STRONG INFRARED EFFECTS IN QUANTUM-GRAVITY

We explore ''quantum cosmological gravity,'' or quantum general relati vity with a nonzero cosmological constant. It is explained why and how QCG can be used reliably in the far infrared, despite the absence of renormalizability. We show that loop corrections to positive Lambda QC G mediate powerful infrared effects for two reasons: first, the theory allows massless gravitons to self-interact via a coupling of dimensio n three; second, the inflationary redshift of the classical background progressively increases the population of soft gravitons. One consequ ence is that QCG must eventually dominate the physics of inflation wit h respect to any phenomenologically confirmed theory, no matter how mu ch stronger the nongravitational couplings may seem. Another consequen ce is that the graviton's on-shell self-energy is negative and infrare d divergent at one loop, thereby inducing a negative infrared divergen ce in the two-loop vacuum energy. We analyze these effects in the cont ext of causal evolution from an initial patch of one Hubble volume whi ch begins inflation at finite times in one of the homogeneous and isot ropic Fock states of free QCG. Up to some tedious but probably managea ble tenser algebra we show that quantum infrared effects exert an ever increasing drag on the background's expansion for as long as perturba tion theory remains valid. A rough estimate of the relaxation time is easily consistent with enough inflation to solve the smoothness proble m. (C) 1995 Academic Press, Inc.