Inloop quantum gravity theory, aPlanck star is a hypotheticalastronomical object, theorized as acompact,exotic star, that exists within ablack hole'sevent horizon, created when theenergy density of acollapsing star reaches thePlanck energy density. Under these conditions, assuminggravity andspacetime arequantized, a repulsive "force" arises fromHeisenberg'suncertainty principle. The accumulation ofmass–energy inside the Planck star cannot collapse beyond this limit because it violates the uncertainty principle for spacetime itself.^[1]

The key feature of this theoretical object is that this repulsion arises from theenergy density, not thePlanck length, and starts taking effect far earlier than might be expected. This repulsive "force" is strong enough to stop the star's collapse well before asingularity is formed and, indeed, well before the Planck scale for distance: for a stellar mass black hole the Planck star would be of the order of10⁻¹² m - for a primordial black hole, the order of10⁻¹⁶ m.^[2] whilst tiny, these scales are many orders of magnitude larger than the Planck length of10⁻³⁵ m. Then too, this allows adequate room for all the information captured inside ablack hole to be encoded in the star, thus avoidinginformation loss.^[1]

While it might be expected that such a repulsion would act very quickly to reverse the collapse of a star, it turns out that therelativistic effects of the extreme gravity of such an object slow down time for the Planck star to a similarly extreme degree. Seen from outside the star'sSchwarzschild radius, the rebound from a Planck star takes approximately fourteen billion years, such that evenprimordial black holes are only now starting to rebound from an outside perspective. Furthermore, the emission ofHawking radiation can be calculated to correspond to the timescale ofgravitational effects on time,^{[clarification needed]} such that the event horizon that "forms" a black holeevaporates as the rebound proceeds.^[1]

Carlo Rovelli andFrancesca Vidotto, who first proposed the existence of Planck stars, theorized in 2014 that Planck stars form inside black holes^[3] as a solution to theblack hole firewall and theblack hole information paradox. Confirmation of emissions from rebounding black holes could provide evidence forloop quantum gravity. Recent work demonstrates that Planck stars may exist insideblack holes as part of a cycle between black andwhite holes.^{[clarification needed][4]}

A somewhat analogous object theorized understring theory is thefuzzball, whichsimilarly eliminates the singularity within a black hole and accounts for a way to preserve the quantum information that falls into a black hole's event horizon.

• List of nearest black holes
• Supermassive black hole
• Intermediate-mass black hole
• Stellar black hole
• Micro black hole
• Neutron star

• Star types
• Hypothetical stars
• Black holes

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• Wikipedia articles needing clarification from April 2019