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Early background microwave radiation question.

  1. Oct 22, 2003 #1
    Why hasn’t the background microwave radiation from the early universe all burnt out by now?

    (Penzias, Wilson, Dicke, Peebles, and George Gamow’s discovery of background radiation is what I am referring to.)

    I’ve heard a couple reasons on what caused the radiation… One about antimatter and matter annihilating each other in the early universe which released energy that can still be viewed today… and another about how the early universe was so hot that the effects could be measured now as microwaves.

    Why is the radiation still there for us to observe?

    My microwave-oven emits microwaves when it is on. And when I turn it off and open the door the waves are gone. (I understand why the microwaves are gone when I open the oven door.)

    If I light a match I will see its light, but when I blow it out I won’t see its light any more. (I understand why this happens too)

    However I don’t understand why the heat from the early universe would still be detectable today…

  2. jcsd
  3. Oct 22, 2003 #2


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    Once a photon is in "flight," it continues to fly across the universe until it hits something. Many of the photons produced at decoupling have not yet hit anything. They have gotten stretched out to longer and longer wavelengths, but haven't actually interacted with anything in nearly 13.7 billion years, until they hit the detector of some enterprising scientist...

    When you blow out your match, the match stops making radiation. The radiation that it already made, however, has escaped the earth, and will continue to fly, forever, until it hits something.

    - Warren
  4. Oct 22, 2003 #3
    Amazing… Thanks for the info…

    Did the photons start out as higher energy waves and lose some energy over the distance they traveled to Earth, or perhaps it has something to do with our instruments causing them to appear stretched out? Or have they always been Microwaves?
  5. Oct 23, 2003 #4
    It is probable that the radiation now being received as microwave background started when the universe changed "state" from opaque to transparent which may have occurred over years as opposed to seconds much less nanoseconds. The spectrum of this radiation was continuous and as that from the equivalent of a black body at an unknown very high temperature in the billion(?) degree kelvin range. If the exact temperature were known, its characteristic peak temperature, now received as the microwave background radiation, would be of immense help to cosmologists in determining the amount of expansion of the universe and maybe even the profile of the rate of expansion which probably varied over time.[?]
  6. Oct 23, 2003 #5


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    Looking at the homogenity and the istropy of the CMBR, you can conclude two things: a)it orginates from a single source as otherwise you would mot epect it to have the same wavelengt b) that source is very far away otherwise we would not expect it to reach the Earth in such an even fashion (for example it is easy to see that a nearby source like the sun does not distribute it's radiation on the Earth evenly). How do you square these two deductions with the fact that it is coming at us from all directions? As it is far away it must of taken a very long time to reach us so we can conclude that when the CMBR was emitted the universe was alot smaller and alot more homogenous than it is now. This is why the CMBR is major evidence for a big bang cosmology.

    What is believed to of happen is that about 300,000 years after the big bang the universe very quickly underwent a phase transition from being opaque (in this stage any photons emitted would be instanly absorbed due to the universes denisty) to a state of being transparent (the photons could now finally escape), this, among other names, is known as the de-coupling era.

    From my calculations it was orginally in about the lower wavelength regions of visible light.
    Last edited: Oct 23, 2003
  7. Oct 23, 2003 #6
    We will always see background radiation: at later times, we will simply see radiation arriving that was emitted from farther away. The radiation was produced everywhere in space, at the same time, so in any direction we will always see some of it, emitted from some sufficiently distant region of space.

    The radiation was not created by matter/antimatter annihilation, but simply because the matter in the universe was hot: ordinary thermal radiation, like a glowing hot object. When the universe cooled, the plasma (free electrons, protons, etc.) bound into neutral atoms, allowing light to propagate freely and turning the universe transparent, as jcsd said. (It's like looking at a cloud: we can see the undersurface of the cloud, but not through it, because inside the cloud the light scatters too much.)

    This photon decoupling happened about 380,000 years ago, when the temperature of the universe was about 3,000 degrees, glowing in the red part of the visible spectrum. The light has since been redshifted down below the visible spectrum, into the microwave range. The amount by which the wavelength of radiation has increased since it was produced is equal to the amount by which the universe has expanded in size during that time: about a factor of 1100.
  8. Oct 24, 2003 #7
    Excellent explanation jcsd, Ambitwistor, chroot, and isaacsgf! Thanks!

    I understood you completely. And have a much better understanding of CMBR now!

    Your answers to my last question leads me to antoher question:

    I understand nothing can travel faster than the speed of light (on a macroscopic scale). Is the rate of the universe’s expansion bound to this speed limit as well? If the universe can not expand faster than C will the cosmic background radiation eventually catch up to all matter and burn out?


    PS: I saw a small discrepancy between what Ambitwistor and JCSD wrote, it was simply a typo I'm betting…decoupling happen about 300,000-300,800 years after the big bang and not 300,000-300,800 years ago right?
  9. Oct 24, 2003 #8


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    Yes, syano, the decoupling occured approximately 300 kyr after the big bang.

    - Warren
  10. Oct 24, 2003 #9
    I would be very interested if you would expand on that…

    How come it is detected today as microwave radiation given that it originally started off as “lower wavelength regions of visible light.” radiation?.... it has something to do with the Doppler effect I’m assuming?


    Chroot thanks for confirming that…What does kyr stand for may I ask? In exchange for your knowledge I’ll tell you what “tla” stands for… three letter acronym :smile:
    Last edited: Oct 24, 2003
  11. Oct 24, 2003 #10


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    kyr = kiloyear = 1,000 years
    Myr = megayear = 1,000,000 years
    Gyr = gigayear = 1,000,000,000 years

    Sorry for using jargon!

    - Warren
  12. Oct 24, 2003 #11
    Well, I don't know about the rate of the expansion of the universe, but recession velocities of distant galaxies theoretically increase without limit the further they are from the observer. This is true for any observer, anywhere in the universe.
    It's metaphor modelling time ( general concepts from Cosmology by Harrison ):
    Space seems to be dynamic.Represent 3D space ( ignore time ) as an expandable ( we can stretch it uniformally ) surface.
    Some might imagine a flat rubber sheet stretched uniformally by pulling the four corners.Of course, the real universe probably has no edges or corners and seemingly expands in 3 spacial dimensions, not 2, but this model is only for visualisation purposes.
    I prefer to imagine the surface as the surface of a balloon.
    Of course, the real universe seems to have 3 spacial dimensions which we are representing as 2--only the surface of the balloon and not the inside of the balloon.
    It is widely accepted that the real universe is homogenous at large scales ( 'large' meaning distances typically the distances between galactic superclusters, or occasionally distances between galactic clusters, but not as 'small' as distances between galaxies ) i.e. it's pretty much the same everywhere, and that the real universe is isotropic i.e. no matter in which direction you look ( from any point in the universe ) it looks pretty much the same in all directions.
    We glue paper discs to the sheet with regular spacing ( homogenous and isotropic ) where the discs represent typical superclusters i.e. clusters of clusters of galaxies of stars.The discs of paper are rigid and do not expand as the rubber expands corresponding to superclusters holding themselves together by mutual gravitational attraction.
    When the rubber surface is stretched by pulling the corners of the sheet, or blowing up the balloon, the universe uniformally expands.
    Say after a given time that the typical distance, s, seperating one disc ( supercluster ) to another is doubled to 2*s = 2s. This is the rate of expansion, which I don't know much about at all.I think it is called the hubble term but don't quote me on that.
    Because the surface expands uniformally, then alltypical distances are doubled. This is shown as a dash- for a distance between discs(superclusters) and a numbers12345 for superclusters showing the distance doubling at constant rate for a few time intervals where the distance doubles uniformally:
    If any observer within any supercluster looks further away, the recession velocity increases since the distances increase without limit for each time interval.
    However, although the recession velocity increases without limit the greater the distance, the actual 'current' highest velocity depends on the size ( age ) of the universe---an expanding universe will at some stage have observers who can not observe the very distant superclusters as their recession velocity exceeds C.
    A very old universe will contain superclusters receding from one another at velocities exceeding C by many orders of magnitude, truly without limit if spacetime persists forever.
    I do not have any idea if the 13.7 billion year universe of ours is yet large ( old ) enough to have these 'photon horizons'.
    General relativiyt allows for dynamism of space, and special relativety only models light in local ( paper disc ) space and so C is maximum speed of something travelling through space but is irrelevant to the qualities of space itself.
  13. Oct 24, 2003 #12
    Sorry, that was indeed a typo: photon decoupling was 380,000 years after the Big Bang.

    As to your other question, relativity places a restriction on how fast matter and radiation can travel through space, but it places no restriction on how fast space itself can expand.

    Also, regardless of whether the expansion of the universe is is greater or less than c, the cosmic radiation will always be present, since no matter how fast the universe has expanded or how old it is, there is always some point in space from which light emitted at photon decoupling would now be reaching us.
  14. Oct 24, 2003 #13
    As I mentioned, the amount by which the light's wavelength has increased is the amount by which the universe has expanded: the expansion of the universe stretches the light waves that travel within it.
  15. Oct 31, 2003 #14
    I understand that there are three 'types' of redshift:
    (1.) Doppler redshift (-ve blueshift) caused by relative motion through space
    (2.) Cosmic expansion redshift caused by the expansion of space.
    (3.) Gravitational redshift caused by, er, gravitation, although some people ( that is, at least me ! ) interpret 2. and 3. as two sides of the same coin it's still convenient ( i.e. necessary ) to seperate them.
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