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50 Giga light years!

  1. Jan 29, 2010 #1
    Ok, someone was kind enough to point out to me recently the obvious fact that the universe must be much larger than what we can see. Suddenly I feel like I'm literally trying to fit all of it into my head!

    I've always had this nagging feeling about inflation, but it's never come up to the surface enough to warrant any investigation. Could it be that inflation was caused simply by the fact that all of that matter (or energy doubling as matter, or whatever) simply couldn't occupy the same points in space? Like the universe inflated until it was large enough to obey the exclusion principle? I can't tell if these are crazed ramblings or just naive questions.

    Anyone want to shed some light? Name a book? A URL?
     
  2. jcsd
  3. Jan 30, 2010 #2

    Chalnoth

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    Well, the issue here is that inflation is a rather complex phenomenon. In the simplest models, what you need is for the majority of the energy density to be in a particular kind of field. This field, due to the way it interacts, must experience a sort of potential energy depending upon its value. And it must have enough potential energy such that it drives an expansion so fast that the expansion slows down how quickly the field is able to roll down to its minimum value. Then, once it reaches that minimum, it has to evaporate into standard model particles very rapidly.

    So it's not quite so simple an idea that it can merely be the result of some simple principle. At least, not so directly.
     
  4. Jan 30, 2010 #3
    Know any good resources on inflation models? At some point I'll probably be asking you to recommend a school and a scholarship resource!
     
  5. Jan 30, 2010 #4

    Chalnoth

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    Unfortunately, no, I'm not really sure what to offer with respect to inflation beyond the Wikipedia article. You might try looking to see if you can find anything from Alan Guth, though. He's the guy that came up with the idea, and has presented a number of quite interesting talks on the subject that can be found online.
     
  6. Jan 30, 2010 #5

    Chronos

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    The observable universe is 13.7 billion years old. You are referring to the 'time now' size of the universe. That is irrelevant.
     
  7. Jan 30, 2010 #6

    Char. Limit

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    If the observable universe is 13.7 billion years old (what day is the universe's birthday?), does that mean that the maximum diameter of the universe is 27.4 gigalightyears?
     
  8. Jan 30, 2010 #7

    MathematicalPhysicist

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    1st of january, but ofcourse. :smile:
     
  9. Jan 30, 2010 #8

    Borek

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    Last edited by a moderator: May 4, 2017
  10. Jan 30, 2010 #9

    Chalnoth

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    Nope. I and others have gone into this in detail elsewhere, but the punchline is that this comes down to the weirdness of curved space-time. What you've said would only be true if we were talking about flat space-time.
     
  11. Jan 30, 2010 #10
    Perhaps we can offer an easier explanation. Using the most powerful telescopes, we are capable of observing galaxies 10 to 13 billion light years away. But the "horizon" of visibility in the cosmos streches more than just some 20 to 26 billion light years because space itself is expanding. So the edge of the observable universe must be at least 46 billion light years away from us (called the comoving distance or the partical horizon). The visible universe, however, is most likely a sphere of 93 billion light years.
     
  12. Jan 30, 2010 #11

    Chalnoth

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    One thing I should mention is that these distances are all a bit arbitrary. How far away something is depends entirely upon what you mean by "distance".

    If you want to get into the nitty gritty details, here is a rundown of all of the major distance measures used in Cosmology:
    http://arxiv.org/abs/astro-ph/9905116

    A couple of examples of the available distance measures are:

    1. Luminosity distance: let's say you want to judge distance by comparing against the brightness of objects. In 3-dimensional space, objects' brightness drops off as 1/r^2. So if we know the intrinsic brightness of something, we can measure how far away it is. The "r" here is the luminosity distance.

    2. Angular diameter distance: if we know how physically large something is, and then compare it to how large it appears on the sky, then that's another measure of distance. Here the angle that the object subtends on the sky drops off as 1/r (for small angles), and this "r" we can use as the angular diameter distance.

    3. Comoving distance: If we take a hypothetical situation where we stop the expansion in such a way that every point in space sees the same CMB temperature, and then send some light rays around, the comoving distance is the time it takes for those light rays to get from place to place.

    It's this last one that people usually use when talking about distances to the public.
     
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