The Supersonic Project aims to explore the effect of the early Universe Stream Velocity on structure formation.

What is the Stream Velocity?

In the standard model of structure formation, due to the baryon-radiation coupling, baryon over-densities at the time of recombination (z ∼ 1020) were about five orders of magnitude smaller than dark matter over-densities. Recently, Tseliakhovich and Hirata (2010) showed that not only the amplitudes of the dark matter and baryonic density fluctuations were different at early times, but so were their velocities. After recombination, the baryons decoupled from the photons, and their subsequent evolution was dominated by the gravitational potential of the dark matter. In the period following recombination, the baryons underwent rapid cooling. At this point, their relative velocity with respect to dark matter, which at recombination was on the order of ∼ 30 km / sec, was, in fact, supersonic. Tseliakhovich and Hirata (2010) also showed that this relative velocity between the baryons and the dark matter remained coherent on scales of a few megaparsecs, and in these regions, it can be modeled as a stream velocity.

Although formally a second-order effect, the stream velocity has far-reaching consequences on a wide variety of cosmological phenomena. In particular, the non-linear effects of the stream velocity on the first structures were subsequently investigated using numerical simulations, as well as analytically, and it was shown that the stream velocity tends to suppress the formation of gas-rich small-scale structures at the early Universe. Additionally, this relative velocity may have nontrivial effects on the cosmological 21 cm signal, the formation of primordial black holes, and even on primordial magnetic fields.

Recently our team predicted the existence of Supersonically induced gas objects (SIGOs) with little to no dark matter (DM) component in patches of the Universe with a non-negligible relative velocity between baryons and the DM at the time of recombination. Our group has recently not only investigated these intriguing objects but also suggested a possible connection between these objects and globular clusters.

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