4,826 research outputs found

### 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.Comment: 21 page

### Density Perturbations of Thermal Origin During Inflation

We study thermally induced density perturbations during inflation. This
scenario is characterized by two thermodynamical conditions: (1) The primordial
perturbations originate in the epoch when the inflationary universe contains a
thermalized heat bath. (2) The perturbations of the inflationary scalar field
are given by the fluctuation-dissipation relation. We show that the spectrum of
the primordial perturbations is of power law, but tilted, and there is a
relation between the amplitude and the index of the power spectrum. Aside from
the mass scale of the inflation, the amplitude-index relation does not depend
on other parameters like $g$-factor. These results are found to be well
consistent with observations of the temperature fluctuations of cosmic
microwave background if the mass scale of the inflation is about $10^{15}$ GeV.
Instead of the purely adiabatic case, the consequent density perturbation is an
admixture of adiabatic and isocurvature one. Therefore, the detection of
super-Hubble suppression of the spectrum would be effective for further
discrimination between the thermally originated models and others.Comment: 21 pages, 7 postscript figures, using revte

### The Flatness of Mass-to-Light Ratio on Large Scales

It has been suggested that the mass-to-light ($M/L$) ratio of gravitationally
clustering objects is scale-independent on scales beyond galaxy clusters, and
may also be independent of the mass of the objects. In this paper, we show that
the scale behavior of $M/L$ ratio is closely related to the scaling of cosmic
structures larger than clusters. The scale dependence of the $M/L$ ratio can be
determined by comparing the observed scaling of richness function (RF) of
multi-scale identified objects with the model-predicted scaling of mass
function (MF) of large scale structures. Using the multi-scale identified
clusters from IRAS 1.2 Jy galaxy survey, we have made comparisons of the
observed RF scaling of IRAS $r_{cl}$-clusters with the MF scalings given by
simulations of three popular models SCDM, LCDM and OCDM. We find that, the M/L
ratio basically is scale-independent from the Abell radius up to about 24
$h^{-1}$Mpc, while it seems to show a slight, but systematical, increase over
this scale range. This result is weakly dependent on the cosmological
parameters.Comment: AAS Latex file, 8 pages+ 4 figures, accepted for publication in ApJ

### Intermittent behavior of cosmic mass field revealed by QSO's Ly_alpha forests

The intermittent behavior of the space-scale distribution of Ly$\alpha$
transmitted flux of QSO HS1700+64 has been analyzed via a discrete wavelet
transform. We found that there are strong indications of intermittency on
scales down to about 10 $h^{-1}$ kpc. These are: 1.) the probability
distribution function of the local fluctuations of the flux is significantly
long-tailed on small scales, and 2.) the local power spectrum of the flux shows
prominent spiky structures on small scales. Moreover, the local power spectrum
averaged on regions with different sizes shows similar spiky structures.
Therefore, the random mass density field traced by the Ly$\alpha$ forests is
rougher on smaller scales, consistent with singular clustering.Comment: Accepted for publication in ApJ Letters, 12 pages, 3 figure

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