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!!