“The final stage in the life of a star,” observes physicist Carlo Rovelli in Seven Brief Lessons on Physics, “where the quantum fluctuations of space-time balance the weight of matter, is what is known as a ‘Planck star’. If the sun were to stop burning and to form a black hole, it would measure about one and a half kilometers in diameter. Inside this black hole the sun’s matter would continue to collapse, eventually becoming such a Planck star. Its dimensions would then be similar to those of an atom.
“The entire matter of the sun condensed into the space of an atom: a Planck star should be constituted by this extreme state of matter. A Planck star is not stable: once compressed to the maximum, it rebounds and begins to expand again. This leads to an explosion of the black hole.
“A black hole is a rebounding star seen in extreme slow motion. It is possible that in the furnace of the first instants of the universe black holes were formed and that some of these are now exploding. If that were true, we could perhaps observe the signals that they emit when exploding, in the form of high-energy cosmic rays coming from the sky, thereby allowing us to observe and measure a direct effect of a phenomenon governed by quantum gravity.”
The Daily Galaxy, Max Goldberg, via Rovelli, Carlo. Seven Brief Lessons on Physics (p. 46/47). Kindle Edition.
Image credit: NASA TESS image of a black hole