Multifield Inflation after Planck: The Case for Nonminimal Couplings (1304.0363v3)

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.

• The paper demonstrates that multifield inflation models with nonminimal couplings naturally evolve to a single-field attractor solution, ensuring robust predictions across diverse initial conditions.
• It shows that key predictions for the spectral index, tensor-to-scalar ratio, and non-Gaussianities align closely with Planck data.
• The study emphasizes that measuring isocurvature perturbations can further test and refine these models, reinforcing their relevance in early-universe cosmology.

Multifield Inflation with Nonminimal Couplings: Insights and Implications

The paper, authored by David I. Kaiser and Evangelos I. Sfakianakis, presents an analysis of multifield models of inflation characterized by nonminimal couplings, following significant observations from the Planck collaboration. This paper explores a class of models that involve multiple scalar fields interacting through nonminimal couplings, which are argued to be essential for a consistent quantum field treatment in curved spacetime. Such models are posited as particularly promising in light of recent empirical data, especially in regard to primordial spectral features.

Key Findings

Kaiser and Sfakianakis demonstrate that multifield inflation models with nonminimal couplings naturally evolve towards a single-field attractor solution over a wide range of couplings and initial conditions. This effectively single-field behavior results in predictions that closely match observational data, including the primordial spectral index ($n_s$), its running ($\alpha$), the tensor-to-scalar ratio ($r$), and non-Gaussianities ($f_{\rm NL}$).

Numerical Results

• Spectral Index ($n_s$): The models predict $n_s \simeq 0.966$ for 60 efolds and $n_s \simeq 0.959$ for 50 efolds, aligning well with Planck data ($n_s = 0.9603 \pm 0.0073$).
• Tensor-to-Scalar Ratio ($r$): The estimated value is $r \simeq 0.0033$ for 60 efolds, suggesting minimal tensor perturbations.
• Running of the Spectral Index ($\alpha$): Calculations yield $\alpha \simeq -5.83 \times 10^{-4}$ for 60 efolds, consistent with Planck's findings indicating negligible running.
• Non-Gaussianity ($f_{\rm NL}$): Predicted values show $\vert f_{\rm NL} \vert < 0.1$, corresponding with the observation $f_{\rm NL} \sim 0$.

Implications

The attractor solution emphasizes the robustness of these models across various initial conditions and coupling values, suggesting broad applicability without extensive fine-tuning. This trait enhances the predictive power and reliability of multifield models as they address outstanding questions in cosmology. Importantly, these models can potentially elucidate the low power observed at large scales in the CMB power spectrum. The amplification of isocurvature perturbations during inflation presents a viable explanation for these discrepancies.

The identification of multifield inflationary models as strong candidates hinges upon future measurements of primordial isocurvature perturbations, which could yield critical insights into the underlying dynamics of the early universe. Particularly, isocurvature spectra may help differentiate among models within this class, thus resolving inherent observational degeneracies.

Future Directions

Through this investigation, multifield inflation models with nonminimal couplings stand poised for further empirical testing. Upcoming CMB polarization experiments pose the potential to validate or refute the predicted tensor-to-scalar ratio. Additionally, advancements in the understanding of isocurvature fluctuations may provide more stringent constraints on multifield theories, potentially guiding research towards novel formulations or refinements of inflationary theory.

In summary, this paper presents a compelling blend of theoretical development and alignment with observational data, thereby reinforcing the viability of multifield models in contemporary cosmological research. The emphasis on nonminimal couplings not only strengthens theoretical consistency but also enhances empirical congruity, offering a promising framework for exploring fundamental aspects of early-universe cosmology.