DEPARTMENT OF PHYSICS / HIP JOINT COLLOQUIA / SEMINARS 2025

  • Tuesday 10 June 2025 at 10:15 in A315 and using Zoom: Reijo Keskitalo (Berkeley)
    Next ten years of CMB observations from the Chilean high desert
    Abstract:     I will discuss aspects of observational cosmology in the context of ground-based CMB experiments in the Chilean Atacama desert. The talk will include overviews of the Simons Observatory and CMB Stage 4 (CMB-S4) with an eye on survey design, simulations and data reduction.
  • Thursday 12 June 2025 at 10:15 in A315 and using Zoom: Marcel Niedermeier (Aalto)
    Re-interpreting tensor networks: quantum-inspired algorithms for hard problems in quantum physics and beyond
    Abstract: Solving problems in many-body quantum physics is hard – as soon as we consider quantum systems made up of multiple constituents, the complexity of the problem grows exponentially with the size of the system. A well-established tool for tackling these systems are different tensor network algorithms, which may reduce the parameter count of the problem to scaling only linearly with the system size, if the entanglement in the system is low. Recently, it has been recognised that the same tensor network algorithms have yet a much wider range of applications to offer. While they are rooted in quantum many-body physics, ultimately, expressing a given quantity as a tensor network can be understood as analogous to the compression of information contained in an image. In this talk, I will discuss the quantics prescription, which allows us to represent arbitrary numerical functions as tensors – and thus make them amenable to a tensor network representation, using the tensor cross interpolation algorithm. This idea is ultimately due to the fact that functions in physics typically exhibit a lot of structure – smoothness, organisation in length scales, etc. – and structure implies compressibility. After a thorough introduction, I will illustrate this idea with two examples. First, I will discuss the solution of non-linear partial differential equations with quantics tensor cross interpolation methods, using the Gross-Pitaesvkii equation as an example. By transforming the initial state and the time-evolution operator into quantics tensor trains, we are able to resolve physical phenomena across length scales separated by seven orders of magnitude within one hour on a single-core PC, thus greatly surpassing the capabilities of conventional numerical methods. Second, I will present an improved way of performing self-consistent mean-field calculations in the context of super-moiré tight-binding models. Here, the attainable system size is in principle severely restricted by the number of mean-field parameters to be calculated in each iteration for every system site. Using a quantics tensor train representation of the mean-field Hamiltonian, we could however show that a high-fidelity approximation of the Hamiltonian can be obtained while reducing the necessary number of parameters up to hundred-fold. Solving mean-field systems with tens of millions of sites is now feasible, thus surpassing the previous state of the art by orders of magnitude. I will conclude by giving an outlook on some ongoing research.
    A. Fumega, M.Niedermeier and J. Lado, 2025 2D Mater. 12 015018.
    Y. Sun, M. Niedermeier, T. Antao, A. Fumega and J. Lado, arxiv:2503.04373 (2025).
    M. Niedermeier, A. Moulinas, T. Louvet, J. Lado and X. Waintal, in preparation (2025).
  • Tuesday 17 June 2025 at 10:15 in A315 and using Zoom: Saga Säppi (Barcelona)
    TBA
    Abstract: TBA
  • Tuesday 24 June 2025 at 10:15 in A315 and using Zoom: Tyler Gorda (Frankfurt and Ohio)
    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