Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How can we see the cosmic background radiation?

  1. Apr 19, 2007 #1
    I am wondering how it is possible that we see CBR. Here's why:
    When the big bang occured, there was a sea of particles that gave off radiation. Now, if we are now made out of those particles, then how are we seeing their radiation now?
    The only way I see this happening is if matter travelled faster than light at some point. How else would we be seeing the radiation from the same particles we are made out of? It would be violating the light-cone rules. Can someone clarify this for me? Thanks
  2. jcsd
  3. Apr 19, 2007 #2
    I think you need look no further than a static (one with no reception) channel on your T.V.
  4. Apr 20, 2007 #3
    What's more, we are looking into the past with CBR. So, we have a long time to separate from those particles, but we are seeing them back in their primitive form much as our particles were.

    Funny: So, those particles may some day very well become part of an ET fixing to hunt us down and exterminate us for unwittingly insulting them with our messages to outer space.
    Indeed, what if those SOS signals you made as a kid playing with your Dad's flashlight actually translate into something insulting enough for ET to start an intergalactic war once ET sees it????

    Oh no.... What have you done?!?!
    Last edited: Apr 20, 2007
  5. Apr 20, 2007 #4


    User Avatar
    Science Advisor

    This is indeed a good question, which luckily has a solution!
    The point here is that when the CMB (Cosmic Microwave Background) raditation was released, the universe was in fact much bigger than what one naively would think if one just lets it expand from a single point with the speed of light.
    First of all, the initial singularity of the Big Bang need not be equivalent with that the universe started out from a single point. Somewhat loosley speaking the singularity just means that the density goes to infinity, not that the volume has to go to zero.
    Secondly, even if the universe is finite in size (a closed universe) and once was much smaller, the period of http://en.wikipedia.org/wiki/Cosmic_inflation" [Broken] saves the day, since during this epoch space expanded much faster than the speed of light (which does not contradict General Relativity). When the CMBR finally was released the space was already huge.
    Last edited by a moderator: May 2, 2017
  6. Apr 20, 2007 #5


    User Avatar
    Science Advisor

    The more fundamental thing to point out (apologies if I'm being too obvious here) is that was are not 'made out of those particles' that gave off the CMB photons that we see today. The stuff we are made of did give off CMB photons, but those photons are now a very very long way away from us. The material that gave of the particular CMB photons is by the same token a very long way away from us.
  7. Apr 23, 2007 #6
    Thanks guys, that really helped.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook