Chiral EFT for Electro-Weak Processes and the Role of the Delta

A major goal of nuclear theory is to explain the wealth of data and peculiarities exhibited by nuclear systems using a fully microscopic approach. In this approach, which we refer to as the basic model of nuclear theory, nucleons interact with each other via many-body (primarily two- and three-body) effective interactions, and with external electroweak probes via effective currents describing the coupling of these probes to individual nucleons and many-body clusters of them. These effective interactions and currents serve as the primary inputs to it ab-initio methods for solving the many-body Schr\"{o}dinger equation associated with the nuclear system under consideration. In this talk, I will discuss recent advances in Quantum Monte Carlo calculations of low-lying spectra and electroweak properties of light nuclei, as well as the equation of state for nucleonic matter. Calculations based on the chiral effective field theory approach will be emphasized.