It is well known that the baryonic component occupies about 4% of
the cosmic energy density. However, the amount of the
baryons observationally identified in galaxies, galaxy
clusters in nearby universe is at most only half of the
cosmic baryonic content. Such unindentified baryonic
component is called "missing baryon" or "dark baryon".
Based on cosmological hydrodynamic simulations of the
large-scale structure in the universe, about 40% of the
baryonic component in the current universe is in the form
of diffuse plasma with a temperature of 105 K
to 107 K associated with the large-scale
structure, and a promissing candidate of the dark
baryon. Such component is called "Warm-Hot Intergalactic
Medium" of WHIM for short.
Galaxy clusters are the largest virialized systems in the
universe, and their typical dynamical timescale is
comparable to the age of the universe. Thus, the abundance
of galaxy clusters is an important clue to the
cosmological parameters. Recently, galaxy clusters are
observed in the radio bands through the Sunyaev-Zel'dovich
(SZ) effect as well as in the X-ray bands, and the
combinations of X-ray and radio observations of galaxy
clusters provide new physical insights to the intracluster
medium (ICM). Furthermore, the physical state of the ICM
in the outskirts of galaxy clusters which had been beyond
the reach of X-ray observation is probed with the
observation using Suzaku satellite.
Neutrinos are the electrically neutral leptons and thought
to be detached from photons in the very early phase of our
universe. In the standard model of elemetary particle
physics, they are assumed to be massless based on the fact
that all neutrinos are left-handed. However, the detection
of the neutrino oscillation in ground-based experiments
turns out that neutrinos are MASSIVE. We can only measure
the squared masses differences of neutrinos in different
mass eigen states through the detection of neutrino
oscillation. Since cosmological neutrinos detached from
photons in the early universe are non-relativistic massive
component in the present universe, it can make physical
impact on the fomation of the large-scale structure in the
universe through the gravitational interaction. One of the
such physical effects is the collisionless damping (aka
free streaming or Landau damping) caused by the large
velocity dispersion of the cosmological neutrnios and
tends to erase the density fluctuation, which is evolved
through the gravitational instability drived by the cold
dark matter. The collisionless damping is actually
difficult to be handled with conventional N-body
simulations. We, instead, explore an alternative approach
other than N-body simulations, the Vlasov simulation,
which directly solves the collisionless Boltzmann equation
or the Vlasov equation.