6,097 research outputs found
Axion Dark Matter and Cosmological Parameters
We observe that photon cooling after big bang nucleosynthesis (BBN) but
before recombination can remove the conflict between the observed and
theoretically predicted value of the primordial abundance of Li. Such
cooling is ordinarily difficult to achieve. However, the recent realization
that dark matter axions form a Bose-Einstein condensate (BEC) provides a
possible mechanism, because the much colder axions may reach thermal contact
with the photons. This proposal predicts a high effective number of neutrinos
as measured by the cosmic microwave anisotropy spectrum.Comment: 4 pages, one figure. Version to appear in Phys. Rev. Lett.,
incorporating useful comments by the referees and emphasizing that photon
cooling by axion BEC is a possibility, not a certaint
Parametric Instability of Supersonic Shear Layers Induced by Periodic Mach Waves
It is suggested that parametric instability can be induced in a confined supersonic shear layer by the use of a periodic Mach wave system generated by a wavy wall. The existence of such an instability solution is demonstrated computationally by solving the Floquet system of equations. The solution is constructed by means of a Fourier-Chebyshev expansion. Numerical convergence is assured by using a very large number of Fourier and Chebyshev basis functions. The computed growth rate of the induced flow instability is found to vary linearly with the amplitude of the mach waves when the amplitude is not excessively large. This ensures that the instability is, indeed, tied to the presence of the Mach waves. It is proposed that enhanced mixing of supersonic shear layers may be achieved by the use of such a periodic Mach wave system through the inducement of parametric instabilities in the flow. © 1991 American Institute of Physics
Remote sensing and GIS-based analysis of cave development in the Suoimuoi Catchment (Son La - NW Vietnam)
Integration of remotely sensed imagery with ground surveys is a promising method in cave
development studies. In this research a methodology was set up in which a variety of remote
sensing and GIS techniques support cave analysis in the tropical karst area of the Suoimuoi
catchment, NW Vietnam. In order to extract the maximum information from different remotely
sensed data, the hue invariant IHS transformation was applied to integrate Landsat multispectral
channels with the high resolution Landsat 7 ETM panchromatic channel. The resulting
fused image was used, after enhancement, to visually and digitally extract lineaments.
Aerial photos evaluated the extracted lineaments. Based on lineament density indices a fracture
zone favorable for cave development is defined. The distance between caves and faults
was investigated as well as the correspondence between the cave occurrence and the fracture
zone
Serological protein profiling of neuroblastoma by ProteinChip SELDI-TOF technology
Serological proteins of neuroblastoma were profiled and analyzed by ProteinChip-SELDI-TOF MS technology with five types of protein chips. By comparing with normal control, a number of protein or polypeptide signals were found significantly and consistently different in their intensities (expression levels) in tumor sera. Interestingly, nine polypeptide peaks in these proteomic features can be simultaneously detected with consistent variations by more than one type of protein chips. None of the expression differences of these nine polypeptides was found in similar comparisons between healthy controls and hepatomas. Preliminary protein identification showed hints for that some of these proteomic alterations may be closely related to the tumorigenesis of neuroblastoma. These results demonstrated the potential of serological biomarker identification for neuroblastoma by ProteinChip-SELDI technology.postprin
Truncated α1-antitrypsin as an infection biomarker for Severe Acute Respiratory Syndrome (SARS)
published_or_final_versio
Some estimates of Wang-Yau quasilocal energy
Given a spacelike 2-surface in a spacetime and a constant future
timelike unit vector in , we derive upper and lower estimates
of Wang-Yau quasilocal energy for a given isometric
embedding of into a flat 3-slice in . The quantity itself depends on the choice of , however the infimum of
over does not. In particular, when lies
in a time symmetric 3-slice in and has nonnegative Brown-York quasilocal
mass \mby(\Sigma), our estimates show that equals \mby (\Sigma). We also study the spatial limit of , where is a large coordinate sphere in a
fixed end of an asymptotically flat initial data set and is
an isometric embeddings of into .
We show that if has future timelike ADM energy-momentum, then
equals the ADM mass
of .Comment: 17 page
Longitudinal filament oscillations enhanced by two C-class flares
In this paper, we report the multiwavelength observations of a very long
filament in active region (AR) 11112 on 2010 October 18. The filament was
composed of two parts, the eastern part (EP) and western part (WP). We focus on
longitudinal oscillations of the EP, which were enhanced by two homologous
C-class flares in the same AR. The C1.3 flare was confined without a CME. Both
EP and WP of the filament were slightly disturbed and survived the flare. After
5 hrs, eruption of the WP generated a C2.6 flare and a narrow, jet-like CME.
Three oscillating threads (thd, thd, thd) are obviously identified
in the EP and their oscillations are naturally divided into three phases by the
two flares. The initial amplitude ranges from 1.6 to 30 Mm with a mean value of
14 Mm. The period ranges from 34 to 73 minutes with a mean value of
53 minutes. The curvature radii of the magnetic dips are estimated to be
29 to 133 Mm with a mean value of 74 Mm. The damping times ranges from
62 to 96 minutes with a mean value of 82 minutes. The value
of is between 1.2 and 1.8. For thd in the EP, the amplitudes were
enhanced by the two flares from 6.1 Mm to 6.8 Mm after the C1.3 flare and
further to 21.4 Mm after the C2.6 flare. The period variation as a result of
perturbation from the flares was within 20\%. The attenuation became faster
after the C2.6 flare. To the best of our knowledge, this is the first report of
large-amplitude, longitudinal filament oscillations enhanced by flares.
Numerical simulations reproduce the oscillations of thd very well. The
simulated amplitudes and periods are close to the observed values, while the
damping time in the last phase is longer, implying additional mechanisms should
be taken into account apart from radiative loss.Comment: 9 pages, 16 figures, 3 tables, accepted for publication in A&
Cosmic axion thermalization
Axions differ from the other cold dark matter candidates in that they form a
degenerate Bose gas. It is shown that their huge quantum degeneracy and large
correlation length cause cold dark matter axions to thermalize through
gravitational self-interactions when the photon temperature reaches
approximately 500 eV. When they thermalize, the axions form a Bose-Einstein
condensate. Their thermalization occurs in a regime, herein called the
`condensed regime', where the Boltzmann equation is not valid because the
energy dispersion of the particles is smaller than their interaction rate. We
derive analytical expressions for the thermalization rate of particles in the
condensed regime, and check the validity of these expressions by numerical
simulation of a toy model. We revisit axion cosmology in light of axion
Bose-Einstein condensation. It is shown that axions are indistinguishable from
ordinary cold dark matter on all scales of observational interest, except when
they thermalize or rethermalize. The rethermalization of axions that are about
to fall in a galactic potential well causes them to acquire net overall
rotation as they go to the lowest energy state consistent with the total
angular momentum they acquired by tidal torquing. This phenomenon explains the
occurrence of caustic rings of dark matter in galactic halos. We find that
photons may reach thermal contact with axions and investigate the implications
of this possibility for the measurements of cosmological parameters.Comment: 38 pages, 1 figur
Study of cavernous underground conduits in Nam La (Northwest Vietnam) by an integrative approach
This paper presents the result of an investigation of underground conduits, which connect the swallow holes and the resurgence of a blind river in the tropical, highly karstified limestone Nam La catchment in the NW of Vietnam. The Nam La River disappears underground in several swallow holes near the outlet of the catchment. In the rainy season this results in flooding upstream of the sinkholes. A hypothesis is that the Nam La River resurges at a large cavern spring 4.5 km east of the catchment outlet. A multi-thematic study of the possible connections between the swallow holes and the resurgence was carried out to investigate the geological structure, tectonics, cave structure analysis and discharge time series. The existence of the underground conduits was also tested and proven by tracer experiments. On the basis of a lineament analysis the location of the underground conduits were predicted. A remote sensing derived lineament-length density map was used to track routes from the swallow holes to the resurgence, having the shortest length but highest lineament density. This resulted in a plan-view prediction of underground conduits that matches with the cave and fault development. The functioning of the conduits was further explained by analysing flooding records of a nearby doline, which turns out to act as a temporary storage reservoir mitigating flooding of the catchment outlet area
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