__article__on Carlo Rovelli and Francesca Vidotto's paper

__"Planck stars"__. What are they?

A star that collapses gravitationally can reach a further stage of its life, where quantum-gravitational pressure counteracts weight. The duration of this stage is very short in the star proper time, yielding a bounce, but extremely long seen from the outside, because of the huge gravitational time dilation. Since the onset of quantum-gravitational effects is governed by energy density ---not by size--- the star can be much larger than planckian in this phase. The object emerging at the end of the Hawking evaporation of a black hole can then be larger than planckian by a factor (m/mP)n, where m is the mass fallen into the hole, mP is the Planck mass, and n is positive. We consider arguments for n=1/3 and for n=1. There is no causality violation or faster-than-light propagation. The existence of these objects alleviates the black-hole information paradox. More interestingly, these objects could have astrophysical and cosmological interest: they produce a detectable signal, of quantum gravitational origin, around the 10−14cm wavelength.

Translated into more comprehensible English? Information does not disappear in black holes, but is rather transformed along with the star into something fantastically dense.

[The authors'] key insight is that quantum gravitational effects prevent the universe from collapsing to infinite density. Instead, the universe ”bounces” when the energy density of matter reaches the Planck scale, the smallest possible size in physics.

That’s hugely significant. “The bounce does not happen when the universe is of planckian size, as was previously expected; it happens when the matter energy density reaches the Planck density,” they say. In other words, quantum gravity could become relevant when the volume of the universe is some 75 orders of magnitude larger than the Planck volume.

Rovelli and Vidotto say the same reasoning can be applied to a black hole. Instead of forming a singularity, the collapse of a star is eventually stopped by the same quantum pressure, a force that is similar to the one that prevents an electron falling into the nucleus of an atom. “We call a star in this phase a “Planck star”,” they say.

Planck stars would be small— stellar-mass black hole would form a Planck star about 10^-10 centimetres in diameter. But that’s still some 30 orders of magnitude larger than the Planck length.

An interesting question is whether these Planck stars would be stable throughout the life of the black hole that surrounds them. Rovelli and Vidotto have a fascinating answer. They say that the lifetime of a Planck star is extremely short, about the length of time it takes for light to travel across it.

But to an outside observer, Planck stars would appear to exist much longer. That’s because time slows down near high-density masses. For such an observer, a Planck star would last just as long as its parent black hole.

Universe Today's Brian Koberlein has a

__still more succinct summary__.

Key is the fact that the Planck star theory predicts that detectable gamma rays would be produced. This is a testable thesis.

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