Large tensor-to-scalar ratio from composite inflation

The claimed detection of the BICEP2 experiment on the primordial B-mode of cosmic microwave background polarization suggests that cosmic inflation possibly takes place at the energy around the grand unified theory scale given a constraint on the tensor-to-scalar ratio, i.e., $r\simeq 0.20$. In this report, we revisit single-field (slow-roll) composite inflation and show that, with the proper choice of parameters and sizeable number of $e$-foldings, a large tensor-to-scalar ratio consistent with the recent BICEP2 results can be significantly produced with regard to the composite paradigms.

Figure 1

The plot shows the relation between the amplitude of the power spectrum ${\mathcal{A}}_s$ and the nonminimal coupling $\xi$ with $10^{-3}\lesssim \xi \lesssim 10^6$ for $\mathcal{N}=50,60$ predicted by model 1. The horizontal bands represent the $1\sigma$ (yellow) and $2\sigma$ (purple) C.L. for ${\mathcal{A}}_s$ obtained from Planck.

Figure 2

The plot shows the relation between the amplitude of the power spectrum ${\mathcal{A}}_s$ and the nonminimal coupling $\xi$ with $10^{-3}\lesssim \xi \lesssim 10^6$ for $\mathcal{N}=50,60$ predicted by model 2. The horizontal bands represent the $1\sigma$ (yellow) and $2\sigma$ (purple) C.L. for ${\mathcal{A}}_s$ obtained from Planck.

Figure 3

The contours show the resulting 68% and 95% confidence regions for the tensor-to-scalar ratio $r$ and the scalar spectral index $n_s$. The red contours are for the $\text{Planck}+\mathrm{WP}+\text{highL}$ data combination, while the blue ones display the BICEP2 constraints on $r$. The plots show the analytical and numerical data predicted by the models examined in this work (1 and 2).