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We perform a suite of cosmological simulations in the LCDM paradigm of theformation of the first structures in the Universe prior to astrophysicalreheating and reionization (15<~z<200). These are the first simulationsinitialized in a manner that self consistently account for the impact ofpressure on the rate of growth of modes, temperature fluctuations in the gas,and the dark matter-baryon supersonic velocity difference. Even with theseimprovements, these are still difficult times to simulate accurately as theJeans length of the cold intergalactic gas must be resolved while alsocapturing a representative sample of the Universe.  Our simulations support the finding of recent studies that the darkmatter-baryon velocity difference has a surprisingly large impact on theaccretion of gas onto the first star-forming minihalos (with masses of ~10^6Msun). In fact, the halo gas is often significantly downwind of such halos andwith lower densities, which delays the formation of the first stars in mostlocations in the Universe. We also show that dynamical friction plays animportant role in the nonlinear evolution of the differential velocity, actingto erase this velocity difference quickly in overdense gas as well as sourcingvisually-apparent bow shocks and Mach cones throughout the Universe.  We use simulations with both the GADGET and Enzo cosmological codes to testthe robustness of these conclusions. We find that particle coupling in GADGETbetween the gas and dark matter particles can result in spurious growth thatmimics nonlinear growth in the matter power spectrum. In a companion paper, weuse the simulations presented here to make detailed estimates for the impact ofthe dark matter--baryon velocity differential on redshifted 21cm radiation. Theinitial conditions generator used in this study CICsASS can be publiclydownloaded.

The formation of the first cosmic structures and the physics of the z~20 Universe