Quantum vacuum phenomena in various backgrounds

In this paper, we will review, through various examples, some ideas connecting the physics of the quantum vacuum and the Casimir effect with the mechanism of symmetry breaking in different backgrounds. In the first example, we will discuss how the quantum vacuum energy is altered by the presence of nonlinearities in the underlying quantum field theory in a way that depends on the spatial dimensionality, as dictated by the Mermin-Wagner-Hohenberg-Coleman theorem. In the second example, we will explore how the combination of boundary effects and (discrete) chiral symmetry breaking can affect the thermodynamical behavior of a system of interacting fermions, and how this is reflected on the Casimir force. Even in the simplest setup of two parallel plates, two interesting things happen: first, the order of the transition through which discrete chiral symmetry may be broken/restored changes from second-order for infinitely large separation to first-order for a finite separation between the boundaries. Second, a peculiar behavior in the fermion condensate occurs, resulting in the appearance of two different phases (massless and massive) in the Casimir force. In the third example, we will also be concerned with the Casimir effect on an interacting fermion background over a string of finite length. Using a self-consistent method, we will show how a nontrivial behavior in the Casimir force arises, displaying a switch from an attractive to a repulsive regime, as a result of the competing effects due to the usual attractive Casimir force and a repulsive component coming from the condensate.