Inflation is the leading theory to describe elegantly the initial conditions that led to structure formation in our universe .
In this paper , we present a novel phenomenological fit to the Planck , WMAP polarisation ( WP ) and the BICEP2 datasets using an alternative parameterisation .
Instead of starting from inflationary potentials and computing the inflationary observables , we use a phenomenological parameterisation due to Mukhanov , describing inflation by an effective equation-of-state , in terms of the number of e-folds and two phenomenological parameters \alpha and \beta .
Within such a parametrisation , which captures the different inflationary models in a model-independent way , the values of the scalar spectral index n _ { s } , its running and the tensor-to-scalar ratio r are predicted , given a set of parameters ( \alpha, \beta ) .
We perform a Markov Chain Monte Carlo analysis of these parameters , and we show that the combined analysis of Planck and WP data favours the Starobinsky and Higgs inflation scenarios .
Assuming that the BICEP2 signal is not entirely due to foregrounds , the addition of this last data set prefers instead the \phi ^ { 2 } chaotic models .
The constraint we get from Planck and WP data alone on the derived tensor-to-scalar ratio is r < 0.18 at 95 \% CL , value which is consistent with the one quoted from the BICEP2 collaboration analysis , r = 0.16 ^ { +0 - 06 } _ { -0.05 } , after foreground subtraction .
This is not necessarily at odds with the 2 \sigma tension found between Planck and BICEP2 measurements when analysing data in terms of the usual n _ { s } and r parameters , given that the parameterisation used here includes , implicitly , a running spectral index .