Evolution of initial spectrum at y = 0 given by Eq. (18)with  − Y_SZ = Y_BEC = 10^-5. |ΔT/T| ≡ |T_eff/T − 1| is plotted, where T_eff is the temperature of a blackbody spectrum with the corresponding intensity at a particular frequency. Bose-Einstein spectrum defined by n(ν) = 1/(e^{hν/k_BT_e + μ} − 1) with negative μ is labeled “-ve BE”, red dashed, double-dotted line in the figure. The distortions are positive in the Rayleigh-Jeans region (labeled “+ve”) on the left side and negative in the Wien region (labeled “-ve”) on the right side of the figure.