Principal Investigator
Nelson J. Nunes

Abstract
Over a decade has passed since supernova observations first indicated a remarkable property of our Universe: it is currently undergoing a period of accelerated expansion. In the intervening time, this acceleration has been confirmed by a range of complementary observational probes, such as those of the cosmic microwave background and of large scale structure and baryonic acoustic oscillations. Yet despite this plethora of observational evidence, the theoretical model that explains this phase of evolution remains unknown. The discovery of present-day cosmic acceleration posed many theoretical problems. The most obvious of which is: what is the matter content, which must violate the strong energy condition, that drives such a phase of evolution? The obvious choice is the cosmological constant, but that leads to a huge fine tuning problem. Why is the required value so small compared to the predicted value in particle physics? This puzzle puts the old cosmological constant problem of why our universe seems not to respond to the expected vacuum energy of particle physics into even sharper focus. Moreover, further problems include the “why now” question of why cosmic acceleration only starts relatively recently in the Universe's history. In dynamical models of acceleration, the “why now” problem reduces to the question of the sensitivity to initial conditions. In this project we aim to take a step towards determining the theoretical model of present-day acceleration by analysing an extremely general class of models, which can have intriguing theoretical properties, to evaluate if they can also be observationally consistent. These models are found within the most general scalar tensor theory of gravity with second order field equations, first discovered by Horndeski in 1974. The ability to write down such a general class ensures that we have not omitted any similar theories and makes us confident of the generality of our conclusions. Our study will focus on the properties of the Horndeski Lagrangian and is divided into two main parts. In the first part we search for a robust mechanism that shields the spacetime curvature from the vacuum energy of particle physics without affecting the radiation and matter epochs of our Universe’s history. We hope to deliver an efficient and viable solution to the cosmological constant problem. In the second part of our study, we will seek to find a viable accelerated expansion of the Universe once couplings of the scalar field to dark matter or to neutrinos are turned on. This is an important step in the understanding of the nature of dark energy and its implications for the formation of structure in the Universe. This study is extremely timely given the current push to understand theoretical models concerning dark energy and the cosmological constant in time for them to be confronted by the data of the Euclid mission.

Start
1 April 2014
End
1 April 2015

Funding Institution
Fundação para a Ciência e a Tecnologia