Skip to main content

Dark energy, non-linearites and effective theories

Periodic Reporting for period 1 - DEFT (Dark energy, non-linearites and effective theories)

Reporting period: 2015-10-01 to 2017-09-30

"The main goal of this project was deepening our knowledge about the dark energy that generates the current acceleration of the Universe. Cosmological observations of many different kinds have shown that the Universe is not just expanding, but it does so faster and faster as time goes by, stretching the fabric of spacetime. The ""engine"" behind this acceleration is called dark energy by cosmologists. Its pull is too weak to be felt at short distances in our daily lives, but its cumulative effect on very large scales is the main drive of cosmological dynamics. In fact, the observations made with telescopes on Earth and orbiting around it indicate that about three quarters of the Universe are actually made of dark energy. Clearly, understanding in detail its properties (about which we still know very little) will allow us to put in perspective our place in the cosmos... And more so given that the matter we are made of only amounts to five percent of the total energy budget of the Universe. The discovery of dark energy, made in 1998 using the light of exploding stars called supernovae, was awarded the Nobel Prize in Physics in 2011. In my MSCA project I have studied how the description of this phenomenon, happening on vast cosmological distances can be connected to the dynamics of continuous media (such as a fluid or a superfluid) that interacts with gravity. It turns out that the propagation of tiny cosmological inhomogeneities along space and time resembles the dynamics of sound waves (i.e vibrational modes) on continuous media. This analogy can be carried forward in a mathematically precise and practical way using the language of effective theories (which has been very successful in many branches of physics) by studying the symmetry properties of spacetime and focusing on the relevant energy regimes. The objective of my project was to pave the way for the interpretation of future cosmological data within this context and tools, in order to learn about the properties of the acceleration of the Universe.

As explained above, my work has shown how the large scale dynamics of the Universe might be easily described using standard tools that have been developed for decades to study a variety of physical phenomena ranging from subatomic particles to condensed matter systems. The project has deepened into the description of dark energy as a continuous medium, showing how the properties of such a system may be used to describe the global properties and dynamics of the Universe at very large scales. It has thus given insights on how to test dark energy from the effects of small fluctuations in the fabric of spacetime and from the distribution of galaxies in space. The framework that I have developed may be used in the near future to analyze and interpret the data of future ground-based and satellite observations of the Universe.

I have also proposed a very minimal extension of the Standard Model of particle physics which aims to connect several long-standing problems of particle physics and cosmology. In particular, I have explored how the homogeneity and isotropy of the Universe, the unbalance between the amount of matter and antimatter, the symmetry properties of the binding force of nuclei, the small mass of the neutrinos and the origin of the dark matter, may all be related in a simple and testable model.

My work has also been devoted to study two other possibilities for dark matter. One, that it may be a composite particle (i.e. made of yet more fundamental particles) connected to the origin of the mass of the Higgs boson. And the other, completely different, that the dark matter might be atomic-sized black holes produced at the very early stages of the life of our Universe.
The output of this project is at the forefront of research on several key issues of fundamental physics: the evolution of the Universe and its current phase of acceleration, the dynamics of galaxies and the constituents of the subatomic world. The impact on the research communities of particle physics and cosmology has been high.
Part of my research results during this period have also been featured on the media (newspapers, radio, etc) and my outreach activities have also helped to disseminate them across a broad range of audiences.