Line of research overview

Quantum gravity phenomenology

Principal Investigator: Stefano Liberati

Abstract: The structure of spacetime at the Planck scale may lead to a breakdown (or modification) of Lorentz invariance at high energies in the form of nonlinear dispersion relations for the fundamental particles. In spite of the fact that the extra Lorentz-breaking terms of power higher than 2 in the momentum are suppressed by inverse powers of the Planck scale, it is possible to observe their effect in relatively low energy interactions (w.r.t. the Planck energy) when these involve thresholds, cumulative effects (such as time delay between photons of different frequencies) or are sensitive to the possible existence of a maximal group velocity for the elementary particles (synchrotron effect). An alternative scenario concerns the possibility that Lorentz invariance is conserved but realising the notion of locality. This is what is for example suggested in quantum gravity models such as String Field theory or Causal Set theory where the standard D’Alambertian appearing in the Klein-Gordon equation for a scalar field is replaced by some function of these operator. Testing this kind of physics is more subtle and requires precision quantum experiments rather than high energies. Finally, quantum gravity effect are supposed to be determinant in resolving singularities at the Big Bang or inside black holes. This resolution can lead to new phenomenology such as bouncing solutions, regular black holes and perhaps even ultra-compact objects. Using multi-messenger astrophysics (in particular GW astrophysics) can then open a window to test these deviations from the standard GR scenario.

Status of project and perspectives: The group has made substantial progress in assessing possible families of regular black holes associated to the regularisation by quantum gravity of the inner singularities of black holes. Among such solutions, we have shown a generic instability of the inner horizon of some regular black hole metrics. We have also extended to the rotating case these and other solutions and discussed the phenomenology associated to black holes with wormhole like interiors (in particular superradiance and quasi-normal modes) or more general Kerr-black hole mimickers. We have produced several phenomenological papers concerning possible observational signatures of these metric alternatives to GR black holes and intend to further pursue these studies in the next future. Also, we have parallelly worked on the possible geometrical implementation in curved spacetime of deformed special relativistic symmetries as a possible remnant of quantum spacetime, considering in this setting also possible phenomenological tests and black hole properties.