Dark Energy and Modified Gravity

Dark energy is the most abundant and yet most mysterious substance in the Universe. The existence of dark energy can be inferred by its effects on the expansion rate of the Universe and on the growth of cosmic structures, but its fundamental nature is still completely unknown. The simplest explanation for dark energy, given by a cosmological constant, is also the most successful in reproducing observational data, but lacks a firm theoretical basis and requires an extremely unnatural fine-tuning. Alternative models are therefore continuously proposed, and a detailed understanding of their specific observational features is necessary in order to compare different scenarios against data.

The standard approach to modelling the late time acceleration of the Universe is to introduce a dark energy component and arrange its properties such that it dominates at late times. However, an alternative approach to describing the current epoch is to postulate that at cosmological distance scales, gravity deviates from its standard General Relativistic description. This can be achieved, for example, by introducing fields which mediate gravity in addition to the standard spin-2 graviton. The aim is to construct a gravitational model which allows for accelerated expansion in the absence of dark energy on large scales, yet approximately reproduces standard Newtonian and Einsteinian gravity on galactic and solar system scales.

Our group conduct research on a variety of modified gravity models, with focus on constraining deviations from General Relativity using both theoretical considerations and observational datasets.

Gravitational effects on the metastability of the Universe ground state 

The ground state of the unified theory of electroweak interactions (the Standard Model) may not be a stable quantum state, depending on the values of some parameters of the theory. The study of the metastability of this ground state is of great relevance, since it may suggest the presence of possible physical phenomena external to the Standard Model. This metastability is characterized by energy scales close to the Planck mass; it is, therefore, natural to wonder if gravitational interactions are relevant for the computation of the decay rate of the ground state. This calculation can be done employing the simplest version of general relativity, namely at the first order (Einstein General Relativity) in an expansion in inverse powers of the Planck Mass and employing a metric invariant under four-dimensional Euclidean rotations. The aim of our research is to study the gravitational effects on the Standard Model ground state metastability, employing different metrics and/or modified gravitational theories.

Nicola Maggiore
Giovanni Ridolfi

PhD Students
Erica Bertolini