Astroparticles & Cosmology

Oriol Pujolàs

The Astroparticles and Cosmology group studies the properties of elementary particles and their interactions in astrophysical and cosmological settings. Many things can be learned about particle physics in these settings because they allow to access processes that are very difficult to reproduce in the laboratory. We are interested in: axion physics, neutrinos (atmospherical and solar), phase transitions in the early universe, dark matter and, of course, dark energy.

Introduction

Astroparticle physics and particle cosmology are recent fields of research at the intersection between particle physics, astrophysics and cosmology. The main research goal is to exploit our knowledge of astrophysical and cosmological phenomena to answer fundamental physics questions. The main research lines in this area include: early universe cosmology, dark matter, axion-like particles, dark matter, gravitation and the application of the AdS/CFT correspondence to condensed matter systems.

During 2017, the work done by the members of the Theory Division in this research area concern early universe cosmology, the role of black holes as dark matter, and the application of the AdS/CFT correspondence for condensed matter phenomena. Among these works, we highlight the following.

Holographic Phonons

In collaboration with M. Ammon (Jena U, Germany) A. Jimenez-Alba (Jena U, Germany), L. Alberte (ICTP Trieste, Italy) and M. Baggioli (Crete U., Greece), O. Pujolas have demonstrated (in JHEP1801(2018)129 and arXiv:1711.03100, to appear in Phys.Rev.Lett.) that black holes can acquire solid properties, when endowed with appropriate charges. We have demonstrated this by computing the black hole mechanical properties such as the black hole elastic modulus and the speed of transverse sound. We found that they relate to each other as dictated by the elasticity theory. This allows to extend the standard AdS/CFT dictionary to materials that display solid properties. As an application, we found that solids which are in a critical (scale-invariant) regime necessarily exhibit a melting transition that is continuous, similar to what occurs in glassy materials.

Figure 1: Location of poles in the transverse retarded Green function in the complex frequency plane, for various temperatures (different colours). The pole at w=0 is the transverse phonon, and it is present at all temperatures.

Figure 2: Comparison of the transverse phonon velocity extracted from the poles of the Green function (black dots) and from from the elasticity mudulus according to elasticity theory (solid lines) for different model parameters (green - orange). Inset: The dependence of the shear elastic modulus on m/T. The fall-off at large temperatures corresponds to the melting transition.