the electrical fluctuations in a receiving antenna that are due to the radio radiation of the sun, the stars, the planets, interstellar media, and other matter.

Regardless of its origin, cosmic noise manifests itself in radio reception in the same manner as thermal noise. Its intensity is defined in terms of the brightness temperature of the sky, which is the equivalent temperature of the hypothetical celestial sphere that envelops the antenna and that has the properties of a black body. The heat-radiation levels of the hypothetical sphere and of the real sources of cosmic noise must be equal in all directions to satisfy the criterion of equivalence. Sky temperature decreases rapidly with the decrease of wavelength λ. Specifically, when detected by an antenna with a broad directivity diagram, mean cosmic noise levels—the noise background of the sky—decrease with λ as λ^–24. When λ ≈ 5 cm, for example, cosmic-noise temperature is approximately 15°K.