18 March 2021, 14:00-15:00: “First Galaxies in Cold, Warm, and Fuzzy Dark Matter Cosmologies” Dr Philip Mocz, Princeton University, NASA Einstein Fellow, Lyman Spitzer Jr Fellow.

Summary: The near-century-old dark matter (DM) problem is one of the most intriguing mysteries in modern physics. I discuss how the first galaxies that form in the Universe are a unique probe for the nature of dark matter. These first objects form in low-mass DM potential wells, probing the behavior of DM on kiloparsec (kpc) scales. I present pioneering simulations of what the young Universe would look like if DM were ultra-light, in the so-called ‘fuzzy dark matter’ (FDM) limit where DM is a ~10^-22 eV boson, and contrast this against Warm and Cold DM models. The simulations highlight the interplay between baryonic physics and unique wavelike features inherent to FDM, including a new nonlinear formation channel for solitons. Future telescopes like the James Webb will soon offer an observational window into this emergent world. I will further discuss a variety of other small-scale astrophysical consequences of FDM due to its unique substructure, which place independent constraints on the FDM particle mass. I present prospects to validate or rule out FDM.

10 December 202014:00-15:00: “Gravitational Wave Astronomy” Prof Nick Stergioulas, Aristotle University of Thessaloniki. The seminar was presented in Greek,

3 December 2020, 14:00-15:00: “Magnetorotational Instabilities in Cylindrical Taylor-Couette Flows” Prof Rainer Hollerbach, University of Leeds, UK

Summary: Taylor-Couette flow is the flow generated between differentially rotating cylinders.  It is among the most fundamental problems in classical fluid dynamics.  One interesting extension is to take the fluid to be electrically conducting, and allow for externally imposed magnetic fields.  One especially interesting result in this case are so-called magnetorotational instabilities (MRIs), whereby the presence of a magnetic field destabilises a flow that would otherwise be stable.  The MRI is believed to play a crucial role in astrophysical accretion disks, where the Keplerian angular velocity profile Omega(r) ~ r^{-3/2} is in exactly this MRI regime.  In this talk I will present the basic theory of some different MRI variants, as well as show some results of liquid metal laboratory experiments.

26 November 2020: “Neutron Star Resonant Shattering Flares as Multimessenger Probes of Nuclear Physics” Dr Dave Tsang, University of Bath, UK

Summary: Neutron stars contain the most extreme physical matter in the universe, and provide a probe of physics inaccessible to normal terrestrial experiments.  The recent success of the LIGO/Virgo gravitational-wave observatories have begun new era of mutlimessenger observations of neutron star mergers which promise to provide new insights into neutron star physics. Resonant Shattering Flares (RSFs) can occur during the gravitational-wave driven inspiral, when the tidal forcing frequency due to a neutron star’s compact companion matches the resonant frequency of the crust-core interface mode, causing the crust to shatter and induce a gamma-ray flare. These RSFs are relatively isotropic and can be observed as precursors to short gamma ray bursts (SGRBs), or as orphan RSFs when the relatively narrow SGRBs are beamed away from the observer. If an RSF can be detected with coincident timing of the gravitational-wave chirp, this can be used to precisely determine the asteroseismic frequency of the core/crust interface-mode, allowing us to provide competitive constraints on nuclear physics parameters, such as the symmetry energy at nuclear saturation and its derivatives. For particular nuclear physics models, this can provide constraints that are significantly tighter than those provided by current experiments.

7 May 2020: “Exploring the Universe Isotropy with Galaxy Clusters” Konstantinos Migkas IMPRS for Astronomy and Astrophysics, Bonn.

27 February 2020: “Study of the Solar Wind with the Complex Plane Strategy” Dr Vasileios Karageorgopoulos, University of Patras.