Inflation Driven by the Galileon Field

We propose a new class of inflation model, $G$ inflation, which has a Galileon-like nonlinear derivative interaction of the form $G\mathbf{(}\varphi ,(\nabla \varphi {)}^2\mathbf{)}\square \varphi$ in the Lagrangian with the resultant equations of motion being of second order. It is shown that (almost) scale-invariant curvature fluctuations can be generated even in the exactly de Sitter background and that the tensor-to-scalar ratio can take a significantly larger value than in the standard inflation models, violating the standard consistency relation. Furthermore, violation of the null energy condition can occur without any instabilities. As a result, the spectral index of tensor modes can be blue, which makes it easier to observe quantum gravitational waves from inflation by the planned gravitational-wave experiments such as LISA and DECIGO as well as by the upcoming CMB experiments such as Planck and CMBpol.

Figure 1

Schematic phase space diagram of $G$ inflation. The line $\dot{H}=0$ does not coincide with the line $c_s^2=0$ in general so that stable violation of the null energy condition is possible.