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Background radiation

  1. Sep 20, 2008 #1
    I write as an intersted ignoramus.

    I have often read that studies of background radiation can take us back to, or give us information of, the earliest stages of the universe, when, as I understand everything was pretty close together. How is it that radiation from 14 billion years ago is reaching us now, when that radiation should have left Earth far behind?
  2. jcsd
  3. Sep 20, 2008 #2


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    Don't think of the bigbang model as an explosion.
    think of the matter which radiated as sitting still
    and the matter which eventually formed the earth and us also as sitting still

    Gen Rel allows for distances between stuff to change, to increase (as one possibility)

    So think of the distance between our matter and that other stuff that radiated, as originally about 40 million LY apart.
    But the distance increased over 100--fold, to about 45 billion, while the light was traveling to us.

    That 1000 fold increase in distance corresponds to the 1000-fold increase in wavelength.

    I have to go to supper. Back later.
    Last edited: Sep 20, 2008
  4. Sep 20, 2008 #3


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    OK. I'm back.
    try this calculator, Lazzini.
    the link, if you ever want it, is in my sig.

    To use it, under "matter density" put 0.27
    Under "cosmological constant" put 0.73
    For Hubble parameter I would put 71.

    After that you can put any redshift in, like 1090 for the background radiation, and press "calculate".

    The background radiation wavelength is now about 1090 times longer than when the radiation was emitted by hot (partly ionized) gas.

    The calculator will tell you how far the gas was that emitted the light, when it emitted it.
    (it will say something like 40 million LY then)
    and it will say how far the same material is is now, on the day we received the radiation.
    (it will say something around 45 billion LY now)

    The speeds the calculator gives are not the speeds that anything is moving. It assumes that our material and their material are both sitting still. the speeds are just the rates that the distance between is increasing.

    they wouldnt make sense as speeds of actual motion because they are several times the speed of light--as you know things don't move that fast :smile:

    but distances certainly can increase that fast :biggrin:

    check the calculator out and, if you have any questions or comments, please let us know.
  5. Sep 20, 2008 #4


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    Lazzini, if I can assume you tried experimenting with the calculator, here is a followup mental exercise. In case you try it, please tell me if it helps at all. This is something to do in imagination.

    Suppose you have a balloon, and a ruler marked out in billions of LY----that is the unit. Maybe in reality a unit is only ten centimeters, but we call it a billion LY.

    You paint a blue spot on the balloon. That is our Milkyway matter, that will eventually form our galaxy including this sun and these planets and living creatures. At the beginning of the story, this matter is hot gas like everything else.

    In addition suppose you paint a red circle around that spot, with radius 0.04 unit. That is the matter that, when everything was hot gas, sent us the light we now get as background radiation.

    (if a unit is 10 centimeters then 0.04 units will be only 4 mm, a little less than half a cm---quite a small circle)

    Now we slowly inflate the balloon 1000-fold. The red circle becomes radius 40 units.
    what we are calling 40 billion LY.

    Just for good measure, suppose we start over again and as well as the blue dot and the red circle we paint a lot of other dots to stand for other patches of matter that eventually will condense to make galaxies (with stars planets etc) like ours. Let them be some other color, like yellow.

    While inflating the balloon again, tell yourself that none of these dots are moving with respect to the process of expansion itself.
    Each dot keeps the same latitude and longitude on the balloon that it always had. The galaxies are stationary, in this picture, but light can move between them.

    Photons of light can swim at a fixed speed across the face of the balloon, and they can travel away from one dot and towards another. (Remember dots themselves are stationary. They don't travel.) The expansion will, of course, interfere some with the journeys the photons make. It will tend to expand the distance the photon has already traveled, which is good, but it will also be expanding the distance that remains for the photon to go, which makes it a challenge to ever arrive at anywhere. With patience it can be done, however: as long as expansion isn't too fast and they have plenty of time, the photons can get from one stationary spot to another stationary destination.

    In reality, galaxies also have some random local motion but it is negligible---such a small fraction of lightspeed that to a good approximation we can regard them as fixed, and only the photons of light move, in this picture.

    And the microwave background photons are the ones that very early on left the red circle matter headed towards the blue dot, and eventually, after some 13 billion years, arrived there.
    Last edited: Sep 20, 2008
  6. Sep 21, 2008 #5
    Many thanks, marcus, for your replies, which I have only just seen. I have so far only skimmed through them, but they obviously deserve more than that.
  7. Sep 23, 2008 #6


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    The CMB is still with us because it was always all was. In an expanding universe this was always the case. Expansion redishifted lightspeed particles, like photons and gravitons.
  8. Sep 23, 2008 #7
    Many Thanks Marcus, for patiently trying to explain this to an ignorant.
    I cannot grasp it but I'm working on it.
    One question.

    At the time of recombination photons started to travel in all different directions.
    I assume this happened everywere at the same time. Then we would expect
    to detect photons that has traveled a shorter distance in the expanding universe
    than others. So, have the CMB photons different redshift?

    Oh, and by the way. The use of the word recombination puzzeles me.
    As I understand it is supposed to describe the recombination of eletrons and
    positrons of our well known ligther atoms. But these particels have never ever
    been combined before. So why the word recombination?
  9. Sep 23, 2008 #8


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    You are right. Everybody realizes it is a bad choice of word, for the reason you point out.
    Scientific terms come about sometimes accidentally and then they take root in the literature.
    Big Bang is another example, we have it because of historical reasons even though it is not a good description of the event (the beginning of expansion).
    It misleads people because it makes them think of the explosion of some kind of bomb in the middle of some empty space, but we keep on using this misleading term, because it caught on. Once a word takes hold in a language it is very hard to correct it.

    No, all the CMB photons have almost exactly the same redshift---about z = 1090.

    The event called recombination happened at a particular time, all at very nearly the same time. The redshift formula is

    z+1 = (size of universe now)/(size of universe then)

    more exactly we should say scale factor a(t) instead of size of universe. the universe may not have a finite size but it does have a scale factor, an increasing function of time, which is built into the usual metric by which we measure distances.

    z + 1 = a(now)/a(then) = a(now when light received)/a(then when light emitted)

    This is the standard equation governing the cosmological redshift which one learns around the first lecture in a cosmology course. It is basic. In effect, the wavelengths of the wave expand along with the distances between galaxies.

    z+1 is the ratio of expansion. If z = 2 it means that the wavelength is 3 times longer.


    the photons all come from stuff that is the same distance away, because the CMB photons from nearer already came and passed by us, and the CMB photons from farther stuff have not gotten here yet.

    All the photons we are detecting at present came from stuff that was around 41 million LY when it emitted (and happened to emit some light in our direction) and that stuff is now about 45 billion LY from us. Neither the stuff nor we have moved. Both are assumed stationary. the distance has simply increased. (as in the balloon analogy---everybody keeps the same latitude longitude, only separation distances increase)

    the location of the stuff whose CMB light we are now receiving is shown in the balloon example earlier by that red circle painted on the balloon at a certain radius from the Milkyway dot that is us. technically the location of the stuff whose light we are getting now as CMB photons is called "the surface of last scattering"

    as time goes on, the surface of last scattering expands outwards, as we hear from matter that is farther and farther away. At the present time it is a sphere of radius 45 billion LY. Make sure this is clear and ask questions if it is not.
    Last edited: Sep 23, 2008
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