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I know the reason we have a CMB is because the photons don't react with any of the matter in the universe, or the mean distance between interactions for photons generated since decoupling is now the length of the observable universe. However, I can't reason why those microwaves don't interact with anything but the microwaves in my kitchen do.

I think it has to do with their relative intensities and the high density of food compared to the low density of the universe, but im not sure. Can anyone help?

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    $\begingroup$ Your average microwave operates on 800-1,000 Watts. That's a healthy amount of power generating a whole lot of microwaves. A low dosage of microwaves does very little, but trillions of them of them will warm up your food pretty quickly. (I'm not sure the approximate number of photons that a microwave oven generates, but it's a whole lot more than you get floating around the universe). $\endgroup$ – userLTK Nov 15 '15 at 5:54
  • $\begingroup$ It's everything to do with relative intensities, and nothing to do with density of food. Per Cosmas Zachos' answer to this question, a microwave oven puts out $\sim 10^{26}$ microwave photons/sec. At any given moment, the volume of a typical microwave oven has $\sim 10^{6}$ CMB photons passing through it (and most of those are probably the wrong frequency to heat your food anyway). $\endgroup$ – Peter Erwin Nov 15 '18 at 8:43
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I think it has to do with their relative intensities and the high density of food compared to the low density of the universe,

This is correct. The microwave ovens are not working with a black body radiation curve with an average of 3K , nor is the heating effect a thermal balance between two black bodies: food and microwave. Bodies in space away from the sun eventually balance their temperature to the CMB temperature.

Instead a high intensity is generated by a "suitable antenna" and the food itself has receptors , quantum mechanical energy levels, that absorb this high intensity wave and turn it into vibrations and heat.

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The reason that there is a CMB is because of the big bang. The photons from the very beginning of our universe has spread (almost)uniformly throughout the universe to give rise to a general noise which we call background.

Now to answer your question, as the universe expanded after big bang the photons got redshifted and their energy decreased. Now the avg temperature of CMB is 3K or -270 celsius. I hope it is obvious that you do not want to cook your food at that temperature.

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  • $\begingroup$ So why are the microwaves in my kitchen warmer? $\endgroup$ – Franco Scopo Nov 15 '15 at 4:59
  • $\begingroup$ Because you generate it that way. Those microwaves has no relation with the CMB. CMB was created in big bang, yours are created in the tungsten wires because of inductance. $\endgroup$ – Ari Nov 15 '15 at 5:02
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If we consider the twin questions, "Why doesn't the Cosmic Microwave Background heat the oceans—and conversely, how is my kitchen microwave able to heat food above 2.7°K?", the answer “boils” down to frequency vs. intensity. Intensity of light is a measure of density, the number of photons within a given space, whereas frequency is an emergent property of the organization of those photons.

While the CMB and the oven appliance both emit photons in the "microwave" electromagnetic frequency range, the latter does so at a vastly greater intensity. The microwave frequency range is specifically resonant with that of the water molecule, which allows heating of food or other materials containing water. Conversely therefore, the CMB does induce heating of the oceans, but the low intensity of the CMB means the effect is probably too minuscule to measure.

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