Stumbling around in the modified gravity landscape

Soham Bhattacharyya (Albert Einstein Institute, Max Planck Institute for Gravitational Physics)

Parameters extracted from Gravitational Wave (GW) data allow observers to quantify certain physical aspects of GW producing sources, like neutron stars (NS) and black holes (BH). In the case of an isolated BH, its physical shape in space, as a freely falling asymptotic observer would deduce from data, combined with its inertial mass, gets encoded in GWs radiated from these sources. In classical terms, a multipolar structure of radiating sources can be established from the GW data, giving a freely falling observer a dynamic view of the ‘horizon’ of a BH as it undergoes a damped oscillation towards a stable shape. However, the dynamical behavior is the opposite when the source is a binary system. Two ‘symmetric’ bodies mutually deform each other to a maximally distorted single compact object, radiating GWs that increase in frequency and amplitude (to specific maximum values) along with the distortion. Using a Zangenbewegung approach towards the maximal distortion (known in GW
 literature as the merger) from both sides (ring-down and inspiral, respectively), I will talk about GR’s predictions and the corresponding predictions made by extended theories of gravitation like f(R) and dynamical Chern-Simons.

Meeting ID:  854 3210 3337      Password: 959078

Ultralocality and the robustness of slow contraction to cosmic initial conditions

Anna Ijjas (Albert Einstein Institute, Max Planck Institute for Gravitational Physics)

I will discuss the detailed process by which slow contraction smooths and flattens the universe using an improved numerical relativity code that accepts initial conditions with non-perturbative deviations from homogeneity and isotropy along two independent spatial directions. Contrary to common descriptions of the early universe, I will show that the geometry first rapidly converges to an inhomogeneous, spatially-curved and anisotropic ultralocal state in which all spatial gradient contributions to the equations of motion decrease as an exponential in time to negligible values. This is followed by a second stage in which the geometry converges to a homogeneous, spatially flat and isotropic spacetime. In particular, the decay appears to follow the same history whether the entire spacetime or only parts of it are smoothed by the end of slow contraction.

Meeting ID:  854 3210 3337      Password: 959078

Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon

Andrew Chael (Princeton University)

Simulated images of a black hole surrounded by optically thin emission typically display two main features: acentral brightness depression and a narrow, bright “photon ring” consisting of strongly lensed images superposedon top of the direct emission. The photon ring closely tracks a theoretical curve on the image plane correspondingto light rays that asymptote to unstably bound photon orbits around the black hole. This critical curve has asize and shape that are purely governed by the Kerr geometry; in contrast, the size, shape, and depth of theobserved brightness depression all depend on the details of the emission region. For instance, images of sphericalaccretion models display a distinctive dark region—the “black hole shadow”—that completely fills the photonring. By contrast, in models of equatorial disks extending to the black hole’s event horizon, the darkest regionin the image is restricted to a much smaller area—aninner shadow—whose edge lies near the direct lensedimage of the equatorial horizon. Using both semi-analytic models and general relativistic magnetohydrodynamic(GRMHD) simulations, we demonstrate that the photon ring and inner shadow may be simultaneously visible insubmillimeter images of M87, where magnetically arrested disk (MAD) simulations predict that the emissionarises in a thin region near the equatorial plane. We show that the relative size, shape, and centroid of thephoton ring and inner shadow can be used to estimate the black hole mass and spin, breaking degeneracies inmeasurements of these quantities that rely on the photon ring alone. Both features may be accessible to directobservation via high-dynamic-range images with a next-generation Event Horizon Telescope.

Meeting ID:  854 3210 3337      Password: 959078

Neutrino transport in neutron star merger simulations

Francois Foucart (University of New Hampshire)

The first observation of a neutron star merger through gravitational waves and electromagnetic (EM) signals has shown us the power of multi-messenger observations. Multiple studies based on these observations have placed useful constraints on the equation of state of dense nuclear matter, while the event itself confirmed that mergers are likely (one of) the sources of r-process elements (e.g. gold, uranium) in the Universe. Many interpretations of these observations require reliable models for kilonovae, the radioactively powered EM transient powered by mergers. The numerical simulations that typically inform kilonovae models however have two important “known unknowns’’, namely the uncertainties due to approximate modeling of magnetic fields and neutrinos. In this talk, I will review the role of neutrinos in neutron star-neutron star mergers, as well as existing approximate transport methods used in simulations. I will also present a Monte-Carlo algorithm recently implemented in the SpEC code, used to perform the first simulations of merging neutron stars that directly attempt to solve Boltzmann’s equation of radiation transport. This scheme is purposely built to be as inexpensive as possible: the cost of a simulation remains comparable to simulations using our best existing approximate transport scheme. I will discuss the trade-offs made to reach that target, and how the scheme may be improved in the future. Related papers: arXiv:2103.16588, arXiv:2008.08089

Meeting ID:  854 3210 3337      Password: 959078


Long term Magneto-thermal evolution of neutron stars: the roles of the Hall drift amb ambipolar diffusion

José A. Pons

It is generally accepted that the nonlinear, dynamical evolution of magnetic fields in the interior of neutron stars plays a key role in the explanation of the observed phenomenology (temperatures, luminosities, spin period and derivative). Understanding the transfer of energy between toroidal and poloidal components, or between different scales, is of particular relevance. In this talk I discuss the general aspects of the long term magnetic and thermal evolution, with particular emphasis in the role of the Hall drift and ambipolar diffusion for typical magnetar conditions

Meeting ID:  854 3210 3337        Password: 959078

Bubble dynamics from holography

David Julián Mateos Solé (Universitat de Barcelona)

Cosmological phase transitions proceed via the nucleation of bubbles that subsequently expand and collide. The resulting gravitational wave spectrum depends crucially on the bubble wall velocity. Microscopic calculations of this velocity are challenging even in weakly coupled theories. I will show how to use holography to compute the wall velocity from first principles in strongly coupled, non-Abelian, four-dimensional gauge theories. No previous knowledge of holography or string theory required.

Meeting ID:  854 3210 3337        Password: 959078

Comparison of Kerr and dilaton black hole shadows: Impact of non-thermal emission

Jan Röder (Institut für Theoretische Physik, Goethe -Universität Frankfurt)

With the Event Horizon Telescope, a very long baseline interferometry (VLBI) array, both temporal and spatial event horizon-scale resolutions needed to observe super-massive black holes were reached for the first time. Current open questions revolve around the type of compact object in the Galactic Center, plasma dynam- ics around it and emission processes at play. The main goal of this thesis is to assess whether it is possible to distinguish between two spacetimes by means of synthetic imaging, under the aspect of different emission models. Extending the studies conducted in the pioneering work of Mizuno et al. 2018, general relativis- tic radiative transfer (GRRT) calculations are carried out on general relativistic magneto-hydrodynamics (GRMHD) simulations of a Kerr and of a non-rotating dilaton black hole. The systems are matched at the innermost stable circular orbit, and both black holes are initially surrounded by a torus in hydrostatic equilibrium with a weak poloidal magnetic field. In order to investigate the plasma dynam- ics, GRMHD simulations were carried out using the “Black Hole Accretion Code” (BHAC). In the literature the ratio between the temperatures of simulated ions and radiating electrons is often taken to be a constant, while in reality it is ex- pected to depend on plasma properties. In radiative post-processing with the code “Black Hole Observations in Stationary Spacetimes” (BHOSS) the temperature ra- tio was therefore parametrized. Additionally, in the jet wall, electrons are believed to be accelerated and should therefore be modeled with non-thermal electrons. To this end, both thermal and non-thermal electron energy distribution functions were employed. Lastly, images were reconstructed from synthetic VLBI data with the “eht-imaging” Python package to study how the effects of the emission models carry over to an observational environment. The most impactful result is the effect of the parameter Rhigh in the temperature ratio parametrization, splitting source structures into torus– and jet dominated configurations. Non-thermal emission turns out to be negligible at the field of view used and for the region it is applied in. Hence, given the present observational capabilities, it is unlikely that it is possible to distinguish spacetimes in observations. The striking visual differences are due to the difference in rotation between the black holes. In synthetic VLBI images, even the difference in shadow size is lost for most configurations. The situation may be improved in the future by a better VLBI array.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

GR-Athena++: puncture evolutions on vertex-centered oct-tree AMR

Boris Daszuta (Friedrich-Schiller-Universität Jena)

`GR-Athena++` is a general-relativistic, high-order, vertex-centered solver that extends the oct-tree, adaptive mesh refinement capabilities of the astrophysical (radiation) magnetohydrodynamics code `Athena++`. To simulate dynamical spacetimes `GR-Athena++` uses the Z4c evolution scheme of numerical relativity coupled to the moving puncture gauge. Stable and accurate binary black hole merger evolutions are demonstrated in convergence testing, cross-code validation, and verification against state-of-the-art effective-one-body waveforms. `GR-Athena++` leverages the task-based parallelism paradigm of `Athena++` to achieve excellent scalability. Strong scaling efficiencies above 95% for up to 1.2×1e4 CPUs and excellent weak scaling up to 1e5 CPUs in a production binary black hole setup with adaptive mesh refinement are measured. `GR-Athena++` thus allows for the robust simulation of compact binary coalescences and and offers a viable path towards numerical relativity at exascale.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

Maximum mass of compact stars from gravitational wave events with finite-temperature equations of state

Armen Sedrakian ( Frankfurt Institute for Advanced Studies)

We conjecture and verify a set of universal relations between global parameters of hot and fast-rotating compact stars, including a relation connecting the masses of the mass-shedding (Kepler) and static configurations. We apply these relations to the GW170817 event by adopting the scenario in which a hypermassive compact star remnant formed in a merger evolves into a supramassive compact star that collapses into a black hole once the stability line for such stars is crossed. We deduce an upper limit on the maximum mass of static, cold neutron stars 2.15+0.100.07MTOV2.24+0.120.10 for the typical range of entropy per baryon 2S/A3 and electron fraction Ye=0.1 characterizing the hot hypermassive star. Our result implies that accounting for the finite temperature of the merger remnant relaxes previously derived constraints on the value of the maximum mass of a cold, static compact star.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

Black hole scalarization

Stoytcho Yazadjiev ( Eberhard Karls University of T ̈ubingen & Sofia University)

In extended scalar-tensor theories, such as
scalar-Gauss-Bonnet gravity, the black holes can undergo spontaneous
scalarization – a strong gravity phase transition triggered by a
tachyonic instability due to the non-minimal coupling between the
scalar field(s) and the spacetime curvature. This very interesting
phenomenon is the only known dynamical mechanism for endowing black
holes (and other compact objects) with scalar hair without altering
the predictions in the weak-field limit. In this talk, I will present
the basic theoretical ideas behind the spontaneous scalarization and
will review some of the recent achievements in the field.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078