SEMESTER 2019 - WINTER
Antonio Enea Romano
Hubble trouble or Hubble bubble?
The recent analysis of low-redshift supernovae (SN) has increased the apparent tension between the value of H0 estimated from low and high redshift observations such as the cosmic microwave background (CMB) radiation. At the same time other observations have provided evidence of the existence of local radial inhomogeneities extending in different directions up to a redshift of about 0.07. About 40% of the Cepheids used for SN calibration are directly affected because are located along the directions of these inhomogeneities. We derive a new simple formula relating directly the luminosity distance to the monopole of the density contrast, which does not involve any metric perturbation. We then use it to develop a new inversion method to reconstruct the monopole of the density field from the deviations of the redshift uncorrected observed luminosity distance respect to the LambdaCDM prediction based on cosmological parameters obtained from large scale observations. The inversion method confirms the existence of inhomogeneities whose effects were not previously taken into account because the 2M++density field maps used to obtain the peculiar velocity for redshift correction were for z≤0.06, which is not a sufficiently large scale to detect the presence of inhomogeneities extending up to z=0.07. The inhomogeneity does not affect the high redshift luminosity distance because the volume averaged density contrast tends to zero asymptotically, making the value of H0 obtained from CMB observations insensitive to any local structure. The inversion method can provide a unique tool to reconstruct the density field at high redshift where only SN data is available, and in particular to normalize correctly the density field respect to the average large scale density of the Universe.
Max Planck Institute, Munich
New effects in Dark Matter production
How was dark matter produced in the early Universe? Answering this question is of great importance since it allows us to predict the expected experimental signatures. I will discuss non-perturbative effects in coannihilation driven freeze-out, i.e. Sommerfeld enhancement and bound state formation in a thermal environment, and analyze the connection with phenomenology. In addition, I will comment on the conditions for freeze-out and point out an alternative production mechanism of dark matter which relies on conversion processes instead of annhilations. Interestingly, this mechanism points towards long-lived particles at could be observed at the LHC.
Max Planck Institute, Heidelberg
Recent developments in large-N beta functions
There has been a growing interest in the past years in computing beta functions in theories with large number of flavour-like degrees of freedom. The singular structure of these large-N beta functions has inspired speculations of possible asymptotically safe realizations. I will review the current status of these computations, and show that by studying the connection with conformal field theory methods, one can obtain new insights on the singular structure that disfavour the UV fixed points relying on the singularities.
Center for Theoretical Physics PAS, Warsaw
Testing the dark matter paradigm, or how to falsify CDM and its alternatives?
While the Earth-base laboratories keep trying very hard to elucidate on the nature of the elusive dark matter particles the other very promising avenue to test and/or falsify potential dark matter candidates resides in astrophysical observations. In this context our own Galaxy - the Milky Way - with its unique set of satellites shows potential to serve as a extraterrestrial laboratory for dark matter. The very physical nature of dark matter particles and especially the differences between the main candidate, the neutralino of Cold Dark Matter (CDM), and its currently strongest competitor, the sterile neutrino of Warm Dark Matter candidate, may lead to significant differences in the properties of dwarf galaxies. Such objects are dominated (by mass) by their host DM haloes and therefore provide an unique view on the physical properties of DM. I shall discuss our recent efforts to use the state-of-the-art galaxy formation hydrodynamical simulation scheme of the EAGLE project as well as high-resolution Copernicus Complexio N-body simulations to study the galaxy formation of Milky Way like systems in CDM and WDM scenarios. Our results render new insights on potential ways to use astronomical observations for falsifying the CDM paradigm and testing its competitors.
Dark Matter Sommerfeld-enhanced annihilation and Bound-state decay at finite temperature
Astrophysical and cosmological observations from galactic to cosmological scales indicate the existence of dark matter. Nevertheless, most of its property still remain to be unknown and hence candidates range from 10-31 to 1050 GeV in its mass scale. Among them, the traditional dark matter candidate, so called WIMP (weakly interacting massive particle) is still attractive because its production mechanism, i.e., thermal freeze-out, naturally explains its abundance and pins down its mass scale to be 1~105 GeV. Moreover, the same interaction required for the thermal freeze-out allows us to detect them directly/indirectly. The key observable for this program is the abundance of dark matter, which is one exceptional parameter we measured very precisely, i.e., within 1% accuracy. Hence, it is desirable to give theoretical prediction of WIMP abundance for a given model within this accuracy. Based on these, I will talk about my recent attempts to refine the calculation of WIMP abundance regarding the Sommerfeld enhancement and bound state formation.
Accessible Lepton-Number-Violating Models and Negligible Neutrino Masses
In this talk, I will first review lepton-number violation (LNV) and lepton-flavor violation (LFV) in the Standard Model and beyond as well as the connection between LNV and LFV. Then, I discuss a complete family of models where lepton number is violated but the generated Majorana neutrino masses are tiny, even if the new-physics scale is below 1 TeV. The phenomenology of these models are explored, including charged-lepton flavor-violating phenomena and baryon-number-violating phenomena. I will identity scenarios where the allowed rates for μ- → e+ conversion in nuclei are potentially accessible to next-generation experiments.
Institut für Theoretische Physik, University of Heidelberg
Towards understanding the gauge hierarchy problem with asymptotically safe gravity
The gauge hierarchy problem is a question why the electroweak scale is so much smaller than the Planck scale. If there is no any new physics between these scales, this question could be rephrased: what has made the Higgs critical at the Planck scale? In this talk, we discuss that asymptotically safe gravity could provide a hint for such a criticality of the Higgs. We focus on the graviton fluctuation on the scalar potential and then see that its effect could generate a large anomalous dimension which changes the scaling behaviour of scalar coupling constants involving the scalar mass term. From this fact, we argue the possibility for the resolution of the gauge hierarchy problem. If we have time, we would mention the prediction of the Higgs mass.