Astrophysical black hole candidates might be horizonless ultracompact objects. Of particular interest is the plausible fundamental connection with quantum gravity. The puzzle is then why we shall expect Planck scale corrections around the horizon of a macroscopic black hole. Taking asymptotically free quadratic gravity as a possible candidate of UV completion of general relativity, I will show how the would-be horizon can be naturally replaced by a tiny interior with only Planckian deviation when the matter distribution is sufficiently dense. The new horizonless object may be the nearly black endpoint of gravitational collapse, and it exhibits intriguing thermodynamic behaviors. The implication for phenomenology is rich. For large objects, gravitational wave echoes in the post-merger phase of binary coalescence provide a smoking gun signal for the new physics. The small objects, on the other hand, behave as cold and stable remnants, and they may constitute all of dark matter. A distinctive phenomenon associated with remnant mergers occurs, predicting fluxes of high-energy astrophysical particles due to the spectacular evaporation of the merger product.

Jing Ren is currently an associate professor at the Theoretical Physics Division of the Institute of High Energy Physics. Her research interests lie in theoretical particle physics and the interface with gravity, with the main focus of understanding new physics beyond the Standard Model and General Relativity. Prior to joining IHEP in 2018, she was a postdoctoral fellow at University of Toronto in Canada. She received the PhD in Physics (2014) and undergraduate degree (2008) from Tsinghua University.