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In loop quantum gravity theory, a Planck star is a hypothetical astronomical object, theorized as a compact, exotic star, that exists within a black hole's event horizon, 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. The accumulation of mass-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 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, 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 might 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 extreme gravity such an object generates slow down time for the Planck star to a similarly extreme degree. Seen from outside the star's Schwartzschild radius, the rebound from a Planck star takes approximately fourteen billion years, such that even primordial black holes are only now starting to rebound from an outside perspective.[2] Furthermore, the emission of Hawking radiation can be calculated to correspond to the timescale of gravitational effects on time, such that the event horizon that 'forms' a black hole evaporates as the rebound proceeds.

The existence of Planck stars was first proposed by Carlo Rovelli and Francesca Vidotto, who theorized in 2014 that Planck stars form inside black holes[3] as a solution to the black hole firewall and black hole information paradox. Confirmation of emissions from rebounding black holes could possibly provide evidence for loop quantum gravity.[2] Recent work demonstrates that Planck stars may exist inside of black holes as part of a cycle between black hole to white hole. [4]

A somewhat analogous object theorized under string theory is the fuzzball, which similarly 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.
References

"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. S2CID 118917980.

Carlo Rovelli (10 December 2018). "Viewpoint: Black Hole Evolution Traced Out with Loop Quantum Gravity". Retrieved 11 December 2018.

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Stars
Formation

Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track

Evolution

Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]

Remnants

White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster

Hypothetical

Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

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Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova

Structure

Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
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Properties

Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system

Earth-centric
observations

Sun
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Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
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Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

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Black holes
Types

Schwarzschild Rotating Charged Virtual Kugelblitz Primordial Planck particle


Black hole - Messier 87 crop max res.jpg
Size

Micro
Extremal Electron Stellar
Microquasar Intermediate-mass Supermassive
Active galactic nucleus Quasar Blazar

Formation

Stellar evolution Gravitational collapse Neutron star
Related links Tolman–Oppenheimer–Volkoff limit White dwarf
Related links Supernova
Related links Hypernova Gamma-ray burst Binary black hole

Properties

Gravitational singularity
Ring singularity Theorems Event horizon Photon sphere Innermost stable circular orbit Ergosphere
Penrose process Blandford–Znajek process Accretion disk Hawking radiation Gravitational lens Bondi accretion M–sigma relation Quasi-periodic oscillation Thermodynamics
Immirzi parameter Schwarzschild radius Spaghettification

Issues

Black hole complementarity Information paradox Cosmic censorship ER=EPR Final parsec problem Firewall (physics) Holographic principle No-hair theorem

Metrics

Schwarzschild (Derivation) Kerr Reissner–Nordström Kerr–Newman Hayward

Alternatives

Nonsingular black hole models Black star Dark star Dark-energy star Gravastar Magnetospheric eternally collapsing object Planck star Q star Fuzzball

Analogs

Optical black hole Sonic black hole

Lists

Black holes Most massive Nearest Quasars Microquasars

Related

Black Hole Initiative Black hole starship Compact star Exotic star
Quark star Preon star Gamma-ray burst progenitors Gravity well Hypercompact stellar system Membrane paradigm Naked singularity Quasi-star Rossi X-ray Timing Explorer Timeline of black hole physics White hole Wormhole

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