The 2nd workshop on "Singularities of general relativity and their quantum fate"

21.05.2018 - 25.05.2018 | Warsaw


Agnieszka Janiuk, Astrophysical Sources of Gravitational Radiation

Abstract: In this talk, I will discuss the possible origin, cosmic environment, and fate of the binary compact objects responsible for gravitational wave radiation detected so far. First, I will present the LIGO assembly of binary black holes that was established since the discovery of GW150914. These systems contain very massive black holes, whose origin poses a puzzle for the stellar evolution models. One of the possibilities is a process of direct collapse of massive stars. The feedback from a rotationally supported innermost parts of the star during the collapse may however help unbind the outer layers and halt accretion. This will have a consequence for both the ultimate mass of the black hole, and its resultant spin. I will also discuss the possible electromagnetic counterparts of the gravitational wave sources. The binary neutron star merger GW170817 was connected with the gamma ray emission observed as a weak short burst, which peculiar properties pose constraints for its progenitor model. Moreover, at lower energies the follow-up surveys have shown the presence of a kilonova emission from the merger’s dynamical ejecta. These ejecta masses are broadly consistent with the estimated r-process production rate, required before to explain the Milky Way isotopes abundances. It is possible that the magnetically driven winds launched due to the accretion in the GRB central ngine may also contribute to the kilonova emission from NS-NS merger. Finally, the electromagnetic signal is in general not expected from a BH-BH merger. However, the wear transient detected by Fermi GBM detector 0.4 seconds after GW 150914 has been generating much speculation. I will briefly present the scenarios that aimed to account for such a coincident signal.

Albin Nilsson, Toward Quantum Gravity Phenomenology

Abstract: Since Lorentz symmetry is a continuous symmetry there is no a priori reason to expect it to hold in the quantum realm. Therefore it is an excellent probe of new physics. In my talk, I will outline some modiffications of general relativity in which Lorentz symmetry is a low-energy emergent symmetry rather than a fundamental constituent of the theory. I will also present some possible observational consequences of such models which could be used to ascertain their validity. Moreover, I will discuss some work done in determining the precise energy scale of symmetry breaking, which is important for phenomenological studies. Finally, I will describe a way to search for Lorentz symmetry breaking in a way which is independent of the cosmological model.

Alex Nielsen, Black hole echoes in gravitational wave data

Abstract: In some models of quantum gravity, non-trivial structure can form at or near black hole horizons. Recent detections of merging black holes now allow observational tests of the nature of these objects and open the possibility of experimental evidence for such structure. A number of different models with different motivations have been proposed, but a generic possibility is that long wavelength gravitational wave signals could reflect off of such structure, giving rise to so-called echo signals. Several groups have searched the LIGO gravitational wave data for echo signals, with different outcomes, and I will critically compare these different results. If these echo signals were found they would provide very real observational data of quantum gravity.

Alexander Kamenshchik, Singularity crossing in classical and quantum cosmology.

We investigate particular cosmological models, based either on tachyon fieldsor on perfect fluids, for which soft future singularities arise in a natural way. Our main result is the description of a smooth crossing of the soft singularity in models with an anti-Chaplygin gas or with a particular tachyon field in the presence of dust. Such a crossing is made possible by certain transformations of matter properties. We compare also different approaches to the problem of crossing of the Big Bang – Big Crunch singularities in isotropic and anisotropic cosmological models. The possible applications of quantum cosmology to the singularity problem are also discussed.

Ana Alonso-Serrano, Black holes information flux

Abstract: We have studied the entropy budget per particle emitted in black hole evaporation process. In order to carry out this calculation we adopted a variant of the “average subsystem” approach, but consider a tripartite pure system that includes the influence of the rest of the universe. If the process is unitary, the entropy is exactly compensated by the “hidden information” in the correlations that we choose not to consider within the specific selected coarse-graining. In addition, the entropy budget should be corrected at the last stages of evaporation, due to quantum gravity effects. We have been shown recently how these effects (expressed in terms of the generalized uncertainty principle) modify the Hawking flux when we approach the Planck size.

Andrew Hamilton, Inflation is followed by BKL collapse in accreting, rotating black holes

Abstract: In 1970 Belinskii, Khalatnikov and Lifshitz argued that the generic outcome of collapse to a spacelike singularity would be chaotic and oscillatory. The conclusion was called into question with Poisson & Israel's 1990 discovery of the mass inflation instability at the inner horizon of a rotating black hole. It has been widely claimed that the generic outcome of mass infation is a weak null singularity. However, this claim is premised on a black hole that remains forever isolated. Real astronomical black holes accrete. Nonlinear general relativistic numerical calculations show that for a black hole that accretes, infation is followed by BKL collapse. This talk will be illustrated by real-time interactive visualizations with the Black Hole Flight Simulator.

Andrzej Góźdź, BKL scenario and its quantization


Artur Miroszewski, Phase space formulation of quantum mechanics:

coherent states and coherent spaces

Abstract: In my talk I will focus on introducing a phase space formulation of quantum mechanics based on unitary irreducible representations of classical phase spaces. The main ingredient of the new formulation is an extension of coherent states in such a way that the fiducial vectors can dynamically evolve in the so called coherent space. In principle, the formulation is equivalent to the Hilbert space formulation, but allows for consistent truncations to reduced phase spaces in which approximate quantum motions can be derived. The approach can be very useful in the domain of quantum cosmology and therefore, the cosmological phase spaces will be introduced to provide new non-trivial results.

Aurélien Barrau, Loop Quantum Gravity and observations

Abstract: Loop quantum gravity is a non-perturbative quantization of General Relativity which is free of singularity and background-invariant. As for all other tentative theories of quantum gravity, the major challenge is the link with experiments. In this talk I will focus on recent developments on the black hole sector and show that there are some promising attempts to predict clear signals. I will also mention some recent results on the cosmological side.

Claes Uggla, Spikes in perturbative cosmology

Abstract: We investigate permanent spikes created by that a subset of worldlines has an initial quasi-isotropic singularity while neighboring worldlines have a vacuum dominated behavior toward the initial singularity. This is done by using approximations based on the conformal Hubble-normalized state space approach to Einstein’s equations and comparing with results obtained in standard cosmological perturbation theory. These approximate results are furthermore compared with exact permanent spike results obtained from some explicit solutions.

Daniele Oriti, Multiple possible resolutions of the cosmological singularity in quantum gravity

Abstract: We illustrate the general ideas and recent results in group field theory condensate cosmology, a framework in which cosmological dynamics emerges as the hydrodynamics of the microscopic quantum gravity degrees of freedom. We then focus on the fate of the classical cosmological singularity, in this context, and discuss two ways in which it may be resolved. We also discuss some conceptual and physical issues associated with this resolution and with the whole framework.

Edward Wilson-Ewing, The loop quantum cosmology bounce as a Kasner transition

Abstract: For the Bianchi type I space-time (vacuum or with a massless scalar field), the loop quantum cosmology bounce can be viewed as a rapid transition between two classical solutions, with a simple transformation rule relating the Kasner exponents of the two epochs. This transformation rule can be extended to other Bianchi space-times under the assumption that during the loop quantum cosmology bounce the contribution of the spatial curvature to the Hamiltonian constraint is negligible compared to the kinetic terms. For the vacuum Bianchi type IX space-time there are transformation rules for how each of the parameters characterizing the Kasner epochs change during the bounce. This provides a quantum gravity extension to the Mixmaster dynamics of general relativity, and may have interesting implications for the Belinski-Khalatnikov-Lifshitz conjecture.

Eugenio Bianchi, White Holes as Remnants: A Surprising Scenario for the End of a Black Hole

Abstract: In the fnal stage of black hole evaporation, when the black hole has shrunk down to a Planck mass, the probability to tunnel into a white hole is no longer suppressed. In this talk I show that this transition results into a long-lived compact object with a large interior. Recent results on singularity resolution in quantum gravity imply that the lifetime of the white hole and the volume of its interior depend on the age of the parent black hole. These features of white-hole remnants provide a new scenario for the resolution of the black- hole information paradox, as argued in arXiv:1802.04264 in collaboration with Christodoulou, D'Ambrosio, Haggard, and Rovelli.

Ewa Czuchry, Integrable Toda system as a novel approximation to the Mixmaster

Abstract: A completely new approach to the study of the dynamics of the Mixmaster universe is presented. Bianchi IX anisotropy potential is approximated by the explicitly integrable periodic 3-particle Toda system. It is shown that the covariant Weyl-Heisenberg integral quantization naturally amplifies the dynamical role of the underlying Toda system by smoothing out the three canyons of the anisotropy potential. Since the relevant eigenfunctions can be constructed, those finding paves the way to a novel perturbative approach to the evidently non-integrable quantum Mixmaster dynamics.

Francesca Vidotto, Planck Stars: the final fate of quantum non-singular black holes and its phenomenological consequences

Abstract: Loop Quantum Gravity presents a spacetime quantum discreteness that devoids the theory from any curvature singularity. The resulting dynamics can be understood in terms of an effective quantum potential that stops the collapse into the central black-hole singularity, then replaced by maximal energy-density region evocatively called a "Planck Star". The collapsing region may undergo quantum fluctuations of the metric such that a black hole turns into an expanding region, i.e. a white hole. In the effective theory, the Planck Star connects these two phases of the hole life. I discuss the results of such a transition: an explosive event, and possibly a "remnant". The final explosion can be detected today for black holes that are sufficiently old and sufficiently small. Primordial black holes, that may have formed in the early universe, satisfy these requirements. I discuss the possibility of observing astrophysical emission from the explosion of primordial black holes in different wavelengths (submillimeter, radio, TeV). These emissions can be discriminated from other astrophysical sources because of a peculiar way the emitted wavelength scales with distance. I conclude illustrating the properties of this kind of black holes as possible dark matter component.

Grzegorz Plewa, Examination of the BKL singularity


Ingemar Bengtsson, Points that arise in Hawking radiation

Abstract: We don’t know what happens when black holes evaporate. Our equations fail. Physical intuition may help. Guided in this way various authors (Haggard and Rovelli, Maudlin, . . . ) have been led to disrupt the structure of the spacetime manifold at special points. In my talk I will examine some of these points.

István Rácz, On the nature of spacetime singularities

Abstract: It is widely held that––likewise in case of known particular examples––some of the physical quantities have to blow up at spacetime singularities. As opposed to this the seminal singularity theorems of Penrose and Hawking––though under very generic and physically plausible conditions––predict only the existence of incomplete, inextendible causal geodesics. In this talk an argument will be presented claiming the that by suitable application of results on global extendibility of geodesically incomplete spacetimes we may get the desired type of strengthening of the conclusions of the classical singularity theorems.

Jakub Gizbert-Studnicki, Quantum gravity on a torus

Abstract: I will discuss a quantum gravity model defined by Causal Dynamical Triangulations (CDT). Identification of phase structure and order of the phase transitions constitute first steps in the quest for a continuum limit of CDT where, following the asymptotic safety conjecture, the resulting theory of quantum gravity becomes nonperturbatively renormalizable. Previous studies of 4-dim CDT focused on a model with fixed spatial topology of a 3-sphere leading to many interesting results, including identification of four distinct phases of geometry, some of which are separated by second (or higher) order phase transition lines. I will present recent results concerning phase diagram of CDT with spatial topology of a 3-torus and discuss properties of the, so called, phase C whose geometry both in spherical and toroidal case shows good semiclassical behaviour. If time permits I will also comment on the recent idea of reintroducing semiclassical coordinates in the CDT setup.

Jakub Mielczarek, From Causal Dynamical Triangulations To Astronomical Observations

Abstract: In the talk we will discuss possible empirical consequences of the Causal Dynamical Triangulations (CDT) approach to quantum gravity. A possibility of using both astrophysical and cosmological observations to test predictions of CDT will be examined. We will show that some scenarios can be empirically falsifiable.

Jakub Rembieliński, On the time evolution and spin EPR correlations of chiral relativistic qubits

Abstract: Qubit as a simplest irreducible set of quantum states of an elementary physical system abounds in the quantum information theory. The notion of a qubit is widely used because of its intuitive character and simplicity. An important example is the analysis of the quantum entanglement of subsystems of more complex quantum systems. In the relativistic quantum theory we have difficulties with the notion of a qubit modelled on the space of states of a massive spinning relativistic particle. In my talk I introduce and analyze a universal and convenient notion of the relativistic chiral qubit that can be applied also in the quantum information theory. In particular, I define a class of manifestly Lorentz covariant unitary time evolutions of the relativistic qubit related to accelerated spinning particles. The obtained results are applied to the analysis of relativistic Einstein – Podolsky – Rosen spin correlations including scenario with the Rindler horizons.

Javier Olmedo, Canonical quantum gravity and the fate of singularities

Abstract: In this talk, I will discuss the fate of singularities of general relativity in the context of canonical quantum gravity. Here, quantum geometrodynamics and, more recently, loop quantum gravity have approached this issue, at least in mini and midisuperspace models. I will report on several advances made so far in cosmological scenarios and spherically sym-metric spacetimes. In all these settings, the genuine quantum geometry of loop quantum gravity provides a natural mechanism for the singularity resolution. I will also discuss potential windows to confront the predictions with future observations.

Jerzy Kijowski, Trautman-Bondi energy and its universality

Abstract: In his Ph.D. thesis (1958) Andrzej Trautman has shown how to calculate amount of energy carried by gravitational waves. This approach was later simplified by Roger Penrose’s definition of “null infinity”. In my talk I show that these phenomena are universal and occur not only in General Relativity but also in any special-relativistic field theory (e.g. in – linear or nonlinear – electrodynamics). For this purpose I use a novel description of the “Scri”, which leads to a further simplification of the theory.

Jerzy Kowalski-Glikman, The Unruh effect without space-time

Abstract: We show that the Unruh effect can be associated with the partitioning of the real line, and derived from the basic representation theory of the group of affine transformations in one dimension. Our result shows that thermal distributions naturally emerge in connecting quantum states belonging to representations related to distinct notions of translational symmetry. The existence of inaccessible regions of configuration space is the key ingredient of the analysis.

José Senovilla, Singularities in warped products: instability of extra dimensions?

Abstract: An analysis of singularity theorems in warped-product spacetimes is presented, based on the existence of compact trapped submanifolds of arbitrary co-dimension. The consequences for the (classical) stability of extra compact dimensions, such as is required in string theory, will be discussed.

Luciano Rezzolla, Binary neutron stars: Einstein's richest laboratory

Abstract: I will argue that if black holes represent one the most fascinating implications of Einstein's theory of gravity, neutron stars in binary system are arguably its richest laboratory, where gravity blends with astrophysics and particle physics. I will discuss the rapid recent progress made in modelling these systems and show how the inspiral and merger of a binary system of neutron stars is more than a strong source of gravitational waves. Indeed, while the gravitational signal can provide tight constraints on the equation of state for matter at nuclear densities, the formation of a black-hole--torus system can explain much of the phenomenology of short gamma-ray bursts, while the the ejection of matter during the merger can shed light on the chemical enrichment of the universe.

Martin Bojowald, Effective field theory of quantum cosmology

Abstract: A discussion of inhomogeneity is indispensable in quantum cosmology, even if one is interested only in the dynamics of homogeneous models in a first approximation. Examples of important issues that cannot be addressed within a pure minisuperspace setting include covariance, stability, robustness, potentially observable phenomena, or possible implications of physical effects such as cosmological analogs of the Casimir force. While a full quantization of inhomogeneous gravity is not available, a framework of effective field theory can be used to provide important ingredients for quantum cosmology. Such a framework also allows one to take into account lessons from the BKL scenario. Based on these new ingredients, in turn, one can formulate conditions on various parameters or other mathematical objects that appear in minisuperspace models. Loop quantum cosmology, in its commonly practiced form, does not obey these conditions.

Nelson Pinto-Neto, The Bohmian approach to quantum cosmology

Abstract: Quantum gravity aims to describe gravity in quantum mechanical terms. How exactly this needs to be done is an open question. Various proposals have been put on the table, such as canonical quantum gravity, loop quantum gravity, string theory, etc. These proposals encounter technical and conceptual problems. In this contribution we focus on canonical quantum cosmology, and we show that many conceptual problems, such as the measurement problem, the problem of time and the problem of space-time singularities, can be overcome by using the de Broglie-Bohm quantum theory. In addition, the Bohmian approach yields a conceptually clear framework to study quantum cosmological perturbations, their quantum-to-classical transition, and the comparison with observations. In particular, we exhibit a non-singular cosmological model with a quantum bounce and a dark energy expanding phase, where the amplitudes and spectra of scalar and tensor perturbations are consistent with observations.

Nick Kwidzynski, Dynamics of general Bianchi IX near the singularity

Abstract: We consider the non-diagonal Bianchi IX universe filled with dust. Particular attention will be given to the asymptotic regime close to the singularity. The classical BKL analysis suggests that the metric becomes effectively diagonal in this asymptotic regime. We shall give numerical support to the validity of the resulting approximated equations of motion.

Piotr Jaranowski, Analytical relativity and the first direct detections of gravitational wave,

Abstract: Recent detections of gravitational waves coming from coalescing compact binaries made by LIGO/Virgo detectors initiated new branches of observational astronomy with enormous scietific potential: gravitational-wave astronomy and multi-messenger astronomy. The crucial role of analytical relativity results for the successful detections of gravitational-wave signals and estimation of their parameters will be explained. The adjective "analytical" means here methods that rely on solving explicit (that is analytically given) ordinary differential equations, contrary to full numerical relativity simulations. Among numerous analytical relativity results the emphasis will be placed on post-Newtonian approximations and on effective one-body approach to relativistic two-body problem.

Przemysław Małkiewicz, Exploring the fate of singularities with affine symmetry and coherent states

Abstract: We discuss a quantum theory of the minisuperspace models of the big-bang/big-crunch singularity. We start with emphasising the affine symmetry and incomplete Hamiltonian flows in the classical models. A suitable quantization based on coherent states is then proposed. We introduce a useful methodology for investigating the respective quantum models and next use it to discuss in some detail the quantum dynamics of the closed Friedmann universe and the anisotropic mixmaster universe. We demonstrate that in both cases the classical singularity is replaced with a quantum bounce. We discuss the essential features of the mixmaster bounce.

Rudolfo Gambini, Emergent Space-time from Path Dependent Observables

Abstract: There exists a renewed interest in the description in terms of observables of gravity. In the 1960's Mandelstam pioneered an approach to the quantization of gauge fields and gravity based on path-dependent Dirac observables. Although some limited success in the computation of graviton propagators was achieved, the formalism proved unwieldy, in particular making it very difficult to characterize if two curves ended in the same point. We note that the use of techniques of the group of loops, in particular the loop and connection derivative, allow to overcome these difficulties, opening new possibilities for the quantization of gravity. In this path-dependent description space-time points become emergent entities.

Stephen Green, Extended I-Love relations for slowly rotating neutron stars

Abstract: Observations of gravitational waves from inspiralling neutron star binaries{suchas GW170817{can be used to constrain the nuclear equation of state by placing bounds on stellar tidal deformability. For slowly rotating neutron stars, the response to a we quadrupolar tidal feld is characterized by four internal-structure-dependent constants called Love numbers. The tidal Love numbers k2 el and k2 mag measure the tides raised by the gravitoelectric and gravitomagnetic components of the applied field, and the rotational-tidal Love numbers fo and ko measure those raised by couplings between the applied field and the neutron star spin. In this work we compute these four Love numbers for perfect uid neutron stars with realistic equations of state. We discover (nearly) equation-of-state independent relations between the rotational-tidal Love numbers and the moment of inertia, thereby extending the scope of I-Love-Q universality. We find that similar relations hold among the tidal and rotational-tidal Love numbers. These relations extend the applications of I-Love universality in gravitational-wave astronomy.

Thomas Dent, Singularity spacetimes and gravitational wave observations

Abstract: Gravitational collapse of compact astrophysical objects - massive stellar cores or hypermassive neutron stars - in Einstein's General Relativity leads to a singularity condition, which is believed to be hidden by the event horizons of astrophysical black holes. The most direct probes of the dynamics of such structures are likely to be transient events where black holes are formed or merge together. In the first observing runs of the Advanced LIGO and Virgo detectors several compact binary merger events have been observed via their gravitational-wave emission. I summarize these observations and the constraints derived on various possible deviations from the predictions of GR.

Tomasz Trześniewski, What happens to spikes after their quantization?

Abstract: TBA

Woei Chet Lim, Spike dynamics near spacelike singularities

Abstract: It is widely known that, near spacelike singularities, most worldlines would undergo the so-called BKL singularity dynamics, which is an in nite se- quence of Kasner saddle states connected by Taub transitions. Some world-lines however would undergo highly inhomogeneous spike dynamics, which is an infinite sequence of Kasner saddle states connected by spike transitions. My contributions in this area are the discovery of the exact solutions which describe the spike transitions, and the development of a numerical zooming technique specially designed for spike simulations. I will illustrate the spike transitions with snapshots of the Hubble-normalised state space trajectories, and point out interesting features.

Przemysław Małkiewicz, Internal clock formulation of quantum mechanics

Abstract: Quantum mechanics is based on an external and unique parameter called time. According to general relativity such na entity does not exist. The evolution of gravitational systems may be expressed in terms of internal degrees of freedom. In my talk I will propose a reformulation of quantum mechanics which removes the absolute time from its formalism and replaces it with an arbitrarily chosen internal degree of freedom, the internal clock. I will focus on the comparison of different internal clock-based quantum dynamics and use quantum cosmological systems to illustrate the results.








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