A Space Mission to Map the Entire Observable Universe using the CMB as a Backlight

TL;DR

This white paper proposes a space mission using the CMB as a backlight to map the universe's gas, mass, and stars through spectral distortions, enabling diverse cosmological studies with high resolution and sensitivity.

Contribution

It introduces the BACKLIGHT space mission concept, aiming to survey the entire observable universe via CMB distortions with unprecedented resolution and sensitivity.

Findings

Potential to map cosmic web components in detail

Enhanced understanding of CMB spectral distortions

Synergy with ground-based CMB experiments

Abstract

This Science White Paper, prepared in response to the ESA Voyage 2050 call for long-term mission planning, aims to describe the various science possibilities that can be realized with an L-class space observatory that is dedicated to the study of the interactions of cosmic microwave background (CMB) photons with the cosmic web. Our aim is specifically to use the CMB as a backlight -- and survey the gas, total mass, and stellar content of the entire observable Universe by means of analyzing the spatial and spectral distortions imprinted on it. These distortions result from two major processes that impact on CMB photons: scattering by electrons (Sunyaev-Zeldovich effect in diverse forms, Rayleigh scattering, resonant scattering) and deflection by gravitational potential (lensing effect). Even though the list of topics collected in this White Paper is not exhaustive, it helps to illustrate the exceptional diversity of major scientific questions that can be addressed by a space mission that will reach an angular resolution of 1.5 arcmin (goal 1 arcmin), have an average sensitivity better than 1 uK-arcmin, and span the microwave frequency range from roughly 50 GHz to 1 THz. The current paper also highlights the synergy of our BACKLIGHT mission concept with several upcoming and proposed ground-based CMB experiments.