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Entropy of the initital state of the universe

  1. Nov 21, 2014 #1
    I saw an old interview with Roger Penrose where at one point he was talking about the degree of organization the universe exhibited at it's initial state. He said the second law of thermodynamics tells us as time passes the universe is becoming more disorderly, which means if we were to go back in time the universe would become more and more ordered/organized. He said the degree of organization the universe was in was so special that the mathematical figure representing/describing it would be at least 10^123. Can someone please explain to me how he came up with this figure. It's a very large number obviously. How does this large number represent degree of order?

    Link to the video (begin at 4:58): http://www.youtube.com/watch?v=pEIj9zcLzp0
    His published paper: accelconf.web.cern.ch/accel...nf/e06/PAPERS/THESPA01.PDF

    In the opening of this paper he mentions initial state numerous times. Later on he says this though:

    "To deal with a spatially infinite universe, I shall assume that we need consider only, say, that comoving portion of the universe that intersects our past light cone. This contains something of the order of 10^80 baryons."

    What time in the universes' history would the "portion of the universe that intersects our past light cone" be? Is he talking about present time? If not, when?
    Last edited: Nov 21, 2014
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  3. Nov 21, 2014 #2

    Doug Huffman

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    About entropy, what value would you assign to a Universal singularity?
  4. Nov 21, 2014 #3
    I don't know. I am a laymen. Was hoping for some insight regarding my OP.
  5. Nov 21, 2014 #4


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    Not sure about the first question, but our past light cone refers to the region of the universe which we can observe due to signals from the universe having enough time to reach us since the beginning of time. The edge of this region is also known as the particle horizon, which is the farthest part of the universe which we can see. Let us assume for a moment that the entire universe is infinite in extent (might be wrong, but we don't know how big it is). We can only see a small region of space around us. As the universe ages, light from farther away will have time to reach us, and our observable universe is getting bigger all the time.

    We can't say how many particles there are in the universe. We can only estimate the particles in the observable part of the universe, which is the space enveloped by our past light cone
  6. Nov 21, 2014 #5

    Doug Huffman

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    The Eddington Number, NEdd, has good provenance.

    As I recall, Barrows and Tipler review it in their The Anthropic Cosmological Principle.
  7. Nov 22, 2014 #6
    If we can only see as far back in time as light takes to travel to us then how do we know what happened a split second after the Big Bang? We are told all these things about what happened in the universe during the first few seconds after the universe underwent inflation and so forth, but how do we know this is we can't observe it? The CMB was roughly 380k years after the Big Bang, which suppose to be as far back as we can see. Is there any good reason to believe the universe is actually much older than 13.79 billion years old?
  8. Nov 22, 2014 #7

    Doug Huffman

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    Paraphrasing E. T. Jaynes (Probability Theory: The Logic of Science) only cause and effect are constrained to the arrow of time, logic is not so.
  9. Nov 23, 2014 #8


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    We can only observe light from the point in the universe's history when the CMB was emitted. But we can make other observations that tell us about events that happened before that. Light is not the only way of observing.
  10. Nov 24, 2014 #9


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    Here is my glib answer... the initial entropy of the universe was 0, the current entropy of the universe is 0, the final entropy of the universe will be 0. BUT... since in QM entropy is not additive you can have non-zero entropies for parts of the universe which do not add to the entropy of the whole. I define entropy as a measure of entanglement with the environment and so the "universe as a whole" with "no environment" has no entropy.
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