We consider a possibility that the Higgs field in the Standard Model ( SM ) serves as an inflaton when its value is around the Planck scale .
We assume that the SM is valid up to an ultraviolet cutoff scale \Lambda , which is slightly below the Planck scale , and that the Higgs potential becomes almost flat above \Lambda .
Contrary to the ordinary Higgs inflation scenario , we do not assume the huge non-minimal coupling , of O ( 10 ^ { 4 } ) , of the Higgs field to the Ricci scalar .
We find that \Lambda must be less than 5 \times 10 ^ { 17 } \text { GeV } in order to explain the observed fluctuation of the cosmic microwave background , no matter how we extrapolate the Higgs potential above \Lambda .
The scale 10 ^ { 17 } \text { GeV } coincides with the perturbative string scale , which suggests that the SM is directly connected with the string theory .
For this to be true , the top quark mass is restricted to around 171 \text { GeV } , with which \Lambda can exceed 10 ^ { 17 } \text { GeV } .
As a concrete example of the potential above \Lambda , we propose a simple log type potential .
The predictions of this specific model for the e -foldings N _ { * } = 50 –60 are consistent with the current observation , namely , the scalar spectral index is n _ { s } = 0.977 \text { - - } 0.983 and the tensor to scalar ratio 0 < r < 0.012 –0.010 .
Other parameters , \text { d } n _ { s } / \text { d } \ln k , n _ { t } , and their derivatives , are also consistent .