The seminar of the Theory Section takes place Fridays in E3-128 and on zoom, starting normally at 12.15

- 09.10.2020,
**David Mota**(UiO): Cosmological and Astrophysical probes of Gravity Theories beyond General Relativity

Abstract: Several modifications to general relativity have been proposed with the aim to explain the nature of dark energy and the accelerated expansion of the Universe. In this talk I will review the present status of modified theories of gravity in the light of astrophysical probes of gravity in the weak-field regime, ranging from stars to cosmological scales. I begin by setting the scene for how theories beyond General Relativity are expected to behave in the different astrophysical systems, as well as their cosmological signatures. With these in hand, I present a range of observational tests with an eye to using the current and next generation of observations for tests of gravity. In particular, I will show how physical observables of the non-linear regime of structure formation are promising probes to constraining theoretical models in the nonlinear dynamics of stars, galaxies, clusters and large scale structure.

- 23.10.2020,
**Tim Linden**(Stockholm U/OKC): Thermal WIMPs on the Brink

Abstract: Weakly Interacting Dark Matter Particles (WIMPs) are among the most well-motivated models for particle dark matter. While these particles can be detected through direct, indirect, or collider experiments, only their indirect annihilation produces a guaranteed signal. Intriguingly, experimental searches using both gamma-rays and cosmic-rays are beginning to close in on this coveted "thermal annihilation cross-section". Intriguingly, several excesses in Galactic center gamma-rays and cosmic-ray antiprotons have been discovered. Even more intriguingly, these excesses may be consistent with each other. In this talk, I will summarize the current state of the field, and argue that there is hope of resolving this puzzle within the next five years.

- 30.10.2020,
**Angelo Ricciardione**(INFN Padua): Characterization of Cosmological Gravitational Waves with the LISA Detector

Abstract: Primordial Gravitational Waves (GWs) represent a key test of inflation and they are a unique tool to explore the physics and the microphysics of the early Universe. After the GW detections by the LIGO/Virgo collaboration the next target of modern cosmology is the detection of stochastic background of GWs. Even if the main probe of primordial GWs is the Cosmic Microwave Background, we will see in this talk how we can extract information about primordial GWs at smaller scale. In particular the space based LISA interferometer, in addiction to detection and characterization of GWs of astrophysical origin, will give compelling information about the cosmological background of GWs. In this talk I will summarise part of the activity developed within the LISA Cosmology working group, and, in particular, I will discuss on the ability of LISA to test primordial well motivated model of inflation and I will discuss about peculiar features of the SGWB, like anisotropy and non-Gaussian. - 20.11.2020,
**Tor Nordam**(NTNU)**:**Eulerian and Lagrangian methods for advection-diffusion problems

Abstract: In applied environmental science, such as oceanography and meteorology, transport problems are a common topic. The focus could be the movement of the ocean and atmosphere themselves, or transport of other substances such as pollutants, algae, fish eggs, etc. Different numerical approaches are used, and it is common to separate between Eulerian methods, where you solve the advection-diffusion PDE directly, and Lagrangian methods, where you simulate the random motion of an ensemble of "particles". The idea in the latter case is that the distribution of particles should evolve like the distribution described by the PDE. Mathematically, Lagrangian methods are numerical solutions of a Stochastic Differential Equation (SDEs), whose Fokker-Planck equation is the advection diffusion equation. In this talk, I will describe and illustrate the Eulerian and Lagrangian approaches, using simple 1D examples. I will discuss strengths and weaknesses of the two different approaches, and the conditions that must be satisfied for the two approaches to be equivalent. Finally, I will present some "pitfalls", where a naïve approach can lead to wrong answers.

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