: Astrophysical black hole candidates might be horizonless ultra-compact 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 as dictated by the dynamics. The new horizonless 2-2-hole, as a quite generic static solution sourced by sufficiently dense matter, may then be the nearly black endpoint of gravitational collapse. In the era of gravitational wave astronomy, echoes in the post-merger phase provide a great opportunity to probe such scenario.
Given the uncertainties associated with the waveform of echoes, I will discuss some model-independent search strategies, where the primary task is to find the time delay between echoes. The search range is then motivated by the Planck scale deviation outside the would-be horizon.