Many efforts to constrain cosmological models focus on measurements of the spectral index and tensor amplitude. However, the tensor amplitude could be vanishingly small in which case the scale dependence of the spectrum, or the running, may actually be more accessible to observations. This paper assesses the ability of the next generation of cosmological experiments to measure the running.
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 21-cm 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, α_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_s and α_s can be constrained by currently envisaged observations, including CMB (Simons Observatory, CMB-S4 and LiteBIRD), optical/near infra-red (DESI and SPHEREx), and 21-cm intensity mapping (Tianlai and CHIME) surveys. We identify optimal combinations of data sets 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.
- Bahr-Kalus, Parkinson and Easther
- Constraining cosmic inflation with observations: Prospects for 2030
- Mon.Not.Roy.Astron.Soc. 520 (2023) 2, 2405-2416 or ArXiV:2212.04115
Forecast CMB-S4 and SPHEREx posterior contours for the running.