Planck's Law Approaching Absolute Zero
Planck's law approximates the spectral radiance emitted by a black body of matter at thermal equilibrium as a function of temperature shown here from 1K (kelvin) to 5770K, the temperature of the emitting surface of the sun according to Harder et al. (2009). The cosmic microwave background (CMB) is thermal radiation observed from space away from suns, planets, and other observed bodies of mass. It has a temperature of 2.72548 ±0.00057 K (Fixsen, 2009) (green line).