Astrophysics

The equations establish a relationship between the geometry of space-time, represented by the Ricci curvature tensor Rµν and the metric tensor gµν while the distribution of matter and energy is represented by the energy-momentum tensor Tµν . The cosmological constant Λ, originally introduced to describe a static universe, now represents the energy density of the vacuum and explains the accelerated expansion of the universe at late times. Thanks to GR, we are able to study and describe relativistic astrophysical phenomena, like accretion processes, gamma-ray bursts or extreme objects such as black holes and neutron stars formation and the gravitational interaction between and surrounding them. However, regardless of the successes of GR, some problems arise leading to question its validity as the true theory of gravity. Consequentially, our research group is also dedicated to investigate alternatives and extensions to GR.

BLACK HOLES

One of the topic the research group focuses on are black holes (BHs). These ob- jects are solutions to the field equations of general relativity, so massive that not even light can escape within a radius called the event horizon. At their center, general relativity predicts the existence of a singularity where the curvature of space-time becomes infinite and the known laws of physics break down, suggest- ing a limitation of the theory as it lacks a quantum description of gravity to deal with these singularities. The first BH solution was discovered by Schwarzschild in 1916, describing a static black hole in a vacuum. Since then, many solutions have been developed, including rotating or charged black holes, and together how these compact objects interact with accretion disks and their formation and therefore matter, or in binary systems. Indirect proof of their existence arrived with phenomena like gravitational waves, recently detected from merging black holes with LIGO and VIRGO, and the Event Horizon Telescope image of the black hole shadow in M87 galaxy. However, black hole physics is still an open field and our research group investigates it by studying how accretion processes around black holes are influenced by the presence of different matter models, by considering the effects of repulsive gravity on the particles surrounding them or by introducing alternative models for these objects, such as regular black holes: solutions that eliminate the singularity and ensure finite space-time curvature. This is achieved by modifying the theory of gravity or by including corrections 1 such as those from non-linear electrodynamics. These modifications are useful for studying how they affect relativistic astrophysical processes. Specifically, the group studies their influence on accretion models and properties such as quasi-normal modes, quantum corrections, and non-linear electrodynamics. 

GAMMA-RAY BURSTS

The CosmoLab group is also focused on high-energy astrophysical sources that can be adopted to map the universe up to very high-redshift regions. Such probes can be gamma-ray bursts (GRBs), the most powerful explosions in the universe that are in the majority found in intermediate/high-redshift domains. These events can be classified into two categories, depending on the duration of the burst: short-GRBs when it lasts less than two seconds or long-GRBs when it lasts longer than two seconds. Also, the intertwining of various GRB observables lead to different linear correlations providing to be essential in order to use them as cosmological distance indicators. These correlations are divided into two main groups depending on the observables we deal with: prompt-emission correlations when prompt emission quantities are considered and prompt-afterglow emission correlations when quantities from both the prompt and afterglow emission are taken into account. However, despite their potentiality to connect early and late epochs when combined with other probes they are affected by a circularity problem jeopardizing their use as distance indicators. The circularity prob- lem is the consequence of the dependence on a background cosmological model through the observable quantities entering the correlations. Circumnavigating this issue is fundamental because GRBs can shed light, e.g., on the behaviour and evolution of dark energy. This is of outmost importance, considering that the recent outcomes by the DESI Collaboration seems to point in the direction of a dynamically evolving dark energy rather than a cosmological constant. Our research group works on way to elude the circularity problem and standardize these objects with the adoption of model-independent techniques among which figure the use of B´ezier polynomials to reconstruct the Hubble rate without the assumption of a cosmological model a priori. 

THEORIES OF GRAVITY

While GR has been successfully proven over the years (e.g. the bending of light by gravity, the detection of gravitational waves, etc.), there are still many open problems, and our research group focuses on some of them. The first is the lack of a unifying framework with quantum mechanics, often referred to as quantum gravity. In fact, GR is not renormalizable, meaning that it cannot describe phenomena on quantum scales, such as near black hole singularities or in the earliest moments of the universe. Furthermore, the nature of dark matter and dark energy, which make up most of the matter in the universe, remains 2 unsolved. These problems have led to the formulation of alternative theories to describe gravity. Among these, we have modified gravity models which extend GR by introducing additional components, to account for dark energy and dark matter or a modified gravitational behaviour. For example, a class of modified gravity models are scalar-tensor theories, in which a scalar degree of freedom is added to the theory to study its effect on the overall dynamics of the problem; indeed, the group studied some of these models such as f(R) theories, teleparallel gravity or Horndeski models. Other examples include alternative models for the dark sector, e.g. an extension of the Chaplygin gas to describe the dark energy or models of dark matter with particular features such as non-standard pressure configurations. This allows us to study features and interactions with modified dynamics, applying them to large/small-scale structures to test deviations from GR. 3