Gravitational Waves from Oscillons after Inflation

We investigate the production of gravitational waves during preheating after inflation in the common case of field potentials that are asymmetric around the minimum. In particular, we study the impact of oscillons, comparatively long lived and spatially localized regions where a scalar field (e.g., the inflaton) oscillates with large amplitude. Contrary to a previous study, which considered a symmetric potential, we find that oscillons in asymmetric potentials associated with a phase transition can generate a pronounced peak in the spectrum of gravitational waves that largely exceeds the linear preheating spectrum. We discuss the possible implications of this enhanced amplitude of gravitational waves. For instance, for low scale inflation models, the contribution from the oscillons can strongly enhance the observation prospects at current and future gravitational wave detectors.

Figure 1

Evolution of the spectrum of GW as a function of the physical momentum $k/(a{H}_i)$. The spectra are obtained from a lattice simulation of the model (1) with $128^3$ points. Table 1 specifies the simulation setup. The lines correspond to the following times, scale factors: $t=573/m_{\varphi }$, $a=1.47$ (purple); $t=5544/m_{\varphi }$, $a=4.29$ (blue); $t=18924/m_{\varphi }$, $a=8.9$ (yellow); $t=38040/m_{\varphi }$, $a=13.81$ (orange); $t=57156/m_{\varphi }$, $a=17.94$ (red).

Figure 2

Snapshot of the energy density during the “oscillon phase,” at $a=5.35$, with energy density contours at $6⟨\rho {⟩}_{\mathcal{V}}$ (blue) and $20⟨\rho {⟩}_{\mathcal{V}}$ (red), obtained from a lattice simulation of model (1) with $v={10}^{-2}{m}_{\mathrm{Pl}}$, $V_0={10}^{-19}{m}_{\mathrm{Pl}}^4$, and $p=6$. The lattice size is $L=0.02/H_i$ with $256^3$ points.

Figure 3

Example predictions for gravitational wave spectra today, obtained from simulations of model (2) with parameters given in Table 1 and using the results at $a_e=15.51$. The spectra are shown for $V_0\simeq (110\mathrm{TeV}{)}^4$ (blue), $V_0\simeq (200\mathrm{TeV}{)}^4$ (green), $V_0\simeq (375\mathrm{TeV}{)}^4$ (orange) and compared to the expected sensitivity curve of the fifth observing run (O5) of the aLIGO–AdVirgo detector network [39].