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We define temperature as average kinetic energy of atoms. We know that outer space has vacuum that is it lacks matter and hence the temperature should be 0 K. But because of CMB radiation, the temperature is not 0. But if there exists no matter then how could there be any temperature?

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  • $\begingroup$ en.wikipedia.org/wiki/Planck%27s_law $\endgroup$ – G. Smith Sep 18 '19 at 18:06
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    $\begingroup$ We define temperature as average kinetic energy of atoms. That definition is too limited. A photon gas can have a temperature. $\endgroup$ – G. Smith Sep 18 '19 at 18:08
  • $\begingroup$ @G.Smith Could you please elaborate or let me know where I could read about what you are trying to say? $\endgroup$ – Mike Victor Sep 18 '19 at 18:14
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    $\begingroup$ I already provided a link to Planck’s law for blackbody radiation. The energy distribution of the CMB photons corresponds to a temperature. You should also read about how a photon gas has thermodynamics, just as a gas of atoms does. The masslessness of photons is irrelevant. $\endgroup$ – G. Smith Sep 18 '19 at 19:02
  • $\begingroup$ physics.stackexchange.com/questions/133985/… $\endgroup$ – BowlOfRed Sep 18 '19 at 20:46
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Although there is almost no matter in deep space, there is still radiation in deep space, and a temperature can be assigned to it. All objects in deep space will both radiate heat and absorb it from the radiation that bathes them. After a while, those objects will come into equilibrium with that radiation, and the rate of emission will then equal the rate of absorption. At that point, the temperature of the object will equal the temperature of the radiation.

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If you put a probe out in space far away from a star and record the temp, it will end up at the CMB temperature. Getting anything colder than that requires active cooling of some flavor.

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