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The Cosmic Background Radiation can be considered as the after-glow of the Big Bang, but does it transmit heat to objects in space? It is thermal radiation and is basically charged photons experiencing a red-shift as the universe experiences motion. As we are all aware, there are three methods of heat transmission: namely, conduction, convection and radiation. In the vacuum of the space-time continuum, there is no mode of convection or objects for conduction. So, if there is a body (Do not consider quantum jitters and fluctuations) will its temperature rise as time passes, considering the Cosmic Background Radiation is the only radiation affecting it?

PS: Please do not use extremely complicated concepts and if you plan on doing so, send links to explain it as I am only 14 and asked this question purely out of my curiosity.

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    $\begingroup$ Charged photons? What are those? $\endgroup$
    – g.kertesz
    Mar 1, 2023 at 9:31
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    $\begingroup$ @g.kertesz This probably means "photons with energy". A photon cannot have an electrical charge, like static electricity, but photons do carry energy. $\endgroup$
    – wizzwizz4
    Mar 1, 2023 at 20:46

4 Answers 4

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Yes, of course. Note however that the Cosmic Microwave Background is very cold (only 2.7 Kelvin), so usually, objects are warmer; then it's the other way around: objects radiate heat and thus cool. But yes, if you took an object, cooled it down to sub-Kelvin temperatures (as in $<1$K), and somehow transported it to outer space, the Cosmic Microwave Background would warm it up a bit.

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  • $\begingroup$ Are sub-kelvin temperatures onlyl theoritical? $\endgroup$
    – OrigamiEye
    Mar 3, 2023 at 8:17
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    $\begingroup$ @OrigamiEye, nope, the world record according to this link ltl.tkk.fi/wiki/LTL/World_record_in_low_temperatures is at 100 pK. (First link google spit up for some search terms, I have not checked if it's true). $\endgroup$
    – AnoE
    Mar 3, 2023 at 8:42
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    $\begingroup$ @OrigamiEye Maybe also interesting. Black holes give off (to our current most accepted theories) Hawking radiation which would, in principle, lead to them loosing energy and evaporating. But if they are large enough (like the ones we found so far), the Hawking temperature is below the CMB, so they are actually cooler than the universe around them. $\endgroup$
    – Cream
    Mar 3, 2023 at 9:49
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    $\begingroup$ @Cream and since they are cooler than the universe around them they are absorbing energy from the CMB and thus growing instead of evaporating. They will evaporate eventually, but only after the universe has cooled below their Hawking temperaure. $\endgroup$
    – N. Virgo
    Mar 3, 2023 at 12:51
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    $\begingroup$ @N.Virgo Yes, just an example for real objects in the universe that are (until now) cooler than the CMB. $\endgroup$
    – Cream
    Mar 3, 2023 at 12:55
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Yes it does. You don't even have to go to space: the heat can be sensed by instruments on the ground. That's how it was discovered. It's right in the title of the discovery paper: A Measurement of Excess Antenna Temperature at 4080 Mc/s.

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In the absence of convection and conduction, you might be tempted to assume that any exposure to radiation would necessarily raise the temperature of an object over time, as it's receiving energy all the time.

But that's missing a step. Every macroscopic object also emits radiation based on its temperature. We don't normally think of this because everyday objects emit infrared radiation that we can't see, and at low enough levels that it doesn't raise our temperature and thus we can't feel it either. But I'm sure you've seen (at least pictures of) metal heated up enough to glow; it actually "glows" even when it's cold, just in infrared where you can't see it.

So every object is both absorbing energy from radiation and emitting it, all the time. It will heat up over time if it is absorbing more energy through radiation than it is emitting, and cool down over time if it is emitting more energy than absorbing. But as an object gets hotter it emits more radiation, and as it gets colder it emits less. If you wait long enough in a constant environment it will eventually reach equilibrium (where it is absorbing and emitting exactly the same amount of energy), and stay at that temperature forever.

The cosmic background radiation is about equivalent to the thermal radiation you'd see if you were surrounded by a shell at 2.7 Kelvin. That means that any object you place in space that is warmer than 2.7 K will be losing more energy in the radiation it emits than it will be absorbing from the cosmic background; it will thus cool down. Only an object that was already colder than 2.7 K would actually be warmed up by the cosmic background, and only until it reaches 2.7 K.

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  • $\begingroup$ Kelvin are not measured in degrees (°). $\endgroup$
    – Alex
    Mar 4, 2023 at 6:22
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    $\begingroup$ @Alex Thanks; I've had that wrong for ages. $\endgroup$
    – Ben
    Mar 4, 2023 at 6:51
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The cosmic background corresponds nearly the radiation emitted from a 3K warm (or cold) black body. So, if you place any object in space, far away from any other sources, it will take the temperature of 3K, when it has reached equilibrium (Kirchhoff-law). This will last longer, if the body absorbs few radiation because if it is nearly white or it will go faster if the body is nearly black.

This is the reason, why it is said, the coldest sites in the universe are in some low temperature laboratories on earth.

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