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