General Relativity & Quantum Cosmology

"After reviewing some key hints and puzzles from the early universe, I will introduce recent joint work with Neil Turok suggesting a rigid and predictive new approach to addressing them. Our universe seems to be dominated by radiation at early times, and positive vacuum energy at late times. Taking the symmetry and analyticity properties of such a spacetime seriously leads to a new formula for the gravitational entropy of our universe, and a picture in which the Big Bang may be regarded as a kind of mirror. I will explain how this line of thought suggests new explanations for a number of observed properties of the universe, including: its homogeneity, isotropy and flatness; the arrow of time (i.e. the fact that entropy increases *away* from the bang); several properties of the primordial perturbations; the nature of dark matter (which, in this picture, is a right-handed neutrino, radiated from the early universe like Hawking radiation from a black hole); the origin of the primordial perturbations; and even the existence of three generations of standard model fermions. I will discuss some observational predictions that will be tested in the coming decade, and some key open questions."

I will review recent work on accelerating spacetimes in 3D, explaining how acceleration manifests for point particles and BTZ black holes, as well as a novel BTZ-like solution in a disconnect region of parameter space. I will also discuss some holographic aspects of the solutions.

I will give an overview of how the Event Horizon Telescope achieves its horizon scale science and what's to come. I will also review selected recent results from the Event Horizon Telescope both on Sgr A* and refined analysis of M87*. A focus will be on analysis aspects that are relevant for any theory / model building along with a few examples. The presentation aims to provide key conceptual aspects relevant to gravity experts who are new to VLBI.

To make progress in developing a quantum theory of gravity, we need to connect candidate theories to observations. I will review ideas on connecting quantum gravity to observations in particle physics, to searches for dark matter and to observations of black holes, in particular with the (next-generation) Event Horizon Telescope.

We are in a new era of gravitational physics in which both gravitational-wave measurements and cosmological observations can be used to test fundamental physics. Moreover, recent computational developments allow us to investigate (at present largely unexplored) regimes where the gravitational force is strong – a very promising area to search for new physics. In this talk I will overview two cases in which we aim to use strong-gravity to learn about the dark components of the universe: the interaction of binary black hole mergers with their dark matter environments and the emission and propagation of scalar gravitational waves, a distinct signature of theories aiming to explain dark energy.

Zoom link:  https://pitp.zoom.us/j/92308918921?pwd=UEk1aEt3bmRxdklqUUpSNlRocldVdz09

We develop the reduced phase space quantization of causal diamonds in $2+1$ dimensional gravity with a nonpositive cosmological constant. The system is defined as the domain of dependence of a spacelike topological disk with fixed (induced) boundary metric. By solving the constraints in a constant-mean-curvature time gauge and removing all the spatial gauge redundancy, we find that the phase space is the cotangent bundle of $Diff^+(S^1)/PSL(2, \mathbb{R})$, i.e., the group of orientation-preserving diffeomorphisms of the circle modulo the projective special linear subgroup. Classically, the states correspond to causal diamonds embedded in $AdS_3$ (or $Mink_3$ if $\Lambda = 0$), with a fixed corner length, that have the topological disk as a Cauchy surface. Because this phase space does not admit a global system of coordinates, a generalization of the standard canonical (coordinate) quantization is required --- in particular, since the configuration space is a homogeneous space for a Lie group, we apply Isham's group-theoretic quantization scheme. The Hilbert space of the associated quantum theory carries an irreducible unitary representation of the $BMS_3$ group, and can be realized by wavefunctions on a coadjoint orbit of Virasoro with labels in irreducible unitary representations of the corresponding little group. A surprising result is that the twist of the diamond boundary loop is quantized in terms of the ratio of the Planck length to the corner length.

Zoom link:  https://pitp.zoom.us/j/94369372201?pwd=NWNsYno3RmZIWUx0LytWZ09PVDVVQT09

I will discuss several issues related to the computation of Poisson brackets in field theories. I will show that by choosing appropriate functional spaces as configuration manifolds, it is possible to avoid the use of distributions. This is relevant, for instance, in the context of Loop Quantum Gravity, where the basic holonomy/flux variables play a central role. I will illustrate the main ideas by using simple examples based on Sobolev spaces.

Zoom link:  https://pitp.zoom.us/j/92794475242?pwd=T2Fjbk1lTXRCNnBxVDZabnJqWlAzUT09

Highly magnetized neutron stars are a source of extreme transients observed in different bands, like the fast radio burst (FRB) and associated hard X-ray burst from the Galactic magnetar SGR 1935+2154. The origin of such outbursts, hard X-rays on the one hand and millisecond duration FRBs on the other hand, is still unknown. We present a global model for various kinds of such magnetar outbursting activities. Crustal surface motions are expected to twist the inner magnetar magnetosphere by shifting the frozen-in footpoints of magnetic field lines. We discuss criteria for the development of various instabilities of 3D twisted flux bundles in the force-free dipolar magnetospheres and compare their energetic properties to observations of magnetar X-ray flares. We then review a recently developed FRB generation mechanism in the outer magnetosphere of a magnetar. The strong magnetic pulse induced by a magnetar flare collides with the current sheet of the magnetar wind, compresses and fragments it into a self-similar chain of magnetic islands. Time-dependent plasma currents created during their collisions produce relatively narrow-band GHz emission with luminosities sufficient to explain bright extragalactic FRBs.

Zoom ink:  https://pitp.zoom.us/j/96469987183?pwd=N1UzVkh6RGZUeWV6TFZJLzk0M3VWZz09

 I will talk about new constructions to the parity-violating sector of gravity that can yield large observables, in particular those from inflation. Within this framework, I will demonstrate the potential for seeking gravitational parity-violation in  uncharted regions of parameter space and give hints to novel methods of probing baryogenesis. I will also comment on a few interesting theoretical aspects regarding inflationary  correlators.

Zoom link:  https://pitp.zoom.us/j/95924890817?pwd=MkRyWmV2a3g1dndPNFhoVlQvb3Ixdz09

After discussing the basics of Carrollian physics, we shall revisit the motion  of massless particles with anyonic spin in the black hole horizons. As recently shown, such particles can move within the horizon of the black hole due to the  coupling of charges associated with a 2-parametric central extension of the 2-dimensional Carroll group to the magnetic field around the black hole -- the so called "anyonic  spin-Hall effect". We shall study several examples of such motions. Of these, the most  interesting is the motion in misaligned (asymptotically uniform) magnetic field around Kerr,  which results in a time dependent motion of Carrollian particles that is reminiscent  of "monarch migration".

Zoom link:  https://pitp.zoom.us/j/97094479078?pwd=SHN6NEU5aE93OXVQYk9aWTVOdzRRUT09

The geometry of kilonovae is a key diagnostic of the physics of merging neutron stars with current hydrodynamical merger models typically showing aspherical ejecta. Previously, Sr II was identified in the spectrum of the only well-studied kilonova AT2017gfo, associated with the gravitational wave event GW170817. In this talk, we show that combining the strong Sr II P Cygni absorption-emission spectral feature and the blackbody nature of the kilonova spectrum, to determine that the kilonova is highly spherical at early epochs. Line shape analysis combined with the known inclination angle of the source also shows the same sphericity independently. The near-spherical geometry suggests early spectra of kilonovae may provide excellent precision cosmic distance measurements using the Expanding Photosphere Method.

Zoom link:  https://pitp.zoom.us/j/97287055430?pwd=bFVyeTRuZHVLcGlDdnhlK1d0OWE1Zz09