Previous seminars
Joint Seminar 1
Speaker: Paul Lasky (Monash University)
Time and Date: 5:00pm in JST / 9:00am in CET, February 24th 2022
Title: What's next in gravitational-wave astronomy?
Abstract: The LIGO and Virgo interferometers have observed almost 100 gravitational-wave signals from collisions of black holes and neutron stars. Meanwhile, pulsar timing arrays around the globe are beginning to see interesting signals that may be attributed to a gravitational-wave stochastic background, potentially from supermassive black hole binaries throughout the Universe, or even primordial gravitational waves from the very early Universe. In this talk, I will review the state of both ground-based gravitational-wave astronomy and pulsar timing arrays. I will also discuss our ability to use current and future gravitational-wave observations to test Einstein's general theory of relativity in the ultra-strong field regime.
Joint Seminar 2
Speaker: Shane O'Sullivan (Dublin City University)
Time and Date: 5:00pm in JST / 9:00am in CET, March 25th 2022
Title: Radio polarimetry and Cosmic Magnetism
Abstract: In this talk, I will introduce the techniques of radio polarimetry and Faraday rotation for the study of cosmic magnetic fields. Faraday rotation is a birefringent effect caused by magnetised plasma along the line of sight, which we measure using the frequency-dependent rotation of the plane of linearly polarized light from radio galaxies (i.e. synchrotron emission). Radio galaxies can be observed throughout the majority of the history of the Universe and are thus excellent beacons for measuring the properties of the cosmic web and their evolution with cosmic time. In particular, I will highlight recent results from the Low Frequency Array (LOFAR) radio telescope. LOFAR is the world's premier low-frequency radio telescope, providing exceptional RM precision, in addition to unrivalled angular resolution, sensitivity and image fidelity, which facilitates the reliable identification of the host galaxy through comparison with optical and infrared data (from which one can then determine the redshift). Our recent work shows how these capabilities are allowing us to transform our understanding of cosmic magnetic fields and are providing a new way to study the properties of filaments and voids of the cosmic web in general.
Joint Seminar 3
Speaker: Luca Visinelli (Tsung-Dao Lee Institute)
Time and Date: 4:00pm in JST / 9:00am in CET, April 27th 2022
Title: Axion Miniclusters in the Milky Way
Abstract: Axion miniclusters (AMCs) are relatively dense, gravitationally bound clumps of dark matter (DM) QCD axions. AMCs have intriguing observational consequences for Earth-based axion detectors, for DM substructure searches with microlensing, and for radio signatures from AMC encounters with neutron stars (NSs). However, the properties of AMCs in the Milky Way may be drastically altered by tidal interactions with ordinary stars. We present Monte Carlo simulations following the evolution of AMCs orbiting in the Milky Way, which can be used to estimate the properties of AMCs throughout the Galaxy today and can be easily recast. We use this information as a key ingredient in estimating the rate, duration, flux, and sky locations of radio signals from axion-photon conversion due to NS encounters with AMCs. The resulting radio transients are within reach of current and future radio telescopes, opening a new avenue for detecting QCD axion DM.
Joint Seminar 4
Speaker: Hiroki Kawai (The University of Tokyo)
Time and Date: 4:00pm in JST / 9:00am in CET, May 25th 2022
Title: An analytic model for the structures in FDM halos
Abstract: Fuzzy dark matter is a hypothetical scalar particle whose mass is around 10^-22 eV, which is one of the alternatives to cold dark matter model to alleviate the small-scale problems. The quantum nature of FDM arises rich phenomena in small scale, such as soliton core and granular structures inside FDM halos.
For the first part in this talk, we focus on the granular structures and show an analytical model of the density profile. Using this model, we calculate a sub-galactic matter power spectrum and compare it with that obtained from the strong lens system SDSS J0252+0039.
Next we study how to determine a soliton core mass for a given halo mass. We compare the core-halo mass relation in our model and that obtained from the previous FDM simulations.
Joint Seminar 5
Speaker: Simona Vegetti (Max Planck Institute for Astrophysics)
Time and Date: 4:00pm in JST / 9:00am in CET, June 22nd 2022
Title: Strong Gravitational Lensing as a Probe of Dark Matter
Abstract: The Cold Dark Matter model for structure formation is currently the most successful at reproducing many observations, but it remains largely untested in the non-linear sub-galactic regime. A clear prediction of this model is that a significant number of low-mass haloes should populate any galaxy and its line of sight. As most of these objects are expected to be completely dark, strong gravitational lensing provides a unique channel to detect them and determine the properties of dark matter by constraining the halo-mass function at the low-mass end.
In this talk, I will review the current status of this field and present the latest observational constraints on the halo mass function. I will then discuss the most significant challenges in successfully constraining the properties of dark matter with strong gravitational lensing and how upcoming observing facilities such as Euclid the SKA and the ELT will revolutionise this field.
Joint Seminar 6
Speaker: David J.E. Marsh (King's College London)
Time and Date: 5:00pm in JST / 9:00am in CET, November 21st, 2022
Title: Axion Miniclusters: Recent Progress and Open Problems
Abstract: If the Peccei Quinn symmetry is broken after inflaiton, then axions are produced by the decay of topological defects. This scenario is attractive, since the axion mass is in principle predictable from the relic density, and favours the range ~0.1-10 meV. Miniclusters offer a route to probe this scenario astrophysically. Miniclusters are formed from the overdensities in the axion field left over after the topological defects decay. I will describe recent efforts to understand the mass distribution and density profiles of miniclusters, using theory and simulation. I will also describe efforts to test the minicluster scenario using gravitational microlensing, and radio transients. Open problems surround both scenarios, related to the possible existence and survival fraction of the densest miniclusters.