Astroparticles & Cosmology

Oriol Pujolàs

The Astroparticles and Cosmology group studies on the cosmology-particle physics interplay, such as extracting information on the nature of the fundamental interactions from astrophysical and cosmological environments. This is a valuable and complementary approach because cosmological and astrophysical systems probe conditions that are very difficult to reproduce in the laboratory. We are interested in: axion physics, phase transitions in the early universe, dark matter, dark energy, neutrinos (atmospherical and solar) and modifications of gravity. Additionally, we are interested in using particle physics techniques for condensed matter problems.

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 2018, 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 and Effective Field Theory methods for solid state physics. Among these works, we highlight the following.

Applications to Condensed Matter

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 showed that the holographic models of solids (with a finite static shear modulus and propagating transverse phonons) accommodate for an explicit breaking of translations, and showed that its effect is to give the phonons a mass term in a fashion very similar to how the quark masses give the pions a mass. Moreover, we showed that the phonon mass obeys a Gell-Mann-Oakes-Renner-like relation. This opens a new approach to understand the effects of disorder on the phonons in solids.

In collaboration with L. Alberte (ICTP, Trieste) and M. Baggioli (Crete U.), PhD student Victor Cancer-Castillo and O. Pujolas have studied how the Effective Field Theory methods can be used to study consistently the nonlinear elastic response of materials, and in particular to obtain universal bounds on the elasticity (the degree of reversible deformation) attainable by materials.

Dark Matter

During 2017-2018, Prof. F. Ferrer (McDonnell Ctr. Space Sci) visited IFAE on sabbatical leave. This visit was instrumental in order to complete a work on Primordial Black Holes and the QCD axion with a number of members of the TH division: E. Masso, G. Panico, O. Pujolas and F. Rompineve. The collaboration lead to a Letter, arXiv:1807.01707, which has recently been accepted for publication in Physical Review Letters, and which we describe below.

Primordial Black Holes from the QCD Axion

Dark Matter might be in the form of supermassive black holes created shortly after the QCD epoch due to the cosmological dynamics of the QCD axion.

We proposed a mechanism to generate Primordial Black Holes (PBHs) which is independent of cosmological inflation. The mechanism takes place slightly after the cosmological QCD phase transition and it relies on the collapse of long-lived string-domain wall networks. This type of topological defect network is naturally realized in QCD axion models with domain wall number $N_{DW}>1$ and with the Peccei-Quinn symmetry broken after inflation. In our framework, Dark Matter is mostly composed of axions in the meV mass range, however, there is also a small fraction, $\Omega_{PBH} \gtrsim 10^{-6} \Omega_{CDM} $ of heavy $M \sim 10^4-10^7 M_\odot$ PBHs. The latter could play a role in alleviating some of the shortcomings of the $\Lambda$CDM model on sub-galactic scales. The scenario might have distinct signatures in ongoing axion searches as well as gravitational wave observatories.

Figure 1: Constraints on $F$ (axion decay constant) and $T_2$ (temperature at network collapse) from Dark Matter overproduction (blue shaded region), and from supernovae cooling (orange shaded). The axion solution to the strong CP problem does not require tuning below the line $\delta = 0.1$. The figure of merit $p$ measures the probability to form a black hole when the temperature was $T_2$, and the (red lines) indicate the mass of the corresponding black hole.

Figure 2: The figure of merit (BH formation likelihood) increases dramatically for structures that collapse at temperatures $T_*$ slightly below the transition temperature $T_2$. For $T_2 \simeq 7 MeV$, we show the figure of merit (dashed lines) and the BH masses.