From what I can understand, Cosmic Microwave Background Radiation (CMBR) is microwave now, but was initially visible light emitted long time ago.

  • The microwave is producing a wave that’s about 12 centimeters long.

  • Now CMBR has right now this (time changing) distribution (peak at 2mm):

By Quantum Doughnut - Own work This diagram was created with gnuplot., Public Domain, Link

So it looks to me that in some time, when its wavelength falls at around 12cm), some of its energy should be transferred to water molecules the way it is in a microwave oven.

  • Is it the case? Will some of the CMBR energy actually transferred to water molecules?
  • Will this energy be too faint to matter at our scale?
  • $\begingroup$ looks related: space.stackexchange.com/questions/20325/… $\endgroup$
    – J. Chomel
    Commented Sep 29, 2017 at 8:18
  • $\begingroup$ i think the CMBR energy is slightly less than 3 kelvin compared to average healthy human at 310 kelvin. $\endgroup$
    – user6760
    Commented Sep 29, 2017 at 8:26
  • 3
    $\begingroup$ Sunlight is roughly 1368 Watts per square meters which is approximately 450 million times more than CMBR at 3 µW (a.k.a microWatts) $\endgroup$
    – user6760
    Commented Sep 29, 2017 at 9:03
  • 5
    $\begingroup$ A microwave boils a cup of water because there's a 1,000 Watt engine generating a boatload of those long wavelength photons over an area about the size of a dinner plate. It's not just the wavelength but the quantity of photons that matters. Maybe somebody could work out the math but I think CMB comes to about 1 watt per acre. That's not going to boil anything except maybe liquid helium. $\endgroup$
    – userLTK
    Commented Sep 29, 2017 at 14:52
  • 3
    $\begingroup$ Keep in mind here that the Sun is a black body spectrum and thus already produces photons in the 12 cm range (that's part of the definition of a black body). You don't see those photons boiling lakes do you? As user6760 has already pointed out, the Sun produces waaay more energy than what is in the CMB so if the Sun can't do it, then the CMB certainly can't. $\endgroup$
    – zephyr
    Commented Sep 29, 2017 at 17:29

1 Answer 1


Not really: microwaves heat mainly by using a lot of radiation, the particular wavelengths matters only for the ability to (somewhat) heat evenly.

You probably know of the Sun, a thing in the sky which shines most brightly in the optical, that is in wavelengths much shorter than the ones in a microwave. Yet it is clearly very capable of heating you and everything around you far above the ambient temperature of space.

So the wavelength of radiation is not crucial for heating (it matters a bit depending on the material), it is the amount of incoming radiation that matters. If the CMB could start to boil water because it was redshifted from 2 mm to 12 cm, it would need to be at such high intensity that water would already be boiling, because water absorbs more radiation of 2 mm wavelength than of 12 cm: enter image description here


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