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The cosmic microwave background (CMB) radiation as a remnant from an early stage of the universe (Big Bang) is filled all space and has a thermal black body spectrum at a temperature of $2.72548±0.00057$ kelvin. Therefore, even the empty scape has an average temperature of 2.7 kelvin, which is greater than the absolute zero temperature (i.e., $0$ ${\rm{K}}$).

Interestingly, the lowest temperature ever recorded in lab is really close to the absolute zero, for example see this link. How is it possible? How can experimental physicists eliminate the thermal effects of CMB noise in such very low-temperature experiments? Is someone here familiar with their experimental techniques or the theoretical basis?

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    $\begingroup$ The thermal radiation in the lab at around 300 kelvin was a much bigger source of heat than the CMB! $\endgroup$ – Andrew Steane Apr 14 at 17:25
  • $\begingroup$ I don't need an answer anymore because in the answers my mistake was made clear to me. The two answers are good with different viewpoints and they complement each other. I don't know which one is better to be chosen as the most helpful ! Both (+1) from me. $\endgroup$ – SG8 Apr 14 at 19:09
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In my kitchen the background radiation is even around 293K, and yet in the fridge there is 273K and below. The second law of thermodynamics does not say that it is impossible to cool down anything below ambient level, but rather only that this does not happen spontaneously, i.e. without supplying energy that feeds a thermodynamic machine.

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Those ultralow temperature experiments are performed inside extremely well-shielded and insulated test chambers, to isolate them from the environment, and those experiments are performed while powerful refrigeration machines are actively pulling out as much heat as possible from inside the test chamber.

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