[Paper Review] Inflation: Theory and Observations

Ana Achúcarro, Matteo Biagetti+20 more|Data Archiving and Networked Services (DANS)|Mar 15, 2022

Cosmology and Gravitation TheoriesPhysics and Astronomy78 citations

TL;DR

This Snowmass white paper surveys the theory and observational prospects of inflation, focusing on primordial gravitational waves, non-Gaussianity, and primordial features, with current constraints and future sensitivities.

ABSTRACT

Cosmic inflation provides a window to the highest energy densities accessible in nature, far beyond those achievable in any realistic terrestrial experiment. Theoretical insights into the inflationary era and its observational probes may therefore shed unique light on the physical laws underlying our universe. This white paper describes our current theoretical understanding of the inflationary era, with a focus on the statistical properties of primordial fluctuations. In particular, we survey observational targets for three important signatures of inflation: primordial gravitational waves, primordial non-Gaussianity and primordial features. With the requisite advancements in analysis techniques, the tremendous increase in the raw sensitivities of upcoming and planned surveys will translate to leaps in our understanding of the inflationary paradigm and could open new frontiers for cosmology and particle physics. The combination of future theoretical and observational developments therefore offer the potential for a dramatic discovery about the nature of cosmic acceleration in the very early universe and physics on the smallest scales.

Motivation & Objective

• Summarize the theoretical understanding of the inflationary era and its observational probes.
• Review how primordial gravitational waves, non-Gaussianity, and features imprint on cosmological observables.
• Assess current constraints and forecast the impact of upcoming surveys on inflationary models.
• Outline experimental prospects and methodological challenges for detecting inflationary signatures.

Proposed method

• Discuss the inflationary background and perturbation theory using the ADM formalism and metric fluctuations.
• Describe the observational imprints of scalar and tensor modes on CMB and LSS.
• Summarize current constraints on primordial gravitational waves, non-Gaussianity, and features.
• Present expected sensitivity thresholds for future CMB, LSS, and gravitational wave measurements.
• Reference the effective field theory approach to inflation as a framework for model-building and data interpretation.

Experimental results

Research questions

• RQ1 What are the theoretical predictions and observational signatures of primordial gravitational waves, non-Gaussianity, and primordial features?
• RQ2 What are the current constraints on the three main inflationary observables (r, f_NL, and features) from CMB and LSS data?
• RQ3 How will upcoming surveys improve sensitivity to these observables and constrain inflationary models?
• RQ4 What energy scales and model-space are probed by potential detections or improved limits on these observables?

Key findings

• The current tensor-to-scalar ratio upper limit is r < 0.035 (95% C.L.).
• Future data could reach r ~ 0.01 and r ~ 0.001 thresholds with CMB experiments.
• Current bispectrum bounds are f_NL^local = -0.9 ± 5.1, f_NL^equil = -26 ± 47, f_NL^ortho = -38 ± 24, showing no evidence for primordial non-Gaussianity.
• Upcoming data may achieve σ(f_NL^local) < 1, enabling discrimination of models with extra light species during or after inflation.
• Cosmological data constrain primordial features to less than ~1% of the scalar amplitude A_s, with future surveys likely improving by one to two orders of magnitude.
• Cross-cutting improvements from CMB spectral distortions and stochastic gravitational wave background measurements are anticipated to enhance inflationary probes.