Tag Archives: cosmological inflation

My new cosmology paper is out!

My latest paper in cosmology is now public! It took many years to write, as it was a single author side-project on a subject that is very different from my main field. My PhD was indeed in particle cosmology, while now I am mostly working on building physics and methods for energy saving in buildings. I am however still relatively active in theoretical physics, and this is meant to be the first article of a comeback into the field.


Here is the abstract:


In the framework of MSSM inflation, matter and gravitino production are here investigated through the decay of the fields which are coupled to the udd inflaton, a gauge invariant combination of squarks. After the end of inflation, the flat direction oscillates about the minimum of its potential, losing at each oscillation about 56\% of its energy into bursts of gauge/gaugino and scalar quanta when crossing the origin. These particles then acquire a large inflaton VEV-induced mass and decay perturbatively into the MSSM quanta and gravitinos, transferring the inflaton energy very efficiently via instant preheating.
Regarding thermalization, we show that the MSSM degrees of freedom thermalize very quickly, yet not immediately by virtue of the large vacuum expectation value of the inflaton, which breaks the SU(3)C×U(1)Y symmetry into a residual U(1). Compared to the case of LLe-type inflaton previously studied, we find an even more efficient energy transfer to the MSSM quanta, due to the enhanced particle content of the supersymmetric (SUSY) multiplet that is coupled to the flat direction. Full thermalization is achieved indeed after only O(40) oscillations.
We also compute the gravitino number density from the perturbative decay of the flat direction and of the SUSY multiplet. In agreement with the literature, the inflaton produces a negligible amount of gravitinos and does not raise any cosmological issues. On the contrary, the fields to which it is coupled are responsible for a severe gravitino overproduction problem, which is caused by their large VEV-induced effective masses. We argue that possible solutions might include non-coherent oscillations of multiple flat directions or fragmentation of the inflaton condensate with formation of Q-balls.


Primordial Inflation and BICEP2 results

March 17th 2014 has been a historical date for all of us cosmologists: the BICEP2 experiment results seem to provide evidence for  both gravitational waves and the inflationary expansion of the early Universe (even though the observed tensor-to-scalar ratio r = 0.20 at 1σ is in tension with the upper bound r<0.11 at 95% C.L. given by a combination of data from Planck, SPT, ACT and WMAP).

But what is cosmological inflation? As I write extensively in my PhD thesis, it was an accelerated (read: exponential) expansion of the Universe, which occurred right after the Big Bang explosion. It lasted from 10−36 seconds after the Big Bang to sometime between 10−33 and 10−32 seconds (even though r=0.2 now indicates that inflation began even earlier than that).

The theory of inflation was originally proposed in 1980 by Alan Guth and by Katsuhiko Sato, as a mechanism for solving some technical problems of the previous Big Bang theory (the so-called “standard Big Bang scenario”), which didn’t assume an accelerated expansion.

This first version of inflation was anyway predicting a too granular Universe, and still needed adjustments. This problem was solved in 1982 by Andrei Linde, and independently by Andreas Albrecht and Paul Steinhardt, in a revised version
that is now called new inflation. The basic idea is that inflation occurred by a scalar field (i.e. a particle) rolling down a potential energy hill. This particle is called the inflaton: it made the Universe expand fast and then, at the end of inflation, it disappeared decaying into the particle spectrum observed today, namely into the stars, galaxies, dark matter…and us 🙂

Most of my past and present work is devoted to this particle production mechanism indeed. I study how the particle spectrum was generated, according to different theories postulating a specific candidate to become this mysterious “inflaton”.
Professors Andrei Linde and Renata Kallosh get acknowledged of the discovery:

I won’t discuss the theory and the discovery any further, I just paste here links to internet sources which discuss these topics:


My PhD thesis http://arxiv.org/abs/1002.2835

Wikipedia: http://en.wikipedia.org/wiki/Inflation_(cosmology)

The BICEP2 paper: http://arxiv.org/abs/1403.3985

Cosmology discussions:




As a last remark, I can say that one thing is certain: after the 2012 detection of the Higgs boson, the 2013 Planck satellite results and this latest discovery of gravitational waves and experimental proof of inflation, we are all definitely living exciting times!