I've read that the temperature of the CMBR is 2.7 degrees K or so and is much cooler than when it started out, due to the expansion of the universe making the wavelength longer. What is meant by the temperature of the CMBR? How do you measure it? Is it determines purely by the wavelength? Is the "photon density" a factor in the temperature? Is the photon density of the CMBR changing over time? i.e. since the "surface of last scattering" is increasing in size, shouldn't the amount of photons that reach us in a given time period be increasing? The observable universe (including the CMBR) is said to homogeneous and isotropic. Is it possible that the CMBR we see is not actually isotropic but the differences are too small to be detectable and that there actually is a "center" to the universe somewhere, outside the observable universe.