2019 seminars

2019 seminars

Click on the talk title to download the presentation (if available).

Spring Term

  • 25.01.2019 (A315 14:15-15:115) 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 cross-sections 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 Gauss-Bonnet gravity
    Abstract: In this talk I will describe some concepts of dynamical systems applied to semi-realistic models of higher curvature (Gauss-Bonnet) cosmology, including linear stability theory, bifurcation theory and coordinate-invariant characterization of chaos.
  • 13.03.2019 Cyril Pitrou (IAP)
    Precision big bang nucleosynthesis with improved Helium-4 predictions
    Abstract : Primordial nucleosynthesis or Big-Bang Nucleosynthesis (BBN) happens during the first 300 seconds of our Universe. It is one of the three evidences for the big-bang 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 Helium-4 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 scale-invariant perturbations. Embedding inflation within a UV-complete 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 string-based 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 Lumi-Pyry 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 self-potential 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.

Autumn Term

  • 04.09.2019 Jussi Väliviita (University of Helsinki and HIP)
    Overview of Planck 2018 results; constraints on primordial isocurvature and tensor perturbations
    Abstract: Planck satellite surveyed the full sky 8 times in 2009 – 2013. The primary target was to measure the temperature and polarization anisotropies of the cosmic microwave background radiation from the largest scales down to very small (0.075°) angular scales, and deduce the implications for cosmological models. The first temperature maps and products were released in 2013, and preliminary polarization results were included in the 2015 publications. Most of the final legacy release papers, were 2015 analysis was improved, were submitted to arXiv and A&A in 2018, while the last of them “Planck 2018 results. V. CMB power spectra and likelihoods” appeared in the end of July 2019. In addition, Planck produced a large amount of astrophysical data. I will briefly discuss the 2018 Planck legacy release, the role of the Helsinki Planck team in the large collaboration, and then focus on the implications for the nature of primordial perturbations.
  • 06.11.19 Gerasimos Rigopoulos (Newcastle)
     ΔN and the stochastic conveyor belt of Ultra Slow-Roll
    Abstract: We analyse field fluctuations during an Ultra Slow-Roll phase in the stochastic picture of inflation and the corresponding non-Gaussian curvature perturbation, fully including the gravitational backreaction of the field’s velocity. By consistently working to leading order in a gradient expansion, we first demonstrate that the momentum constraint of General Relativity prevents the field velocity from having a stochastic source, unlike what a naive application of the separate universe picture might seem to suggest. We then focus on a completely level potential surface, V=V0, extending from a specified exit point øe, when slow roll may resume or, alternatively, inflation ends, to ø→ + ∞. We compute the fluctuation in the number of e-folds N required to reach øe, which directly gives the curvature perturbation. We find that, if the field’s initial velocity is high enough, all points exit through øe and a finite curvature perturbation is generated. On the contrary, if the initial velocity is low, some points enter an eternally inflating regime despite the existence of øe. In that case the probability distribution for N, although normalizable, does not possess finite moments, leading to a divergent curvature perturbation.

    The talk will be based on the recent paper 1910.08487.

  • 04.12.19 Susana Cebrián Guajardo (Zaragoza, the ANAIS experiment)

     Testing DAMA/LIBRA result with ANAIS-112 experiment at the Canfranc Underground Laboratory in Spain
    Abstract: DAMA/LIBRA observation of an annual modulation in the detection rate compatible with that expected for dark matter particles in the Milky Way’s halo is for about two decades one of the most puzzling experimental results in the field. It has neither been confirmed nor ruled out in a model independent way, despite the impressive improvement in sensitivity in dark matter direct detection experiments based on new detector technologies combined with strong background suppression. ANAIS experiment, using 112.5 kg of sodium iodide as target, is taking data smoothly at the Canfranc Underground Laboratory in Spain since August 2017 aiming at testing the observation by the DAMA/LIBRA experiment using the same target and technique.

    The first model independent testing of this result has been presented this year searching for modulation in the ANAIS-112 experiment [1]; recently, up to two years of data (220.69 kgxy) have been analyzed [2]. Results are compatible with the absence of modulation and incompatible with DAMA/LIBRA signal at 2.6σ; in particular, the best fits for the modulation hypothesis give amplitudes Sm=-0.0029±0050 cpd/kg/keV and -0.0036±0.0054 cpd/kg/keV for the [2-6] and [1-6] keV energy regions, respectively. These results are in agreement with our estimated sensitivity for the accumulated exposure, supporting our projected goal of reaching a 3σ sensitivity to the DAMA/LIBRA result in less than three years of data-taking from now. Before unblinding the data, the whole analysis procedure was fixed [3], the background of the experiment thoroughly studied [4] and the expected sensitivity for the five-year operation evaluated [5].

    In this seminar, after posing the DAMA/LIBRA conundrum, the ANAIS-112 experiment will be firstly described presenting the set-up, performance, analysis methods and background model developed. Then, the obtained results from the annual modulation analysis will be shown and their implications and the future prospects will be discussed.

    [1] First results on dark matter annual modulation from ANAIS-112 experiment, J. Amaré et al, Phys. Rev. Lett. 123 (2019) 031301.
    [2] ANAIS-112 status: two years results on annual modulation , J. Amaré et al, arXiv:1910.13365v2 [astro-ph.IM].
    [3] Performance of ANAIS-112 experiment after the first year of data taking, J. Amaré et al, Eur. Phys. J. C (2019) 79:228.
    [4] Analysis of backgrounds for the ANAIS-112 dark matter experiment, J. Amaré et al, Eur. Phys. J. C (2019) 79:412
    [5] ANAIS-112 sensitivity in the search for dark matter annual modulation, I. Coarasa et al, Eur. Phys. J. C (2019) 79:223.


  • 17.12.19 Hyun Min Lee (CERN and Chung-Ang University)
    Tuesday in A315 at 14:15
    Relaxation of four-form fluxes for Higgs mass, cosmological constant and flattening inflaton potential
    Abstract: We consider the cosmological relaxation of the Higgs mass and the cosmological constant due to the four-form fluxes in four dimensions. We present concrete models of a singlet scalar field with four-form couplings where the Higgs mass is relaxed to a right value as well as the Universe reheats to a sufficiently high reheating temperature after the last membrane nucleation. We also discuss some interesting features in the cases of singlet scalar fields with non-minimal or minimal couplings to gravity. We also discuss the implications of the non-minimal four-form coupling to gravity for chaotic inflation models with a pseudo-scalar field. The non-minimal four-form coupling to gravity flattens the inflaton potential at large field values, respecting the shift symmetry in the effective Lagrangian. We show the success of inflationary predictions and robustness of the results against higher order terms and address the reheating in these scenarios.