Hello, I am sorry. I apologize for my poor English. [ Implicit assumption of CMBR? ] It is not certain whether this kind of experiment has already been conducted. Still, it need be tested whether the wavelength distribution of 3000k radiation cooling down to 2.7K is completely identical to that of the radiation from a 2.7K black body. Case-1 3000K black body -->cooling down black body -->2.7K black body --> 2.7K radiation (thermal equilibrium state?) -------------------> (thermal equilibrium state?) Case-2 3000K black body --> 3000K radiation --> cooling down radiation(2.7K) (thermal equilibrium state?) ------------> (adiabatic expansion and redshifted?) Can we completely trust that case-1 is equal to case-2? For the experiment, 600K~1200K radiation needs cooling down to 300K or so, and the resulting values need be compared with those of radiation from the black body has a 300K.