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Probably dumb questions

  1. May 6, 2004 #1


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    I'm probably missing something really basic (Doh!), but if it is true that the universe is expanding at an accelerated rate, does it not follow that our visible universe is inevitably shrinking?

    If I understand the concept of inflation, there can be galaxies that are currently visible that are presently receding from us at faster than the speed of light. If expansion is accelerating, though, the objects at the limits of our visibility will redshift further and further and over the course of billions of years they will disappear from our view.

    Now for the hard part: If the universe is infinite and there are infinite numbers of galaxies in every dimension, could the redshifted light of the infinite population of distant galaxies average out and be responsible for the cosmic microwave background radiation? I understand that the CMBR is statistically smooth, but as the resolution of our telescopes improve, will we find enough "lumpiness" in the temperature profiles to reconsider the "Big Ban" Echo" model?

    I guess I'm thinking that Olber's Paradox wasn't such a bad idea, but that given the nature of electromagnetic radiation, the "bright as the Sun" night sky just needed to be put in the context of "bright in WHAT frequencies".
  2. jcsd
  3. May 7, 2004 #2


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    I am sure your questions are not dumb (for me your questions are difficult, but lets try…).

    First you should understand the difference between event horizon and particle horizon.

    The particle horizon is the present distance light can travel from the beginning of time up to now. This is usually referred when the term ‘observable universe’ is used.

    The event horizon is the present distance beyond which light will never reach us in future.

    You should notice the difference: the particle horizon may be great. But, if there is a strong expansion of space, the light from sources located at a moderate redshift at the present will never reach us in the future. In future we will still have a great observable universe, but those objets will be beyond of it’s limits then.

    An accelerated expansion leads the event horizon to shrink. The particle horizon grows always.

    I highly recommend this:
    Inflation and the Cosmic Microwave Background, Charles H. Lineweaver
    http://arxiv.org/astro-ph/0305179 [Broken]

    Regarding your question about the CMB: I think the light from distant sources does not fit with the observed black-body spectrum. If I recall correctly, Ned Wright gives arguments against such an hypothesis in its online cosmology tutorial. May be others could elaborate more here.

    Last edited by a moderator: May 1, 2017
  4. May 19, 2004 #3


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    if the universe is infinite, it will take an infinite amount of time for the infinite amount of background light to reach us.
  5. May 19, 2004 #4


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    Welcome to Physics Forums, turbo-1!
    Don't worry - - your questions are fine.

    It is true that with the accelerated expansion, eventually distant galaxies will be invisible from our standpoint. Even though our "visible universe" (the distance we can see based on the speed of light & the age of the universe) keeps increasing (1 light year per year), the accelerated expansion of space will eventually clear out anything interesting to see in that region. It may just be our local group of galaxies (or less).
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