Scale Invariance from Modified Dispersion Relations

In this paper, inspired by the investigations on the theory of cosmological perturbations in Ho\v{r}ava-Liftshit cosmology, we calculated the spectrum of primordial perturbation leaded by a scalar field with modified dispersion relation \omega\sim k^z/a^{p-1}, in which z is the critical exponent and p is generally not equal to z. We discussed that for fixed z, if the spectrum is required to be scale invariant, how should p depend on the background evolution. We concluded that there is always a room of parameters for the generation of scale invariant spectrum.