Location: Park 3.01

C5 - Wednesday 14:00-15:40 (Mairi Sakellariadou)

Constraining the Temperature of Astrophysical Black Holes through Ringdown Detection


Adrian, Ka Wai Chung (Theoretical Particle Physics & Cosmology Research Group. Department of Physics, King’s College London)

Quantum gravity predicts that a black hole of mass $ M $ radiates at a low temperature of $ sim 10^{-8} frac{M_{odot}}{M} rm K $.
Nevertheless, such low temperature has been conceived to be unobservable for astrophysical black holes.
Therefore, experimental studies of black-hole radiation have been largely confined to analogous black holes.
Here we propose a novel method to measure the temperature of astrophysical black holes through detecting their ringdown phases, without assuming a specific dependence of the temperature on the mass and spin of black hole.
In this talk, I will present the constraints on the temperature of the remnant of detected gravitational-wave events.
To our best knowledge, we obtain the first constraints of the temperature of astrophysical black holes.
Our constraints of black-hole temperature are consistent with the current prediction by quantum gravity and does not provide evidence of possible quantum-gravity signatures.
Our work suggests that gravitational-wave detection can constrain the strength of quantum-gravity effects of astrophysical black holes.

Predictions for the universe from unimodular quantum gravity


Natascha Riahi (Faculty of Physics, University of Vienna)

Unimodular gravity represents a theory practically equivalent to General Relativity, but with a different
canonical structure. For the special case of reduced models any constraint is absent and therefore a time-evolution emerges
naturally for unimodular quantum cosmology.
We investigate the results of quantization of homogeneous and isotropic models
in the framework of unimodular theory. We present the conditions for a late-time classical time evolution.
Further we discuss possible quantum corrections to the scale factor and finally examine the stochastic geodesic equation
for past light cones in order to draw conclusions regarding the horizon problem.

Extra dimension of zero length in gravity theory


Sandipan Sengupta (Physics, IIT Kharagpur)

A dynamical theory of vacuum gravity based on an extra dimension of vanishing proper length is introduced and explored. The natural framework where this could
be done is provided by the degenerate tetrad solutions of first order gravity theory in five dimensions. From the most general such solution of the field equations as obtained, an emergent Einsteinian theory in four dimensions is recovered. Based on the associated nonpropagating fields, we set up a purely geometric model of the galactic `halo’ and propose a potential resolution to the `dark matter’ problem. (Reference: arXiv:1908.04830 [gr-qc], S. Sengupta, 2019)

C6 - Wednesday 16:10-17:50 (Mairi Sakellariadou)

Einstein-Dirac solitons: fermion self-trapping & evolution


Peter Leith (School of Physics & Astronomy, University of St Andrews, UK)

Einstein-Dirac solitons, first introduced by Finster, Smoller & Yau in [1], are spherically symmetric, static, bound-state solutions of the coupled Einstein-Dirac equations. These particle-like, Planck-scale objects consist of neutral fermions bound together solely by their gravitational attraction, and prevented from collapse by the uncertainty principle. Since the Einstein-Dirac system is not fully quantum (in the sense that Quantum Field Theory is not involved), solutions are readily tractable, and importantly the back-reaction of matter on the space-time metric is not neglected. A study of such systems could provide indications as to how to proceed to a working theory of Quantum Gravity.

Analysis of these objects has hitherto been confined to solutions with only small numbers of particles and a relatively low central compression. Our work extends such analysis to systems involving large ensembles of particles, with high central redshift, for which the effect of back-reaction becomes evident in the form of fermion self-trapping. For these solutions, the resulting spacetime becomes extremely compact, and a region containing a series of circular null geodesics (photon spheres) appears, characterised by ‘bottlenecks’ in the spacetime optical geometry. Within this region, the constituent fermions appear to behave like massless particles, with their wavefunctions exhibiting a series of well-defined peaks centred around these bottlenecks. A shell-like structure therefore arises for the bound state, consisting of spatially separated, trapped ‘pockets’ of fermions.

A time-dependent dynamical analysis of the evolution of Einstein-Dirac solitons is also presented for the first time. While low-redshift solutions are found to be stable, numerical simulations suggest that the unstable high-redshift solutions can have two distinct fates – gravitational collapse to form a black hole type object, or a supernova-type explosion, in which fermionic matter is ejected to infinity. Einstein-Dirac solitons appear therefore to exhibit similar behaviour to astrophysical objects, but on the Planck scale.

[1] Felix Finster, Joel Smoller, and Shing-Tung Yau. Particlelike solutions of the Einstein-Dirac equations. Physical Review D, 59(10):104020, 1999.

Slow-roll inflation and the swampland


Kunihito Uzawa (Kwansei Gakuin University)

In this talk, we show how the swampland conjecture, which has recently been attracting attention in string theory and gravity theory, relates to the slow-roll condition of inflation. First, we will briefly explain the reason why we focus on the swampland conjecture to investigate the dynamics of inflation. Next, we discuss the behavior of the scalar field describing the evolution of the inflationary scenario from the viewpoint of the swampland conjecture. Finally, we present the implications of energy conditions on cosmological compactification solutions of the higher-dimensional Einstein field equations, and show the relation between the slow-roll condition in the inflationary scenario and the swampland criterion.

C7 - Thursday 14:00-15:40 (Mairi Sakellariadou)

Shadows without singularity


Aaron Held (Institute for Theoretical Physics, Heidelberg University)

Black-holes in GR are unphysical objects with singularities. Resolving the singularity requires a repulsive force. With a minimal set of assumptions, the generic impact of singularity-resolving physics on both non-spinning and spinning black-hole shadows will be presented. Singularity resolution tied to local curvature scales generically shrinks the shadow size and imprints characteristic features on the shadow-shape of spinning black holes. Moreover, we find that the ground-breaking observations of the EHT are not yet able to exclude horizonless objects. This leaves open a wide parameter space of unexpected but admissible modified gravitational dynamics to resolve the singularity. Many different quantum-gravity models result in structurally similar such quantum-improved and singularity-free black-hole spacetimes.

Perturbations of a quantum cosmological spacetime


Artur Miroszewski (National Centre for Nuclear Research)

With the use of canonical General Relativity and coherent state quantisation procedure one obtains a quantum model of spacetime. For cosmological spacetimes such models produce singularity avoidance scenarios, resulting in a smooth evolution of the universe undergoing the Big Bounce. Introducing quantum perturbations to the quantum cosmological spacetime allows to consider interesting observables such as primordial amplitude spectra, and to study how they are influenced by genuinely quantum properties of the cosmological background. Discussed theoretical effects may in principle probe the quantum era of the universe and should be further developed.

Hawking radiation in view of Generalized and Extended Uncertainty Principles


Mariusz Dabrowski (Institute of Physics, University of Szczecin)

I will first discuss the impact of Generalized Uncertainty Principle (GUP) onto the Bekenstein entropy and the Hawking temperature and show how GUP influences the Hawking radiation leaving a remnant of a radiating black hole. I will also show that when GUP is applied, the Hawking radiation does not necessarily have to be sparse when a black hole approaches the Planck mass. In the second part of my talk, I will present the influence of Extended Uncertainty Principle (EUP) on the Bekenstein entropy and the Hawking radiation for Rindler and cosmological spacetimes. I will also comment on the possibility to bound the GUP and EUP parameters from cosmological data.

C8 - Thursday 16:10-17:50 (Mairi Sakellariadou)

Hadamard renormalisation for charged scalar fields


Visakan Balakumar (School of Mathematics and Statistics, University of Sheffield)

The Hadamard renormalisation method provides a powerful and axiomatic approach to renormalising the stress-energy tensor in the study of quantum fields in curved spacetime. This procedure has been developed by Decanini and Folacci for massive neutral scalar fields in a general spacetime of arbitrary dimension. Motivated by the study of superradiant scattering in Reissner–Nordström black holes, we extend their work to include charged scalar fields in spacetimes with a classical, background gauge field and explicitly demonstrate the Hadamard renormalisation procedure in four dimensions.

Recent results on LIV studies using the MAGIC telescopes


Tomislav Terzić (Department of Physics, University of Rijeka)

Certain candidates for the theory of quantum gravity predict a non-trivial dispersion relation for photons, an effect dubbed Lorentz Invariance Violation (LIV). As a consequence, the speed of photons may depend on their energy. This tiny effect might be accumulated for very high energy (VHE, E > 100 GeV) photons crossing cosmological distances, resulting in a different time-of-flight (ToF) compared to low energy photons, presumably emitted simultaneously at the source. Using a flare from the blazar Mrk 421 detected by the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes in 2014, we set the strongest limits so far on the quadratic correction for the dispersion relation of the photon using the ToF method.
Moreover, the MAGIC Collaboration recently observed for the first time photons above 200 GeV from the gamma-ray burst GRB 190114C. This is by far the highest energy detected from a GRB. Results of a LIV study on these data will be presented as well.