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Why do we observe only microwaves in the background radiation?

  1. Mar 20, 2014 #1
    Why aren't the other bands (visible, UV, etc.) in the electromagnetic spectrum also seen today as part of the background radiation?
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  3. Mar 20, 2014 #2


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    When the CMB was first emitted, the plasma filling all of space was at a temperature of around 3,000 kelvin and much of the radiation was in the visible range. However, over the last 13 billion years the expansion of space has stretched out the radiation until the wavelengths now fall mostly in the microwave range.
  4. Mar 20, 2014 #3


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    The CMB is now stretched by a factor of 1090, as Drakkith noted. That 3000K emission is the only known EM radiation source in the universe at age 380,000 years.
  5. Mar 21, 2014 #4
    If you are familiar with the theory of black body radiation, you realize that at a given temperature all matter radiates a broad spectrum of emr. Since any detection device is comprised of matter, there is an intensity of emr which is simply not differentiable from equipment noise. (not to mention external sources). Take a look at the recent report from antarctica on the CMB polarization. It took them 3 years to make sure what they thought they were seeing wasn't an artifact. Basically, its the same reason as why we couldn't see the meteor that struck Russia (till it was right on top of us) ...its called "glare".
  6. Apr 24, 2014 #5
    If the background radiation that we see today were in the visible range (for example red), would the night sky be red instead of black?
  7. Apr 24, 2014 #6


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    The light would still be very faint, as there's very little energy density of the CMB now. I doubt you would be able to see it even if it was in the visible spectrum.
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