Current seminars

Abstract: The threshold of black hole formation in parametrized families of collapse models exhibits many of the features of a phase transition. Universality and scaling behaviour are generically observed and a picture of critical phenomena emerges, completely analogous to that seen in statistical mechanical systems.
Most of the studies of critical collapse have been performed in the context of spherical symmetry where a relatively clear picture of the phenomenology has arisen. In particular, the critical solutions that are found tend to be (locally) unique, with some additional symmetry, and with a single unstable mode in perturbation theory.
Going beyond spherical symmetry in this field has proven to be quite challenging, and an overall understanding of the nature of non-spherical critical collapse remains elusive. Nonetheless, some progress has been made recently on models involving the collapse of pure gravitational waves as well as electromagnetic waves. My talk will focus on these developments.

Abstract: We construct the first four-dimensional multi-black hole solution of general relativity with a positive cosmological constant. The solution consists of two static black holes whose gravitational attraction is balanced by the cosmic expansion. These static binaries provide the first four-dimensional example of non-uniqueness in general relativity without matter.

Abstract: We try to develop a method for constructing ''Love symmetry'' generators in rotating black hole spacetimes of general dimensions. After revisiting the 4D Kerr and 5D Myers–Perry cases, we apply it generalized Lense-Thirring spacetimes which describe a wide variety of slowly rotating black hole metrics in any number of dimensions.

Abstract: We consider 4-dimensional, Ricci-flat and asymptotically flat (AF) manifolds with metrics of either Lorentzian or Euclidean signature, and with a 1-parameter group of isometries whose Killing tangent vectors have bounded length at infinity. If non-flat we call such Lorentzian solutions stationary AF black holes (SBHs), and the Euclidean ones S1-AF instantons (SGIs). The latter play a role in the path integral approach to Quantum Gravity which we review briefly. We then recall known examples of SBHs and SGIs. We continue with explaining the "nuts and bolts" classification of SGIs which is the Euclidean counterpart to stationary and static black hole horizons. We next review the Kerr uniqueness results, which read that analytic, connected SBH must be Kerr, and that SGIs with 2 nuts must be Euclidean Kerr. We also discuss the family of SGIs found by Chen and Teo which have 3 nuts and no real Lorentzian counterparts. As a step towards a conjectured uniqueness result for this family, we show that any SGI with "Chen-Teo topology" must be "half algebraically special". We finally sketch uniqueness results for stationary black holes and S1-instantons with are locally asymptotically flat rather than AF.

Abstract: In my talk, a null geometric approach to the quasilocal Brown-York formalism will be used to calculate, within a bounded gravitating physical system, the flux of energy through a dynamical horizon of a non-stationary spacetime. This is done by varying the total Hamiltonian of the system (bulk part plus boundary part) so as to derive an integral law describing the rate of change of mass and/or radiant energy escaping through the dynamical horizon. The results obtained in this way, as is shown, lead to previously unrecognized correction terms, including a bulk-to-boundary inflow term that leads to corrective extensions of Einstein's quadrupole formula in the large sphere limit and to quasilocal corrections to Bondi's mass-loss formula, the latter occurring in the null geometric context. Specific applications in the areas of gravitational wave physics and the theory of tidal heating and deformation effects are discussed.

Abstract: One of the important extensions of Riemann geometry is Weyl geometry, which is essentially based on the ideas of conformal invariance and nonmetricity. A similar non-Riemannian geometry was proposed by Erwin Schrödinger in the late 1940s, in a geometry which is simpler, and (probably) more elegant than the Weyl geometry. Even it contains nonmetricity, the Schrödinger connection preserves the length of vectors under parallel transport, and thus seems to be more physical than the Weyl connection. Interestingly enough, Schrödinger's approach did not attract much interest in the field of gravitational physics. It is the goal of the present talk to reconsider the Schrödinger geometry as a potential candidate for a gravitational theory extending standard general relativity. We consider a gravitational action constructed from a length preserving non-metricity, in the absence of torsion, and investigate its variation in both Palatini and metric formalisms. While the Palatini variation leads to standard general relativity, the metric version of the theory adds some non-metricity dependent extra terms in the gravitational Einstein equations, which can be interpreted as representing a geometric type dark energy. After obtaining the generalized Friedmann equations, we analyze in detail the cosmological implications of the theory, by considering two distinct models, corresponding to a dark energy satisfying a linear equation of state, and to conserved matter energy, respectively. We compare the predictions of the Weyl-Schrödinger cosmology with a set of observational data for the Hubble function, and with the results of the ΛCDM standard paradigm. The Weyl-Schrödinger cosmological models give a good description of the observational data, and, for certain values of the model parameters, they can reproduce almost exactly the predictions of the ΛCDM model. Hence, the Weyl-Schrödinger theory represents a simple, and viable alternative to standard general relativity, in which dark energy is of purely geometric origin.