Abstract
Very compact objects probe extreme gravitational fields and may be the key to understand outstanding puzzles in fundamental physics. These include the nature of dark matter, the fate of spacetime singularities, or the loss of unitarity in Hawking evaporation. The standard astrophysical description of collapsing objects tells us that massive, dark and compact objects are black holes. Any observation suggesting otherwise would be an indication of beyond-the-standard-model physics. Null results strengthen and quantify the Kerr black hole paradigm. The advent of gravitational-wave astronomy and precise measurements with very long baseline interferometry allow one to finally probe into such foundational issues. We overview the physics of exotic dark compact objects and their observational status, including the observational evidence for black holes with current and future experiments.
Highlights
The discovery of the electron and the known neutrality of matter led in 1904 to J
Together with observations of phenomena so powerful that could only be explained via massive compact objects, the theoretical understanding of black holes (BHs) turned them into undisputed kings of the cosmos
It is tacitly assumed that such “quantum gravity effects” are relevant only near the Planck scale: at lengths P ∼ G /c3 ∼ 10−35 m, the Schwarzschild radius is of the order of the Compton wavelength of the BH and the notion of a classical system is lost
Summary
The crushing of matter to infinite density by infinite tidal gravitation forces is a phenomenon with which one cannot live comfortably. From a purely philosophical standpoint it is difficult to believe that physical singularities are a fundamental and unavoidable feature of our universe [...] one is inclined to discard or modify that theory rather than accept the suggestion that the singularity occurs in nature. No testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavors to establish
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