25,632 research outputs found
Deficient Reasoning for Dark Matter in Galaxies
Astronomers have been using the measured luminosity to estimate the {\em
luminous mass} of stars, based on empirically established mass-to-light ratio
which seems to be only applicable to a special class of stars---the
main-sequence stars---with still considerable uncertainties. Another basic tool
to determine the mass of a system of stars or galaxies comes from the study of
their motion, as Newton demonstrated with his law of gravitation, which yields
the {\em gravitational mass}. Because the luminous mass can at best only
represent a portion of the gravitational mass, finding the luminous mass to be
different or less than the gravitational mass should not be surprising. Using
such an apparent discrepancy as a compelling evidence for the so-called dark
matter, which has been believed to possess mysterious nonbaryonic properties
and present a dominant amount in galaxies and the universe, seems to be too far
a stretch when seriously examining the facts and uncertainties in the
measurement techniques. In our opinion, a galaxy with star type distribution
varying from its center to edge may have a mass-to-light ratio varying
accordingly. With the thin-disk model computations based on measured rotation
curves, we found that most galaxies have a typical mass density profile that
peaks at the galactic center and decreases rapidly within of the
cut-off radius, and then declines nearly exponentially toward the edge. The
predicted mass density in the Galactic disk is reasonably within the reported
range of that observed in interstellar medium. This leads us to believe that
ordinary baryonic matter can be sufficient for supporting the observed galactic
rotation curves; speculation of large amount of non-baryonic matter may be
based on an ill-conceived discrepancy between gravitational mass and luminous
mass which appears to be unjustified
Excitation of nonlinear ion acoustic waves in CH plasmas
Excitation of nonlinear ion acoustic wave (IAW) by an external electric field
is demonstrated by Vlasov simulation. The frequency calculated by the
dispersion relation with no damping is verified much closer to the resonance
frequency of the small-amplitude nonlinear IAW than that calculated by the
linear dispersion relation. When the wave number increases,
the linear Landau damping of the fast mode (its phase velocity is greater than
any ion's thermal velocity) increases obviously in the region of in which the fast mode is weakly damped mode. As a result, the deviation
between the frequency calculated by the linear dispersion relation and that by
the dispersion relation with no damping becomes larger with
increasing. When is not large, such as , the nonlinear IAW can be excited by the driver with the linear frequency
of the modes. However, when is large, such as
, the linear frequency can not be applied to exciting the
nonlinear IAW, while the frequency calculated by the dispersion relation with
no damping can be applied to exciting the nonlinear IAW.Comment: 10 pages, 9 figures, Accepted by POP, Publication in August 1
Anti-Stokes scattering and Stokes scattering of stimulated Brillouin scattering cascade in high-intensity laser-plasmas interaction
The anti-Stokes scattering and Stokes scattering in stimulated Brillouin
scattering (SBS) cascade have been researched by the Vlasov-Maxwell simulation.
In the high-intensity laser-plasmas interaction, the stimulated anti-Stokes
Brillouin scattering (SABS) will occur after the second stage SBS rescattering.
The mechanism of SABS has been put forward to explain this phenomenon. And the
SABS will compete with the SBS rescattering to determine the total SBS
reflectivity. Thus, the SBS rescattering including the SABS is an important
saturation mechanism of SBS, and should be taken into account in the
high-intensity laser-plasmas interaction.Comment: 6 pages, 5 figure
Existence problem of proton semi-bubble structure in the state of Si
The fully self-consistent Hartree-Fock (HF) plus random phase approximation
(RPA) based on Skyrme-type interaction is used to study the existence problem
of proton semi-bubble structure in the state of Si. The
experimental excitation energy and the B(E2) strength of the state in
Si can be reproduced quite well. The tensor effect is also studied. It
is shown that the tensor interaction has a notable impact on the excitation
energy of the state and a small effect on the B(E2) value. Besides, its
effect on the density distributions in the ground and state of
Si is negligible. Our present results with T36 and T44 show that the
state of Si is mainly caused by proton transiton from orbit to orbit, and the existence of a proton
semi-bubble structure in this state is very unlikely.Comment: 6 pages, 3 figures, 3 table
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