Future Probes of the Primordial Scalar and Tensor Perturbation Spectra: Prospects from the CMB, Cosmic Shear and High-Volume Redshift Surveys

TL;DR

This paper discusses how future cosmic microwave background, cosmic shear, and large-scale redshift surveys can improve measurements of primordial scalar and tensor perturbation spectra, shedding light on early universe physics.

Contribution

It evaluates the potential of upcoming observational methods to precisely measure the primordial spectra and their parameters, including tensor-to-scalar ratio and spectral tilts.

Findings

Tensor tilt measurement error decreases with r, from 0.1 at r=0.001 to 0.02 at r=0.1.

CMB and cosmic shear observations can reduce errors on spectral tilt and running to 10^{-3}.

Achieving further improvements may require large-volume spectroscopic redshift surveys with about 10^9 galaxies.

Abstract

Detailed study of the scalar and tensor perturbation spectra can provide much information about the primordial fluctuation-generator, be it inflation or something else. The tensor perturbation spectrum may be observable through its influence on CMB polarization, but only if the tensor-to-scalar ratio, r = T/S, is greater than about 10^{-5}. The tensor tilt can be measured with an error of sigma(n_T) that decreases with r from 0.1 at r=0.001 to 0.02 at r = 0.1. Current CMB constraints on the scalar perturbation spectrum can be improved by higher--resolution CMB observations and/or by tomographic cosmic shear observations. These can both shrink errors on the tilt (n_S) and running (n_S'= dn_S/d\ln k) to the 10^{-3} level. Stunning as these results would be, it may become very desirable to improve upon them an order of magnitude further in order to study the expected departures from n_S' = 0. Such improvements are likely to require observation of three--dimensional clustering over very large volumes. Unfortunately, to get down to the 10^{-4} level will require a sparse spectroscopic redshift survey with about 10^9 galaxies spread over a volume less than but comparable to that of the observable Universe.