Colloquium

I will summarize recent progress in uncovering the analytic structure of scattering amplitudes. The overarching theme will be exploiting new intricate ways of analytically continuing time, extending beyond the Wick rotation. I will highlight a broad range of applications: from high-precision calculations in particle physics, through computations of gravitational waves, to formal topics in the scattering of strings.

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The recent breakthrough in the detection of gravitational waves (GWs) from merging black hole (BH) and neutron star (NS) binaries by advanced LIGO/Virgo has generated renewed interest in understanding the formation mechanisms of merging compact binaries, from the evolution of massive stellar binaries and triples in the galactic fields, dynamical interactions in dense star clusters to binary mergers in AGN disks. I will review different aspects of the dynamical formation channels, and discuss how observations of spin-orbit misalignments, eccentricities, masses and mass ratios in a sample of merging binaries by aLIGO can constrain these formation channels. The important roles of space-borne gravitational wave detectors will also be discussed.

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Gravitational-wave observatories have established a new field of transient astronomy. The most recent LIGO-Virgo-KAGRA catalog, GWTC-3, identified 90 merging binaries, which range from a double neutron star with a total mass of 2.7 at 40 Mpc (GW170817) to a double black hole with a total mass of 150 at 5.3 Gpc (GW190521). These observations have many connections to nuclear astrophysics. They are revealing the remnants of stellar evolution and supernovae in merging binary systems, they are constraining event rates and astrophysical environments for heavy-element nucleosynthesis, and they are illuminating the dense matter dynamics inside the cores of merging neutron stars. Here, I will describe the imprint of dense matter on gravitational waves, the implications of existing observations for nuclear physics, and some prospects for the coming years including the science potential of proposed next-generation observatories like Cosmic Explorer.

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Zoom link https://pitp.zoom.us/j/99015121355?pwd=NStOc2srbEJXdW9aSTJJbDk4RWZhdz09

Abstract TBA

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Zoom link https://pitp.zoom.us/j/98123499206?pwd=QWlHcFBJWE8zRTlyT1A5WUVsQS9NUT09

Atmospheric characterisation of habitable-zone exoplanets is a major frontier of exoplanet science. The detection of atmospheric signatures of habitable Earth-like exoplanets is challenging due to their small planet-star size contrast and thin atmospheres with high mean molecular weight. Recently, a new class of habitable sub-Neptune exoplanets, called Hycean worlds, have been proposed, which are expected to be temperate ocean-covered worlds with H2-rich atmospheres. Their large sizes and extended atmospheres, compared to rocky planets of the same mass, make Hycean worlds significantly more accessible to atmospheric spectroscopy. Several temperate Sub-Neptunes have been identified in recent studies as candidate Hycean worlds orbiting nearby M dwarfs that make them highly conducive for transmission spectroscopy with JWST. Recently, we reported the first JWST spectrum of a possible Hycean world, K2-18 b, with detections of multiple carbon-bearing molecules in its atmosphere. In this talk, we will present constraints on the atmospheric composition of K2-18 b and on the temperature structure, clouds/hazes, atmospheric extent, chemical disequilibrium and the possibility of a habitable ocean underneath the atmosphere. We will discuss new observational and theoretical developments in the characterisation of candidate Hycean worlds, and their potential for habitability. Our findings demonstrate the unprecedented potential of JWST for characterising Hycean worlds, and temperate sub-Neptunes in general, and open a new era of atmospheric characterisation of habitable-zone exoplanets with JWST.

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Zoom link https://pitp.zoom.us/j/98012554989?pwd=b0pCYkIvYmd2Y2hueUExQXBNVG8vZz09

Long-range entangled quantum matter encompasses a wealth of fascinating phenomena including fractionalization and criticality.  I will show how quantum dynamics involving measurements can both enable new kinds of long-range entangled states and facilitate their realization on quantum simulators.  In the first part, I will illustrate how competing measurements along with unitary time evolution can give rise to distinct universality classes of non-equilibrium criticality.  In the second part, I will show how measurements and unitary evolution conditioned on the measurement outcomes (“adaptive quantum circuits”) enable efficient preparation of long-range entangled matter.  Finally, I will demonstrate how environmental measurement (decoherence) can remarkably enrich quantum critical pure states, giving rise to renormalization group flows between quantum channels with important implications on the entanglement structure of the resulting critical mixed states.  

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Zoom link https://pitp.zoom.us/j/97087115629?pwd=NHdlWG9oM0xHUDIzU05sUWRKdWlFdz09

I argue that the answer is yes, by reviewing the history and current status of such a theory.  Since 1982, I have been developing a series of such theories, beginning in 1982 with an N --> infinity limit of 2+1 dimensional matrix model (the IAS model), through another N --> infinity, T --> 0 limit of a BFSS model.  During this time our work was complemented by Adler's Trace model and others.   

Beginning in 2012, Cortes and I developed a different approach to an relational quantum cosmology by adding intrinsic energy momentum to Sorkin et al's causal set models.  The addition of energy- momentum as intrinsic variables opens up a new mechanism for the emergence of space, and spacetime, plus interacting relativistic particles.  Note that the warm phase is purely algebraic, you need no prior existence of any space to get other dimensions to emerge.   In 2021 I discovered how to derive quantum non-relativistic many body theory from what has since been called the Causal Theory of Views.  Finally in papers since we report progress on the construction of special and general relativistic  Causal Theory of Views.

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Zoom link: https://pitp.zoom.us/j/95891848248?pwd=TUpWK2RWbU9GTGxZS1lMeS81QWp1dz09

In this colloquium, the communicative boundary between science and society in Africa will be explored, using physics as an illustration. Different types of contact zones and scenes will be discussed, as well as the societal impact of staging and narrative formats. Based on our ongoing engagement project Making Science the Star, an overview of the relationship between sender and receiver will be presented, and recipes for tuning this interaction to unlock untapped potential in predominantly non-scientific communities will be analyzed.

DISCLAIMER: This talk contains Season1 Episode2 from the show series Science In The City. This is used with permission from Dr. Stéphane Kenmoe, Producer of the series and Presenter of this talk.

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Bio: Stéphane Kenmoe got a Ph.D. in Physics from the Max Planck Institute for Iron Research in Germany in 2015. He is presently a habilitation candidate at the Faculty of Chemistry at the University of Duisburg-Essen, Germany. He is also a science popularizer on TV and on social media, a science writer and a novelist. In 2020, he produced the movie ‘’Science in the City’’ (Science dans la Cité) in Cameroon. He is very active in networking for the promotion of early career African scientists and for connecting science and society. He has won many awards for his engagement, among which the 2020 Diversity Prize for Academic Leadership at the University of Duisburg-Essen, the Falling Walls Award for Science Engagement in 2021 and the Award for Prototypes of Humanity of the Dubai Authority for arts and culture in 2022. Since 2022, he is the chief editor of the African Physics Newsletter, an electronic quarterly about physics in Africa published by the American Physical Society. 

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Zoom link: https://pitp.zoom.us/j/92725978684?pwd=Yys0ci9JdE0zS054SGxyaWoxZkdUUT09

The universe has turned out to be simpler than expected on both very small and very large scales. We propose a minimal, highly predictive framework connecting particle physics to cosmology. Instead of introducing an ``attractor” phase such as inflation we extrapolate the observed universe all the way back to the initial singularity where we impose a CPT symmetric boundary condition via a generalization of the method of images. If the hot plasma in the early universe is perfectly conformal, so is the singularity. The cosmos may then be analytically extended to a ``mirror image” universe prior to the bang. Using this new boundary condition we calculate the gravitational entropy for cosmologies with radiation, matter, Lambda and space curvature, finding it favours spatially flat, homogeneous and isotropic universes with a small positive cosmological constant in accord with observation. To maintain conformal symmetry, we include unusual Dim-0 (dimension zero) fields. They improve the SM’s coupling to gravity, cancelling the vacuum energy and two local “Weyl” anomalies, without introducing additional propagating modes. They also cancel pathologies introduced into the graviton propagator by loops of SM particles. Cancellation requires precisely 3 generations of SM fermions, each with a RH neutrino. It also requires a composite Higgs, presumably built with the Dim-0 fields. One of the RH neutrinos, if stable, is a viable candidate for the dark matter which will be tested soon. The Dim-0 fields source scale-invariant curvature perturbations in the early universe. Subject to two simple but crucial theoretical assumptions, the amplitude and spectral tilt match the observations with remarkable accuracy. (See arXiv:2302.00344 and references therein).

Zoom Link: https://pitp.zoom.us/j/95784600151?pwd=dkppa2s3ZDM4NG5yb0ZVV2w5SXErdz09

Complementing the spectacular breakthroughs in gravitational wave and multi-messenger astronomy, advancements in our theoretical understanding and modelling of the strong gravity are essential to apprehending the nonlinear dynamics of spacetime, and to unlocking the full potential of the observations. I will illustrate the scope of new physics that might be probed by black holes, neutron stars, and other such systems, including testing general relativity and strong gravity signatures of new particles, and describe some recent developments that allow for uncovering novel phenomena and making detailed predictions in this regime.

Zoom Link: https://pitp.zoom.us/j/96180256322?pwd=LzEyd3ZBWGdZeDR6MjB1dGpLWFRpUT09

There are fascinating new insights we could achieve if we succeeded in connecting quantum gravity to particle physics in the visible and the dark universe. However, making such a connection is challenging, because the typical scale of quantum gravity is believed to be far from the typical scales in particle physics. I will exploit lever arms that could make it possible to bridge this gap in scales.  To provide concrete examples for these ideas, I will review recent results on the proton lifetime in quantum gravity, the effects of asymptotically safe quantum gravity on properties of Standard Model matter and the effects of asymptotically safe quantum gravity on simple models of dark energy and dark matter.

Zoom Link: https://pitp.zoom.us/j/96545839038?pwd=UG1IVzJPWUZGa2ovOFhzdHBhOFhOdz09