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Inflation & Time before t=0

  1. Dec 6, 2014 #1
    Im reading Mukhanov, cosmology foundations, right now (excelent book, by the way). As many others, he gives some reasoning behind the origin of inflation, and there he says that there had not been enough time for the different patches of the universe to get in contact and justify the homogeneity of the observable universe. In all these reasonings, It seems that it is supposed that there is not time before t=0, but from what I understood, t=0 is not necessary the time when time started. It only is the time when our physical theories break down. There could be time before t=0 but the rules should be different. So, there could be infinite time before t=0, or at least enough time to make our observable universe homogeneous, and we loose one important reason behind inflation.
    So, are the reasoning behind the homogeneity & Inflation essencially wrong? (surely not, but what am I not seeing??)

    Thanks in advance
  2. jcsd
  3. Dec 6, 2014 #2


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    That's sort of what inflation is: it's a model of what happened before the standard big bang theory took over.

    And no, just saying, "something happened before the standard big bang theory," isn't good enough. A specific model is required if one is to say anything meaningful at all. And that's what inflation is.
  4. Dec 7, 2014 #3
    Thanks for your reply. Nevertheless, some books seem to say something else. I will write what Mukhanov says in page 226-227.
    "...The present homogeneous, isotropic domain
    of the universe is at least as large as the present horizon scale, ct0 ∼ 10^28 cm.

    Initially the size of this domain was smaller by the ratio of the corresponding scale
    factors, ai/a0. Assuming that inhomogeneity cannot be dissolved by expansion, we
    may safely conclude that the size of the homogeneous, isotropic region from which
    our universe originated at t = ti was larger than

    li ∼ c*t0*ai/a0

    It is natural to compare this scale to the size of a causal region lc ∼ c*ti :

    li/lc∼ t0/ti*ai/a0 (5.2)

    To obtain a rough estimate of this ratio we note that if the primordial radiation
    dominates at ti ∼ tPl , then its temperature is TPl ∼ 10^32 K. Hence

    (ai/a0) ∼(T0/TPl ) ∼ 10^−32

    and we obtain

    li/lc∼ 10^17/10^−43*10−32 ∼ 10^28.

    Thus, at the initial Planckian time, the size of our universe exceeded the causality
    scale by 28 orders of magnitude. This means that in 10^84 causally disconnected
    regions the energy density was smoothly distributed with a fractional variation not
    exceeding δε/ε ∼ 10^−4. Because no signals can propagate faster than light, no
    causal physical processes can be responsible for such an unnaturally fine-tuned
    matter distribution.
    Assuming that the scale factor grows as some power of time, we can use an
    estimate a/t ∼ a' and rewrite (5.2) as

    li/lc∼ ai'/a0' (5.4)

    Thus, the size of our universe was initially larger than that of a causal patch by
    the ratio of the corresponding expansion rates. Assuming that gravity was always
    attractive and hence was decelerating the expansion, we conclude from (5.4) that
    the homogeneity scale was always larger than the scale of causality. Therefore, the
    homogeneity problem is also sometimes called the horizon problem..."

    Here ai, ti and such are those variables when the universe temperature was TPl. So, the horizon problem here arises because he measures the time only after ti=tPl and he deduces that from that point there was not time for some regions to be in equilibrium.

    Am I saying it right? The horizon problem, stated like in Mukhanov, is such a big problem only if we think that there was not time before ti=tPl.

  5. Dec 7, 2014 #4


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    Cosmological interpretations aside, how do we determine the age of anything? Looking a little closer to home, how do we know the age of earth? We examine and date the most ancient rocks on the planet based on various radioactive decay products. The answer turns out to be about 3.8 billion years. If we similarly examine meteorites in the solar system the oldest turn out to be about 4.56 billion years. If we examine the ratio of various elements in the spectrographs of stars in the universe, the most ancient of these measures about 14.4 billion years old (give or take a couple billion). After averaging a number of various such indicators we find the oldest things in the universe to be about 13 billion years old. Allowing for the uncertainty in these measurements, we can say with fair certainty that 13.7 billion years is an entirely reasonable estimate for the age of the universe. It is less reasonable to assume the age of the universe much exceeds this without assuming something inexplicable prevented the universe from aging over an indeterminate period of time. See http://www.astro.ucla.edu/~wright/age.html for further discussion.
  6. Dec 7, 2014 #5


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    It's a problem no matter what, as long as you don't modify the expansion history.
  7. Dec 7, 2014 #6
    thanks Chronos for your answer, but I dont think that you are making a point. All the measures that you (and your link) provide, depend on the average life of certain atoms and stuff like that, but we already know that in t=tPl=0 there was not even one nucleai of a Hydrogen atom. I mean, I think that, due to the Big Bang theory chronology, mostly dependent on CMB observations, it's obvious that all the other measures will give less than (+-)13.5 Gyrs.

    Thanks all the same!!
  8. Dec 7, 2014 #7
    Chalnot, perhaps I express myself wrong. I agree with you, it is a problem, but what bothers me is that Mukhanov sort of says ("there should have been inflation cause, otherwise, different patches could not have been in contact from tPL to t*" -here t* is the first time where we know that traditional Big Bang theory works, I dont know it exactly, lets say 10^-6 seconds).
    From this line of reasoning, I dont see that it is necessary that Inflation should have existed (at least as clear as Mukhanov sees that). I just see that it is a good possibility. I follow the necessity of Inflation if there is no time before t=0 but given that we dont know that, I just see that inflation it is just that, a good possibility.

    Ps: Just to make my point clear, I dont want to sound arrogant, Im reading this awesome book and Inflation looks like an incredible theory and a beautiful story. I just want to be convinced that it is not just a good possibility but, at least apparently, the only good possibility.

    Ps2: sorry for my english.
  9. Dec 7, 2014 #8


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    Aren't you asking for too much, pulp?
    Maybe it is wrong to be convinced that it is "the only good possibility".
    There are reputable physicists who think there are other good possibilities and are exploring them.
    As I recall there is Robert Brandenberger at McGill U in Canada, he has many collaborators and works on something called "matter bounce"
    There are also Sergei Odintsov, and Jaume de Haro at Barcelona
    There is Paul Steinhardt at Princeton. These people study a variety of models which can dispense with inflation.
    It is not hard to find papers by numerous other physicists who work in quantum cosmology where GR is quantized and this eliminates the singularity so that time goes back before the start of expansion. The research literature in quantum cosmology is large, on the order of 100 papers a year. Over half of these use cosmic models where there is a bounce (quantum effects cause gravity to repel at high density, so following our universe history back in time leads to prior a contracting phase).

    I think it has not been determined yet that inflation is the "only good" possibility. So it seems unwise for anyone to try to convince themselves that it is. Or perhaps instead of "unwise", the right word is "unscientific" :D

    If you do this:
    you get many hits and the second one is this:
    The Matter Bounce Alternative to Inflationary Cosmology
    by RH Brandenberger - ‎2012 -Jun 19, 2012 - Astrophysics > Cosmology and Nongalactic Astrophysics ... Observational signatures which will allow the matter bounce to be distinguished ...
    Last edited: Dec 7, 2014
  10. Dec 7, 2014 #9


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    It's not necessary. But it does solve the horizon problem neatly. There are other models that also solve the horizon problem in different ways.

    But it is necessary to do something beyond the standard big bang theory which assumes General Relativity and some amount of normal matter, dark matter, and radiation. That universe makes no sense because it is impossible for most of the universe to have been in causal contact before the emission of the CMB.
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