When is Colder Thermal Radiation Incident Upon a Warmer Body?

In my recent articles on thermal radiation physics (here and here), I showed that the consensus physics on thermal radiation is wrong in its belief that black body radiators near one another and in a state of equilibrium emit the same radiation as they would when isolated from one another and surrounded by an environment at T = 0K.  That is, they do not each emit power per unit area as given by the Stefan-Boltzmann Law when they are in the vicinity of one another in a state of equilibrium.

How then is it that one can detect the Cosmic Microwave Background Radiation with a detector at 4K when the Cosmic Microwave Background Radiation is that of a black body radiator at 2.73K?  This might at first blush seem to be an argument against my claims.  It may not be a very sound argument, however, due to the very unusual circumstances of detecting this radiation from gigantic distances and long ago when there could not be an equilibrium condition between the detection location on the Earth and the matter which emitted the radiation.

The Cosmic Microwave Background Radiation is believed to be radiation left over from the Big Bang.  The remaining radiation has had little in the universe that can absorb it since the universe cooled to the temperature at which electrons and protons could form hydrogen atoms.  The really long wave radiation left now is not absorbed by hydrogen atoms, which are about all that fill most of the space in the universe.  This microwave background radiation is cooling with time as the universe expands.

This microwave background radiation is now commonly detected by transition edge detectors using the rapid increase in resistance with temperature increase as a superconductor warms through the transition to the normal state at temperatures such as 0.1K.  A map of this background radiation is shown below:

Why does this microwave radiation background not violate my explanation of thermal radiation when it was originally detected by Arno Penzias and Robert Wilson using a detector warmer than 2.73K?  One reason is that this radiation was not emitted from a cooler body to a warmer body.  It is in fact the radiation from much hotter matter and it has been in the form of photons flying about the universe for eons of time.  The Earth did not even exist when this thermal radiation was emitted, so the relative electromagnetic field energy density at the surface of a detector on the Earth to that adjacent to some matter from which this radiation was emitted is not at all in play.  No equilibrium condition between them has ever existed.

Let us take a case of thermal radiation emitted from an actual body in the universe today far from Earth.  Any photons emitted because that body has a temperature higher than that somewhere else in the universe at a particular instant in time could be in flight for a long time.  Let us suppose that the warmer Earth moves into the trajectory of that long ago emitted photon and as a result the photon strikes a warmer detector on Earth.  There is every reason to believe this could happen.

These are two cases where one expects to detect radiation with a lower characteristic black body radiation temperature than a sensor might have on Earth.  Indeed, such radiation can and does bombard our planet.  This does not invalidate the theory on thermal radiation between two bodies in equilibrium with one another that I have presented.  In the context of the Earth's surface emission of infrared radiation and its absorption of infrared radiation emitted by greenhouse gases, colder greenhouse gases do not emit radiation which is absorbed by the Earth's warmer surface.  There is no such back radiation contributing to catastrophic man-made global warming.

Updated 25 March 2018 to emphasize that a cooler body in equilibrium with a warmer body does not emit radiation which is incident upon the warmer body.