  We investigate the vorticity of the IGM velocity field on large scales with
cosmological hydrodynamic simulation of the concordance model of LCDM. We show
that the vorticity field is significantly increasing with time as it can
effectively be generated by shocks and complex structures in the IGM.
Therefore, the vorticity field is an effective tool to reveal the nonlinear
behavior of the IGM, especially the formation and evolution of turbulence in
the IGM. We find that the vorticity field does not follow the filaments and
sheets structures of underlying dark matter density field and shows highly non-
Gaussian and intermittent features. The power spectrum of the vorticity field
is used to measure the development of turbulence in Fourier space. We show that
the relation between the power spectra of vorticity and velocity fields is
perfectly in agreement with the prediction of a fully developed homogeneous and
isotropic turbulence from 0.2 to 3 h^{-1} Mpc at z~0. This indicates that
cosmic baryonic field is in the state of fully developed turbulence on scales
less than about 3 h^{-1} Mpc. The random field of the turbulent fluid yields
turbulent pressure to prevent the gravitational collapsing of the IGM. The
vorticity and turbulent pressure are strong inside and even outside of high
density regions. In IGM regions with 10 times mean overdensity, the turbulent
pressure can be on an average equivalent to the thermal pressure of the
baryonic gas with a temperature of 10^5 K. The fully developed turbulence would
prevent the baryons in the IGM from falling into the gravitational well of dark
matter halos. Moreover, turbulent pressure is dynamical and non-thermal, which
makes it different from pre-heating mechanism as it does not affect the thermal
state and ionizing process of hydrogen in the IGM.
