I am trying to understand why the simple inflationary model with potential $V(\phi) \propto \phi^2$ is disfavored compared to models predicting a smaller tensor-to-scalar ratio.
To do so I am reading Planck 2015 results. In particular, I focused on "Power law potentials" subsection.
The issues I have are
- I do not understand why the validity of the model can be determined by the value of the $B$ mode.
- Why the $B$ mode values $\ln B = −11.6$ and $\ln B = −23.3$ for the cubic and quartic potentials, respectively, are regarded as "strongly disfavored" and $\ln B = −4.7$ for the quadratic potential as "moderately disfavored"? What I mean is: what is the threshold value at which we can consider the potential as favored and why?
