Multifield Inflation after Planck: The Case for Nonminimal Couplings

TL;DR

Multifield models of inflation with nonminimal couplings are in excellent agreement with the recent results from Planck and can amplify isocurvature perturbations, which could account for the low power recently observed in the cosmic microwave background power spectrum at low multipoles.

Abstract

Multifield models of inflation with nonminimal couplings are in excellent agreement with the recent results from {\it Planck}. Across a broad range of couplings and initial conditions, such models evolve along an effectively single-field attractor solution and predict values of the primordial spectral index and its running, the tensor-to-scalar ratio, and non-Gaussianities squarely in the observationally most-favored region. Such models also can amplify isocurvature perturbations, which could account for the low power recently observed in the CMB power spectrum at low multipoles. Future measurements of primordial isocurvature perturbations and the tensor-to-scalar ratio may serve to distinguish between the currently viable possibilities.

Figures (2)

• Figure 1: Potential in the Einstein frame, $V (\phi^I )$. Left: $\lambda_\chi = 0.75 \> \lambda_\phi$, $g = \lambda_\phi$, $\xi_\chi = 1.2 \> \xi_\phi$. Right: $\lambda_\chi = g = \lambda_\phi$, $\xi_\phi = \xi_\chi$. In both cases, $\xi_I \gg 1$ and $0 < \lambda_I , g < 1$.
• Figure 2: Left: Field trajectories for different couplings and initial conditions (here for $\dot{\phi}_0 , \dot{\chi}_0 = 0$). Open circles indicate fields' initial values. The parameters $\{\lambda_\chi, g,\xi_\chi, \theta_0 \}$ are given by: $\{0.75 \> \lambda_\phi, \lambda_\phi, 1.2\> \xi_\phi, \pi/4 \}$ (red), $\{\lambda_\phi, \lambda_\phi, 0.8 \> \xi_\phi, \pi/4 \}$ (blue), $\{\lambda_\phi, 0.75 \> \lambda_\phi, 0.8\xi_\phi, \pi/6 \}$ (green), $\{\lambda_\phi, 0.75 \> \lambda_\phi, 0.8\xi_\phi, \pi/3 \}$ (black). Right: Numerical vs. analytic evaluation of the slow-roll parameters, $\epsilon$ (numerical = blue, analytic = red) and $\eta_{\sigma\sigma}$ (numerical = black, analytic = green), for $\lambda_\phi = 0.01$, $\lambda_\chi = 0.75 \> \lambda_\phi$, $g = \lambda_\phi$, $\xi_\phi = 10^3$, and $\xi_\chi = 1.2 \> \xi_\phi$, with $\theta_0 = \pi / 4$ and $\dot{\phi}_0 = \dot{\chi}_0 = +10 \> \vert \dot{\phi}_{\rm SR} \vert$.