Why CMB backlight? CMB is a ubiquitous backlight for all the objects in our observable Universe. The faint imprints of their interactions with the baryons and the total mass distribution within the cosmic web can inform us about such diverse topics as the epoch of reionization, evolution of cosmic thermal energy, relative motion of the matter on largest scales, the location of the hottest plasma in the Universe, and masses of gravitationally bound halos up to arbitrarily high redshifts. Many of these "secondary" CMB effects have been considered by theoreticians for the past several decades, but only with the advent of the current generation of CMB experiments their enormous potentials are coming to reality.
Our aim is to outline and further explore, in the context of the Voyage 2050 process, the scientific potential of using CMB as a universal backlight to study the cosmic matter distribution and its evolution. A high-sensitivity, full-sky survey with arcminute angular resolution and frequency coverage spanning the millimeter and sub millimeter wavebands would map the distribution of essentially all baryonic and dark-matter structures in the observable Universe, and measure the peculiar motion of matter within the cosmic web. Such a complete matter census would be transformational.
After leaving the surface of last scattering, CMB photons interact with intervening matter primarily via two processes: (1) scattering by electrons in ionized plasma, called the Sunyaev-Zeldovich (SZ) effect; and (2) deflection by gravitational potential wells along the cosmic web. There are several variants of the SZ effect. The thermal SZ (tSZ) traces the total of gas pressure in the baryons associated with large-scale structures along the line of sight. Galaxy clusters are the most dominant contributors, however, with the sensitivity level of upcoming missions, tSZ will probe also the unbound gas along cosmic filaments [ref]. The kinetic SZ (kSZ) traces line-of-sight gas momentum with respect to the CMB. As the large-scale velocities directly trace the growth of structures in the linear regime, kSZ is a highly-promising probe for the dark energy and alternate gravity models [ref]. Relativistic effects (rSZ) subtly alter the tSZ frequency spectrum and can be used to directly measure gas temperatures (e.g., without relying on X-ray data) [ref]. There are additionally non-thermal SZ (ntSZ) signals that can constrain the particle composition of exotic plasmas, such as within radio bubbles driven by AGN feedback or giant radio halos within clusters, and help to constrain the magnetic field strength in combination with low-frequency radio data. Lastly, polarized SZ (pSZ) signal probes cluster transverse motions and internal substructure, with its own suite of astrophysical applications [ref].
Gravitational potentials lens the CMB backlight [1–3]. By cross-correlating CMB lensing maps with visible tracers, such as galaxies and clusters, we can partition the lensing signal into redshift slices, a process known as “lensing tomography,” and measure the growth of structure back to redshifts of a few,well past the point where galaxy cosmic shear becomes ineffective due to lack of background sources.On smaller scales, the lensing effect probes deflections by strong localized over-densities, enabling determination of cluster masses out to redshifts beyond the reach of galaxy shear measurements (z >2) ; this technique will be essential for using high-z clusters as a cosmological tool. CMB lensing can also be used to detect cluster’s transverse motions through the moving-lens effect [5, 6], complementing kinetic and polarised SZ measurements of the cosmic velocity field.The CMB provides an ideal backlight for these studies because: (1) it originates from a known redshift; (2) its spectrum at emission is known to be an almost perfect blackbody; and (3) its statistical properties are well defined. It is a new and powerful tool for a comprehensive census of matter in the Universe.
 Planck Collaboration 2013 XVII, A&A571, A17 (2014), 1303.5077.
 B. D. Sherwin, A. van Engelen, N. Sehgal, M. Madhavacheril, G. E. Addison, S. Aiola, R. Allison,N. Battaglia, D. T. Becker, J. A. Beall, et al., Phys. Rev. D95, 123529 (2017), 1611.09753.
 G. Simard, Y. Omori, K. Aylor, E. J. Baxter, B. A. Benson, L. E. Bleem, J. E. Carlstrom, C. L. Chang,H. M. Cho, R. Chown, et al., ApJ860, 137 (2018), 1712.07541.
 R. Laureijs, arXiv e-prints arXiv:0912.0914 (2009), 0912.0914.
 S. Yasini, N. Mirzatuny, and E. Pierpaoli, ApJL873, L23 (2019), 1812.04241.
 S. C. Hotinli, J. Meyers, N. Dalal, A. H. Jaffe, M. C. Johnson, J. B. Mertens, M. M ̈unchmeyer, K. M.Smith, and A. van Engelen, arXiv e-prints arXiv:1812.03167 (2018), 1812.03167.