2 ms·

> In short, an object would come to thermal equilibrium in space at the 3K CMB temperature only if the CMB were the only radiation source in the object's sky. B

> In short, an object would come to thermal equilibrium in space at the 3K CMB temperature only if the CMB were the only radiation source in the object's sky. But in actual fact it never will be. I still don't see if this answer my question, assuming CMB is the only source, but the photon density has diminished significantly, would there be enough CMB photons to keep the object from going lower than 3K from emitting blackbody radiation? The gist of my question is: Will there ever be a time where there are not enough CMB to heat the blackbody? Just like solar photons at 5500K, there are not enough of them to heat stuff at Earth to the same temp.

pdonis 6y ago
> assuming CMB is the only source, but the photon density has diminished significantly, would there be enough CMB photons to keep the object from going lower than 3K from emitting blackbody radiation? Obviously not, since if the photon density has diminished significantly, the temperature of the CMB will diminish significantly too. The temperature of the CMB gets lower as the universe expands. When the CMB was originally emitted, some 300,000 years or so after the Big Bang, its temperature was around 3000K, not 3K. It has cooled to 3K because the universe has expanded by a factor of about 1000. It will continue to cool as the universe expands further. > Will there ever be a time where there are not enough CMB to heat the blackbody? Meaning, to heat the black body to the same temperature as the CMB at whatever time you are considering? No. The CMB is unlike all other radiation sources in that it is everywhere, and its effective temperature is the same everywhere. You can't be "at a distance" from the CMB the way you can from a star or a galaxy.