Does a Photon have Temperature?

The idea about the Cosmic Microwave Background CMB radiation is conveyed to the public by authoritative sources as follows starting at the creation of the Universe with a Big Bang:

• After about 380,000 years when the Universe had cooled to around 3000 Kelvin,  photons were able to move unhindered through the Universe: it became transparent.
• Over the intervening 14 billion years, the Universe has expanded and cooled greatly. Due to the expansion of space, the wavelengths of the photons have grown (they have been ‘redshifted’) to roughly 1 millimetre and thus their effective temperature has decreased to just 2.7 Kelvin. 
• These photons fill the Universe today (there are roughly 400 in every cubic centimetre of space) and create a background glow that can be detected by far-infrared and radio telescopes.

We meet the idea that photons are moving through space like some form of particles with effective temperature of 2.7 K filling the Universe as an after-glow of Big Bang. 

But the concept of photon lacks real physics. Light does not consist of a stream of light particles named photons, but is an electromagnetic wave phenomena and as such can have a frequency and an amplitude/intensity. An emitter of light like the Sun has a temperature, while the light emitted is characterised by its spectrum as intensity vs frequency. A spectrum can give information about the temperature of the emitter with the Planck spectrum the spectrum of an ideal blackbody at a certain temperature with in particular a high-frequency cut-off scaling linearly with temperature. 

Emitted light can be recieved by an antenna through resonance recording the frequency. It is also possible to record the temperature of an emitter by connecting the antenna to a form of radiation thermometer reading temperature from radiative equilibrium, in the same way as a common thermometer reads the temperature of a source by direct contact/equilibrium.  

But is more difficult to read a spectrum since properties of emissivity, transmissivity and absorptivity as well as view angles enter. In the absence of information a Planck spectrum is often assumed, but most emitters do not have blackbody spectra.

A Big Bang emitter at 3000 K is thus postulated with an after-glow received as a blackbody spectrum of 3 K with frequency reduced and wave length increased by a factor of 1000 into far-infrared. 

What is effectively measured is a combination of temperature and intensity, which shows up as a perfect blackbody spectrum. The message is that this is an after-glow of Big Bang, thus giving evidence to Big Bang: If there is an after-glow there must have been some glow to start with = Big Bang. More precisely, it is variations letting the antenna sweep the sky, which are measured and have to be given a physical meaning as some variability of the Early Universe. 

The basic idea is thus that photons have been traveling through empty space for 14 billion years under a stretching of a factor 1000 but no other influence, and that collecting these photons gives a picture of the Early Universe. This appears as a lofty  speculation cleverly designed as to prevent inspection because both theory and instrumentation are hidden in mist. Here is the main picture from The Music of the Big Bang by Amedo Balbi: 

The source is thus gone since 14 billion years, while the after-glow still surrounds us and can be measured. This is mind boggling. 

Let us compare with the picture presented as Computational Blackbody Radiation, where emitter and receiver establish contact by resonance of electromagnetic waves and so take on the same temperature by reaching radiative equilibrium, in the same way as two distant tune forks can find an equilibrium.

What about the time delay between emitter and receiver from finite speed of light? If a light source is switched on, it will take some time before it reaches a receiver. Is it the same when a light source is switched off? Do you feel being warmed even a while after the fire is dead? What about a solar eclipse? Does it take 8 minutes before we feel the cold? 

In any case, the connection between Big Bang which is gone since 14 billion years and a proclaimed after-glow, which we can enjoy today from the presence of about 400 photons in every cubic centimetre of space at 3 K, appears as science fiction to me at least. 

Radiation as electromagnetic waves needs a source to sustain over time. If the Big Bang source to CMB disappeared 14 billion years ago, the electromagnetic waves have so to speak have a life of their own over very long time, like a tsunami wave sweeping the Pacific long after the earth quake source has disappeared. Here the ocean acts as a physical medium carrying the energy, while a corresponding medium for electromagnetic waves as an aether has no physical presence. The energy is thus carried by the source some of which is transmitted to the receiver in resonance.