We reconsider non-minimal \lambda \phi ^ { 4 } chaotic inflation which includes the gravitational coupling term \xi \mathcal { R } \phi ^ { 2 } , where \phi denotes a gauge singlet inflaton field and \mathcal { R } is the Ricci scalar .
For \xi \gg 1 we require , following recent discussions , that the energy scale \lambda ^ { 1 / 4 } m _ { P } / \sqrt { \xi } for inflation should not exceed the effective UV cut-off scale m _ { P } / \xi , where m _ { P } denotes the reduced Planck scale .
The predictions for the tensor to scalar ratio r and the scalar spectral index n _ { s } are found to lie within the WMAP 1- \sigma bounds for 10 ^ { -12 } \lesssim \lambda \lesssim 10 ^ { -4 } and 10 ^ { -3 } \lesssim \xi \lesssim 10 ^ { 2 } .
In contrast , the corresponding predictions of minimal \lambda \phi ^ { 4 } chaotic inflation lie outside the WMAP 2- \sigma bounds .
We also find that r \gtrsim 0.002 , provided the scalar spectral index n _ { s } \geq 0.96 .
In estimating the lower bound on r we take into account possible modifications due to quantum corrections of the tree level inflationary potential .