Talks

Cosmic-ray-driven instabilities play a crucial role in particle acceleration at shocks and during the propagation of GeV cosmic rays in galaxies and galaxy clusters within the self-confinement picture of CR transport. These instabilities amplify magnetic fields, which, in turn, scatter cosmic rays and thus self-regulate their transport. This leads to a strong coupling between the collisionless cosmic ray population and the thermal background plasma, implying potentially significant dynamic feedback. In this presentation, I discuss a recent discovery of a new cosmic ray-driven instability, referred to as the intermediate-scale instability, which triggers comoving ion-cyclotron electromagnetic waves at sub-ion skin-depth scales. Its growth rate is notably faster compared to the ion gyro scale (streaming) instability, which is commonly assumed to be the dominant instability in the self-confinement picture. Therefore, this new instability could play a vital role in the transport of cosmic rays in galactic and stellar environments. I then explore the implications of this instability for electron acceleration at non-relativistic shocks. Through Particle-in-cell (PIC) simulations, it is demonstrated that the new instability triggers the dominant mechanism for efficient electron acceleration at parallel electron-ion shocks, addressing a persistent issue with electron injection at these shocks. The PIC simulations also reveal that the common practice of using reduced ion-to-electron mass ratios in shock simulations, which artificially suppresses the intermediate instability, not only hinders electron acceleration but also leads to incorrect electron and ion heating in downstream and shock transition areas.

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Zoom link https://pitp.zoom.us/j/91367222746?pwd=REpEdXE3ZGdLeVQ1bnh0NldIQktWQT09

Out of the many lessons quantum mechanics seems to teach us, one is that it seems there are things we cannot experimentally have access to. There is, indeed, a fundamental limit to our ability to experimentally explore the world. In this work we accept this lesson as a fact and we build a general theory based on this principle. We start by assuming the existence of statements whose truth value is not experimentally accessible. That is, there is no way, not even in theory, to directly test if these statements are true or false. We further develop a theory in which experimentally accessible statements are a union of a fixed minimum number of inaccessible statements. For example, the value of truth of the statements a and b is not accessible, but the value of truth of the statement “a or b" is accessible. We do not directly assume probability theory, we exclusively define experimentally accessible and inaccessible statements and build on these notions using the rules of classical logic. We find that an interesting structure emerges. Developing this theory, we relax the logical structure, naturally obtaining a derivation of a constrained quasi-probabilistic theory rich in structure that we name theory of inaccessible information. Surprisingly, the simplest model of theory of inaccessible information is the qubit in quantum mechanics. Along the path for the construction of this theory, we characterise and study a family of multiplicative information measures that we call inaccessibility measures. arXiv:https://arxiv.org/abs/2305.05734

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Zoom link https://pitp.zoom.us/j/91350754706?pwd=V1dVdGM3Zk9MNkp4VlpCYUoxbXg3UT09

For a simple Lie algebra \mathfrak{g}, Kazhdan-Lusztig correspondence states that for certain values of the level k, there is an equivalence between two braided tensor categories: the category of modules of the affine Lie algebra of \mathfrak{g} at level k and the category of modules of the quantum group of \mathfrak{g} at q=e^{\pi i/k}. I will report on recent work to appear with T. Creutzig and T. Dimofte proving such a statement for a class of Lie superalgebras. These Lie superalgebras and their affine VOAs arise from the study of boundary conditions in 3d \mathcal{N}=4 abelian gauge theories. I will also explain how the corresponding supergroups act on the category of matrix factorizations.

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Zoom link https://pitp.zoom.us/j/92338197336?pwd=OG40V2p6V0FmalNCZnV2ZFF1NHAzZz09

One of the fascinating things about Black Hole Thermodynamics is how we often approach it in a very classical fashion, yet - if real - it is an intrinsically quantum phenomenon. I'll use the illustration of thermodynamics of an accelerating black hole to highlight some of these issues, and I'd like to raise questions on how we derive the First Law, and how unique this derivation is.

Black holes contain, deep in their interior, theoretical evidence of the failure of general relativity. A series of fundamental results, starting from the 1965 Penrose singularity theorem, proved that physically realistic initial conditions will unavoidably produce a singular black hole spacetime. It is generally expected that a full theory of quantum gravity should remove the singularities that appear in general relativity. However, the lack of proper understanding of the dynamical laws dictating the evolution of spacetime and matter in these extreme situations hinders the extraction of predictions in specific models. I will discuss, in a model-independent manner, the different possibilities that singularity regularization may open. I will then focus on fundamental open issues that need to be addressed to obtain viable nonsingular black hole candidates.

In this talk I present our solution to the information paradox published in Phys.Rev.Lett. 128 (2022) 11, 111301 and Phys.Lett.B 827 (2022) 136995 (see EPL 139 (2022) 4, 49001 for a review). Long wavelength quantum gravitational effects allow the interior state of the black hole to influence Hawking radiation, leading to unitary evaporation. I explain why the Mathur theorem is evaded due to the complex nature of the Hawking radiation superposition state. --- The presenter will be joining via Zoom for this talk.

How did the universe begin? How did it evolve to what we see now?

There was a time when few people believed such questions could even be posed in scientific terms. Now, as increasingly precise instruments deliver their treasure trove of data, the answers may be within reach.

On Wednesday, October 25, Perimeter Director Emeritus Neil Turok will tackle this intriguing topic in a Perimeter Institute Public Lecture, “Secrets of the Universe: Hiding in Plain Sight?”