Large-field inflation and the cosmological collider

Large-field inflation is a major class of inflation models featuring a near- or super-Planckian excursion of the inflaton field. We point out that the large excursion generically introduces significant scale dependence to spectator fields through inflaton couplings, which in turn induces characteristic distortions to the oscillatory shape dependence in the primordial bispectrum mediated by a spectator field. This so-called cosmological collider signal can thus be a useful indicator of large field excursions. We show an explicit example with signals from the “tower states” motivated by the swampland distance conjecture.

Figure 1

The real part of the mode functions with time dependent masses (5) (dark blue). In all plots we take $\alpha =1$ and ${\epsilon }_{*}=0.02$. For comparison, the corresponding constant-mass mode functions are shown in gray curves.

Figure 2

The correction to scalar tilt, $\mathrm{\Delta }{n}_s$, from an intermediate state with time-dependent mass given in (5), plotted on the plane of $\sigma$ mass $m$ at a reference scale and the mixing parameter $\mu$ between $\sigma$ and $\delta \varphi$. In this plot we fix $\epsilon =1.25\times {10}^{-3}$, which corresponds to $r=0.02$ ($<0.02$) when $\mu =0$ $(>0)$. The lower-left corner with $\sqrt{m^2+{\mu }^2}<3H/2$, where no oscillation signals occur, is excluded.

Figure 3

The shape function (17) from the process (15). In both panels we take $m_{*}=\mu =3H$ and ${\lambda }_3=H$. The gray shades indicate the expected frequency ${\nu }_{\mu }=\sqrt{m_{*}^2+{\mu }^2}$ for constant mass in the squeezed limit. The blue and red curves show the signals from time-dependent masses, in which both the amplitude and the frequency of the oscillations change visibly with $k_1/k_3$.