# Gravity seminars 2023

November 14, 2023 |
Maria Charisi, Vanderbilt University Abstract: A Pulsar Timing Array (PTA) is a galactic-scale detector that relies on precision timing of milli-second pulsars. As of this summer, all major PTA collaborations have found evidence of a low-frequency gravitational wave background. The most likely origin of this background is a population of supermassive black hole binaries (SMBHBs) formed in galaxy mergers. I will present the exciting recent results from the North American Nanohertz Observatory for Gravitational waves (NANOGrav) collaboration, and their meaning for extragalactic astronomy. I will also describe the next major milestone, which is likely the detection of an individual resolved binary. These systems, which should stand above the background, are also expected to be bright sources of electromagnetic emission and can be detected as quasars with periodic variability. I will summarize the status of current electromagnetic searches and their challenges. Finally, I will discuss prospects for combining electromagnetic and gravitational-wave data and bringing the first multi-messenger detection of an SMBHB within closer reach. |

October 31, 2023 |
Roman Rafikov, University of Cambridge Title: Cluster tides, stellar dynamics and LIGO/Virgo gravitational wave sources Abstract: |

October 17, 2023 |
Josu Aurrekoetxea, University of Oxford Title: Strong-gravity signatures from relics of the early universe Abstract: Some of the most well-motivated extensions of the standard model of particle physics and general relativity introduce extra fundamental fields that play a key role in mechanisms of the early universe. These phenomena, in turn, lead to the formation of structures that can leave imprints and persist to this day. In this talk I will discuss the use of numerical relativity to study the strong-gravity signatures of these relics, to probe the physics of the early universe. |

June 13, 2023 |
Miguel Zumalacarregui, Albert Einstein Institute Title: On the propagation of gravitational waves: diffraction, dispersion and birefringence Abstract: Just like light, gravitational waves (GWs) are deflected and magnified by the large-scale structure of the Universe, a phenomenon known as gravitational lensing. Their low frequency, phase coherence and capacity to propagate with no absorption makes GWs an ideal signal in which to observe wave-propagation phenomena. I will describe how GWs deflected by cosmic structures produce diffractive, wave-optics phenomena, whose measurement will allow us to infer the properties of galactic and dark matter halos. For GWs in strong gravitational fields, such as the vicinity of a massive black hole, their propagation depends on the frecuency (dispersion) and polarization (birefringence) through the gravitational spin-hall effect. I will describe how observations of sources near central black holes of galaxies may enable the observation of dispersive GWs. While birefringence might be too suppressed to observe in Einstein’s general relativity, alternative theories predict that the two GW polarizations travel at different speeds near massive objects. Searches for this effect provide one of the most stringent tests of gravity so far. |

May 30, 2023 |
Donal O’Connell, University of Edinburgh Title: Classical Gravity from Quantum Amplitudes Abstract: The era of high-precision gravitational data requires a demanding new level of precision in gravitational theory. In this talk I will discuss the use of methods from quantum field theory to address the challenge. These methods totally reframe our theoretical approach to gravity, making gravity look like two copies of electrodynamics. We will see that the Schwarzschild metric is a "double copy" of the Coulomb charge, and that gravitational waveforms can be determined without ever using the Einstein equation. |

May 23, 2023 |
Vijay Varma, Albert Einstein Institute Title: Numerical relativity surrogates and recent applications for gravitational wave astronomy Abstract: Data-driven methods like surrogate modeling can capture the full physics of binary black hole simulations, including complex effects like spin precession and recoil kicks. As our gravitational wave detectors continually improve, physically complete models are crucial to accurately extract valuable astrophysical information from signals of merging black holes. In this talk, I will present the latest advances in surrogate modeling, including an ongoing effort to extend these models to low-mass signals using information from post-Newtonian theory. In addition, I will present the latest constraints on kick and spin measurements from GWTC3, obtained using surrogate models. These results include the first individual event showing strong signs of precession and a large kick, along with the first individual event showing strong support for negative effective spin. Observational constraints on spins and kicks play an important role in understanding how black holes form in nature, which I will discuss briefly. |

May 16, 2023 |
Zoltan Haiman, Columbia University Title: Gravitational wave and electromagnetic signatures of binary Abstract: Binary black holes (BHBs) embedded in dense gas hold the promise of so-called "multi-messenger astrophysics": when they are detected both through gravitational waves (GWs) and electromagnetic (EM) observations, they will enable novel science. This is true both for massive BHBs, whose GWs will be detectable by the future LISA satellite and by on-going pulsar timing arrays (PTAs), as well as for stellar-mass BHBs detected through ground-based GW detectors. In both cases, identifying the coalescing binaries through their EM signatures will help clarify their astrophysical origin and yield novel probes of cosmology, fundamental physics, and accretion physics. In this talk, I will describe how circumbinary gas may produce characteristic EM signatures for both massive and stellar-mass BHBs, based on analytic models as well as hydrodynamical simulations. I will also argue that in both cases, some coalescing binaries may have already been detected in optical surveys, providing clues about their origin. |

May 2, 2023 |
Anna Lisa Varri, University of Edinburgh Title: Rotating stellar systems and their black holes Abstract: The study of self-gravitating spheroidal rotating bodies is a classical fluid dynamics problem with a distinguished history, yet its kinetic counterpart has rarely been explored. As an example, I will present a family of self-consistent kinetic equilibria describing uniformly rotating, axisymmetric quasi-isothermal stellar systems. Such equilibria define a singular perturbation Vlasov-Poisson problem with a free boundary which can be approached by means of an asymptotic expansion based on the rotation strength parameter. I will then illustrate an extension to the case of self-consistent equilibria with a central black hole. Explicit asymptotic results are obtained over three nested regimes surrounding the sharp transition between equilibria dominated by the mass of the host stellar system or by the mass of the central black hole. I will conclude by discussing the astrophysical implications of these results for current multi-messanger explorations of black hole demographics. |

April 18, 2023 |
Adam Pound, University of Southampton Title: Progress in gravitational self-force theory: recent advances in modelling asymmetric binaries Abstract: As gravitational-wave detectors become more sensitive to lower frequencies, they will increasingly detect binaries with smaller mass ratios, larger spins, and higher eccentricities. In this talk I describe how gravitational self-force theory, when combined with a method of multiscale expansions, provides an ideal framework for modelling these systems. The framework proceeds from first principles while simultaneously enabling rapid generation of waveforms on a timescale of milliseconds. I discuss the state of the art in this method: nonspinning, quasicircular waveforms at second perturbative order in the mass ratio. I present progress toward extending this second-order model to include spins and to include the final merger and ringdown. I also discuss the domain of validity of these models, focusing on their accuracy for mass ratios in the intermediate regime ~1:10 to 1:100. |

March 7, 2023 |
Katy Clough, Queen Mary University of London Title: Black holes in dark matter environments - the impact of initial data |

February 7, 2023 |
Marc Casals, Leipzig University, Germany Title: Black hole spectroscopy: stability, censorship and Love Abstract: The investigation of how black holes react to field perturbations helps us understand better these enigmatic objects in the Universe. The decomposition into frequency modes of the field perturbations of black holes, analogously to atomic spectroscopy, is particularly useful. In this talk, we will present three different results obtained from such spectroscopy studies of rotating (Kerr) black holes. The first result is on the linear stability properties of Kerr, including a horizon instability in the maximally-rotating limit. The second result is evidence for the violation of Penrose’s strong cosmic censorship hypothesis for charged Kerr black holes in a Universe with a positive cosmological constant (de Sitter), thus indicating a possible breakdown of the predictability of Einstein’s equations inside these black holes. Time allowing, we will also present a last result on the deformability of Kerr black holes under an external static gravitational field, as measured by the so-called Love numbers. |

January 10, 2023 |
Prof. Thomas Baumgarte, Bowdoin College Title: Critical Phenomena in Gravitational Collapse Abstract: Critical Phenomena, including the appearance of universal scaling laws and critical exponents in the vicinity of phase transitions, appear in different fields of physics and beyond. Critical phenomena in gravitational collapse to black holes were first observed by Matt Choptuik about 30 years ago - a seminal discovery that launched an entire new field of research. While many aspects of critical collapse are well understood in the context of spherical symmetry, much less is known for systems that break spherical symmetry. In particular, this includes the perhaps most intriguing case, namely the collapse of vacuum gravitational waves. In this talk I will discuss the appearance of scaling laws and self-similarity close to the onset of black hole formation. I will review a number of attempts to reproduce early reports of these phenomena for the collapse of gravitational waves, leading up to results from current simulations. Based on these recent results I will suggest how our notion of criticality may have to be generalized in order to account for these phenomena in the absence of spherical symmetry. |