Instant preheating - art. no. 123523.

We describe a new efficient mechanism of reheating. Immediately after rolli ng down the rapidly moving inflaton field phi produces particles chi, which may be either bosons or fermions. This is a nonperturbative process which occurs almost instantly; no oscillations or parametric resonance is require d. The effective masses of the chi particles may be very small at the momen t when they are produced, but they "fatten" when the field phi increases. W hen the particles chi become sufficiently heavy, they rapidly decay to othe r, lighter particles. This leads to an almost instantaneous reheating accom panied by the production of particles with masses which may be as large as 10(17)-10(18) GeV. This mechanism works in the usual inflationary models wh ere V(phi) has a minimum, where it takes only a half of a single oscillatio n of the inflaton field phi, but it is especially efficient in models with effective potentials slowly decreasing at large phi as in the theory of qui ntessence. [S0556-2821(99)07912-6].