N-body Simulations Model Gravitational Wave Emission from Nonspherical Collapse

This paper investigates the dynamics of nonspherical overdense patch collapse during an early matter-dominated era and the associated production of gravitational waves (GWs) using a semirelativistic N-body framework. The collapsing patch is initialized through a Zel'dovich deformation of a homogeneous sphere and evolved in an Einstein-de Sitter background. The emitted signal is computed directly from the numerical quadrupole evolution. The study demonstrates that a reliable prediction of the signal requires a fully numerical treatment of the nonlinear collapse dynamics. Fitting-based procedures and Zel'dovich-based estimates fail to capture the post-shell-crossing evolution and can over/under-estimate the emitted power of the GWs. The dominant contribution arises from peaks of relatively modest height, around ν ≈ 3, while a larger variance significantly enhances the signal. By varying the horizon mass and reheating temperature, the present-day GW spectra are mapped to the sensitivity bands of different classes of detectors. The signal can populate a broad range of frequencies, from pulsar timing arrays to very high-frequency experiments, showing that GWs from nonspherical collapse can provide a probe of the pre-BBN thermal history. This research contributes to our understanding of the early universe and the formation of structures.

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