How much cosmological information can be measured?

TL;DR

This paper discusses the finite amount of cosmological information obtainable from observations, emphasizing the limitations of Gaussian perturbations and the potential of future 3D surveys to improve parameter constraints.

Contribution

It quantifies the maximum extractable cosmological information from current and future observations, highlighting the finite nature of accessible data even with advanced surveys.

Findings

Current microwave observations are close to the maximum accessible information.

Future 3D surveys can significantly increase information but remain limited.

Total effective signal-to-noise ratio on parameters is capped around 10^7.

Abstract

It has become common to call this the "era of precision cosmology," and hence one rarely hears about the finiteness of the amount of information that is available for constraining cosmological parameters. Under the assumption that the perturbations are purely Gaussian, the amount of extractable information (in terms of total signal-to-noise ratio for power spectrum measurements) is the same (up to a small numerical factor) as an accounting of the number of observable modes. For studies of the microwave sky, we are probably within a factor of a few of the amount of accessible information. To dramatically reduce the uncertainties on parameters will require three-dimensional probes, such as ambitious future redshifted 21-cm surveys. However, even there the available information is still finite, with the total effective signal-to-noise ratio on parameters probably not exceeding $10^7$. The amount of observable information will increase with time (but very slowly) into the extremely distant future.