Cosmology Seminars
We have weekly seminars dedicated to Cosmology and Astroparticle Physics (we also have other regular weekly meetings to discuss the latest results in our field). Former presentations are available from the past cosmology seminars pages. Please write to the contact below to join the mailing list.
Time: Wednesdays 14:1515:15, during term time.
Place: room A315 (3rd floor of the Physicum building)
Format: 50′ + 10′ for questions
Contact: Sofie Marie Koksbang
Scheduled Seminars
Spring Term


 25.01.2019 (A315 14:1515:115) Abhishek Atreya (CAPSS, Bose Institute)
On Dark Matter Self Interactions, Viscosity and Cosmic Expansion
Abstract: In this talk we focus on the self interacting dark matter (SIDM) paradigm which can provide us with a consistent explanation of certain astrophyiscal observations that are in conflict with the cold dark matter (CDM) paradigm. We will make use of the constraints on SIDM crosssections from astrophysical observations, to estimate the mean free path of the dark matter particles. Assuming thermalization within this volume we estimate the shear viscosity (ζ) and bulk iscosity (η) of SIDM, within kinetic theory formalism. The dissipation due to viscosity affects the solution of Einstein’s equation through energy conservation. In a simplified model, we calculate this change and try to argue that σ/m constraints on SIDM provide us with sufficient viscosity to contribute significantly to the observed cosmic cceleration at present epoch. We next calculate H(z) and q(z) within the simplified model, and perform χ2 analysis, using the the cosmic chronometer data, to estimate the best fit model parameters . The best fit values also explain type IA supernova data quite well.  20.02.2019 Shinsuke Kawai (Sungkyunkwan U.)
Dynamical systems in GaussBonnet gravity
Abstract: In this talk I will describe some concepts of dynamical systems applied to semirealistic models of higher curvature (GaussBonnet) cosmology, including linear stability theory, bifurcation theory and coordinateinvariant characterization of chaos.  13.03.2019 Cyril Pitrou (IAP)
Precision big bang nucleosynthesis with improved Helium4 predictions
Abstract : Primordial nucleosynthesis or BigBang Nucleosynthesis (BBN) happens during the first 300 seconds of our Universe. It is one of the three evidences for the bigbang model, together with the cosmological expansion and the Cosmic Microwave Background (CMB). Now that the number of neutrino families and the baryonic densities have been fixed by laboratory measurements or CMB observations, it has become a parameter free model. Hence, it is widely used to exclude or constrain extension of the standard cosmological model. Since the most recent measurements of primordial deuterium and Helium4 abundances reach the percent level in precision, it is necessary to obtain a similar precision in theoretical predictions. I will first review the basic physics of primordial cosmology and BBN and then review the various effects which need to be taken into account at that epoch in order to obtain such precision.  03.04.2019 William Kinney (University at Buffalo & Stockholm University)
Swamp Creatures of the Inflationary Landscape
Abstract: Cosmological inflation provides an elegant framework for explaining initial conditions for a flat, homogeneous universe with Gaussian, nearly scaleinvariant perturbations. Embedding inflation within a UVcomplete theory has presented challenges, including eternal inflation and the possibility of a multiverse within the landscape of String Theory. I will discuss current ideas and issues in inflationary model building, including eternal inflation and recently proposed “swampland” conjectures for consistency with stringbased UV completions.  17.04.2019 Thomas Collett (ICG, Portsmouth)
Strong gravitational lensing as a cosmological probe
Absract: Local measurements of the expansion rate are in tension with those inferred from observations of the distant Universe. Is this the first sign of new physics or merely a sign of systematic errors within individual probes? This key question remains unsolved, because there are only a handful of established probes. Here I will talk about how strong gravitational lensing offers a new window on precision cosmology, shining a new light on the dark Universe. I will present strong lensing constraints on the expansion rate of the Universe and the equation of state of dark energy. I will also show how lensing combined with stellar dynamics yields the most precise test to date of the validity of General Relativity on extragalactic scales.  15.05.2019 Eemeli S. Tomberg (University of Helsinki)
Preheating in Palatini Higgs inflation
Abstract: Preheating is a process at the end of inflation, where energy is transferred from the inflaton field to Standard Model particles. I discuss preheating in Higgs inflation in the Palatini formulation of general relativity and compare it to the standard metric formulation. In the metric case, preheating in Higgs inflation proceeds through the production of massive gauge bosons which decay into fermions. However, in the Palatini formulation, the leading preheating channel is the tachyonic production of Higgs particles. This tachyonic preheating is very efficient, leading to a short reheating period, which affects the model’s predictions for cosmological parameters such as the CMB observables.  29.05.19 LumiPyry Wahlman (University of Helsinki)
Can we Probe Gravitational Degrees of Freedom through Inflation?
Abstract: Higgs inflation is an intriguing model because of its minimalistic nature and predictions, which agree with observations. However, loop corrections to the Higgs selfpotential can foil the flat plateau and corrections to the gravitational action can altogether destabilise pure Higgs inflation. Furthermore, we must specify the gravitational degrees of freedom. In the usual formulation of General Relativity, the metric is the only free variable, but we could take both the metric and the connection to be independent degrees of freedom, as is the case in Palatini formulation. I will discuss the differences of these two approaches in the context of Higgs inflation.
 25.01.2019 (A315 14:1515:115) Abhishek Atreya (CAPSS, Bose Institute)
Autumn Term
