Composite and holographic axions

The idea that low energy fields such as Gravitons or Axions can be 
thought of as composite particles has been entertained since a long 
time. We revisit this idea motivated by the AdS/CFT correspondence. We 
focus in the case of axions and show that hidden sectors coupled to SM 
fields may provide an emergent axionic field that is a composite of the 
hidden fields (instanton density). This is a more general phenomenon 
beyond holographic theories. We study the general properties of such an 
"emergent axion", without a PQ symmetry. The construction goes beyond 

Inflation and Geometry

Inflation, an era of accelerated expansion of the universe prior to the radiation phase, constitutes the paradigm of primordial cosmology. Within this paradigm, the simplest single-field slow-roll models economically explain all curent data. However, the sensitivity of inflation to Planck scale physics, and the fact that ultraviolet completions of inflation invariably involve extra fields coupled to the inflaton, indicate that these models constitute at best a phenomenological description that emerges from a more realistic physical framework.

Dark energy after gravitational wave observations

The observed accelerated expansion of the Universe opens up
the possibility that general relativity is modified on cosmological
scales. While this has motivated the theoretical study of many
alternative theories that will be tested by the next generation of
cosmic large-scale structure surveys, I will show that the recent
observations of gravitational waves by LIGO/Virgo has dramatic
consequences on these theories.

Horndeski and the Sirens

Mergers of compact objects, such as black holes and neutron stars, have been nicknamed standard sirens, by analogy with electromagnetic standard candles, because their waveform directly gives access to their distance. When an electromagnetic counterpart is observed, such sources thus allow us to construct a Hubble diagram, just as supernovae. Recently, the gravitational-wave Hubble diagram has been argued to be a key probe of alternative theories of gravity, such as Horndeski models.

BMS flux-balance laws

Asymptotically flat spacetimes admit both supertranslations and Lorentz transformations as asymptotic symmetries known as BMS symmetries. Furthermore, they admit super-Lorentz transformations, namely superrotations and superboosts, as outer symmetries associated with super angular momentum and super-center-of-mass charges. In this talk, we present the flux-balance laws for all such (extended) BMS charges in terms of radiative multipole moments at future null infinity.

Vacuum stability without Supersymmetry: Brane dynamics, bubbles and holography

n this talk I will present some recent results on instabilities of anti-de Sitter flux compactifications in effective field theories arising from non-supersymmetric string models, namely the USp(32) and U(32) orientifold models and the SO(16) x SO(16) heterotic string. We focus on non-perturbative instabilities, framing the vacua in terms of brane stacks, analyzing their back-reacted geometry and reproducing AdS its near-horizon limit.

D-brane model building and phenomenology

I will present a review on D-brane realizations of the Standard Model and their phenomenological consequences. 
First, I will provide the basic ingredients of D-brane (open string) models which are useful for model building. 
Next, I will use them in order to embed the Standard Model in open string vacua. 
Finally, I will discuss new gauge fields and new matter particles which are predictions of the D-brane realizations of the Standard Model described above.

Miniworkshop on Holographic QCD at high densities, neutron stars and gravitational waves

This one-day workshop will bring together theorists working on the AdS/CFT correspondence, nuclear physics, and General Relativity for an interdisciplinarly exchange aimed at developing a consistent holographic picture of QCD matter at high density, from model building to the description of compact objects and observation via gravitational wave signals. Invited speakers include: Umut Gursoy (Utrecht U); Matti Jarvinen (Utrecht U. and Tel Aviv U.); Niko Jokela (Helsinki U.); Micaela Oertel (LUTH); Eric Gourgoulhon (LUTH). 


Singularity avoidance, clocks and primordial power spectrum

There are ways to cure the singularity problem in cosmology
by means of quantum effects. In the process, one encounters
the question of time, which can for instance be addressed by
using an internal degree of freedom. Applying such a method
to an anisotropic Bianchi I model where the shear serves as a
clock, I show that some generic features can be obtained for
the trajectory of the average scale factor, all avoiding the
singularity and reaching similar asymptotic states, deemed
semi-classical. Using a trajectory approach yields similar


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