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We discuss a special class of quantum gravity phenomena that occur on thescale of the Universe as a whole at any stage of its evolution. These phenomenaare a direct consequence of the zero rest mass of gravitons, conformalnon-invariance of the graviton field, and one-loop finiteness of quantumgravity. The effects are due to graviton-ghost condensates arising from theinterference of quantum coherent states. Each of coherent states is a state ofgravitons and ghosts of a wavelength of the order of the horizon scale and ofdifferent occupation numbers. The state vector of the Universe is a coherentsuperposition of vectors of different occupation numbers. To substantiate thereliability of macroscopic quantum effects, the formalism of one-loop quantumgravity is discussed in detail. The theory is constructed as follows:Faddeev-Popov path integral in Hamilton gauge -> factorization of classical andquantum variables, allowing the existence of a self-consistent system ofequations for gravitons, ghosts and macroscopic geometry -> transition to theone-loop approximation. The ghost sector corresponding to the Hamilton gaugeensures of one-loop finiteness of the theory off the mass shell. TheBogolyubov-Born-Green-Kirckwood-Yvon (BBGKY) chain for the spectral function ofgravitons renormalized by ghosts is used to build a self-consistent theory ofgravitons in the isotropic Universe. We found three exact solutions of theequations, consisting of BBGKY chain and macroscopic Einstein's equations. Thesolutions describe virtual graviton, ghost, and instanton condensates and arereproduced at the level of exact solutions for field operators and statevectors. Each exact solution corresponds to a certain phase state ofgraviton-ghost substratum. We establish conditions under which a continuousquantum-gravity phase transitions occur between different phases of thegraviton-ghost condensate.

Macroscopic effect of quantum gravity: graviton, ghost and instanton condensation on horizon scale of the Universe