Cosmology
During the past 100 years, physics has made unexpected discoveries in the study of the Universe, leading to the development of what is now known as Precision Cosmology, or simply Cosmology. Specifically, our group focuses on both Theoretical and Observational Cosmology.
Today, Cosmology is based on the Big Bang Standard Model, with General Relativity as a background. However, this model is not able to give a fully consistent description of the Universe. In this respect, two main paths are explored: either extending and completing the Standard model or investigating physics beyond it.
Cosmology is thus full of puzzles, tensions and open questions, and our research is actively addressing these challenges. Below, there is a brief overview of our main research topics. For more technical details about our work, please refer to our Publications.
Cosmological Inflation
One of the research topic explored by our group in theoretical cosmology is Inflation. It refers to a period in the very Early Universe during which spacetime underwent an exponential acceleration. This theoretical paradigm emerged during the 1970s and 1980s, and despite its fundamental importance in explaning why and how the Universe has evolved into its current state, there is still no consensous within the scientific community towards a single model that can univocally describe this phenomenon. In addition, dealing with the Early Universe requires a multidisciplinary approach that combines theoretical and observational cosmology, relativistic quantum information, gravitation theories and quantum field theory in curved spacetime. This makes Inflation a fervent and exciting area of research.
Late Time Cosmic Acceleration
Observations show that the expansion of our Universe is accelerating at late times. The ΛCDM model, also known as the concordance model, is currently regarded as the simplest cosmological model that fits all observational data reasonably well.
This paradigm suggests that dark energy, interpreted as a constant term Λ, i.e., the cosmological constant, drives the acceleration of the Universe, dominating over matter and radiation in the current epoch.
However, the concordance model faces two fundamental challenges: the cosmological constant problem and cosmological tensions. To address these issues, dark energy can also be interpreted as a dynamical scalar field that evolves throughout the history of the Universe.
We explore various alternatives to the ΛCDM model to describe the Universe's accelerated expansion and alleviate the cosmological constant problem and cosmological tensions.
One such alternative is the quasi-quintessence field, a scalar field in which the kinetic term does not contribute to the pressure, thereby alleviating the cosmological constant problem.
Additionally, we consider non-minimal couplings between dark energy and gravity to better explain the behavior of the Universe at late times.
Dark Energy Effects on Structure Formation
Our research group also focuses on the exploration of dark energy, the force responsible for the universe's accelerated expansion. We investigate time-dependent equations of state (EoS) for dark energy, especially those featuring rapid and recent transitions inspired by Horndeski scalar-tensor theories. Utilizing an advanced generalization of the spherical collapse formalism that incorporates both background expansion and pressure perturbations, we examine how dynamic dark energy scenarios affect the formation and growth of cosmic structures, ranging from galaxies to galaxy clusters. Our numerical analyses indicate that in models where dark energy asymptotically approaches the cosmological constant (freezing models), the sigma eight parameter may decrease by approximately 8%, presenting a promising solution to existing cosmological tensions. Furthermore, these models predict unique signatures in the nonlinear matter power spectrum and the current distribution of massive galaxies. Our findings enhance the understanding of dark energy's role in the cosmos and offer critical insights that can be tested through large-scale surveys like the Euclid mission.
Dark Energy Stability Analysis
Another research topic pursued by our group involves studying the stability properties of dark energy models within different gravity scenarios.
Stability analysis is a technique used to examine the behavior of dark energy at the cosmological background level, determining its characteristics at late times. By employing this approach, we identify the critical points of various dark energy systems, modifying gravity frameworks, introducing couplings between dark energy and the gravity sector, dark energy and dark matter, and more.
The characteristics of these critical points are determined through linear stability analysis, which enables us to identify the attractor points of the systems.
When linear stability analysis is insufficient, we proceed with numerical analysis using phase-space graphs, or we employ advanced techniques, such as the center manifold theorem, to go beyond linear analysis.
Specifically, we have applied stability analysis to Einstein, teleparallel, and symmetric teleparallel theories of gravity, showing that dark energy can mimic the cosmological constant at the end of its dynamics.
Additionally, we studied a different type of dark energy scalar field, known as quasi-quintessence. Through stability analysis, we discovered that this type of scalar field can unify the dark sector, providing a model in which dark energy behaves as dark matter.