Relic gravitational waves produced after preheating

TL;DR

This paper identifies a robust mechanism for generating a stochastic background of relic gravitational waves during preheating after inflation, arising from the resonant decay of a coherently oscillating scalar field and its non-linear rescattering. It combines analytical estimates with large-scale lattice simulations to quantify the gravitational bremsstrahlung emitted by fluctuations in two-field chaotic inflation and the pure λφ^4 model, linking production to post-inflationary parameters such as φ_ch, m, H_ch, and the resonance strength q. The results indicate that a sizable GW background with Ω_GW h^2 up to ~10^{-12} could be produced at frequencies around 10^4–10^5 Hz in plausible models, with potential detectability in certain parameter regimes and model variants. The work highlights the importance of post-inflationary dynamics and suggests that more complex inflationary scenarios (e.g., hybrid inflation or domain-wall networks) could yield even stronger, potentially observable gravitational-wave backgrounds.

Abstract

We show that gravitational radiation is produced quite efficiently in interactions of classical waves created by resonant decay of a coherently oscillating field. For simple models of chaotic inflation in which the inflaton interacts with another scalar field, we find that today's ratio of energy density in gravitational waves per octave to the critical density of the universe can be as large as 10^{-12} at the maximal wavelength of order 10^{5} cm. In the pure $λφ^4$ model, the maximal today's wavelength of gravitational waves produced by this mechanism is of order 10^6 cm, close to the upper bound of operational LIGO and TIGA frequencies. The energy density of waves in this model, though, is likely to be well below the sensitivity of LIGO or TIGA at such frequencies. We discuss possibility that in other inflationary models interaction of classical waves can lead to an even stronger gravitational radiation background.

Figures (3)

• Figure 1: Variances of fields $X$ (solid curve) and $\phi$ (dotted curve) as functions of conformal time in the model with massless inflaton for $\lambda=10^{-13}$ and $q=30$.
• Figure 2: Power spectrum of the field $\phi$ for the same model as in Fig. 1, output every period at the maxima of $\varphi_0(\tau)$; $k$ is rescaled comoving momentum (see text).
• Figure 3: Today's spectral density of gravitational waves in the pure $\lambda \phi^4$ model (solid line) and in the model where interaction $g^2 \phi^2 X^2$ with a massless scalar field $X$ is added; the dashed line corresponds to $q=30$, and the dotted line corresponds to $q=105$, where $q =g^2/4\lambda$. We used $g_{*}/g_{0}=100$.