Supersonic relative velocity between dark matter and baryons: A review

Abstract

Our understanding of astrophysical and cosmological phenomena in recent years has improved enormously thanks to precision measurements of various cosmic signals such as Cosmic Microwave Background radiation, emission of galaxies and dust, spectral lines attributed to various elements etc. Despite this, our knowledge at intermediate redshifts (10 < z < 1100) remains fragmentary and incomplete, and as a consequence, various physical processes happening between the epochs of hydrogen recombination and reionization remain still highly unconstrained. Moreover, some important fragments of the theoretical description that are less decisive for the Universe today, but that had an important impact at intermediate redshifts, have been omitted in some of the studies concerning the Universe at high redshifts. One such neglected phenomenon, only recently brought to the attention of the community by D. Tseliakhovich and C. M. Hirata (2010) [1], is the fact that after hydrogen recombination the large-scale baryons and dark matter fluctuations had supersonic relative velocities: the central topic of this review. The relative velocities between dark matter and baryons formally introduce a second order effect on the standard results and thus have been neglected in the framework of linear theory. However, when properly considered, the velocities yield a non-perturbative contribution to the growth of structures which is then inherited by the majority of cosmic signals coming from redshifts above z ∼ 10, and in certain cases may even propagate to various low-redshift observables such as the Baryon Acoustic Oscillations measured from the distribution of galaxies. At higher redshifts, the supersonic velocities have thus strong impact affecting the abundance of M ∼ 10 6 M⊙ halos in an inhomogeneous way, hindering the formation of first stars, leaving traces in the redshifted 21-cm signal of neutral hydrogen, as well as having other important contributions at high redshifts all of which we review in this manuscript.

The BOSS–WiggleZ overlap region – I. Baryon acoustic oscillations

“…The different velocities of dark matter and baryons after decoupling means that there is a relative velocity between the two components (Tseliakhovich & Hirata 2010;Fialkov et al 2014). The relative velocity can allow baryons to escape the dark matter potentials and prevent the formation of the first stars in regions with high relative velocity (Fialkov et al 2012;McQuinn & O'Leary 2012;Naoz, Yoshida & Gnedin 2012).…”

Baryonic Processes in the Large-Scale Structuring of the Universe

“…Dark Matter however, which decoupled much earlier, had already started evolving under its own gravity and was thus already somewhat inhomogeneously distributed. Since Dark Matter dominates the matter content of the Universe, the primordial gas fell into the Dark Matter gravitational potential wells already formed at these early times (see Tseliakhovich & Hirata, 2010;Fialkov, 2014, for an account of the effects of the relative velocity between Dark Matter and baryons).…”

Streaming Velocities and the Baryon Acoustic Oscillation Scale

“…It was soon realized that these ingredients implied that small, high-redshift galaxies whose abundance was modulated by the streaming velocity would show an unusual BAO signature [43], and in some models the BAO signature in the pre-reionization 21 cm signal could be strongly enhanced relative to the strength of the BAOs in the matter clustering alone [38,44,45]. Moreover, if lowredshift galaxies have any memory of the streaming velocity, then low-redshift BAO measurements could be biased [43,46].…”