Primordial power spectra of Eddington-inspired Born-Infeld inflation in strong gravity limit

We investigate the scalar and the tensor perturbations of the ${\phi }^2$ inflation model in the strong-gravity limit of Eddington-inspired Born-Infeld (EiBI) theory. In order to consider the strong EiBI-gravity effect, we take the value of $\kappa$ large, where $\kappa$ is the EiBI theory parameter. The energy density of the Universe at the early stage is very high, and the Universe is in a strong-gravity regime. Therefore, the perturbation feature is not altered from what was investigated earlier. At the attractor inflationary stage, however, the feature is changed in the strong EiBI-gravity limit. The correction to the scalar perturbation in this limit comes mainly via the background matter field, while that to the tensor perturbation comes directly from the gravity ($\kappa$) effect. The change in the value of the scalar spectrum is little compared with that in the weak EiBI-gravity limit, or in general relativity. The form of the tensor spectrum is the same as that in the weak limit, but the value of the spectrum can be suppressed down to zero in the strong limit. Therefore, the resulting tensor-to-scalar ratio can also be suppressed in the same way, which makes the ${\phi }^2$ model in EiBI theory viable.

Figure 1

Numerical plot of $D_k$ for $a_0=1$, ${\psi }_0=-{10}^{-12}$, $m={10}^{-5}$, and $\kappa ={10}^{13}$. The value of ${\tau }_{*}$ is taken as the lower bound from Eq. (45), and is $k$ dependent. (In Fig. 1 in Ref. [8], it was fixed as ${\tau }_{*}=1$ for all $k$ modes.) For the parameters used in this plot, ${\tau }_{*}\sim \mathcal{O}(1)$. Even with the $k$ dependence in ${\tau }_{*}$, the shape of $D_k$ is not changed much, and still exhibits a peculiar rise for $k{\tau }_{*}<1$. It approaches unity for $k{\tau }_{*}>1$.