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Parsing Time in the Very Early Universe

  1. Aug 20, 2010 #1
    Presently we can effectively understand the Universe down to the Plank Time - 10-43 seconds. That's pretty small, but obviously within that time interval a large number of interactions between particles occurred. Before that time we need a better theory of quantum gravity to unify the four fundamental forces.

    But it seems to me that at least at some point, it is pointless to continue. At some miniscule interval, no interactions in the universe occurred. What is that interval? I don't know whether it is 10-45 seconds, or whether it is 10-4500 seconds.

    In the very early uniiverse, given the incredible densiities, I would suspect that the interval is a lot smaller than it is today. Time slows down in a large gravitational field, and of course particles were crammed in a lot closer than they are today.

    I've never heard of any discussion of this topic in the professional or amateur literature, but perhaps someone out there knows.


  2. jcsd
  3. Aug 20, 2010 #2
    You make a good point but I think your answers are not yet and may never be decidable in the fields of cosmology/physics so to some extent the following is now off topic.

    Even the act of writing down a number like the Planck scale let alone what you might do with it to propagate causal events in some theory of the universe assumes that number theory in some sense can exactly represent what reality is. If you accept that you are only approximating reality, then the concept of how small the time step is has no theoretical relevance to what is real or not (but may have relevance in terms of the accuracy of the approximation.)

    Some of the issues you raise are more relevant to the fields of information theory and computability. We don't even know if reality does work "exactly" like a mathematical theory as compared to "being approximated" by a mathematical model. Even if the real world does in some sense exactly conform to a mathematical theory there remain very real issues of how reality "computes" the solution to this model. Are the computations reversable or not? Does nature compute to infinite precision or not? If so can transcendental or irrational numbers exist in nature in exact form, or not?

    You might argue that the probabilistic nature of QM would indicate this thinking is not relevant to cosmology, but the same philosophical arguments apply to the calculations done on the probabilities used in Quantum theory!

    By the way I love the play on words in your title. I assume the allusion to parsing/passing was a deliberate one.
  4. Aug 21, 2010 #3


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    Provided we discover the correct theory of quantum gravity and have an accurate model for what happened at the earliest times, we would then have a correct description of the behavior of our universe for any interval of time.

    This does beg the question as to whether or not if, when we discover quantum gravity, whether or not that theory will also break down at some scale. We obviously won't know until we have that theory before us.
  5. Aug 21, 2010 #4


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    Here's a listing of the relevant research (keyword "quantum cosmology") since 2006.

    http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE%3E2006&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

    The list is ordered by citation count---how many times the article has been cited in other research papers (a rough measure of how valuable/influential it has been).

    The database produces over 300 papers (date > 2006). It covers all different approaches and types of QC. But if you look at the top 50 or 100 papers (the most cited ones in other words) you see there is one main approach that is getting the most attention from researchers.

    You will see how to play around with the date, if you want, and check out more recent years, like change to date > 2008. Or go back to the 1990s if you want to see what quantum cosmology research looked like earlier. The database lets you change settings and try different keywords etc.

    In the main QC model there is no singularity. The quantum model differs from the classical within a few planck time units of the start of expansion. But by a few tens of Planck times, or anyway by 100 Planck time units, it converges to the classical picture. So roughly speaking it reproduces what we expect classically after some 30-100 time units.

    There are very slight differences which some researchers are predicting will be seen at high resolution in the map of the CMB (cosmic microwave background)---in the power spectrum of CMB temperature variation. One will need space instruments to see if those predicted traces of the QC model are there or not. One of the main authors in this line of QC testing research is Aurelien Barrau. There are half a dozen, but he is one whose papers you could look for if interested.

    They run QC computer models, as well as using equation models, to study the period before during and after the start of expansion. Some of the results are fairly robust in the sense that you get qualitatively the same behavior even if you vary the input parameters somewhat. Its a new field in a phase of rapid growth.
    Last edited by a moderator: May 4, 2017
  6. Aug 21, 2010 #5


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    Er, uh, what list?
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