**Upcoming seminars**

**Thursday May 17, 2pm**, room 110

Marios Petropoulos (CPHT, Ecole Polytechnique) : **TBA**

**Thursday May 24, 2pm**, room 110

Hayato Motohashi (YITP, Kyoto University) : **Constructing degenerate higher-order theories**

Scalar-tensor theories serve models for inflation and dark energy. Many efforts have been made recently for constructing the most general scalar-tensor theories with higher-order derivatives in their Lagrangian. Since higher-derivative theories are often associated with Ostrogradsky ghost which causes unbounded Hamiltonian, it is important to clarify how to evade it. In this talk, I will explain construction of healthy degenerate theories with higher-order derivatives which circumvent Ostrogradsky ghost. The method also allows us to construct ghost-free theories with derivatives higher than second order in Lagrangian.

**Past seminars**

**Thursday April 12, 2pm**, room 110

Matt Lewandowski (IPhT, Saclay) : **The EFT of Dark Energy and the EFT of Large-Scale Structure**

In the next few years, we are going to probe the low-redshift universe with unprecedented accuracy. Among the various fruits that this will bear, it will greatly improve our knowledge of the dynamics of dark energy. A particularly useful description of dark energy is through the Effective Field Theory of Dark Energy, which assumes that dark energy is the Goldstone boson of broken time translations. Such a formalism makes it easy to ensure that predicted signatures are consistent with well-established principles of physics. Since most of the information resides at high wavenumbers, it is important to be able to make predictions at the highest wavenumber that is possible. The Effective Field Theory of Large-Scale Structure (EFTofLSS) is a theoretical framework that has allowed us to make accurate predictions in the mildly non-linear regime. In this talk, I will discuss both the EFTofLSS and the EFTofDE, including the extension of the EFTofLSS to include the non-linear effects of Horndeski-type dark energy in the quasi-static limit.

**Thursday March 22, 2pm**, room 110

Sebastian Garcia-Saenz (IAP Paris) : **Vortex lines in fluids - from string theory**

Thin vortices in fluids and superfluids admit an effective field theory (EFT) description in which they can be treated as one-dimensional extended objects, or strings. In this theory, vortex lines can interact among each other as well as with the "bulk" fluid by the exchange of phonons, whose dynamics can be described by a simple scalar effective theory. The coupling between vortices and sound is however more complicated, as it requires switching to a magnetic-type dual formulation in which the phonon field is encoded by a gauge two-form. After a review of the theory and its applications, I will discuss recent work where we show that the EFT can be recast into the simpler scalar field language, in a description that is valid for closed string configurations and for bulk modes of wavelength much longer than the typical string size. This is essentially a systematic expansion of the original EFT in which the string is regarded as a point-like source with degrees of freedom given by the string’s multipole moments. As an illustration we compute the sound emitted by an oscillating vortex ring, a calculation that in our field theory language reduces to a simple Feynman diagram to leading order.

**Thursday February 1, 2pm**, room 110

Marc Casals (CBPF, Rio de Janeiro) : **Rotating black holes : spectroscopy, late-time tails and instabilities**

**Thursday January 18, 11am**, room 110

Hiromu Ogawa (Rikkyo University, Tokyo) : **Anti-screening of the Galileon force around a hole of a disk**

The Vainshtein mechanism is known as an efficient way of screening the fifth force around a matter source in modified gravity.
This has been verified mainly in highly symmetric matter configurations. To study how the Vainshtein mechanism works in a less symmetric setup, we numerically solve the scalar field equation around a disk with a hole in the cubic Galileon theory. We find, surprisingly,that the Galileon force is enhanced, rather than suppressed, in the vicinity of the hole. This anti-screening effect is larger for a thinner, less massive disk with a smaller hole. At this stage oursetup is only of academic interest and its astrophysical consequences are unclear, but this resultimplies that the Vainshtein screening mechanism around less symmetric matter configurations is quite nontrivial.

**Thursday November 23, 2pm**, room 110

Gabriel Jung (LPT) : **Non-Gaussianity in two-field inflation beyond the slow-roll approximation**

In this talk, I will discuss the level of bispectral non-Gaussianity produced in two-field inflation models with standard kinetic terms. Even though the Planck satellite has so far not detected any primordial non-Gaussianity, it has tightened the constraints significantly, and it is important to better understand what regions of inflation model space have been ruled out, as well as prepare for the next generation of experiments that might reach the important milestone of Delta f_NL(local) = 1.
I will present our results on the non-Gaussianity parameter f_NL in the case of sum potentials and show that it is very difficult to satisfy simultaneously the conditions for a large f_NL and the observational constraints on the spectral index n_s in the slow-roll approximation. I will then discuss the case of monomial potentials and show explicitly the small region of parameter space in which this is possible.
Finally, I will extend these results beyond the slow-roll approximation and illustrate them with two explicit inflation models.

**Thursday October 19, 3pm**, room 110

Shinji Mukohyama (Yukawa Institute, Kyoto) : **Towards a new scenario of inflationary magnetogenesis**

In the context of a U(1) gauge theory nonminimally coupled
to scalar-tensor gravity, we find a cosmological attractor solution
that represents a de Sitter universe with a homogeneous magnetic
field. The solution fully takes into account backreaction of the
magnetic field to the geometry and the scalar field. Such a solution
is made possible by scaling-type global symmetry and fine-tuning of
two parameters of the theory. If the fine-tuning is relaxed then the
solution is deformed to an axisymmetric Bianchi type-I universe with
constant curvature invariants, a homogeneous magnetic field and a
homogeneous electric field. Implications to inflationary
magnetogenesis are briefly discussed.

**Thursday October 12, 2pm**, room 110

Lukas Witkowski (APC Paris) : **Super-Planckian scalars ?**

Scalar fields with super-Planckian field ranges play an important role in models of large-field inflation and are also employed in cosmological solutions to the electroweak hierarchy problem. One difficulty in constructing such models is that for super-Planckian field values quantum-gravitational effects cannot be ignored. In particular, ultraviolet physics is expected to constrain the field range over which scalars can exhibit a monotonic potential, which in turn has consequences for models of inflation. To study theories of super-Planckian scalars some knowledge of ultraviolet physics is thus necessary. String Theory is a candidate theory of Quantum Gravity and a suitable framework for addressing this question. Another approach is based on the Weak Gravity Conjecture - an as yet unproven theorem regarding consistent theories of Quantum Gravity. In this talk I want to report on recent progress in the study of super-Planckian scalars. In particular, I will explain how the existence of super-Planckian scalars is increasingly called into question by both String Theory and the Weak Gravity Conjecture.

**Thursday September 28, 2pm**, room 110

Sergey Sibiryakov (CERN) : **Beta-functions and asymptotic freedom in (2+1)-dimensional Horava gravity**