Gravitational-optical gradient lens, comoving with radiation, is formed in observer’s frame of reference of time and spatial coordinates (FR) due to evolutional decrease of average density of matter in the Universe as well as due to evolutional decrease of refraction index of interstellar medium. This diverging lens and Hubble gravitational lens together form virtual image of all infinitely far points of Euclidean background space of FR, comoving with expanding Universe, on its focal surface, which is the imaginary observer horizon. Events that take place in different points but simultaneous in observer’s FR are nonsimultaneous in cosmological time of FR, commoving with Universe, due to Universe expansion. Therefore world point of imaginary Big Bang is present in observer’s intrinsic space at every moment of his proper time. This point and observer’s dislocation point are the opposite poles of four-dimensional hypersurface of observer’s space. When gradient lens is not taken into account one may come to a conclusion that Hubble lens forms the horizon of cosmological past (imaginary observer horizon) in vacuum external solutions of equations of gravitational field when cosmological constant is nonzero. This also leads to spatial homogeneity of the negative power of global gravitational lens and, consequently, this leads to a linear dependence of red shift of radiation spectrum of astronomical objects on the distance to those objects. However, when gradient lens is taken into account this dependence becomes nonlinear and corresponds to accelerated expansion of the Universe, while imaginary observer horizon of cosmological past degenerates into point of imaginary Big Bang of the Universe. This is similar to degeneration of the imaginary horizon of cosmological future (Schwarzschild sphere) in internal solution of equations of gravitational field.