Constraining Cosmic Inflation with Observations: Prospects for 2030

TL;DR

This paper forecasts how upcoming cosmological observations from CMB, optical, near-infrared, and 21cm surveys can constrain inflationary models, especially the spectral index and running, even if primordial gravitational waves are not detected.

Contribution

It provides detailed forecasts for constraining inflationary parameters using future observational data, highlighting optimal dataset combinations and implications of non-detection of gravitational waves.

Findings

Forecasted constraints on spectral index and running from upcoming surveys.

Optimal dataset combinations for constraining inflationary parameters.

Potential to inform inflation models even without detecting primordial gravitational waves.

Abstract

The ability to test and constrain theories of cosmic inflation will advance substantially over the next decade. Key data sources include cosmic microwave background (CMB) measurements and observations of the distribution of matter at low-redshift from optical, near-infrared, and 21cm intensity surveys. A positive detection of a CMB B-mode consistent with a primordial stochastic gravitational wave background (SGWB) is widely viewed as a smoking gun for an inflationary phase. Still, a null result does not exclude inflation. However, in a significant class of inflationary scenarios, a low SGWB amplitude is correlated with a more significant running, $\alpha_s$, in the primordial density perturbations than is seen with the simplest inflationary potentials. With this motivation, we forecast the precision with which the spectral index $n_{\rm{s}}$ and $\alpha_{\rm{s}}$ can be constrained by currently envisaged observations, including CMB (Simons Observatory, CMB-S4 and LiteBIRD), optical/near infra-red (DESI and SPHEREx), and 21cm intensity mapping (Tianlai and CHIME) surveys. We identify optimal combinations of datasets for constraining the running and show that they may yield additional and informative constraints on the overall inflationary parameter space if the SGWB remains undetected.