It has recently been suggested that the Standard Model Higgs boson could act as the inflaton while minimally coupled to gravity – given that the gravity sector is extended with an \alpha R ^ { 2 } term and the underlying theory of gravity is of Palatini , rather than metric , type .
In this paper , we revisit the idea and correct some shortcomings in earlier studies .
We find that in this setup the Higgs can indeed act as the inflaton and that the tree-level predictions of the model for the spectral index and the tensor-to-scalar ratio are n _ { s } \simeq 0.941 , r \simeq 0.3 / ( 1 + 10 ^ { -8 } \alpha ) , respectively , for a typical number of e-folds , N = 50 , between horizon exit of the pivot scale k = 0.05 { Mpc } ^ { -1 } and the end of inflation .
Even though the tensor-to-scalar ratio is suppressed compared to the usual minimally coupled case and can be made compatible with data for large enough \alpha , the result for n _ { s } is in severe tension with the Planck results .
We briefly discuss extensions of the model .