A space mission to map the entire observable universe using the CMB as a backlight

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 free electrons and atoms (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 μK-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.

Inverse Gertsenshtein effect as a probe of high-frequency gravitational waves

“…In the future, a number of precision surveys of CMB and its spectral distortions could improve the constraints on HFGWs beyond the GHz regime. Here we consider the expected capabilities of proposed missions such as Polarized Radiation Interferometer for Spectral disTortions and INflation Exploration (PRISTINE) [152], the Primordial Inflation Explorer (PIXIE) [153,154] and its next-generation concept Super-PIXIE, as well as the scheme Voyage 2050 [155], assumed to be a few times more sensitive than Super-PIXIE. We take the foreground-marginalized error budget for temperature measurements of these missions [152] and compute the upper limits of HFGW energy density Ω GW (f ) within the corresponding detectors' frequency bands.…”

Snowmass2021 Cosmic Frontier: Cosmic Microwave Background Measurements White Paper

“…This "Voyage 2050" process defined the science themes for the next three large-class (L-class, ∼ 1 billion Euro) missions, but did not yet define the specific mission concepts. A coordinated series of whitepapers proposed science themes on spectral distortions [96], on using the CMB as a backlight for mapping structures and their baryon content [97], on cosmology via probing structure evolution across cosmic times with line-intensity mapping [98], and on an L-class mission to target them all, with compromises to be made between the different science cases [99]. The Final recommendations from the Voyage 2050 Senior Committee [100] selected "New Physical Probes of the Early Universe" as one of the science themes, and recommended "a Large mission deploying gravitational wave detectors or precision microwave spectrometers to explore the early Universe at large redshifts."…”

Cosmology from the kinetic polarized Sunyaev Zel'dovich effect

“…In what follows we provide forecasts for the 'direct-detection' prospects for the kpSZ effect using CMB-S4, CMB-HD and PIXIE experiments. In addition to these upcoming LiteBIRD [73,74] and proposed ESA Voyage 2050 [75,76] surveys can also potentially improve the prospects of detecting this effect directly on large scales by cross-correlating with velocity templates.…”