Sunday, 28 September 2014

Kaleidoscope PRD

A short post to share that one of the figures in our paper on the combined effect of self-interacting dark matter and dark acoustic oscillations has been chosen to appear in the "Kaleidoscope" of the journal were it was published: Physical Review D. Follow this link to check it out!

Tuesday, 3 June 2014

Simulating SIDM with dark acoustic oscillations

After many conversations with particle physicists and cosmologists working on particle models of dark matter, I became aware of the possibility of additional interactions on the dark sector (other than self-interactions) that could affect the formation of galactic systems.

As I mentioned in an earlier post, if dark matter is 'charged' (in its own sector), then self-interactions can be understood as a Coulomb-like interaction (analogous to Rutherford scattering). The carriers of this new force in the dark sector can be thought of as 'dark photons', and thus one would also expect scattering to occur between the dark photons and the dark matter (analogous to Thomson scattering). It turns out that under the right conditions, this scattering is substantial and quite relevant in the early Universe, it provides a source of pressure that counteracts gravity stalling the growth of the primordial dark matter structures. Once the Universe expands and cools down, the probability of scattering becomes lower and the pressure vanishes. From that point onwards, only dark matter self-interactions are relevant. The phenomenon I just described is entirely analogous to what happens to ordinary matter and photons during the early Universe, it is well known and has been verified by observations, the signature of the balance of radiation pressure and gravity is called baryon acoustic oscillations. In analogy, the phenomenon in the dark sector is called 'dark acoustic oscillations' (DAOs).
This new interaction might be a viable mechanism to supress the formation of low-haloes (more suceptible to be affected by this 'radiation pressure' since their gravitational pull is lower than more massive haloes). This in turn implies a reduction in the abundance of dwarf galaxies, which would alleviate a common problem of the CDM model that tipycally predicts a population of dwarf galaxies that is more abundant than observed.

Motivated by this possibility, we have performed computer simulations of the cosmological formation and evolution o dark matter structures for models that have both self-interactions and DAOs. Our results are a proof of concept of the potential of these models to address the shortcomings of the CDM model, purely by introducing new dark matter physics. Follow this link to read all about our study in a recent paper.

Thursday, 29 May 2014

The first galaxy in the world within SIDM!!

Together with Mark Vogelsberger, Christine Simpson and Adrian Jenkins, we have performed the first simulation in the world within the cosmological context of Self-Interacting Dark Matter!!

In the figure below, I show an schematic description of what we have done. In previous works, we had studied the distribution of dark matter in a SIDM cosmology. In this work, we have added the physics of the gas and stars (hydrodynamics, star formation, supernovae,...) using the code AREPO.  The ordinary matter and SIDM are coupled through gravity and the end result is a dwarf galaxy that globally looks similar to the analogous simulated galaxy in CDM, but that in detail differs substantially. In particular we found that the stars are centrally distributed in a way that mimics the flat dark matter distribution of their SIDM halo.

This a promising result: the signature of dark matter collisions could be imprinted in the stellar distribution of dwarf galaxies. In this link you can read the full paper. 

Friday, 21 March 2014


In a series of posts I have given some background information about the exciting possibility that dark matter might be self-interacting. Thanks to the award of a Marie Curie Fellowship, I will be able to continue my research on SIDM by exploring different questions related to galaxy formation/evolution in SIDM.

Most significantly, together with a team of international collaborators, we will try to simulate, for the first time, the formation of a galaxy in the context of SIDM. So far, all studies based on SIDM have been done exclusively at the level of considering the growth of dark matter structures only. We aim at extending these studies by including the physical processes that are responsible for the formation of stars, their evolution and death as supernovae (in the case of the most massive stars).

I'll try to keep this blog updated with our latest results.

Thursday, 20 March 2014

Dark matter haloes in SIDM

In the last post I briefly described how galaxies form within dark matter haloes, which form in time due to gravity starting from the initial fluctuations (seeds) that we can observe in the CMB. If dark matter can collide with itself, then collisions prevent the accumulation of dark matter in the center of haloes (galaxies), distributing energy among adjacent regions. The prediction in SIDM is that haloes have substantially lower central densities (see figure).

There is observational evidence coming from the dynamics of stars in nearby low-mass (dwarf) galaxies (e.g. the satellites of the Milky Way), that these galaxies inhabit haloes with central densities lower than expected from a pure CDM model. In the figure, I show schematically the results my collaborators and I obtained from SIDM numerical simulations (these results are published in this article).

Thursday, 13 March 2014

Dark matter haloes in CDM

One of the reasons why we believe dark matter exists is that it is necessary to understand how galaxies came to be starting from the primordial conditions in the Early Universe. The 'oldest' light in the Universe we can observe is called the cosmic microwave background (CMB). It comes from a time when the Universe was approximately 380,000 years old, quite an early epoch compared to the age of the Universe today, nearly 14 billion years. From the CMB we can infer that at the time the Universe was very homogeneous, containing regions with only small differences in temperature and density. With time, the regions with the highest density pulled gravitationally the matter around it and grew ever larger in time. We need the additional gravitational pull of a substantial amount of dark matter to explain how the primordial tiny density fluctuations in the CMB grew large enough into very dense galactic systems.

The dark matter fluctuations end up evolving into extended dark matter structures called dark matter haloes, which are supported against gravitational collapse by the large velocity dispersion of the dark matter it contains. Galaxies form within these haloes from the condensation of gas into the centre given rise to the formation of stars. A galaxy like our own, the Milky Way, is surrounded by a dark matter halo that extends ten times beyond the size of the visible galaxy.

The figure shows the result of a numerical simulation that models (starting from the conditions in the CMB) the growth of a halo like the one around our own galaxy (credit, Springel et al. 2008). The halo is comprised of a smooth distribution of dark matter and a plethora of smaller dark matter clumps (called subhaloes). The Milky Way galaxy would live in the centre of the halo, while the Milky Way satellites would live within the different subhaloes.

The CDM model makes two key predictions for the galaxy population: a large abundance of subhaloes and large dark matter densities towards the centre of haloes/subhaloes. The latter is not the case in SIDM as I will discuss in the next post.

Friday, 28 February 2014

Self-Interacting Dark Matter

When I talk about uncovering the nature of dark matter as a particle, I mean that we would like to know the basic properties of the dark matter particle (or particles) such as its mass and the type of interactions it possess. As I mentioned in the previous post, available interactions with ordinary matter are very much constrained, but is possible that dark matter is part of a rich dark sector with its own forces. For instance, one could think of dark matter as being 'charged' and interact with each other via a Coulomb-like interaction, just as electrons/protons interact with each other. Of course, this charge is not the ordinary charge but it only affects particles in the dark sector. See for example this paper for an example of the modeling of this idea.

These type of 'self-interactions' between dark matter particles are allowed by observations to a significant level. They can be large enough to impact the formation and evolution of galaxies. The proposal of self-interacting dark matter (SIDM) became in fact quite popular at the turn of the millennium with a famous paper by D. Spergel and P. Steinhardt as a way to modify the standard CDM paradigm and obtain predictions that were seemingly in better agreement with the observed inner dynamics of galaxies.

The SIDM idea attracted me specially since it might be possible to look for signatures of non-gravitational dark matter interactions in the observed properties of galaxies, a possibility that is absent in the CDM model. I will talk more about SIDM and galaxies in the next post.

Thursday, 20 February 2014



This is a first post to kickstart this blog about my recently funded research project: The impact of collisional dark matter in galaxy formation: Time for a paradigm shift? This project is being funded by the European Union via a Marie Curie Fellowship.

In a series of posts I will describe the background and main objectives of the project. In the following, a few remarks about the elusive dark matter, the main topic of my scientific research.


There is substantial evidence from astronomical observations that the vast majority of matter in the Universe is 'dark', i.e. it doesn't emit electromagnetic radiation at any observable level. The nature of dark matter is one of the greatest mysteries of modern physics, and although we know very little about its properties, we know that it must be a new particle beyond the standard model of particle physics (or perhaps a whole new set of particles and forces in an invisible dark sector!!).

All we know about dark matter comes from its gravitational effects, in particular, we know its existence is fundamental to explain how galaxies form and evolve. However, if dark matter only interacts gravitationally, our quest to understand its particle nature would be essentially hopeless, we hope for additional interactions that could give us clues about the dark sector. As I mentioned above, we know that dark matter is quite dark, so far it has remained invisible to our telescopes, but it might still interact albeit too feebly, with ordinary matter. Indeed, there are significant efforts to discover dark matter laboratories on Earth by either producing directly in the Large Hadron Collider or by looking for signals of its collisions with different targets in several experiments (e.g. LUX).

Discovering non-gravitational dark matter interactions would be a breakthrough in our understanding of this mysterious form of matter. For our theories of how galaxies form and evolve however, this type of interactions with ordinary matter are irrelevant, they are too weak to play a role in this process. This fact, has led to the common hypothesis that dark matter is, for these purposes, collisionless. Indeed this is one of the key hypothesis of the current paradigm, the Cold Dark Matter model.

But there is one possibility that remains open that could change substantially our understanding of galaxy formation/evolution, and could provide clues about the dark matter nature. This possibility is that of non-gravitational interactions between the dark matter particles themselves, without a direct connection with ordinary matter... More about this in the next post!!