Laura Relic gravitational waves from primordial gravitational collapses Xiang-Xi Zeng, Zhuan Ning, Zi-Yan Yuwen, Shao-Jiang Wang, Heling Deng, Rong-Gen Cai Hide abstract | Show figures | Show BibTeX | Show discussion | View PDF | 2504.11275v1 A large primordial density perturbation of the Hubble scale will gravitationally collapse, generating an outgoing sound shell whether or not a primordial black hole (PBH) is formed. In this Letter, we report a new source of the stochastic gravitational wave background induced by the collision of sound shells in the early Universe. The peak frequency and amplitude in the GW spectrum depend on the Hubble horizon and the abundance of sound shells. Abundant density perturbations would lead to GW backgrounds potentially detectable for future pulsar timing arrays and ground-based/space-borne detectors. For those perturbations that collapse into PBHs, future null detection of the corresponding high-frequency GW background could put new observational constraints on those PBHs that have already evaporated. Authors' comments: 5 pages + Supplemental Material, 5 figures, with an accompanying long paper to appear soon