Sommerfeld Theory Colloquium (ASC) LudwigMaximiliansUniversität München

 Vetenskap
The Arnold Sommerfeld Center for Theoretical Physics organizes regular colloquia about topics of current interest in the field of theoretical physics.

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Reexamining Cosmic Acceleration
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 nonHubble `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.

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Branes, Islands, and Massive Gravitons
Quantum Gravity in Antide Sitter space coupled to a nongravitating bath has been the setting for novel approaches to the blackhole information paradox. Works from 2 decades ago in the context of RandallSundrum 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.

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Conjectures on Quantum Gravity and their Realisation in String Theory
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 socalled 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.

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Playing with the building blocks of the Universe
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.

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Influence of the fermionic exchange symmetry in the 1particle picture
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 fewfermion 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 HartreeFock 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.

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Precision at the LHC: why and how
With Run II, the LHC experiment already reached an unprecedented
level of precision compared to previous hadron colliders. The amount
of data collected in Run III and the HighLuminosity run will increase
quickly and dramatically. I will discuss specific challenges to theorists
that must be overcome to provide predictions that have the precision
necessary to match the accuracy of data. A few novel ideas to exploit
this precision directly or indirectly will also be discussed.