<P> where c is the speed of light, ħ is the reduced Planck constant, k is Boltzmann's constant, G is the gravitational constant and M is the mass of the black hole . These predictions have not yet been tested either observationally or experimentally . </P> <P> The big bang theory is based upon the cosmological principle, which states that on large scales the Universe is homogeneous and isotropic . According to theory, the Universe approximately a second after its formation was a near - ideal black body in thermal equilibrium at a temperature above 10 K . The temperature decreased as the Universe expanded and the matter and radiation in it cooled . The cosmic microwave background radiation observed today is "the most perfect black body ever measured in nature". It has a nearly ideal Planck spectrum at a temperature of about 2.7 K. It departs from the perfect isotropy of true black - body radiation by an observed anisotropy that varies with angle on the sky only to about one part in 100,000 . </P> <P> The integration of Planck's law over all frequencies provides the total energy per unit of time per unit of surface area radiated by a black body maintained at a temperature T, and is known as the Stefan--Boltzmann law: </P> <Dl> <Dd> P / A = σ T 4, (\ displaystyle P / A = \ sigma T ^ (4) \,) </Dd> </Dl>

Describe a practical demonstration of a black body