Quantum gravity phenomenology:

Suddhasattwa Brahma, Fudan University: Deformation of classical spacetimes in loop quantum gravity

Quantum corrections appearing from loop quantum gravity (LQG) tend to deform the notion of covariance in the quantum theory leading to the emergence of non-Riemannian spacetimes. The main physical effect of this seems to be signature change with deeply interesting consequences. In this talk, I shall discuss how this might lead to constraining LQG from phenomenology by relating quantization ambiguities to observable quantities. Finally, how these deformations can be avoided (using certain quantization choices) shall also be briefly demonstrated.

Anuj Kumar Dubey, Physics Department, Assam University, Silchar , India: Gravitational Redshift in Kerr-Newman Geometry Using Gravity’s Rainbow

Gravitational redshift is generally reported by most of the authors without considering the influence of the energy of the test particle using various spacetime geometries such as Schwarzschild, Reissner-Nordstrom, Kerr and Kerr-Newman geometries for static, charged static, rotating and charged rotating objects respectively. In the present work, the general expression for the energy dependent gravitational redshift is derived for charged rotating body using the Kerr-Newman geometry along with the energy dependent gravity’s rainbow function. It is found that the gravitational redshift is influenced by the energy of the source or emitter. One may obtain greater correction in the value of gravitational redshift, using the high energy photons. Knowing the value of gravitational redshift from a high energy sources such as Gamma-ray Bursters (GRB), one may obtain the idea of upper bounds on the dimensionless rainbow function parameter (ξ). Also there may be a possibility to introduce a new physical scale of the order of ξ.

Aaron Held, Institute for Theoretical Physics, Heidelberg University: A weak-gravity bound from the matter content in asymptotic safety

Asymptotic safety conjectures a combined UV fixed-point for matter and gravity. I will present how the existence of an ultraviolet completion for interacting Standard-Model type matter can put constraints on the viable microscopic dynamics of asymptotically safe quantum gravity. A fundamental constraint -- the weak-gravity bound -- is rooted in the destruction of quantum scale-invariance in the matter system by strong quantum-gravity fluctuations. As the effective gravitational interaction strength grows quantum gravity induces new UV-divergences in the matter interactions. Beyond a critical strength of gravity, no Standard-Model type matter can be incorporated in a UV-complete model of asymptotic safety any longer.

Viqar Husain, University of New Brunswick: Low energy Lorentz violation from modified dispersion at high energies

Many quantum theories of gravity propose Lorentz violating dispersion relations of the form ω=|k|f(|k|/M), with recovery of approximate Lorentz invariance at energy scales much below M. We show that a quantum field with this dispersion predicts drastic low energy Lorentz violation in atoms modeled as Unruh-DeWitt detectors, for any f that dips below unity somewhere. This test may rule out all Lorentz violating theories. (Based on VH, J. Louko, PRL 116 (2016) 061301)

Niccolò Loret, University of Rome La Sapienza: k-Poincaré non-Riemannian properties

The approaches to the Quantum Gravity problem through space-time noncommutativity have found in the Hopf algebras a powerful tool (see for instance [arXiv:hep-th/0306013]) to formalize Planck-scale deformed space-time symmetries. The phenomenological implications of these deformations are still debated as the physical interpretation of many Hopf algebras features is unclear.
A few years ago, it was formulated the so called Relative Locality interpretation [arXiv:1101.0931] which formalizes the symmetry algebra deformation as an effect of some momentum-space curvature. From this point of view Hopf algebras could be re-expressed just as Riemannian geometry in momentum-space.
This idea, however, is not entirely accurate, and in this talk I will show how Hopf algebras actually hold several non-Riemannian (and apparently nonlocal) features. As a guidance I will use the simple k-Poincaré (bicrossproduct basis) model which was already employed to unravel the duality [arXiv:1305.5062] between the energy-dependent photon velocity in some Quantum Gravity phenomenology models and the redshift in an expanding space-time. In doing so, I will be able to trace-back the modified composition-law for momenta (due to nontrivial Hopf algebras coproducts), as well as apparent violations of the energy-momenta conservation laws, to the relations of distant observers in deSitter space-time.

Jakub Mielczarek, Jagiellonian University: Compact phase spaces and loops 

Recently, a new class of field theories called Nonlinear Field Space Theory (NFST) has been proposed. Within the approach, the standard field phase spaces are generalized to the non-affine manifolds. The compact case associated with finite dimensionality of Hilbert space is especially worth considering. Moreover, the spherical field phase space geometry reveals an interesting relation with the spin physics, leading to the so-called Spin-Field Correspondence. As an example, the duality is applied to the Heisenberg XXZ model, which turns out to be related with the Klein-Gordon field. Consequences of the correspondence, especially in the domain of cosmology are outlined. Furthermore, relationship between the spherical phase space under consideration and the polymer quantization techniques are discussed. Based on this, perspectives of generalizing (in the spirit of NFST) both LQC and LQG to the compact phase space case, are discussed.   

Christian Pfeifer, University of Tartu, Institute of Physics, Laboratory for Theoretical Physics: Covariant momentum dependent spacetime geometry: The kappa-Poincare dispersion relation on curved spacetimes

The kappa-Poincare dispersion relation is one of the most studied dispersion relation in the context of quantum gravity phenomenology. It emerges as Casimir operator of the kappa-quantum deformation of the Poincare algebra and can be connected to quantum gravity by identifying the deformation parameter kappa with the inverse of the Planck length. Phenomenological studies about the observable consequences on particle motion induced by the kappa-Poincare dispersion focused so far on maximally symmetric spacetimes. In this talk we will construct covariant kappa-deformations of generically curved spacetimes, which have the property that locally they look like Minkowski spacetime equipped with the kappa-Poincare dispersion relation. This can be understood as generalisation from local Lorentz invariant spacetimes used in general relativity to local kappa-Poincare invariant spacetimes. Having established the notion of local kappa-Poincare spacetimes we study the kappa-deformations of Schwarzschild and FLRW geometry including observable effects of the deformation on the photon sphere, the redshift and the time of arrival of photons.

Michele Ronco, Sapienza University of Rome and INFN: Spacetime-noncommutativity regime of Loop Quantum Gravity

A recent study by Bojowald and Paily provided a path toward the identification of an effective quantum-spacetime picture of loop quantum gravity, applicable in the “Minkowski regime,” the regime where the large-scale (coarse-grained) spacetime metric is flat. A pivotal role in the analysis is played by loop-quantum-gravity-based modifications to the hypersurface deformation algebra, which leave a trace in the Minkowski regime. We here show that the symmetry-algebra results reported by Bojowald and Paily are consistent with a description of spacetime in the Minkowski regime given in terms of the κ-Minkowski noncommutative spacetime, whose relevance for the study of the quantum-gravity problem had already been proposed for independent reasons.

Giacomo Rosati, Dipartimento di Fisica, UniversitĂ  di Cagliari & INFN, Sezione di Cagliari Cittadella Universitaria, 09042 Monserrato, Italy: Testing Planck-scale in-vacuo dispersion with gamma-ray-burst neutrinos and photons

The recent data on astrophysical neutrinos provided by the IceCube telescope offer a striking opportunity to test in vacuo dispersion of ultra-relativistic particles propagating in quantum spacetime scenarios inspired by phenomenological approaches to quantum gravity. Recently (Phys.Lett.B761(2016)318) we proposed a novel method of investigation of these effects based on a statistical analysis of the correlation between the energy (~O(100TeV)) of an observed neutrino and the difference between the time of observation of that neutrino and the trigger time of a GRB. We find a surprisingly high correlation, and estimate a very low probability for the effect to be produced accidentally by (still unknown) source features or background neutrinos. In a following study (arXiv:1612.02765), accepted for publication in Nature Astrophysics, we compare the neutrino analysis with a similar analysis for GRB photons (E~O(10GeV)), showing that the two features are roughly compatible with a description such that the same effects apply over four orders of magnitude in energy.

Tomasz Trześniewski, University of Wroclaw: LQG, the signature-changing Poincare algebra and spectral dimension

It has been observed that quantum corrections to the hypersurface deformation algebra (which describes local diffeomorphisms of spacetime), predicted within loop quantum gravity, may lead to the changing signature of spacetime metric. In the linearized case the (quantum-corrected) hypersurface deformation algebra becomes simply (a deformation of) the Poincare algebra of spacetime symmetries. By assuming certain reasonable conditions we obtained a specific form of the deformation, characterized by the signature change, and considered it as our toy model. We showed that, in the most natural Ansatz, such a deformed Poincare algebra can be extended by the standard Heisenberg algebra of phase space variables. Consequently, the model is characterized by deformed Lorentz transformations and an invariant energy scale, as well as a non-trivial invariant measure on momentum space. Moreover, we also studied a fictitious diffusion process on spacetime endowed with these deformed symmetries and calculated the corresponding spectral dimension. Depending on the deformation parameter, we either find the constant dimension of spacetime or the small-scale dimensional reduction to one. The latter result agrees with the ultralocal limit of the symmetry algebra and the asymptotic silence scenario from cosmology.