DEPARTMENT OF PHYSICS / HIP JOINT COLLOQUIA / SEMINARS 2025

  • Thursday 17 April 2025 at 10:15 in A315 and using Zoom: Jacopo Ghiglieri (Nantes)
    Thermal axion production in the early universe
    Abstract:     I will introduce the axion as a solution to the strong CP problem and show how axion models give rise to two separate cosmological populations of axions: a condensate of zero-momentum modes acting as dark matter and a “hot” ensemble of thermally-produced axions acting as dark radiation and thus subject to present and future constraints from the effective number of neutrinos Neff. I will concentrate on this latter population and show how the contribution to Neff and its uncertainty can be quantified from thermal QCD dynamics above its crossover transition. In more detail, I will show how the leading-order thermal axion production rate has been computed in the past for axion momenta of the order of the temperature and I will show how different schemes for the incorporation of the collective dynamics of soft gluons extrapolate differently into the regime of softer axion production, thus giving us a first quantitative handle on the theory uncertainty of the rate. I will also show how popular gauge-dependent resummations in the thermal gravitino and axion literature are actually plagued by IR divergences stemming from an improper handling of the chromomagnetic sector. Finally, I will show how, upon solving the Boltzmann equation, the theory uncertainty on the axion rate translates to an uncertainty on Neff.
  • Tuesday 6 May 2025 at 10:15 in A315 and using Zoom: Andreas Schmitt (Southampton)
    TBA
    Abstract:     TBA
  • Thursday 8 May 2025 at 10:15 in A315 and using Zoom: Björn Garbrecht (Munich)
    TBA
    Abstract:     TBA
  • Tuesday 13 May 2025 at 10:15 in A315 and using Zoom: Viatcheslav Grishin (European Spallation Source, Lund)
    TBA
    Abstract:     TBA
  • Tuesday 20 May 2025 at 10:15 in A315 and using Zoom: Helime Ruotsalainen (Helsinki)
    TBA
    Abstract:     TBA
  • Tuesday 3 June 2025 at 10:15 in A315 and using Zoom: Zoltan Fodor (Wuppertal)
    TBA
    Abstract:     TBA
  • Tuesday 19 August 2025 at 10:15 in A315 and using Zoom :  Viatcheslav Mukhanov  (LMU Munich)
    Gravitationally Dominated Instantons
    Abstract:     We study the decay of the false vacuum in the regime where the quantum field theory analysis is not valid, since gravitational effects become important. This happens when the height of the barrier separating the false and the true vacuum is large, and it has implications for the instability of de Sitter, Minkowski and anti-de Sitter vacua. We carry out the calculations for a scalar field with a potential coupled to gravity, and work within the thin-wall approximation, where the bubble wall is thin compared to the size of the bubble. We show that the false de Sitter vacuum is unstable, independently of the height of the potential and the relative depth of the true vacuum compared to the false vacuum. The false Minkowski and anti-de Sitter vacua can be stable despite the existence of a lower energy true vacuum. However, when the relative depth of the true and false vacua exceeds a critical value, which depends on the potential of the false vacuum and the height of the barrier, then the false Minkowski and anti-de Sitter vacua become unstable. We calculate the probability for the decay of the false de Sitter, Minkowski and anti-de Sitter vacua, as a function of the parameters characterizing the field potential.
  • Tuesday 26 August 2025 at 10:15 in A315 and using Zoom :  Mordehai Milgrom  (Weizmann Institute of Science)

    The Bekenstein Memorial Lecture  on his 10th Anniversary:

    Parting with dark matter by departing from Newton and Einstein – modifying the dynamics as an alternative to dark matter

    Abstract: Yaacov (Jacob) Bekenstein made major contributions to the MOND research program. The MOND paradigm proposes that Newton’s laws of dynamics (gravity and inertia), as well as the theory of general relativity, need to be modified when dealing with very small accelerations, such as are found in galactic systems and the Universe at large. MOND proposes different dynamics that account for the behavior of these systems, and the large anomalies they exhibit, without invoking the presence of dark matter, which is required by standard dynamics. I will expound MOND, with its basic axioms, its main predictions and their confrontation with observations, and its underlying formal theories proposed to date, with special emphasis on Yaacov’s contributions.

  • Tuesday 2 September 2025 at 10:15 in A315 and using Zoom : Mikhail Katsnelson (Nijmegen)
    TBA
    Abstract:     TBA

 

Hopefully the up to 50 min + 10 min discussion long seminar/colloquium will be understandable to a wide audience.
Contacts: Keijo Kajantie (keijo kajantie at helsinki fi) [ HIP seminar],
Jaakko Annala (jaakko annala at helsinki fi) [cosmo seminar]

New Nordic series of seminars over the web, link: https://sites.google.com/view/nordictheory/home
For more information, and to subscribe to email announcements, contact Oscar Henriksson (oscar henriksson at helsinki fi)

Other related seminars

Friday 10-12 seminar series in D114:Astrophysics seminar
Mathematical Physics Seminar and Workshop series Wed 14-16 in Exactum C123.
InstituteQ Events and Seminars