Statistical and dynamical decoupling of the IGM from Dark Matter
  The mean mass densities of cosmic dark matter is larger than that of baryonic
matter by a factor of about 5 in the $\Lambda$CDM universe. Therefore, the
gravity on large scales should be dominant by the distribution of dark matter
in the universe. However, a series of observations incontrovertibly show that
the velocity and density fields of baryonic matter are decoupling from
underlying dark matter field. This paper shows our attemps to unveil the
physics behind this puzzle. In linear approximation, the dynamics of the baryon
fluid is completely governed by the gravity of the dark matter. Consequently,
the mass density field of baryon matter $\rho_b({\bf r},t)$ will be
proportional to that of dark matter $\rho_{\rm dm}({\bf r},t)$, even though
they are different from each other initially. In weak and moderate nonlinear
regime, the dynamics of the baryon fluid can be sketched by Burgers equation. A
basic feature of the Burgers dynamics is to yield shocks. When the Reynolds
number is large, the Burgers fluid will be in the state of Burgers turbulence,
which consists of shocks and complex structures. On the other hand, the
collisionless dark matter may not show such shock, but a multivalued velocity
field. Therefore, the weak and moderate nonlinear evolution leads to the
IGM-dark matter deviation. Yet, the velocity field of Burgers fluid is still
irrotational, as gravity is curl-free. In fully nonlinear regime, the vorticity
of velocity field developed, and the cosmic baryonic fluid will no longer be
potential, as the dynamics of vorticity is independent of gravity and can be
self maintained by the nonlinearity of hydrodynamics. In this case, the cosmic
baryon fluid is in the state of fully developed turbulence, which is
statistically and dynamically decoupling from dark matter. This scenario
provides a mechanism of cohenent explanation of observations.
