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Cosmic Microwave Background Radiation

The CMBR is the afterglow of the Big Bang, a uniform thermal radiation permeating the universe, detected at a temperature of approximately 2.7 Kelvin.
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The statement of the theorem

The Cosmic Microwave Background Radiation (CMBR) is modeled as a perfect blackbody spectrum. The energy density ρrad\rho_{rad} of this radiation is given by:\nρrad=aradT4\rho_{rad} = a_{rad} T^4where TT is the temperature and arad=4σca_{rad} = \frac{4\sigma}{c} is the radiation constant. The temperature T(z)T(z) at redshift zz is related to the present temperature T0≈2.7 KT_0 \approx 2.7 \text{ K} by the adiabatic expansion law:\nT(z)=T0(1+z)T(z) = T_0 (1 + z)This relationship confirms the thermal nature and redshift dependence of the afterglow.
Source: Wikipedia