Fab Four: When John and George Play Gravitation and Cosmology

Figure 10

Cosmological evolution of 𝑞, 𝑄_𝜑, 𝑐²_𝜑, 𝑄_𝑆, 𝑐²_𝑆, 𝑄_𝑇, 𝑐²_𝑇 with the scale factor. Top: the case 𝜖=−1 and 𝛾=1 corresponds to a “double inflation.” The scenario is pathological in many respects: singularity in the scalar field velocity (see (a)) while the scalar field EoS is imaginary. Also (see (b), the 𝑦 axis is logarithmic): 𝑐²_𝑇<0 and there are periods for which 𝑐²_𝑠<0 and 𝑄_𝑇<0. Bottom: the case 𝜖=1 and 𝛾=−1. The model is well behaved but does not accelerate the expansion, that is, as in the John alone model. The universe is actually in a super stiff regime and hence in a highly decelerating phase.

(a)

(b)

(c)

(d)