Abstract Gibbons and Hawking ( Phys. Rev. D , 15 (1977) 2738) have shown that the horizon of de Sitter space emits radiation in same way as event black hole. But actual cosmological horizons are not horizons, except space. Nevertheless, this paper proves Gibbons' Hawking's formula an exact result for any flat expanding with strictly positive Hubble parameter. The gives visual intuitive insight ...