Time: 2pm

Place: 2N36

# Events

Time: 2pm

Place: 4N9

Title: Black Holes in Higher-Derivative Gravity and Einstein-Scalar Gravity

Title: Incidence geometry and resolutions of Weierstrass models

Time: 2pm

Place: 4N9

Abstract: In this talk, I will present several new results in the geometry of elliptic fibrations:

In the first part of the talk, I will introduce an incidence geometry described by a representation of a given Lie algebra and explain how it contains a natural poset structure reminiscent of a Gelfand-Tsetlin pattern. This poset provides a simple counting of flats and ensembles of the incidence geometry. For an elliptic fibration, it is understood as a counting of (partial) crepant resolutions. In a physical context, the counting is interpreted as a detailed enumeration of mixed Coulomb-Higgs branches of a five dimensional gauge theory with 8 supercharges. In particular, I will present a generating function counting the mixed Coulomb-Higgs phases of a U(n) gauge theories with matter in the fundamental and anti-fundamental representations.

In the second part of the talk, I will describe the network of crepant resolutions of a SU(2) SU(3) gauge theory modeled by an elliptic fibration. This provides a fully resolved description of models studied recently by Grassi-Halverson-Shanesonn-Taylor. I will explain how representation theory aspects are retrieved by a simple analysis of intersection numbers.

Time: 2pm

Place: 2N36

Title: *Resurgence in quantum field theory: handling the Devil's invention.*

Abstract:Renormalized perturbation theory for QFTs typically produces divergent series, because the series coefficients grow factorially at high order. It has been a historical challenge to understand the asymptotic nature of perturbative series, and it has been unclear in what precise sense semiclassical expansions capture the physics of even weakly-coupled QFTs. I will discuss a recent conjecture that the semiclassical expansion of path integrals for asymptotically free QFTs yields well-defined answers once the implications of resurgence theory are taken into account. Resurgence theory relates expansions around different saddle points of a path integral to each other, and has the striking practical implication that the high-order divergences of perturbative series encode precise information about the non-perturbative physics of a QFT. These ideas will be discussed in the context of several QCD-like theories, where systematic semiclassical control over the dynamics is achieved using adiabatic compactifications on a circle. Fitting a conjecture by ’t Hooft, understanding the origin of the notorious renormalon divergences of perturbation theory of asymptotically-free QFTs allows us to see the microscopic origin of the mass gap of these QFTs in the semiclassical domain.

Time: 2pm

Place: 2N36

Title: *New Ekpyrotic Quantum Cosmology*

Abstract: This talk will present recent results in the quest to construct a viable cosmological model devoid of singularities and infinities. More specifically: the no-boundary proposal provides a compelling theory for the initial conditions of our universe. In this context, the recently discovered ekpyrotic instantons describe the creation of a classically contacting universe out of nothing. One notable consequence is that the ekpyrotic attractor can explain how the universe became classical. However, up to now these instantons ended in a big crunch singularity. We show how this can be remedied by adding higher-derivative terms, allowing a smooth bounce into an expanding phase to take place. Remarkably, and although there is a non-trivial evolution during the bounce, the wavefunction of the universe is ``classical'' in a WKB sense just as much after the bounce as before. These new non-singular instantons can thus form the basis for a fully non-singular and calculable (ekpyrotic) history of the universe, from creation until now.

Time: 2pm

Place: 2N36

Time: 2pm

Place: 4N36

Title: Shifting around cosmology

Time: 2pm

Place: 2N36

Title: Logarithmic Corrections to Black Hole Entropy V2.0

Time: 2pm

Place: 2N36

Title: New ghost-free matter couplings and derivative interactions in massive gravity and multigravity

Time: 2pm

Place: 2N36

Time: 2pm

Place: 2N36

Title: Living beyond the edge -- Higgs inflation, metastable vacua, and importance of radiative corrections

Time: 2pm

Place: 2N36

Abstract: The measurements of the Higgs mass and the top Yukawa coupling indicate that we live in a very special Universe, at the edge of the absolute stability of the electroweak vacuum. While fully stable situation allows for simple Higgs Inflationary setup, boundary and unstable situations turn out to be also very interesting, and also allow for inflation and successful Universe evolution. The key point is taking into account radiative corrections to the couplings at the energy scale $M_P/\xi$. Then, the inflationary potential can appear even for metastable SM vacuum, and the temperature potential at preheating prevents the Universe to settle in the high scale vacuum.

Title: Self-dual strings of six-dimensional SCFTs

Time: 2pm

Place: 2N36

Abstract: A distinctive feature of six-dimensional superconformal field theories is the existence of self-dual strings whose interactions encode the SCFTs' dynamics. A deep understanding of these SCFTs is still lacking, primarily due to a poor understanding of the properties of their strings, which are intrinsically strongly-coupled and can form bound states. I will explain how in several cases (including the theory of N+1 parallel M5 branes and the theory of a small E8 instanton) the strings can be described by suitable two dimensional quiver gauge theories. In particular, this makes it possible to compute the elliptic genera of these strings and of their bound states. For SCFTs for which a quiver gauge theory description is not available, I will explain how topological string theory still provides a way to study the corresponding strings.

Title: Cosmological billiards and hyperbolic Weyl groups

Time: 2pm

Place: 2N36

Abstract: The BKL analysis of spacelike singularities in cosmology gives evidence for a chaotic behavior of the four-dimensional spacetime metric near the singularity. This can be understood as a billiard motion in the Weyl chamber of a hyperbolic Kac-Moody algebra, which is an infinite-dimensional extension of the Lie algebra sl(2,R). Thus the billiard motion is controlled by the Weyl group of the hyperbolic algebra, which can be realized as the modular group PGL(2,Z). The billiard description can be applied also to 11-dimensional supergravity, the low energy limit of M-theory. In this case sl(2,R) is replaced by the exceptional Lie algebra E8, with the hyperbolic extension E10. I will in my talk explain how the modular realization of the Weyl group can be extended to the E10 case by generalizing the rational integers Z to an integer version of the eight-dimensional algebra of octonions.

Title: Cosmology in Massive GravityLand

Time: 2pm

Place: 2N36

Abstract: The last years have seen a renewed interest in theories of massive gravity. They represent an infra-red modification of gravity where the gravitational force weakens at very large scales. Heuristically, they provide the playground to understand a possible modification of GR which could potentially provide a dynamical solution to the cosmological constant problem. In this talk I will discuss a number of theoretical aspects of massive gravity theories, focusing on the relevance of the so-called Vainstein mechanism, both at the classical and the quantum level. I will also discuss how we can use cosmology from the early and late universe to constrain these theories. For example, what does the Cosmic Microwave Background Radiation know about the graviton mass?

Title: From the High Scale to the LHC in the Minimal SUSY B-L Model

Time: 2pm

Place: 2N36

Title: Flux Compactifications Grow Lumps

Time: 2pm

Place: DRL 4N9

Title: The Universe as a Cosmic String

Time: 2 pm

Place: DRL 2N36

Abstract: We are investigating modifications of general relativity that are operative at the largest observable scales. In this context, we are investigating the model of brane induced gravity in 6D, a higher dimensional generalization of the DGP model. As opposed to different claims in the literature, we have proven the quantum stability of the theory in a weakly coupling regime on a Minkowski background. In particular, we have shown that the Hamiltonian of the linear theory is bounded from below. This result opened a new window of opportunity for consistent modified Friedmann cosmologies. In our recent work it is shown that a brane with FRW symmetries necessarily acts as a source of cylindrically symmetric gravitational waves, so called Einstein-Rosen waves. Their existence essentially distinguishes this model from its codimension-one counterpart and necessitates to solve the non-linear system of bulk and brane-matching equations. A numerical analysis is performed and two qualitatively different and dynamically separated classes of cosmologies are derived: degravitating solutions for which the Hubble parameter settles to zero despite the presence of a non-vanishing energy density on the brane and super-accelerating solutions for which Hubble grows unbounded. The parameter space of both the stable and unstable regime is derived and observational consequences are discussed: It is argued that the degravitating regime does not allow for a phenomenologically viable cosmology. On the other hand, the super-accelerating solutions are potentially viable, however, their unstable behavior questions their physical relevance.

Title: Particle Production from Cosmic Strings

Time: 1pm

Place: 4N9

Abstract: Cosmic strings form when an Abelian symmetry is spontaneously broken. How can we determine if the universe is permeated by a network of cosmic strings, and what would it teach us about physics beyond the Standard Model? If the symmetry breaking scale is high (say GUT scale), then the strings are so massive that they should have been detected through lensing of the cosmic microwave background. Lighter strings cannot be exposed by gravitational probes, and we must look for evidence of them through their particle emission. For instance, radiation from the string may induce an electromagnetic cascade that is detected on Earth in the form of a diffuse gamma ray flux. I will discuss cosmic strings in a generic “hidden sector” extension of the Standard Model, which has often been studied in the context of dark matter, collider phenomenology, and electroweak baryogenesis. The Standard Model fields couple to the cosmic string, and I’ll show how this leads to particle production, and I’ll also discuss the associated cosmological and astrophysical signatures.

Title: Unwinding the Landscape

Time: 2pm

Place: DRL 2N36