Throughout the years, I have concentrated my research on mainly three subjects within the fields of Astroparticle Physics and Cosmology.
Without in any way trying to attempt to a review or a complete account of the field or of my contribution, here a list and a short description.
Primordial Nucleosynthesis (BBN)
Dark Matter
Ordinary matter only accounts for approximately 15% of the total matter in the Universe. The remaining 85% is made of something it does not interact with light, and yet exerts gravity. We call it dark matter, we know it is there, but we don't know what it is. Actually, we know what is not. Thanks to a host of astrophysical probes (among which: Cosmic Microwave Background, Galaxy Cluster mergers) we know it can not be any of the particles of the Standard Model, the building bricks of the Universe we perceive on a daily basis, and of which stars, planets, and ourselves are made of. We also know the pervades the whole Universe, and it is concentrated within galaxies of different sizes in different ways, making the bulk of their matter content, thus generating most of the gravitational force that keeps them together, preventing the stars to escape in all directions.
The game today is to try and understand what exactly this Dark Matter is: if it can not be one of the particles we know, it must be something else, there must be physics beyond the Standard Model. This does not mean that the Standard Model is wrong, but just that is incomplete, not enough to describe the entire Universe. Theorists have figured out several ways to extend the Standard Model self-consistently: preserving the part that works, and make predictions that include a stable particle that interacts through gravity with the ordinary matter. This is done today with a host of probes: from colliders where we try to produce the Dark Matter by smashing ordinary particles into each other at very high energies (you may have heard about the LHC, where the Higgs boson has also been detected for the first time), to underground experiments where we expect the Dark Matter particle to leave a signal when recoiling over the material in the experiment itself.
I have worked in several areas of the Dark Matter searches field: from how some dark matter candidates may have affected the birth, life and death of the first stars, to understanding which is the imprint that annihilation of dark matter particles would leave in the CMB, to the latest studies on the actual presence and distribution of dark matter in our own Galaxy, the Milky Way. You can find the actual papers pointed out in my Publication list, and of course contact me directly for more details.
Primordial Nucleosynthesis
When the Universe is very young and still very hot, when quarks have gotten together into protons and neutrons but the latter have not decayed yet, and electrons are too excited to combine with protons to form atoms, protons and neutrons still have enough energy to smash into each other to generate heavier elements.
This occurs all around the Universe, which is expanding quite fast, and at the same time cooling down as a consequence of this expansion. The process of nuclear fusion goes under the name of Primordial Nucleosynthesis (also known as Big Bang Nucleosynthesis, BBN), and it builds up Deuterium and Tritium (heavy Hydrogen) from ordinary protons and neutrons, Helium and some traces of Lithium, by which time the Universe has become too cold for the reactions to continue.
The nuclei produced in this way stay around until the First Stars forms, and from that new elements are generated, cooked in their hot interiors. The amounts of the “primordial elements” are a peculiar signature of the properties of the expanding Universe, and as such they have been used for several decades to test our Cosmological Model.
I studied this mostly with the group in my hometown, publishing among others a Physics Report review available here, and the code to compute the nucleosynthesis process, PArthENoPE
I also gave my (unrequested) opinion on the Lithium problem, expressing it as that an agnostic phenomenologist, see it here
First Stars
As the Universe expands and becomes colder, electrons and nuclei recombine, thus forming the atoms, and then keep cooling down and expanding. They settle into the potential wells generated by the small density anisotropies, which are gravitationally dominated by Dark Matter. While the Universe keeps expanding and cooling some of this potential wells gather enough material that the gravitational attraction wins over the cosmological expansion, and the material inside it undergoes gravitational collapse.
The gas therein heats up, as it acquires the kinetic energy of the potential well, and in an interesting fashion this brings to further cooling and fragmentation, a process that eventually leads to the formation of stars. At this stage they are the very First Stars ever in the Universe, and literally the first light to shine in an Universe which is otherwise cold, and possibly quite boring.
Given to the peculiar conditions these First Stars (sometimes referred to as Population III stars) form in (compared to those of modern stars, sorry, some more study is needed here from your part), they are supposed to have peculiar properties, such as being typically more massive, and possibly hotter than their modern analogues. We still don’t know for sure, as this happened a very long time ago and these stars are either dead or missing, and one other interesting games we are playing (astrophysicists together with cosmologists, together with many others) is to find them, and identify them for sure.
It would be a very marvelous thing to be able to tell “I was able to identify the stars that shed the first light ever in the entire Universe. And I can now tell how they looked like.” But as the First Stars are very elusive, we are playing the game with a host of possible observables, including some that may not seem too obvious at first. I have tried to understand what is the neutrino trace they leave behind, as well as worked out a scenario in which some peculiar candidates of Dark Matter interact with the environment and heavily affect the stars' formation, as well as their life and death. I wrote a critical overview on the subject which you can find here and in my opinion not much has changed since then.
There is much more on the plate, but for that you have to read the papers, and ask me questions. Don't hesitate to contact me!