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Microwave radiation proves what?

  1. May 1, 2008 #1
    Ok, so as I understand it microwave radiation can be detected from 'outer space'
    but what does that prove?

    I really don't see why it should be evidence of a big bang, absolutely none whatsoever,
    infact microwaves would probably one of the last things on my list as evidence.

    What is the reasoning that supposedly make it plausible as evidence of a big bang?
     
  2. jcsd
  3. May 1, 2008 #2

    Wallace

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    Far too many too list all at once, but the key ones are:

    * The spectrum of the CMB is a perfect blackbody, which cannot be produced by any open system (one that can exchange energy with something outside the system). What is the only true closed system in the Universe? The Universe itself! Thus the CMB is the relic radiation of when the Universe was much denser and hotter than today

    * The temperature implied by the peak of the Black Body spectrum is exactly what would be expected if it was the relic radiation from the hot, dense early Universe.

    * The peak in the temperature of the spectrum is incredibly uniform across the sky. It is not plausible for this radiation to be caused by objects such as stars, planets, galaxies etc since it would be much more lumpy in this case. Even gas clouds are too inhomogeneous. It therefore must have been produced by something that was very very uniform. The early universe in Big Bang theory was very uniform.

    * There are very small fluctuations in the temperature across the sky that have been measure by many telescopes to great precision. The angular power spectrum ( which is a measure of the statistical pattern of the blobs of slightly different temperatures) can be predicted from theory and it matches the data very well. In Big Bang theory, the very slight differences in density from place to place in the early universe (that cause these slightly different temperature blobs) were the origin of the structure in the Universe today. So the universe starts out close to uniform, but slightly overdense regions attract more and more material, due to gravity, which naturally causes a uniform distribution of matter to cluster in galaxies etc. We can model this formation of structure and the 'amplitude' of structure ( how non uniform the matter in the universe is) in the early universe implied by the CMB and the amplitude of structure we observe today match the model predictions very well.

    I could go on for a while, but these are just a few of the big ticket items. I encourage you to do some further reading on this. A good, non-technical, introduction I can recommend would be 'The Big Bang' by Simon Singh (last name might be spelled differently?)
     
  4. May 1, 2008 #3
    Firstly what has a black body got to do with microwaves?

    I am not too sure what a black body is but neither of ideas as to what one one is
    make sense.

    Wikipedia's fairly lenghty article on black bodys makes no mention of closed sytems
    whatsoever, and even if it did I am not convinced of it's relavance anyway.
     
  5. May 1, 2008 #4
    And how was that calculated?
    What temperatures would we have expected and why?
     
  6. May 1, 2008 #5

    mgb_phys

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    A blackbody emits a particular range of wavelengths depending only on it's temperature.
    The colder it is the longer the peak wavelength.
    So a star or a light bulb at 6000deg emits visible light. A red hot piece of metal emits red light. You emit infrared and an object at 3deg above absolute zero emits microwaves.
     
  7. May 1, 2008 #6
    3deg above zero is hardly an 'afterglow' to me, that's like, more like evidence
    of a big freeze than a big bang.

    I fail to see how that can be considered as evidence of a big bang.
     
  8. May 1, 2008 #7

    russ_watters

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    What wasn't explained is that the reason for the wavelength we detect is redshift. Objects that are far away are redshifted and looking at the CMB is like looking at the opaque universe itself, not long after the big bang.
     
    Last edited: May 1, 2008
  9. May 1, 2008 #8

    Wallace

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    All bodies emitted thermal radiation with a spectrum that follows the Black Body form. The peak of this spectrum depends on the temperature of the body. For instance, you are currently emitting black body radiation with in the infra-red, corresponding to your body temperature of around 315 Kelvin. However, your spectrum will not be a perfect black body since you are also reflecting light from other sources, and the clothes and skin of your body will absorb some of that light more that others. Therefore the only way for a body to emit the 'pure' thermal spectrum is for it to not be influenced by any external bodies.

    There are two concept here. The first is known as 'decoupling' of baryons and photons and the second is the effects of redshift. These two things together tell us the expected temperature we observe today.

    Firstly decoupling. The very early universe was so hot and dense that hydrogen that made up the majority of matter in the universe at the time was completely ionised. That is to say since it was so hot the electrons were not bound around the hydrogen nuclei as in a neutral atom, but the electrons and protons were all moving separately. There was also a lot of photons (light) bouncing around. Now, as the universe expands the density and temperature of this hot soup of ionised hydrogen cooled. We know from experiments the temperature and pressure at which hydrogen becomes ionised and hence when the universe dropped below this temperature the electrons 'recombined' with the protons to form neutral (non ionised) hydrogen gas. This is important because while the gas was ionised, the photons could not travel far before hitting the electrons that were free. Once the universe became neutral photons could travel much more unhindered. This is known as 'decoupling' since this is when the radiation (light) and matter decoupled. What it also means though is that the energy distribution of these photons was as perfect black body spectrum, with the peak corresponding to the known temperature that hydrogen ionises at. Thus we can predict the temperature.

    The second point is redshift. As the universe expands, photons released when the universe was smaller have a lower energy when viewed by observers at a late time, when the universe is larger. This is a prediction (that has been confirmed experimentally in labs as well as through observations). As the photons redshift, the form of the black body spectrum remains the same but the implied temperature changes since the peak of the spectrum moves. This modifies the expected measured temperature but in a way that we can predict.

    I encourage you to do some further reading on this as there is a great wealth of information out there. A forum such as this can help you with bits you don't understand or have further questions with but to get a good, coherent and well structured explanation of the whole picture it is not the best place.
     
  10. May 1, 2008 #9
    So what values are used to get to this redshift?
    Why are we only seeing microwaves?
     
  11. May 1, 2008 #10
    The strong evidence for the BB is Hubble's* discovery that the galaxies are all receding apart from one another. It's very simple: 1. (like hospital emergency ward staff) you've heard of using Doppler (or red-) shift to determine velocities (from various spectral fingerprints known to chemists); 2. it is elementary to correlate this with distance, as measured by performing simple triangulation (a.k.a. parallax) or just by noting that the loudness of a particular type of gunshot depends on how far you are from the firing of it (i.e. finding calibrated "standard candles").

    Frankly it just isn't necessary to consider the CMB as evidence "of" BB. If you did succeed in understanding the field, which is technical and broad, you would find that CMB details thoroughly "support" BB (and answer further questions); but as an independent proof of BB I imagine one would find the arguments too circular. This thread is like a straw man.

    * Not "hawkins" - you were conflating Dawkins (evangelical atheist) with Hawking (black hole theoretician).
     
  12. May 1, 2008 #11
    Well it's not my straw then as didn't put it forward in the first place as supporting evidence,
    I am merely asking about the science surrounding the 'straw' so to speak.
    But as this radiation apparently exists it seems perfectly reasonable to ask questions
    relating to its origins.

    I mean it seems a rather narrow band. What original temperatures does it indicate?
    Not that hot apparently, would we not expect to see a wider range of temperatures?
    I mean it was a hotter before and cooler after, so should we not expect to see a much
    broader range of background radiation?

    Also I am not sure I mentioned hawkins in the tread.
     
  13. May 1, 2008 #12

    Wallace

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    Read once again my post #8. It explains why the CMB has a uniform temperature, since all the CMB photons were released at a single time in the universes history when the whole Universe was the same temperature.
     
  14. May 2, 2008 #13

    cristo

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    Exactly. The important point, which you have said above, but I want to emphasize, is the fact that the photons emitted at the same time in the past in a blackbody spectrum, when redshifted, take the form of a blackbody spectrum today, just at a lower temperature.

    Who's "hawkins"? :confused:
     
  15. May 2, 2008 #14

    russ_watters

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    Wallace explained this above. More briefly, though, every hot object emits light in a bell-shaped curve who'se peak corresponds to the temperature. Thus there is a limited range of frequencies. The peak frequency calculated for 'decoupling', redshifted for the calculated age of the universe (from redshift data for galaxies) corresponds to the frequency of microwave radiation observed.
     
  16. May 2, 2008 #15
    The Big bang cosmological model of the universe's creation and current state actually predicts the presence of a backgound radiation and its current temperature.

    Basically its the flash that occured when the universe became visible at about 380,000 years after the initial big bang event.

    Photons were able to decouple when the temperature got low enough (although it was still extremely hot at the 380,000 year mark)

    It took over 50 years for sceintists to detect this weak signal at all (WMAP)

    Also, the BB model also predicted that this background radiation should be non-uniform or grainy which would reflect what we see in the night sky today. It shold also be found where ever you pointed your measuring devices or detectors.

    These predictions are quite astonishing and provide strong evidence for the BB and the infaltion theory proposed by Alan Guth!
     
  17. May 2, 2008 #16
    It don't think his explaintion was adaquate to answer the question I asked.
    The same goes for your explaination, it appears to be expalining something I did not ask, at best.
     
  18. May 2, 2008 #17

    cristo

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    I really don't see how neither Wallace's nor Russ's post answered your question. Perhaps it is that you do not quite grasp the answers. If this is the case, then please ask a more specific question related to their answers.
     
  19. May 3, 2008 #18

    cristo

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    The OP in this thread was not genuine, thus I'm locking this thread. If anyone else has a genuine question on this topic, please feel free to start another thread.
     
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