My research
What the heck am I doing in my research?
List of papers
My research path
As an undergraduate student, I studied physics at the University of Pisa, Italy, where I graduate in December 1999, working with prof. Elio Fabri on Regge Calculus. I will never forget him, as he made me love the elegance of General Relativity.
As a graduate student, I started doing my research in Syracuse in July of 2001. Since then I have been working with my SUPERvisor Prof. Mark Trodden and other collaborators on theoretical cosmology and particle physics. It has been a lot of fun!
I have been a postdoc at the University of Sussex, Brighton, UK for two years, mainly working with Prof. Mark Hindmarsh. My experience in the UK has been short but fruitful. My second postdoc was done in Louvain, Belgium, working with Prof. Ringeval and Prof. Gerard. An experience I really enjoyed a lot!
I have worked for two years as postdoc at the Department of Physics & Astronomy at the Tokyo University of Science at Tokyo, Japan with Prof. Shinji Tsujikawa. The position was a special fellowship called JSPS, which treats postdoctoral researchers very well, under many points of view. I enjoyed working in Japan with Shinji.
For two years I have been a Lecturer in Theoretical Physics at The Institute for Fundamental Study (IF), Naresuan University, Phitsanulok, Thailand.
Now, I am an associate professor at the Institute for Theoretical Physics (YITP), Kyoto University, Kyoto, Japan. I have got another chance in my life to sink in the never-ending mystery of Japan.
Minimal theories of gravity
I have recently worked with prof. Mukohyama (and other collaborators) on the possibility that theories of gravity could be minimal, i.e. only having two gravitational degrees of freedom (as in General Relativity). Among several possibilities, we have then introduced two models which aim to describe gravity and its phenomenology at large scales and late times.
Massive (bi)-gravity
I have recently worked with prof. Nakamura and prof. Tanaka on the possibility that bigravity could be detected via graviton oscillations. With prof. Mukohyama I have exploired the possibility to make massive single gravity a consistent theory by introducing a new coupling for the quasilaton field.
Primordial non-gaussianities
Shinji and I started working on this subject, in order to set constraints to Galileon-like theories during inflation. Since the speed of propagation for the scalar field changes, and it can become smaller than one, one can realize large non-gaussianities in the CMB spectrum that can be tested against observations. The task was really tough, as many complex calculations were involved. By developing some techniques in Maple, I was able to calculate non-gaussianities for a very large class of theories, the most general 2nd order scalar-tensor theories.
Galileon gravity
Recently a new possibility has emerged to try to explain Dark Energy. In particular, a new matter field which possesses a kind of Galilean symmetry on Minkowski spacetime, has been studied in order to see whether such a field could excape Solar System constraints, via the Veihnstein mechanism, and if the same field could account for the acceleration of the universe. Together with Prof. Tsujikawa, I found a tracker solution present for this field on FLRW backgrounds, on which the physics of the Galileon becomes easy and elegant.
Gauss-Bonnet gravity
A possibility of modifying gravity consists of choosing the gravity Lagrangian as a function of R, the Ricci scalar. However, one can also choose the Lagrangian as R+f(G), where G is the so-called Gauss-Bonnet scalar, which is a quantity quadratic in the Riemann tensor. The model seem to be a reasonable possibility. In fact, together with Shinji, I showed that it is possible to define models which can have a viable cosmological background and, at the same time, satisfy solar system constraints.
Afterwards, we showed that these theories suffer from instabilities regarding scalar cosmological perturbations. The result is general for any function f(G) whose background are close to General Relativity at early times. I believe this constraint is sufficient to put a very strong constraint on these theories.
Extra dimensions and Dark Energy
It is possible to construct classical models of extra-dimensions based on Field Theory and General Relativity. The goal is to gain deeper understanding into these systems based on tractable and well known theories. In particular, the so-called Randall-Sundrum (RS) models in various dimensions can be modelised as hyper-dimensional topological defects. Our present studies concern the realisation of the Dvali-Gabadadze-Porrati mechanism inside hyper-dimensional monopoles, in which four-dimensional gravity can be obtained by trapping gravitons.
Dark Energy
The recent acceleration of the universe is explained in the standard model by the presence of a non-vanishing cosmological constant. However, one may also question the validity of General Relativity on length scales that have never been so accurately tested so far. However, it is not trivial to modify gravity and to build at the same time a model which can survive all the experimental tests. In collaboration with S. Capozziello, we looked for cosmological exact solutions for modifications of gravity in the form f(R) where f possesses a constant of motion during the evolution of the universe. In the future we plan to look for the subset of solutions which can describe experimental data from radiation domination up to today's accelerated expansion.
I have been working recently on ghost issues on Modified Gravity Models, the ones which use inverse powers of cMy rvature invariants in order to achieve cosmic acceleration. For this project I worked withProf. M. Hindmarsh, and M. Trodden. The problem is that for these models de Sitter is not a stable vacuum. The late-time cosmological attractors are in fact power law solutions. It becomes important then to see the stability of a general FRW background about these backgrounds. A detailed analysis shows that both the scalar and the tensor modes of the metric can become ghosts, superluminal or classically unstable modes. A nice paper, in my opinion, which set strong constraints on these models.
I applied similar bounds as those treated before on the so called Gauss-Bonnet cosmology with Prof. de Carlos, and G. Calcagni. The difference in these models is that one has only one independent scalar degree of freedom.
When I was a graduate student at Syracuse I worked on a “hot” topic: to try to modify standard General Relativity in order to describe the acceleration of our universe. For this project I worked with Prof. Sean Carroll, Vikram Duvvuri, Damien Easson, Mark Trodden and Prof. Michael Turner. We looked for a modification of the Einstein-Hilbert action of gravity by including a general function of different invariants formed only by using the metric tensor. I liked this project a lot and also I had the chance to meet Vikram (Ciao Vikram!). If I were you and would like to read a paper of mine, I would read this one.
In this previous project of mine I had the pleasure to work with Sean Carroll and Mark on the possibility for the equation-of-state parameter w, to be less than -1. We tried to put constraints on these such theories by pointing out the presence of quite a big fine tuning for a large class of theories. In this project I could learn a lot on scalar-tensor theories and also to appreciate the nice weather in southern California (Santa Barbara).
Baryogenesis
My last project with Mark has been about baryogenesis in the context of scalar tensor theories. I investigated the possibility that a scalar field non minimally coupled with gravity could be responsible for the presence of asymmetry between baryons and anti-baryons. This was a project after I attended TASI 2004, a summer school that quite influenced me in my understanding of physics.
On this topic I also started my research here in Syracuse as a graduate student. This was a very useful time to learn and understand how to work in the theoretical cosmology environment. [Also the English, at the beginning, was a worry of mine]. For this project I worked with Mark Trodden and Salah Nasri to try to use a quintessence scalar field to achieve at the same time Dark Energy behavior, Inflation and Baryogenesys.
Topological defects
In my second project I worked with Prof. Mark Bowick and Mark Trodden. We studied a particular class of topological defects in gauge theories, namely the Dirichlet topological defects. They are defects which end on other defects, for example a string that ends on a domain wall. We gave the profile for some solutions of these topological objects [Some high-resolution plots can be found and downloaded at Dirichlet]
Gravitational Waves
As a graduate student I had to take an experimental class where I was supposed to make an experiment. I chose an experiment that could be related to my research as much as possible. Luckily in the physics department of Syracuse University there is a group that works on Gravitational Waves whose leader is Prof. Peter Saulson. I have been working on a project with Peter to understand if the annealing of some fibers could help reduce the noise involved in the mirrors of the LIGO experiment.