Planck stars (1401.6562v4)

Abstract: 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 $(m/m_{\scriptscriptstyle P})^n$, where $m$ is the mass fallen into the hole, $m_{\scriptscriptstyle P}$ 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 $10^{-14} cm$ wavelength.

• The paper introduces Planck stars as high-density phases where quantum gravitational pressure counteracts collapse, preventing singularities in black holes.
• The paper derives scaling relations (with exponents 1/3 and 1) to predict star sizes and suggests observable signatures from Hawking evaporation in the GeV range.
• The paper proposes that Planck stars offer a viable resolution to the black hole information paradox by allowing stored information to be released without singularity formation.

Review of "Planck Stars" by Carlo Rovelli and Francesca Vidotto

The paper "Planck Stars" by Carlo Rovelli and Francesca Vidotto proposes a compelling extension to the conventional understanding of gravitational collapse and black hole physics by introducing the concept of "Planck stars." It addresses one of the vexing problems of theoretical physics: the fate of information in black holes as they evaporate due to Hawking radiation.

Key Contributions and Theoretical Insights

Rovelli and Vidotto introduce the concept of Planck stars as a phase in the lifecycle of a gravitationally collapsed star where quantum gravitational effects counteract gravitational collapse. Unlike conventional end-states of black holes, which are thought to shrink to Planckian sizes, Planck stars are posited to be considerably larger during this phase. The researchers argue that quantum gravitational pressure can counteract gravitational forces at energy densities reaching the Planck scale, before the object reaches Planckian dimensions. They provide arguments for scaling the star size using factors $(m/m_{\scriptscriptstyle P})^n$, with $n = 1/3$ and $n = 1$, where $m$ is the mass of the object.

This framework offers a potential solution to the black hole information paradox by postulating that the gravitational collapse leads to a phase of high-density but non-singular compressed core, avoiding singularities and permitting information storage until its release back into the universe.

Implications for Astrophysics and Quantum Gravity

The existence of Planck stars could have profound astrophysical and cosmological implications. The release of information once the Hawking radiation has caused the horizons to meet presents a non-singular resolution to the fate of black hole information, aligning with certain entropy bounds while adhering to principles derived from loop quantum gravity. The proposition also suggests observational prospects, notably through potential signals detectable from primordial black holes in the late stages of Hawking evaporation, manifesting as a phenomenon of “quantum gravity in the sky” at wavelengths around $10^{-14}$ cm.

Moreover, Rovelli and Vidotto's work hints at an observable bridge across the large gap between Planckian and cosmological scales, predicting that the Planck star's internal quantum effects could lead to a detectable signal in the GeV range. This positions Planck stars as potentially verifiable objects via cosmic observations, offering experimental windows into the elusive domain of quantum gravity.

Considerations and Future Directions

While the Planck stars model offers intriguing solutions to ongoing theoretical challenges, several aspects necessitate further exploration. The dynamics of their formation, stability, and decay warrant detailed investigation, particularly through the lens of competing quantum gravity theories. Future work might focus on refining the internal structure of Planck stars, exploring their potential observational signatures, and developing their implications within the broader context of loop quantum gravity and alternative cosmological models.

In summary, the paper "Planck Stars" provides a significant theoretical advancement in quantum gravity research, proposing a plausible resolution to the black hole information paradox and setting the stage for potential observational tests for phenomena long shielded from empirical scrutiny. Its propositions offer pathways to bridge conceptual gaps between quantum mechanics and general relativity, contributing to the evolving dialogue concerning the deep structure of spacetime.