In loop quantum gravity, A Planck star is a hypothetical astronomical object that is created when the energy density of a collapsing star reaches the Planck energy density. Under these conditions, assuming gravity and spacetime are quantized, there arises a repulsive 'force' derived from Heisenberg's uncertainty principle. Namely, if gravity and spacetime are quantized, the accumulation of mass-energy inside the Planck star cannot collapse beyond this limit because it violates the uncertainty principle for spacetime itself.
The key feature of this theoretical object is that this repulsion arises from the energy density, not the Planck length, and starts taking effect far earlier than might be expected. This repulsive 'force' is strong enough to stop the collapse of the star well before a singularity is formed, and indeed, well before the Planck scale for distance. Since a Planck star is calculated to be considerably larger than the Planck scale for distance, this means there is adequate room for all the information captured inside of a black hole to be encoded in the star, thus avoiding information loss.
While it would be expected that such a repulsion would act very quickly to reverse the collapse of a star, it turns out that the relativistic effects of the very extreme gravity such an object generates would slow down time for the Planck star to a similarly extreme degree. Seen from outside, the rebound from a Planck star of stellar mass would take approximately 14 billion years, such that even primordial black holes would be only now starting to rebound from an outside perspective.  Furthernomore, the emission of Hawking radiation can be calculated to correspond handily to the timescale of the gravity effects on time, such that the event horizon that 'forms' a black hole naturally evaporates as the rebound proceeds.
The idea arose in 2014 when Carlo Rovelli and Francesca Vidotto proposed that Planck stars are naturally formed inside black holes. The theory, if correct, would resolve the black hole firewall and black hole information paradox as well as possibly provide evidence for loop quantum gravity in the form of emissions from rebounding black holes. Recent work also demonstrates that planck stars may exist inside of black holes as part of a cycle between black hole to white hole.
- "New Type of Star Emerges From Inside Black Holes". Physics arXiv Blog. 5 February 2014. Retrieved 8 September 2018.
- Resonance Science Foundation Research Team. "Planck Stars: Quantum gravity research ventures beyond the event horizon". Retrieved 10 December 2018.
- Rovelli, Carlo; Vidotto, Francesca (2014). "Planck stars". International Journal of Modern Physics D. 23 (12): 1442026. arXiv:1401.6562. Bibcode:2014IJMPD..2342026R. doi:10.1142/S0218271814420267.
- Carlo Rovelli (10 December 2018). "Viewpoint: Black Hole Evolution Traced Out with Loop Quantum Gravity". Retrieved 11 December 2018.