Research Interests

Particle Cosmology

My research area is Particle Cosmology. Cosmology is the study of the Universe as a system. Modern Cosmology is based on the Hot Big Bang (HBB) model, which provides a successful description of the Universe history as far back as the first second of its existence. However, the HBB leaves many questions unanswered. The answers hide in the elusive primordial era, when the Universe was so dense and hot that its evolution was determined by High Energy Physics phenomena. In fact, since the energy scale of the cosmology in the Early Universe is much higher than the scale of electroweak unification, one needs to employ Physics beyond the Standard Model to investigate the remaining mysteries. Particle Cosmology uses what is known or conjectured about the fundamental interactions in the context of Particle Theory for the study of the Early Universe.

Cosmic Inflation

One of my main areas of research is Cosmic Inflation. This can be defined as a period of accelerated expansion of space in the Early Universe. Inflation is a compelling solution to the problems of the HBB that have to do with why the Universe appears to be so big and so uniform on very large scales. It also provides an elegant way to generate the small perturbations in the density of the Universe, which are necessary for the formation of structures such as galaxies and galactic clusters. The existence and the characteristics of these Primordial Density Perturbations (PDP) have been observationally determined because they reflect themselves on the Cosmic Microwave Background radiation (CMB) by generating anisotropies and polarisation.

My research regarding Inflation aims to construct and study the dynamics of realistic inflationary models based on Particle Theory. I am also interested in the generation of the PDP during Inflation, which can be explored by investigating the behaviour and evolution of suitable fields, belonging to simple extensions of the Standard Model. These fields may or may not be related to inflationary expansion itself (if not they are called Curvatons). At this level there is also considerable interface between cosmology and String Theory, Branes and Large Extra Dimensions. It should be noted that recent, precise CMB observations from the Planck satellite, have confirmed the basic predictions of Inflation. Hence, Cosmic Inflation is now considered a necessary extension of HBB cosmology.

Quintessential Inflation

Another open issue for cosmologists today is the recent observation that the Universe seems to be engaging into an era of accelerated expansion at present. This is attributed to a mysterious Dark Energy substance, whose nature and origin is unknown. The simplest choice of Dark Energy is a Cosmological Constant. However, such a constant has to be incredibly fine-tunned (by more than a hundred of orders of magnitude) to explain the observations. Alternatively, it has been suggested that the Universe is undergoing a late period of inflation, driven by a scalar field called Quintessence; the fifth element after Dark Matter, neutrinos, photons and baryons. In my research I concentrate on the possibility that the field that drives Cosmic Inflation in the Early Universe is the same with Quintessence. The study of such Quintessential Inflation is not only economical but has the advantage of using a single theoretical framework to determine the global evolution of the Universe from extremely early times until the present. As such, information from Dark Energy observations may shed some light in early Universe physics at high energies. Also, the fact that the electroweak energy scale (probed by collider experiments such as the LHC) is roughly the geometric mean of the Planck energy scale (corresponding to gravity) and the Dark Energy scale implies that, through quintessential inflation, it is possible to obtain feedback from the early and late Universe on particle physics phenomenology.

Primordial Gravitational Waves

Cosmic Inflation is expected to generate a stochastic background of primordial gravitational waves (PGW). This a generic prediction of inflation, which is why the observation of PGWs will be a smoking gun of this scenario. Even though there are many forthcoming gravitational wave observations (e.g. LISA, Advanced LIGO, Einstein Telescope, Cosmic Explorer etc.), inflationary PGW are difficult to observe because they are very faint. However, this depends on the history of the Early Universe. Indeed, it is possible that, some time before the formation of light nuclei by the process of Big Bang Nucleosynthesis (BBN), the Universe was dominated not by radiation (as suggested by the HBB) but by a substance with a stiff equation of state. In this case, part of the PGW spectrum is enhanced and can become readily observable in the near future. In particular, in Quintessential Inflation, such a period follows naturally the inflationary phase. The form of the enhanced PGW spectrum, if observed, can provide crucial information about the Early Universe, which can shed light into the theoretical background of Cosmic Inflation and beyond.

Primordial Black Holes

Primordial Black Holes (PBHs) are a natural outcome of many inflationary scenarios, as they are formed by rare spikes in the spectrum of primordial density perturbations, generated by inflation. PBHs can be the Dark Matter in the Universe, which makes up about 25% of the Universe budget at present. They could also seed the supermassive black holes which reside at the centres of galaxies and are responsible for Active Galactic Nuclei. Their formation depends not only on the details of inflation but also on the conditions in the post-inflation Universe. If they are small enough, they are expected to evaporate through the emission of Hawking radiation, which could heat the Universe after inflation but might also have many other effects on the CMB radiation. After the direct observations of binary black hole mergers, there is intense interest in exploring the cosmology of PBHs.

Primordial Magnetic Fields

Another area of my research is the possible generation of large-scale Primordial Magnetic Fields (PMFs) in the Early Universe. Such fields can be responsible for the observed intergalactic magnetic fields as well as the magnetic fields of the galaxies themselves. One can generate PMFs only in out-of thermal equilibrium conditions, because they break isotropy. Such opportunities exist during Phase Transitions in the Early Universe (at the breaking of symmetries of the fundamental interactions) and during Cosmic Inflation (when the Universe is supercooled because any preexisting entropy is inflated away). In my research I investigate how the process of particle production during inflation can generate primordial magnetic fields which are strong and coherent ehough to explain galactic and inter-galactic magnetism.