Talks

I will discuss two versions of the simplicial Lorentizian path integral, namely the (Lorentzian) quantum Regge and the spin foam version. I will do so in the simple context of de Sitter cosmology. This simple example will reveal the important role of light cone irregular configurations in the simplicial path integral — I will show that these can either lead to an exponentially enhanced or an exponentially suppressed amplitude. I will then highlight an important difference between the spin foams and quantum Regge path integral, which affects the probability for the creation of the (de Sitter) universe.

Warped extra dimensions were originally introduced as a way of addressing the hierarchy problem. But insights from these scenarios have extended to black hole physics, cosmology, and even mechanisms for addressing issues in purely four dimensions. We investigate this scenario and some compelling lessons. The presenter will be joining via Zoom for this talk.

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