Primordial Non-Gaussianities from Inflation Models (1002.1416v3)

Abstract: This is a pedagogical review on primordial non-Gaussianities from inflation models. We introduce formalisms and techniques that are used to compute such quantities. We review different mechanisms which can generate observable large non-Gaussianities during inflation, and distinctive signatures they leave on the non-Gaussian profiles. They are potentially powerful probes to the dynamics of inflation. We also provide a non-technical and qualitative summary of the main results and underlying physics.

• The paper demonstrates how modifications in inflaton dynamics yield sizeable non-Gaussian features observable in the CMB.
• It categorizes inflation models by their distinct non-Gaussian signatures, linking theory with potential observational constraints.
• Analyses highlight that both non-canonical kinetic terms and multi-field interactions provide promising avenues for understanding early universe physics.

Overview of Primordial Non-Gaussianities from Inflation Models

The paper under examination presents a comprehensive discussion on the generation of primordial non-Gaussianities from inflationary dynamics, offering insights into various mechanisms that could produce observable large non-Gaussianities in the cosmic microwave background (CMB). Primordial non-Gaussianities hold paramount importance as they provide clues on the dynamics of inflation and serve as potential probes for understanding the very early universe.

The paper clarifies the inflationary scenario, which resolves several conundrums of the Big Bang cosmology, particularly the isotropy and homogeneity of our universe. Inflation postulates that the universe underwent a phase of accelerated expansion driven by a scalar field, the inflaton, smoothing out any initial irregularities. Quantum fluctuations during this period are believed to be the seeds of the large-scale structure we observe today. In standard single-field slow-roll inflation models, these fluctuations are Gaussian to a good approximation. However, deviations from Gaussianity could be induced by various processes and offer insights into complex inflationary dynamics.

Mechanisms of Large Non-Gaussianities

The paper categorizes inflationary models capable of generating significant non-Gaussianities based on features introduced in the primordial inflaton dynamics:

• Higher Derivative Kinetic Terms: In scenarios where non-canonical kinetic terms dominate dynamics, significant non-Gaussianities with an equilateral shape can emerge. These equilateral bispectra arise due to interactions among modes of similar wavelengths, a condition seen in models like Dirac-Born-Infeld (DBI) inflation. Here, non-Gaussianity parameters such as $f_{NL}$ can become sizeable when the inflaton's sound speed is small.
• Sharp and Periodic Features: Temporal features in the inflaton potential or in the internal space can evoke considerable non-Gaussianities. Sharp features lead to a sinusoidal running in the bispectrum, while periodic features cause resonant signals recognizable through their unique oscillations in momentum space.
• Non-Standard Vacua: Besides the canonical Bunch-Davies vacuum, alternate initial states of quantum fluctuations can introduce distinctive non-Gaussian signatures, especially enhancing correlations in "folded" triangle configurations in momentum space.
• Massive and Massless Isocurvatons: Multifarious inflationary dynamics can also emerge by considering multiple fields. Massive isocurvatons create "intermediate" shapes between local and equilateral forms. In contrast, massless fields with significant non-linear evolution manifest as large local non-Gaussianities, characterized by their predominance in the squeezed triangle limit.

Implications of Non-Gaussianities

Each type of non-Gaussian signature provides different insights into the physics driving inflation. The equilateral forms suggest strong kinetic interaction, while local forms are indicative of multifield interactions or curvaton dynamics. The resonant and feature-induced forms highlight the utility of non-Gaussianities in investigating transient events or deviations from the typical inflationary trajectory.

Non-Gaussianity studies complement traditional power spectrum analyses. These investigations not only refine our model of inflation but also help set constraints on candidate models and the underlying microphysics. The possibility that non-Gaussian features arise at levels detectable by present or upcoming observational missions like the Planck satellite provides a compelling incentive for theoretical investigation.

As theoretical modeling and observational capabilities advance, the paper advocates for the discrimination of inflation models based on the intricacies of primordial non-Gaussianities and their distinctive signatures, paving the way for profound insights into the inflationary era and the fundamental theories governing the early universe.