Strong Gravity

Over forty detections of binary-black-hole mergers have been made during the first three observing runs of the LIGO and Virgo detectors. With this larger number of measurements of increasing accuracy, many of the remarkable predictions of general relativity for strongly curved, dynamical spacetimes will be able to be studied observationally. In this talk, I will discuss one class of strong-gravity phenomena, called gravitational-wave memory effects, which are predictions of general relativity that are most prominent in systems with high gravitational-wave luminosities, like binary black holes. Memory effects are characterized by changes in the gravitational-wave strain and its time integrals that persist after a transient signal passes by a detector. I will summarize the computation of these effects and the prospects for current and planned future gravitational-wave detectors to detect memory effects from black-hole mergers; in particular, there could be evidence for the memory effect in just a few years of advanced LIGO, Virgo, and KAGRA data at their design sensitivities. I will also review what observing gravitational-wave memory effects can teach us about the symmetries and conserved quantities around isolated systems like binary-black-hole mergers. Time permitting, I will present results on memory effects in scalar-tensor theories of gravity and on subleading memory effects.

Core-Collapse Supernovae are a fantastic laboratory to study fundamental physics.  The messengers from the core, neutrino and gravitational waves, carry a wealth of information about the dynamics, thermodynamics, underlying physics, and structure of massive stars at the end of their lives.  In this talk, I will discuss some of the ways we can use supernovae to probe this physics.  From quark-hadron phase transitions emitting unique gravitational wave and neutrino signals, to neutrinos telling us precise information about the distance to and progenitor mass (!) of galactic supernovae, even before the optical signal is emitted.  I will also talk about current efforts to model the population of core-collapse supernovae using parameterized 1D models.

 

The X-ray source Cygnus X-1 was discovered by sounding rockets in 1964 and is perhaps the oldest and best known "black hole candidate".   Its celestial coordinates were poorly known until 1971, when simultaneous radio flares and X-ray flares were observed which enabled the X-ray source to be associated with a 9th magnitude O-star known as HDE 226868.  This star was subsequently shown to be a single-lined spectroscopic binary with an orbital period of about 5.6 days.  However, owing to difficulties with measuring the system geometry and the distance to the source, the mass of the dark companion remained elusive.  In 1974, Stephen Hawking and Kip Thorne famously made a wager on the nature of the dark companion in Cygnus X-1, with Hawking wagering that the dark companion was not a black hole.  Eventually the consensus was that Cygnus X-1 does indeed contain a black hole, and Hawking conceded the bet in 1990.    In the following years many estimates for the black hole mass appeared in the literature, and most of them had large uncertainties, mainly owing to the difficulty of measuring the distance to the source.  In this talk I will describe how we were able to use VLBI radio observations to measure a precise distance to the source, and how, in turn, this precise distance allowed us to obtain precise masses for the component stars in Cygnus X-1.  Using the refined distance and geometry of the binary, it is possible to measure the relativistic spin parameter a* of the black hole using the X-ray continuum fitting technique.  We find that the black hole spin is nearly maximal with  a* > 0.9985 (3σ).

LIGO's successful detection of gravitational waves has revitalized the theoretical understanding of the angular momentum carried away by gravitational radiation. An infinite-dimensional supertranslation ambiguity has presented an essential difficulty for decades of study. Recent advances were made to quantify the supertranslation ambiguity in the context of binary coalescence. In this talk, we will present the first definition of angular momentum in general relativity that is completely free from supertranslation ambiguity. The new definition of angular momentum is derived from the limit of the quasilocal angular momentum we defined previously.

The talk will be based on my latest two papers 2103.02616 and 2103.05700. 
In the first part, I will present my GRMHD simulation of a neutron-star post-merger disk with neutrino fast flavour conversion included dynamically. The fast conversion of neutrinos can lead to flavour space equipartition ubiquitously on the time scale as short as 1ns. Due to the reduction of the number density of electron and anti-electron neutrino, the ejecta becomes more neutron rich. The final r-process nucleosynthesis sees an enhanced abundance of heavy elements close to the solar values. A similar effect may allow for increased lanthanide production in collapsars.
In the second part, I will present fast magnetic energy dissipation through the collision of Alfven waves with anti-aligned magnetic fields. The collision produces a current sheet sustained by an electrical field breaking the MHD condition. Particles entering the current sheet are accelerated following a relativistic variation of Speiser orbit and escape with higher energy. This mechanism can dissipate a large fraction of wave energy, nearly 100% when the wave magnetic field equals the background magnetic field. The fast dissipation may occur in various objects, including magnetars, jets from accreting black holes, and pulsar wind nebulae.

Combining information from the first gravitational wave detected gamma-ray burst, GRB 170817 with observations of cosmological GRBs holds important lessons for understanding the structure of GRB jets and the required conditions at the emitting region. It also re-frames our understanding of more commonly observed phenomena in GRBs, such as X-ray plateaus, and sets our expectations for future observations. I will present different lines of argument suggesting that efficient gamma-ray emission in GRBs has to be restricted to material with Lorentz factor > 50 and is most likely confined to a narrow region around the core. GRB jets viewed slightly beyond their jet cores, result in X-ray plateaus that are consistent with observed light-curves and naturally reproduce correlations between plateau and prompt emission properties. For jets viewed further off-axis (that are expected to be detected as future GW triggered events) we provide new analytical modelling that reveals two different types of light-curves that could be observed (single or double peaked) and outlines how the underlying physical properties can be recovered from such observations.

Motivated by Feynman's early proposal, several schemes have been proposed recently to explore the quantum nature of gravity using table-top experiments. The key idea behind them is to study whether gravity can lead to non-classical quantum correlations between two objects. These experiments, if successful, can test different models of quantum gravity in the Newtonian limit. In this talk, I will discuss one such scheme based upon optomechanics that couples light to mechanical oscillators mediated by quantum radiation pressure. The non-classicality of gravity is witnessed by the quantum squeezing of light.

Light rays can orbit the photon shell of a black hole many times before escaping to infinity.  This means that a distant observer can see a successive series of "echo"  images, each separated in time by the photon shell orbital period, and each dimmer than the previous.  I will present a study of light echos using coherent autocorrelation functions sourced by fluctuating matter in accretion flows.  I will demonstrate that coherent autocorrelation functions are peaked at integer multiples of the photon shell orbital period.  Furthermore, I will argue that the power in echos from supermassive black holes is too small to be observed on Earth.

Zoom Link: https://pitp.zoom.us/j/92751681169?pwd=V0hQeUMwaWtTQjQ3UzdxekJiT0lmQT09

The detections of gravitational waves (GWs) from merging binary black holes (BHs) have motivated many studies on the dynamical formation of such compact black hole binaries (BHBs). Recent works have suggested that tertiary-induced merger via Lidov–Kozai (LK) oscillations may play a significant role in producing the BHBs detected by the LIGO/VIRGO collaboration. I will talk about the dynamics of merging compact binaries near a rotating supermassive black hole (SMBH) in a hierarchical triple configuration, including various general relativistic (GR) effects. I will show that these GR effects significantly increase the merger fraction of BHBs and produce a wide range of spin orientations when the BHB enters LIGO band. I will also discuss the hierarchical BH mergers in multiple systems, focusing on the constraints on the formation of GW190412, GW190814 and GW190521-like events.

General Relativity remains to this day our best description of gravitational phenomena. Nonetheless, issues such its quantization and cosmological constant problem suggest Einstein’s theory might not be final theory of the gravitational interaction. Motivated by these questions, theorists have proposed a myriad of extensions to General Relativity over the decades. In this seminar, I will focus on theories with extra scalar fields. In particular, I will describe how some of these theories can evade Solar System constraints and yet yield to new effects in the strong-gravity regime of compact objects, i.e. neutron stars and black holes. This is achieved through a process known as spontaneous scalarization, in which a compact object growths 'scalar hair' once certain conditions are met and remains 'bald' otherwise. I will review the basics of this effect and then focus on recent efforts in understanding it for black holes both in isolation and in binaries.

The defining feature of a classical black hole horizon is that it is a "perfect absorber". Any evidence showing otherwise

would indicate a departure from the standard black-hole picture. Due to the presence of the horizon, black holes in binaries exchange

energy with their orbit, which backreacts on the orbit. This is called tidal heating. Tidal heating can be used to test the presence of a horizon.

I will discuss the prospect of tidal heating as a discriminator between black holes and horizonless compact objects, especially supermassive ones, in LISA.

I will also discuss a similar prospect for distinguishing between neutron stars and black holes in the LIGO band.