## About this Research Topic

Potential observables of quantum-gravitational effects on the dynamics of general relativistic astrophysical ultra-compact objects would be a breakthrough, which can head off into pathways to building a sound quantum theory of gravity. If the field theory's current view on how nature works is correct, any dynamically dominant quantum effect on a background gravitational field should manifest as an inextricable modification of the physical properties of spacetime itself. So far, we have been unable to uncover new hints from nature (e.g., understanding the absence of right-handed neutrinos would be huge step forward, an issue related to neutrinos being either Majorana or Dirac particles), so that attempting to quantize general relativity along the same lines as in the Standard Model is a blocked road, since the theory is non-renormalizable within the perturbation theory scheme. Hence, resorting to observations of astrophysical sources seems the due task. This might constitute a nonpareil laboratory – like that provided by the early Universe – to unveil measurable quantum-gravitational effects and helping to pave the road to quantum gravity.

To formulate a sound theory of quantum gravity, an issue to cope with is the general relativity’s background independence – a property that, if overlooked, can lead us astray. The fundamental reason is that in quantum field theory we are used to considering that the fields live on a fixed background, whilst the equations of general relativity rather determine the background itself. Loop Quantum Gravity gets past this constraint by recasting the gravitational field dynamics in a particular fixed background from the perspective of tiny (~ħ) changes to its shape, asseverating en passant that the spacetime structure is interwoven by quantum threads, a spin network that builds up a sort of spin foam. For Schwarzschild spacetime the loop quantum fluctuations’ coherent states break down its spherical symmetry. Alternatively, the Asymptotic Safety approach takes the spacetime metric field as the dynamical variable, satisfying diffeomorphism invariance symmetry. The Wilson's functional renormalization group -governing the dependence on the energy scale of the average effective action- is employed to find non-perturbative stable fixed points in the space of couplings of general gravity theories (this approach preserves spacetime symmetry). Evidence of such points have been widely reported, of which is worth mentioning the impressive result from applying the asymptotic safety of gravity techniques to the Standard Model for predicting the Higgs boson mass with remarkable accuracy.

This Research Topic welcomes both experimental and theoretical reviews, and unpublished research articles addressing the potential observability of quantum-gravitational realizations in the dynamics of relativistic ultra-compact astrophysical systems, and related fields such astroparticle physics and astrophysical cosmology.

Themes which can be covered include:

• Recent and new advances connecting quantum gravity and relativistic astrophysics

• Radiation processes during continual gravitational collapse

• Gravity-spin coupling in outgoing particles from collapsing star cores

• Catastrophic phase transitions in neutron stars and potential observables of quantum gravity

• Quantum gravity breach of the strong equivalence principle in relativistic astrophysical systems

• Quantum gravity effects in galactic black hole candidates

• Active galactic nuclei: quantum-gravitational effects in accretion disks and jet polarization

• Gravitational waves as carriers of signatures of quantum gravity: violation of Lorentz symmetry, polarization modes from extended theories of gravity (include longitudinal modes, overtones, etc.), phenomenon of birefringence, pseudoscalar configurations violating parity, etc.

• Table-top experiments to search for the quantum nature of gravity

• Foundations of quantum gravity

**Keywords**:
quantum gravity, relativistic astrophysics, continual gravitational collapse, radiation processes, ultra-compact stars, black holes, neutron or quark stars, galactic black hole candidates, active galactic nuclei, accretion disks, jet polarization

**Important Note**:
All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.

Potential observables of quantum-gravitational effects on the dynamics of general relativistic astrophysical ultra-compact objects would be a breakthrough, which can head off into pathways to building a sound quantum theory of gravity. If the field theory's current view on how nature works is correct, any dynamically dominant quantum effect on a background gravitational field should manifest as an inextricable modification of the physical properties of spacetime itself. So far, we have been unable to uncover new hints from nature (e.g., understanding the absence of right-handed neutrinos would be huge step forward, an issue related to neutrinos being either Majorana or Dirac particles), so that attempting to quantize general relativity along the same lines as in the Standard Model is a blocked road, since the theory is non-renormalizable within the perturbation theory scheme. Hence, resorting to observations of astrophysical sources seems the due task. This might constitute a nonpareil laboratory – like that provided by the early Universe – to unveil measurable quantum-gravitational effects and helping to pave the road to quantum gravity.

To formulate a sound theory of quantum gravity, an issue to cope with is the general relativity’s background independence – a property that, if overlooked, can lead us astray. The fundamental reason is that in quantum field theory we are used to considering that the fields live on a fixed background, whilst the equations of general relativity rather determine the background itself. Loop Quantum Gravity gets past this constraint by recasting the gravitational field dynamics in a particular fixed background from the perspective of tiny (~ħ) changes to its shape, asseverating en passant that the spacetime structure is interwoven by quantum threads, a spin network that builds up a sort of spin foam. For Schwarzschild spacetime the loop quantum fluctuations’ coherent states break down its spherical symmetry. Alternatively, the Asymptotic Safety approach takes the spacetime metric field as the dynamical variable, satisfying diffeomorphism invariance symmetry. The Wilson's functional renormalization group -governing the dependence on the energy scale of the average effective action- is employed to find non-perturbative stable fixed points in the space of couplings of general gravity theories (this approach preserves spacetime symmetry). Evidence of such points have been widely reported, of which is worth mentioning the impressive result from applying the asymptotic safety of gravity techniques to the Standard Model for predicting the Higgs boson mass with remarkable accuracy.

This Research Topic welcomes both experimental and theoretical reviews, and unpublished research articles addressing the potential observability of quantum-gravitational realizations in the dynamics of relativistic ultra-compact astrophysical systems, and related fields such astroparticle physics and astrophysical cosmology.

Themes which can be covered include:

• Recent and new advances connecting quantum gravity and relativistic astrophysics

• Radiation processes during continual gravitational collapse

• Gravity-spin coupling in outgoing particles from collapsing star cores

• Catastrophic phase transitions in neutron stars and potential observables of quantum gravity

• Quantum gravity breach of the strong equivalence principle in relativistic astrophysical systems

• Quantum gravity effects in galactic black hole candidates

• Active galactic nuclei: quantum-gravitational effects in accretion disks and jet polarization

• Gravitational waves as carriers of signatures of quantum gravity: violation of Lorentz symmetry, polarization modes from extended theories of gravity (include longitudinal modes, overtones, etc.), phenomenon of birefringence, pseudoscalar configurations violating parity, etc.

• Table-top experiments to search for the quantum nature of gravity

• Foundations of quantum gravity

**Keywords**:
quantum gravity, relativistic astrophysics, continual gravitational collapse, radiation processes, ultra-compact stars, black holes, neutron or quark stars, galactic black hole candidates, active galactic nuclei, accretion disks, jet polarization

**Important Note**:
All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.