Restoring the sting to metric preheating

TL;DR

The paper investigates whether preheating can amplify super-Hubble metric perturbations and whether inflationary initial conditions can suppress this effect. By analyzing generalized two-field potentials with couplings beyond the canonical $\frac12 g^2\phi^2\chi^2$ and performing numerical simulations, the authors show that the proposed suppression mechanism is not generic: metric preheating can robustly amplify large-scale perturbations in realistic models. They identify two classes—Class I (nonzero $\chi$ vev during inflation) and Class II ($\chi$ light during inflation but heavy during preheating)—where suppression is avoided and amplification persists, with significant implications for adiabatic and isocurvature modes, non-Gaussianity, and potentially primordial magnetic fields. Overall, metric preheating appears to leave observable imprints on the power spectrum and CMB, underscoring the need for more realistic preheating models in inflationary cosmology.

Abstract

The relative growth of field and metric perturbations during preheating is sensitive to initial conditions set in the preceding inflationary phase. Recent work suggests this may protect super-Hubble metric perturbations from resonant amplification during preheating. We show that this possibility is fragile and sensitive to the specific form of the interactions between the inflaton and other fields. The suppression is naturally absent in two classes of preheating in which either (1) the vacua of the non-inflaton fields during inflation are deformed away from the origin, or (2) the effective masses of non-inflaton fields during inflation are small but during preheating are large. Unlike the simple toy model of a $g^2 φ^2 χ^2$ coupling, most realistic particle physics models contain these other features. Moreover, they generically lead to both adiabatic and isocurvature modes and non-Gaussian scars on super-Hubble scales. Large-scale coherent magnetic fields may also appear naturally.

Figures (3)

• Figure 1: Growth of $|k^{3/2}\Phi_k|$, $|k^{3/2}\zeta_k|$, $|k^{3/2}\delta\phi_k|/M_{\rm pl}$ and $|k^{3/2}\delta\chi_k|/M_{\rm pl}$ with $\tilde{g}/g = 10^{-2}$, $k/ma_0 \sim 10^{-23}$ and $q = 3.8 \times 10^5$. Inset: Including the detailed evolution during inflation.
• Figure 2: As in Fig. 1, but for a scale that is within the Hubble radius at the start of preheating ($mt_0\sim 20$), with $k/a_0H_0 =10$.
• Figure 3: The time to nonlinearity for super-Hubble perturbations as $\tilde{g}/g$ increases from $10^{-4}$ to $10^{-2}$ ($q = 3.8 \times 10^5$). On average, $t_{\rm nl}$ decreases rapidly as $\tilde{g}/g$ increases. Inset: a zoom with $5\times 10^{-4}\leq\tilde{g}/g\leq 5\times 10^{-3}$.