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Cosmic Microwave Background (CMB)

The afterglow of the Big Bang, providing crucial evidence for the hot, dense early universe and supporting inflationary theory.
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The statement of the theorem

The Cosmic Microwave Background (CMB) radiation field I(ν,t)I(\nu, t) is characterized by a perfect blackbody spectrum at time tt, defined by the Planck function: I(ν,T)=2hν3c21ehν/(kBT)−1I(\nu, T) = \frac{2h\nu^3}{c^2} \frac{1}{e^{h\nu/(k_B T)} - 1} The temperature T(t)T(t) evolves with the scale factor a(t)a(t) according to the adiabatic cooling law: T(t)=T0(a0a(t))T(t) = T_0 \left( \frac{a_0}{a(t)} \right) where T0T_0 is the present-day temperature and a0a_0 is the scale factor today. The observed spectrum confirms this relationship, providing evidence for the thermal history of the early universe.
Source: Wikipedia