Black holes (and cosmic strings)

Partly settled. Some of it is understood, some is still argued about.

Ginzburg wrote that black holes' existence and astrophysical importance were 'beyond doubt' already in 1999, though he noted lensing/orbital evidence alone could not fully distinguish a true GR black hole from an exotic compact alternative; he included cosmic strings in problem #25 only with a question mark, calling them unobserved and speculative. Since then the case for real black holes has become far stronger: LIGO/Virgo have detected dozens of black-hole mergers with waveforms matching GR's predictions, and the Event Horizon Telescope produced actual images of the shadows of M87* (2019) and Sagittarius A* (2022), consistent with GR. Cosmic strings, however, have never been detected — searches via gravitational lensing and CMB signatures have found nothing — and are now considered at most a minor, speculative scenario.

A black hole is a place where general relativity predicts its own description breaks down, and in 2019 humanity finally took a picture of one.

References

• What problems of physics and astrophysics seem now to be especially important and interesting (thirty years later, already on the verge of the XXI century)?, V L Ginzburg, Physics-Uspekhi 42 (4) 353-373, 1999
• First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole, Event Horizon Telescope Collaboration, The Astrophysical Journal Letters 875, L1, 2019