. See baryogenesis (hydrogen, helium etc.) The recent B-waves etc are also in agreement. The recent planch data narrows down the possible inflation models to models with a single scalar with low kinetic in the metrics. This reduced significantly the number of viable inflationary models. In the encyclopedia erratus on arxiv.com this reduced the number of viable models from over 60 to 17. The recent B-mode findings may narrow this down further.

    With all these observations its now a question of which model matches closest rather than if it occured

    had the wrong name for the Encyclopdia

    Last edited: Mar 26, 2014
  4. Mar 26, 2014 #3
    You will find this article a gem for your question as it better covers what I wrote above.


    This older thread has also some decent coverage

    Last edited: Mar 26, 2014
  5. Mar 27, 2014 #4


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    Inflation does not predict the CMB -- it is a consequence of a hot dense universe which doesn't necessarily include inflation. You also seem to be indicating that inflation predicts BBN yields -- it doesn't do this either. Don't conflate inflation with big bang cosmology.

    Why do you say only single scalars remain? Multifield models still work fine. What do you mean by "low kinetic in the metrics?"

    For the OP: essentially, there are three predictions of garden-variety inflation:

    1) Adiabatic density perturbations
    2) Superhorizon correlations
    3) Gaussian fluctuations

    One could argue that primordial gravity waves (B-modes from tensor fluctuations) should be up there, but their presence depends on the energy scale of inflation (it is entirely possible that inflation happened at an energy scale too low to create observable B-modes). If primordial gravity waves are produced at a detectable level, then a 4th prediction becomes relevant:

    4) Red tilt to the gravity wave spectrum

    The first 3 of these have been verified, and the last might be within a few years. The last is especially important for distinguishing inflation from other theories of early universe structure formation (e.g. string gases, LQC, matter bounce).

    I don't include resolution of the horizon, flatness, and monopole problems in my list because these are essentially questions of initial conditions, and it's not imperative that they be resolved dynamically through a process like inflation.

    Even with the B-mode detection, there is still a massive degeneracy in the inflationary model space, regardless of what people might say. If we restrict ourselves to single field canonical inflation, then yes, it's exciting that we can begin to distinguish between models. But there's no a priori reason to restrict ourselves to single field canonical inflation outside of an arbitrary sense of (potentially misapplied) simplicity.

    Here are two papers describing two sources of degeneracy: ignorance of how the density perturbation was generated (http://arxiv.org/abs/arXiv:1011.0434) and ignorance of the nature of the inflaton kinetic term (http://arxiv.org/abs/arXiv:1212.4154).

    The relevance of detectable B-modes to the degeneracy problem is discussed here: http://arxiv.org/abs/arXiv:1009.3741

    Yes, these are all my own papers but to my knowledge nobody else has written much on the degeneracy issue (probably because it's so depressing).
    Last edited: Mar 27, 2014
  6. Mar 27, 2014 #5
    perhaps I need to be more clear in my wording. The uniformity of the CMB requires inflation. At least thats the impression I got over numerous articles.

    Inflation doesn't cause BBN. However it sudden change in volume is a key factor that allows particles to drop out of thermak equilibrium. Without inflation the ratio of hydrogen and deuterium etc would be different.

    The low kinetic term is from the planch paper and is a direct qoute from that paper. Coincidentally its in the last link I posted. In that thread it was pointed out to me that multifield models have been shown as not as accurate. Coincidentally you also posted your degeneracy papers in the same thread
  7. Mar 27, 2014 #6


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    OK, thanks for clarifying.
    I'm not so sure about this. Inflation leads to an effectively empty universe -- there is extreme supercooling during the expansion and no particle interactions that I know of are relevant during this time. The study of BBN, and the calculations of the yields and elemental ratios, is based on the radiation-dominated universe that follows inflation. Inflation provided the blank slate only. Does this make sense?
    I see. As I mentioned in that thread, multifield models have some generic expectations, but the absence of these signatures by no means rules them out. This is the basis of the degeneracy problem that I apparently haven't been shy about promoting here :tongue: Sorry about that.
  8. Mar 27, 2014 #7
    No problem on the multifield makes sense that there is still validity...

    The blank slate doesn't make sense however.

    Here is why from what I've been studying. Prior to inflation the hot dense state during the planch epoch can be described as an ideal gas with particle antiparticle annihilations in thermal equilibrium.
    The metrics describe photons which is its own anti particle. Due to the small volume. The spin of the photon gives two degrees of freedom. S=2.
    in my textbook "particle physic of the Early universe this is describef as a Bose-Einstein condensate with a fermi-dirac distribution.
    they then show a small change in volume with a cosmological value. Id have to look at what they used. they had a few other particles drop out of equilibrium then went to describe inflation. However they refer to it as a phase transition.

    the supercooling I understand. However doesnt inflation also cause a reheating? wouldnt a reheating require particles?
    I realize that after inflation we no longer have
    thermal equilibrium due to the large volume.
    however inflation occurs approximately during the beginning of the electroweak epoch which is also described as a perfect fluid in equilibrium?
  9. Mar 27, 2014 #8


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    Inflation effectively dilutes pre-existing matter and radiation densities to zero -- during inflation, the stress energy is dominated by the vacuum energy of the scalar field. The thermodynamics of the pre-inflationary universe are of course important for understanding how inflation got started (whether or not the universe was in thermal equilibrium is an important question, for example, because it itells us about the stability of the inflaton and the likely phase transitions it might undergo.) That said, once inflation gets underway, we can erase all that. Inflation is fantastically good at erasing its past -- especially if it goes on for long enough.

    Yes, when inflation comes to end, the universe reheats. This is done through the decay of the inflaton into all the other particles it couples to (think couplings in the Lagrangian, not particles literally in existence during inflation, since these are being rapidly diluted away). The blank slate prepared by inflation ensures that this reheating occurs in a flat, homogeneous universe -- this is effectively the hot big bang. It is at this time that we study the conditions for BBN.
  10. Mar 27, 2014 #9
    Ah ok I gotcha that makes sense thanks again.

    Edit.. Actually your reply helped me out more than I initially realized. I was always stumped as to why Weinberg's "Cosmology" textbook placed BBN at a starting temperature of 10^9 k. As well as to why he only covered thermal equilibrium from 10^16k. Where Griffith discussed higher temperature ranges..

    The particle physics of the early universe went a little further however only went as far as the radiation dominant era prior to recombination. Which coincidentially both textbooks agree that prior to recombination. Processes were still in thermal equilibrium. during recombination they both describe 2 step combinations as expansion rate was too great for single step reactions to be of sufficient density to affect the overall thermodynamics.
    Last edited: Mar 27, 2014
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