If you are interested in cosmology, there is a series of papers by Gorkavyi building a cyclic universe cosmological model that's entirely based on Einstein's ge

If you are interested in cosmology, there is a series of papers by Gorkavyi building a cyclic universe cosmological model that's entirely based on Einstein's general relativity and doesn't require any quantum gravity theory. It's based on black holes mergers with mass loss and convertion of mass into gravitational waves, that later get captured by black holes again. https://pos.sissa.it/335/039/ https://pos.sissa.it/335/039/ https://academic.oup.com/mnras/article/476/1/1384/4848298 https://academic.oup.com/mnras/article/476/1/1384/4848298 https://academic.oup.com/mnras/article/461/3/2929/2608669 https://academic.oup.com/mnras/article/461/3/2929/2608669 https://www.sao.ru/Doc-k8/Science/Public/Bulletin/Vol76/N3/ASPB285.pdf https://www.sao.ru/Doc-k8/Science/Public/Bulletin/Vol76/N3/A... (this one is available only in Russian for now)