91̽»¨

Abstract:

Current cosmological data seem to show that dark energy is evolving in time and that it possibly crossed the phantom divide in the past. So far the only theories that lead to such a behavior involve a nontrivial coupling between dark energy, in the form of a scalar field, and the gravitational or matter sector. We show that there is another possibility involving both a nontrivial kinetic sector in a cubic Galileon theory and a scalar field potential that breaks the Galileon shift symmetry, which can lead to a similar phenomenology on large scales. We perform a full Bayesian analysis using the latest cosmological data, including DESI DR2 baryonic acoustic oscillation measurements, type Ia SNe measurements from DESY5, Union3, and Pantheon+, and cosmic microwave background data from Planck and ACT. We find that it is statistically strongly favored over a universe dominated by a cosmological constant (with a Bayes factor of $\log B≃6.5$ ). Yet, as with other nonminimally coupled theories, it has severe ancillary gravitational effects. These can be mitigated to some extent, but as with other viable theories, the penalty is ever more elaborate scalar field models of dark energy.

Abstract:

The late 20th and early 21st century became an extraordinary era for the Savilian chair and the astrophysics subdepartment at 91̽»¨. It coincided with the golden age of physical cosmology, in which the study of the origin and evolution of the Universe was transformed from a speculative and data-starved backwater, looked upon with some suspicion by astronomers and physicists alike, to a precise, data-driven, and well-resourced field, a jewel in the crown of modern physics. By chance, the Savilian professors in post during that period, George Efstathiou (from 1988 to 1998) and Joseph (‘Joe’) Silk (from 1999 to 2010), were two of the leading figures in the genesis of the modern age of cosmology, instrumental in laying down the groundwork for the revolution that was to come.

Abstract:

The current cosmological model, known as the \Lambda $\mathrm{Âì}$ -Cold Dark Matter model (or \Lambda $\mathrm{Âì}$ CDM for short) is one of the most astonishing accomplishments of contemporary theoretical physics. It is a well-defined mathematical model which depends on very few ingredients and parameters and is able to make a range of predictions and postdictions with astonishing accuracy. It is built out of well-known physics – general relativity, quantum mechanics and atomic physics, statistical mechanics and thermodynamics – and predicts the existence of new, unseen components. Again and again it has been shown to fit new data sets with remarkable precision. Despite these successes, we have yet to understand the unseen components of the Universe and there has been evidence for inconsistencies in the model. In these lectures, we lay the foundations of modern cosmology.