Sommerfeld Theory Colloquium (ASC)

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Searching for Cosmic Strings in New Observational Windows Jun 10, 2026

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Many particle physics theories beyond the Standard Model (BSM) admit topologically stable cosmic string solutions. If Nature is described by such a theory, a network of strings will form in the early universe and persist to the present time. The strings carry energy and hence lead to characteristic signatures in many observational windows. Searching for signals of strings in the sky can lead to new constraints on BSM models. Conversely, cosmic strings may also help solve some current mysteries in astrophysics, e.g. the origin of super-massive black holes.

Particle physics: Plan B Jun 10, 2026

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Particle physics is at the crossroads. The last particle firmly predicted by the Standard Model (SM) has been discovered. In recent years many of its interactions with other known particles have been experimentally studied, again confirming the SM's predictions in large detail. However, despite this impressive success, we are sure that the SM is incomplete. The Standard Model can not explain dark matter, neutrino masses, and matter-antimatter asymmetry of the Universe, and we do know what particles are responsible for these phenomena — their masses, interaction strength, life time etc. I will discuss how the synergy with Cosmology and Astrophysics can help to limit these uncertainties and discuss several concrete examples which can drastically change the situation in particle physics in the next 5-7 years.

Re-examining Cosmic Acceleration Jun 10, 2026

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Type Ia supernovae are standard (isable) candles so observing them out to cosmological distances reveals the change of the Hubble parameter with redshift. Such observations have been interpreted to mean that the expansion rate of the universe is accelerating, as if driven by a Cosmological Constant. However reanalysis of the data shows that the inferred cosmic acceleration is anisotropic and aligned with the CMB dipole - so is likely an artefact due to our being untypical observers embedded in a local non-Hubble `bulk flow’. Moreover the usual kinematic interpretation of the CMB dipole is rejected at 4.9 sigma as the corresponding dipole in the distribution of distant quasars is much bigger than expected. The implications of these surprising findings will be discussed.

Branes, Islands, and Massive Gravitons Jun 10, 2026

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Quantum Gravity in Anti-de Sitter space coupled to a non-gravitating bath has been the setting for novel approaches to the black-hole information paradox. Works from 2 decades ago in the context of Randall-Sundrum braneworlds makes it clear that this system necessarily describes massive gravitons: as soon as one couples AdS gravity to a bath, the graviton gets a mass. These two phenomena are intrinsically linked. We'll review old work that led to this conclusion, and present novel results that elucidate the role of the graviton mass in the determination of the Page curve for evaporating AdS black holes.

Conjectures on Quantum Gravity and their Realisation in String Theory Jun 10, 2026

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A central question in fundamental physics is when an effective field theory can be consistently coupled to gravity at high energies. Over the years, various necessary conditions for this to be possible have been conjectured. String theory is a proposed framework for a quantum gravity theory and hence allows us to quantitatively test and further develop such ideas. In this colloquium I will discuss this with special emphasis on the so-called Weak Gravity Conjecture and the Swampland Distance Conjecture, or its refinement dubbed Emergent String Conjecture. Among other connections to mathematics, we will see how these proposed general principles naturally follow from the geometry of string compactifications near the boundary of moduli space and are deeply routed in string dualities. This includes situations with a minimal amount of supersymmetry in four dimensions.

Playing with the building blocks of the Universe Jun 10, 2026

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The conventional models of high energy physics place the elementary particles on a fixed or slowly evolving, nearly flat, spacetime geometry. The space foam predicted for quantum theory of gravity by A.Wheeler and S.Hawking gives a very different picture of the small scale structure of spacetime, with Planck size black holes popping out of vacuum here and there. Recent discoveries in astronomy and astrophysics, the detection of gravitational waves, exploration of Sgt A* , have confirmed the existence of enormous black holes, yet quantum size black holes remain the subject of theoretical amusement. I describe a vast class of statistical mechanical models, which, on the one hand, perform exact calculations in supersymmetric field theories in various dimensions, whose thermodynamic limit, on the other hand, can be interpreted as describing fluctuating geometry, an `emergent gravity'. The configurations of these models are the arrangements of (hyper)cubes, hence the title.

Influence of the fermionic exchange symmetry in the 1-particle picture Jun 10, 2026

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In a recent breakthrough, a complete set of constraints on fermionic occupation numbers, extending Pauli’s original exclusion principle, has been found. We provide an introduction into this new research field. In particular, we show that those generalized Pauli constraints are approximately saturated in various few-fermion systems, i.e. the vector of occupation numbers lies close to the boundary of the allowed region. Striking implications of this quasipinning phenomenon are revealed and discussed: These are the concept of robust quantum information, a hierarchy of natural extensions of the Hartree-Fock ansatz and the existence of an exchange force emerging from the fermionic exchange symmetry. Finally, we explain how these novel research concepts offer a universal perspective on fermionic quantum systems.

Quantum Gravity and the Swampland Jun 10, 2026

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String theory seems to offer an enormous number of possibilities for low energy physics. The huge set of solutions is often known as the String Theory Landscape. In recent years, however, it has become clear that not all quantum field theories can be consistently coupled to gravity. Theories that cannot be ultraviolet completed in quantum gravity are said to be in the Swampland. In this talk, I'll discuss some conjectured properties of quantum gravity, evidences for them, and their applications to cosmology and particle physics.

Hunting for the stochastic gravitational-wave background: Implications for astrophysics, high energy physics, and theories of gravity Jun 10, 2026

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I will first define the stochastic gravitational-wave background (SGWB) and highlight the method we are using to detect it in the presence of correlated magnetic noise. I will then discuss astrophysical (compact binary coalescences) and cosmological (cosmic strings, first-order phase transitions) sources and report on the current constraints imposed from a non-detection during the last observing run of the LIGO/Virgo/KAGRA collaboration. I will also address the question of a simultaneous estimation of astrophysical and cosmological SGWB. Then I will present a search for circularly polarised SGWB and its relation to early universe cosmology. Finally, I will discuss how the SGWB can provide tests for gravity theories, including quantum gravity proposals.

Topological Phase Transitions in Population Dynamics Jun 10, 2026

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Topological phases were discovered in condensed matter physics and recently extended to classical physics such as topological mechanical metamaterials. Their study and realization in soft-matter and biological systems has only started to develop. In this talk we discuss how topological phases may determine the behavior of nonlinear dynamical systems that arise, for example, in population dynamics. We have shown that topological phases can be realized with the anti-symmetric Lotka-Volterra equation (ALVE). The ALVE is a paradigmatic model system in population dynamics and governs, for example, the evolutionary dynamics of zero-sum games, such as the rock-paper-scissors game [1], but also describes the condensation of non-interacting bosons in driven-dissipative set-ups [2]. We have shown that for the ALVE, defined on a one-dimensional chain of rock-paper-scissors cycles, robust polarization emerges at the chain’s edge [3]. The system undergoes a transition from left to right polarization as the control parameter passes through a critical value. At the critical point, solitary waves are observed. We found that the polarization states are topological phases and that this transition is indeed a topological phase transition. Remarkably, this phase transition falls into symmetry class D within the “ten-fold way” classification scheme of gapped free-fermion systems, which also applies, for example, to one-dimensional topological superconductors. Beyond the observation of topological phases in the ALVE, it might be possible to generalize the approach of our work to other dynamical systems in biological physics whose attractors are nonlinear oscillators or limit cycles. [1] J. Knebel, T. Krüger, M. F. Weber, and E. Frey, Phys. Rev. Lett. 110, 168106 (2013). [2] J. Knebel, M. F. Weber, T. Krüger, and E. Frey, Nature Communications 6, 6977 (2015). [3] J. Knebel, P. M. Geiger, and E. Frey, Phys. Rev. Lett. (in press) [arXiv:2009.01780].

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