Gravitational waves from asymmetric oscillon dynamics?

It has been recently suggested that oscillons produced in the early universe from certain asymmetric potentials continue to emit gravitational waves for a number of $e$-folds of expansion after their formation, leading to potentially detectable gravitational wave signals. We revisit this claim by conducting a convergence study using graphics processing unit (GPU)-accelerated lattice simulations and show that numerical errors accumulated with time are significant in low-resolution scenarios, or in scenarios where the run-time causes the resolution to drop below the relevant scales in the problem. Our study determines that the dominant, growing high frequency peak of the gravitational wave signals in the fiducial “hill-top model” by Antusch et al., [Phys. Rev. Lett. 118, 011303 (2017).] is a numerical artifact. This finding prompts the need for a more careful analysis of the numerical validity of other similar results related to gravitational waves from oscillon dynamics.

Figure 1

Gravitational wave spectra generated over the course of pseudospectral simulations with $N=64$ (orange), $N=128$ (green), and $N=256$ (blue), overlaid with a finite-difference simulation at $N=128$ (dotted black). The first panel corresponds to the point in each simulation when $a(t)=64/(L_i{m}_{\varphi })=3.35$, the second panel when $a(t)=128/(L_i{m}_{\varphi })=6.7$, and the third when $a(t)\lesssim 256/(L_i{m}_{\varphi })=13.45$. Note that we have not removed the peaks that appear at highest-frequencies in each simulation that come from numerical noise near the Nyquist frequency. These peaks correspond to a well-known issue with lattice simulations. Note that the location as well as amplitude of the growing peaks (not just the Nyquist peak) are resolution-dependent, hence not robust physical signatures in the spectrum.

Figure 2

Gravitational wave spectra today for $N=64$ (orange), $N=128$ (green), and $N=256$ (blue), overlaid with a finite-difference simulation at $N=128$ (dotted black). We have assumed that gravitational wave production stops at $a=16$ in our simulations, and is followed by radiation domination. We note that apart from the “shallow” peak at $f\sim 5\times {10}^8$, the rest of the high frequency peaks are numerical artifacts.