Metric preheating and limitations of linearized gravity. II

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  • 15 January 1999
Abstract
Recently it was shown that explosive growth of the field fluctuations during preheating must be accompanied by resonant amplification of scalar metric fluctuations, since the two are united by Einstein's equations. This ``metric preheating" pumps the field fluctuations, increasing particle production, and leading to gravitational rescattering effects even at linear order. In multi-field models with strong preheating, metric perturbations become nonlinear, with the strongest amplification on superhorizon scales. This amplification is causal and due to the superhorizon coherence of the inflaton condensate. It leads to resonant growth of entropy perturbations and invalidates use of the linearized Einstein field equations, irrespective of the amount of fine-tuning of the initial conditions. This has serious implications for cosmic micrwowave background (CMB) anisotropies and primordial black hole formation. We investigate the (q,k) parameter space in a simple two-field model, and introduce the time to nonlinearity, i.e., the timescale for the breakdown of the validity of the linearized Einstein equations. Mode-mode coupling and turbulence tend to re-establish scale-invariance but these are limited by causality. We discuss ways to escape the above conclusions, including preheating solely to fermions. The exclusion principle damps the amplification of metric perturbations significantly. Finally we rank known classes of inflation from strongest (chaotic and strongly coupled hybrid inflation) to weakest (hidden sector, warm inflation) affected by the resonances.

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