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Cosmic Background Radiation - why is it still here?

  1. Apr 26, 2010 #1
    If the Cosmic background radiation was emitted at approximately 384,000 years after the big bang, and it travels the speed of light, with mattter moving at a slower speed, how is it possible that the cosmic rays are still with us and haven't already passed beyond us 13 billion years later? (assuming a point source creation of the universe).
    Last edited: Apr 26, 2010
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  3. Apr 26, 2010 #2


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    Aha. Well, the creation of the CMB has nothing directly to do with the 'creation of the universe', which is not addressed by the big bang theory. The CMB was emitted everywhere in the universe at the time of decoupling. The universe is a big place! Today, we are getting CMB photons that were emitted from far away in the universe.
  4. Apr 26, 2010 #3


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    OOPS! EDIT I see Powell already answered while I was thinking what to say! His responses are consistently helpful* and reliable, so you can go with that and just keep asking him questions. My reply is unnecessary now but having two does no harm so I will just leave it as an alternative.

    the quick way to understand that is to google "wright balloon model" and watch a short computer animation of a simplified universe where all existence is concentrated on the 2D surface of an expanding sphere.

    You will see galaxies (slowly whirling white things) and photons traveling in the space between the galaxies (colored wiggles).

    In that simplified simulation all the things are 2D, and live in the 2D surface. There is no existence inside or outside the sphere, all space and matter are in that infinitely thin 2D world.

    Then you still have the problem of mentally extending the analogy to 3D but at least the balloon model gets you on the right track.

    In today's space there is no one spot that we can point to and say "expansion started outwards from there----it all started from that spot."

    when you watch the animation you will see that the CMB photons never go away. they just thin out as space expands, and their wavelength gets longer. But every galaxy continues to have them around it. At later times the incoming photons just came from stuff that was farther away, so they took longer to get here.

    You can also look at the princeton.edu link in my signature at the end of this post. It is a Scientific American article that has helped a lot of people. It is called "misconceptions about the big bang". The idea that it was an explosion of matter outwards from some spot that we could locate in today's space, like a bomb exploding, that is a common misconception that they deal with in that SciAm article and show you how to get out of that mental trap. Popularizations have done the lay audience a great disservice by promoting that exploding bomb idea, it is not standard mainstream cosmology at all.

    But try the animation and see if it helps! When you google "wright balloon model" you get
    please let me know if it doesn't work on your computer.

    *concise, expert and to the point. Great going! and thanks, if you happen to read this.
    Last edited: Apr 26, 2010
  5. May 3, 2010 #4
    Ok well I am kind of tired of the 'expanding balloon theory'. All models I am aware of point back to a point at the big bang of smthg like a singular point the size of an atom where the big bang occurred and that matter precipitated at around 384,000 years after the big bang ... hydgrogen and stuff. Now I don't care how fast space expands after this point in time, according to all that I understand (which obviously admittedly is not a whole lot), matter can not get to the speed of light ... so now I have a whole lot of photons being emitted from somewhere around this region ... moving at the speed of light and all the matter in the universe has to be travelling slower than the speed of light. Correct? And even if the universe were 384,000 light years across, though the space is expanding at some greater than light speed, but matter is limited to slower than light speeds, that the photons haven't passed us up and exited ahead of us to the 'great beyond' or whatever. Another perhaps important question is do cosmic rays show any red shift? I mean after all if some are emitted ahead of me and are on collision course with me they would have a blue shift, while others catching up from behind would show some kind of red shift right? I mean if all these photons have been here since the big bang when the universe was 384K light years across some of those photons would have to be coming from behind and some from ahead the way I see your explanation.
  6. May 4, 2010 #5


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    That's not a theory, it's a model. An analogy. And you should still try to understand it, because it's meant to address some of the misconceptions you have about the theory.
    Imagine you are one of the dots on the balloon's surface.
    Imagine that your whole universe is contained within said surface (as a two-dimenisional analogy, of course, not as a description of real 3D space). There is nothing outside the surface, not even the spatial dimension that is needed for the analogy.

    You see that things ahead of you are moving away just the same way as things behind you?
    And that these directions are arbitrary, as you're supposed not to move along the surface?
  7. Jun 9, 2010 #6
    The source of the C.M.B.R. surrounds us in the same way that an eggshell surrounds the yolk. This is why it is both omni-directional and still arriving, and will continue so to do even it is no longer being created. It does not emanate from the region, (or the time), that saw the creation of our universe.
  8. Jun 11, 2010 #7
    So how many years from now will the wavelength of the incoming CMBR be longer than the longest wavelength of light we can currently detect?

    Also bapowell said "Today, we are getting CMB photons that were emitted from far away in the universe." When will we receive the last photons from the edge of the universe, the edge that existed 384,000 years after the BB, the farthest of "far away"? or is that edge already outside the visible universe, assuming it will keep slowly expanding?
  9. Jun 11, 2010 #8


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    There is no 'edge' to our universe (at least that we know of). Rather, we should speak of the 'edge' of the observable universe. The CMB photons that we are receiving right now today are coming from the edge of the observable universe. Each CMB photon we receive on earth, whether it is arriving today, 1 million year ago, or a month from now, was born ~400,000 years after the big bang. The ones we receive tomorrow just originated a little further from earth than the ones we receive today.
  10. Jun 11, 2010 #9
    Right, and the entire observable universe was the size of a quarter at one time. The way I'm understanding what your saying is every cubic inch of the universe created CMB photons during the recombination event. The photons we received in the past were born closer to the earth, and the ones we receive later were born farther from the earth. But since there is a limited amount of matter in the universe, you can only go "a little further" for so long until a little further is the edge of the observable universe, the edge of everything that existed inside the size of a quarter; after this point there is no more matter to have recombined to create a photon, right?

    From http://en.wikipedia.org/wiki/Observable_universe: [Broken]

    "While special relativity constrains objects in the Universe from moving faster than the speed of light with respect to each other, there is no such constraint when space itself is expanding. This means that the size of the observable universe could be smaller than the entire universe; there are some parts of the Universe which might never be close enough for the light to overcome the speed of the expansion of space, in order to be observed on Earth. Some parts of the Universe which are currently observable may later be unobservable due to ongoing expansion"

    So instead of asking why we can still see the CMB, I would ask when will the CMB disappear from view. Either from the last viewable remnants of it being pushed outside our observable universe, or from the photons reaching us having been redshifted to a wavelength larger than we can detect (say 30,000 miles). I'm pretty sure the current models of expansion show that eventually only our galaxy will be within the observable universe, so the CMB will disappear someday. In a static universe, or even in a slowly expanding one, I would expect there to be a day when every CMB photon has passed our frame of reference. Considering there was a finite number of them made, there should be a finite time for them to pass us.

    BernieM, you said "Another perhaps important question is do cosmic rays show any red shift? I mean after all if some are emitted ahead of me and are on collision course with me they would have a blue shift, while others catching up from behind would show some kind of red shift right?"

    First, cosmic rays are different from the CMB; cosmic rays are mostly individual protons and helium nuclei and I think most or all of them originate in our galaxy, and close to us. Assuming you meant to ask if the CMB blue and redshifted, the answer is yes. http://www.astr.ua.edu/keel/galaxies/cmbr.html The blue/red ying-yang image is the blueshift/redshift of the CMB. We can deduce our velocity relative to the CMB background which is 630km/s towards the blue direction of that picture.
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  11. Jun 14, 2010 #10


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    Sorry for the late reply. Everything you say is correct under the assumption that there is a finite amount of matter in the universe. This might not be true. If it is true (i.e. the whole universe is of finite size), then surely the CMB will eventually disappear from view from our location in the cosmos. Of course, one cannot say when this day will come, since we have no idea how large the actual universe is.

    In regards to the Wikipedia entry: in a decelerating cosmology, given infinite time, all points in the universe will become observable. While there is a Hubble length (the distance at which objects begin to recede at speeds greater than c), the Hubble length is itself growing faster than c! What this means is that while there is, say, a galaxy that is receding at a speed greater than c today (it's just beyond the Hubble length), it will be receding at a speed less than c tomorrow (since the Hubble length has grown beyond its position). In this way, eventually the whole universe becomes observable. Therefore, if the universe is indeed infinite, we'll never run out of CMB photons.

    Now, that applies to a decelerating universe. How about accelerating universes? Current data suggests that we indeed live in such a universe -- the so-called LCDM cosmology (L stands for the cosmological constant, but we don't know at present if it is indeed constant -- it might be dynamical in which case it's called dark energy -- forgive me if you know all this already.) In a general accelerating spacetime, the Hubble length still increases, but at a rate smaller than c. And so, that galaxy from last paragraph that is outside the Hubble sphere today, will still be outside tomorrow, and ditto for all eternity. The Hubble radius becomes an event horizon -- it defines a limit beyond which things will never be observed by us on Earth. In this case, the CMB will indeed one day disappear, because our observable universe (defined by its particle horizon) will eventually grow beyond the event horizon.
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