Ultrahigh-energy gamma rays and gravitational waves from primordial exotic stellar bubbles Yi-Fu Cai, Chao Chen, Qianhang Ding and Yi Wang Eur. Phys. J. C, 82 5 (2022) 464 Published online: 20 May 2022 DOI: 10.1140/epjc/s10052-022-10395-w