Cosmic Microwave Background Radiation vs Big Bang?

This is a continuation of a previous post on the same topic. The European Space Agency ESA sends this message to the people of Europe and the World:

• The Cosmic Microwave Background (CMB) is the cooled remnant of the first light that could ever travel freely throughout the Universe.
• Scientists consider it as an echo or 'shockwave' of the Big Bang. Over time, this primeval light has cooled and weakened considerably; nowadays we detect it in the microwave domain.

More precisely, CMB is reported to be measured by the FIRAS Far Infrared Absolute Spectrophotometer (FIRAS) instrument on the COBE satellite as a very small temperature variation (18 $\mu K$) over a uniform background of a perfect blackbody spectrum at 2.725 $K$. The main difficulty is to isolate a very weak signal from very far away from more nearby signals including signals from the Earth atmosphere and oceans.  

To understand the technology of the measurement, which is not easy, we take a look at the FIRAS instrument to see what it contains:

 What we see is in particular the following:

• Sky Horn collecting input from the Sky.
• Xcal reference blackbody used for calibration of Sky Horn input.
• Ical reference blackbody for internal calibration.
• Ical is equipped with two germanium resistance thermometers (GRT).
• Xcal is monitored by three GRTs.
• FIRAS = Far Infrared Absolute Spectrophotometer.

The output of FIRAS consists of:

• A very small temperature variation of size 0.00002 K over a background of 2.725 K.
• The measured background spectrum is a perfect Planck blackbody spectrum. 

CMB spectrum as perfect Planck blackbody spectrum. But low frequencies in the far infrared spectrum are missing! 

We see warning signs: 

• Very high precision is reported!
• Perfect Planck blackbody spectrum is reported. But far infrared is missing. 
• Calibration to nearly perfect real blackbodies is made. 
• Temperature of 3 K from very far reported.  
• Spectrum as radiative flux is reported (spectrophotometer).

More understanding comes from plotting the spectrum in terms of frequency:

We here see the COBE-FIRAS (blue) measures intensity at maximum around 200 GHz and a bit beyond for higher frequencies in the cut-off region, while the more essential part of the spectrum in the far infrared is missing. The intensity maximum around 200GHz according to Planck's law corresponds to a temperature of about 3 K, which however, since the essential part of the spectrum is missing, may as well correspond to much higher temperature at much lower emissivity.

In previous posts we have reminded that measurement of temperature is possible by establishing radiative equilibrium between source and instrument/thermometer, but it requires disturbances between source and instrument to be small, which poses a challenge to directly measuring temperature of CMB from very far. 

The alternative in this case is to report temperature from spectrum. But directly measuring radiative flux/spectrum can be even more challenging. Typically this is done (using bolometers and pyrometers) by measuring temperature, and then computing radiative flux/spectrum using Planck's law under assumptions hard to verify. This makes assessing CMB to a very daunting task from a mix of measurement and computation of temperature and radiative flux.

The scenario is thus:

• If a correct full spectrum is measured, a temperature can be determined from the frequency of maximal intensity. 
• If only temperature is given, determining spectrum as radiative flux intensity, requires post processing. 
• A measured/computed temperature of 3K attributed to a very far away source may be misleading.
• Robitaille suggesting that the true origin of the the 3 K CMB is the oceans of the Earth at 300 K.  

To sum up, we have on the table: 

1. Very speculative Big Bang BB.
2. CMB with questionable credibility, maybe noise from Earth Ocean,  

The argument by mainstream physicists/cosmologists is now that since the main role of CMB is to serve as main evidence of Big Bang, and CMB shows to serve this role in such an excellent way, it gives credibility to CMB by being connected to something very big. BB thus supports CMB, which gives support to BB. 

One possibility is then that both BB and CMB are real phenomena The other possibility is that both are free speculations by scientists in search of a mission. What is your impression? 

PS Has COBE-FIRAS detected the same thing as WMAP and PLANCK further away from the Earth:

Which picture is most credible? The more details, the more credible? What happens with small details over time according to the 2nd Law?