31,880 research outputs found
Comment on "Limits on the Time Variation of the Electromagnetic Fine-Structure Constant in the Low Energy Limit from Absorption Lines in the Spectra of Distant Quasars"
In their Letter [Phys. Rev. Lett. 92, 121302 (2004)] (also [Astron.
Astrophys. 417, 853 (2004)]), Srianand et al. analysed optical spectra of
heavy-element species in 23 absorption systems along background quasar
sight-lines, reporting limits on relative variations in the fine-structure
constant: da/a=(-0.06+/-0.06) x 10^{-5}. Here we demonstrate basic flaws in
their analysis, using the same data and absorption profile fits, which led to
spurious values of da/a and significantly underestimated uncertainties. We
conclude that these data and fits offer no stringent test of previous evidence
for a varying alpha.
In their Reply (arXiv:0711.1742) to this Comment, Srianand et al. state or
argue several points regarding their original analysis and our new analysis. We
discuss these points here, dismissing all of them because they are demonstrably
incorrect or because they rely on a flawed application of simple statistical
arguments.Comment: 1+2 pages, 1 EPS figure. Page 1 accepted as PRL Comment on
arXiv:astro-ph/0402177 . Further details available in arXiv:astro-ph/0612407
. v2: Added critical discussion of Reply from Srianand et al.
(arXiv:0711.1742
Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe
The Standard Model of particle physics assumes that the so-called fundamental
constants are universal and unchanging. Absorption lines arising in molecular
clouds along quasar sightlines offer a precise test for variations in the
proton-to-electron mass ratio, mu, over cosmological time and distance scales.
The inversion transitions of ammonia are particularly sensitive to mu compared
to molecular rotational transitions. Comparing the available ammonia spectra
observed towards the quasar B0218+357 with new, high-quality rotational
spectra, we present the first detailed measurement of mu with this technique,
limiting relative deviations from the laboratory value to |dmu/mu| <
1.8x10^{-6} (95% confidence level) at approximately half the Universe's current
age - the strongest astrophysical constraint to date. Higher-quality ammonia
observations will reduce both the statistical and systematic uncertainties in
these measurements.Comment: Science, 20th June 2008. 22 pages, 5 figures (12 EPS files), 2
tables, including Supporting Online Material; v2: Corrected reference for
laboratory mu-variation bound
A computational model for three-dimensional incompressible wall jets with large cross flow
A computational model for the flow field of three dimensional incompressible wall jets prototypic of thrust augmenting ejectors with large cross flow is presented. The formulation employs boundary layer equations in an orthogonal curvilinear coordinate system. Simulation of laminar as well as turbulen wall jets is reported. Quantification of jet spreading, jet growth, nominal separation, and jet shrink effects due to corss flow are discussed
Comparison of serum immunoglobulin G half-life in dairy calves fed colostrum, colostrum replacer or administered with intravenous bovine plasma.
In calves, passive immunity of immunoglobulins can be acquired through ingestion of colostrum or colostrum replacers. Plasma can been used to supplement immunoglobulins in healthy or sick calves. Serum half-life of colostral derived immuglobulin G (IgG) is estimated to be 20 days. Half-life of IgG is important in determining response to antigens and timing of vaccination in calves. To date studies evaluating half-life of colostrum replacer or plasma derived IgG are lacking. The objectives of this study were to compare the serum half-life of IgG derived from colostrum, colostrum replacer and plasma in dairy calves reared up to 35 days of age. Thirty Jersey calves were randomly assigned to receive colostrum or colostrum replacer by oroesophageal tubing or plasma by intravenous administration. Serum samples were collected at 2, 5, 7, 10, 14, 21, 28 and 35 days. Serum IgG concentrations were determined by radial immunodiffusion. The results indicated that half-life for IgG in colostrum fed (28.5 days) or plasma transfused calves (27.3 days) was longer than colostrum replacer fed calves (19.1 days). Further studies are required to evaluate pathogen specific immunoglobulins in order to recommend vaccination timing in calves fed colostrum replacers
Single-pulse broad-band rotational CARS thermometry of cold N2 gas
Coherent anti Stokes Raman scattering (CARS) from the pure rotational Raman lines of N2 was employed to measure the instantaneous (10 nsec) rotational temperature of the gas at room temperature and below. An entire rotational CARS spectrum was generated by a single laser pulse using a broad bandwidth dye laser and was recorded on an optical multichannel analyzer. A best fit temperature obtained for individual experimental spectra by comparison with calculated spectra. Good agreement between CARS temperatures and thermocouple temperatures was observed
The Third Dimension of Reading the Sugar Code by Lectins
Coding of biological information is not confined to nucleic acids and proteins. Endowed with the highest level of structural versatility among biomolecules, the glycan chains of cellular glycoconjugates are well-suited to generate molecular messages/signals in a minimum of space. The sequence and shape of oligosaccharides as well as spatial aspects of multivalent presentation are assumed to underlie the natural specificity/selectivity that cellular glycans have for endogenous lectins. In order to eventually unravel structure-activity profiles cyclic scaffolds have been used as platforms to produce glycoclusters and afford valuable tools. Using adhesion/growth-regulatory galectins and the pan-galectin ligand lactose as a model, emerging insights into the potential of cyclodextrins, cyclic peptides, calixarenes and glycophanes for this purpose are presented herein. The systematic testing of lectin panels with spatially defined ligand presentations can be considered as a biomimetic means to help clarify the mechanisms, which lead to the exquisite accuracy at which endogenous lectins select their physiological counterreceptors from the complexity of the cellular glycome
Time evolution of the fine structure constant
We present a short review of the current quasar (QSO) absorption line
constraints on possible variation of the fine structure constant, alpha =
e^2/(hbar*c). Particular attention is paid to recent optical Keck/HIRES spectra
of 49 absorption systems which indicate a smaller alpha in the past (Murphy et
al. 2001, Webb et al. 2001). Here we present new preliminary results from 128
absorption systems: da/a = (-0.57 +/- 0.10) x 10^{-5} over the redshift range
0.2 < z < 3.7, in agreement with the previous results. Known potential
systematic errors cannot explain these results. We compare them with strong
`local' constraints and discuss other (radio and millimetre-wave) QSO
absorption line constraints on variations in alpha^2 * g_p and alpha^2 * g_p *
m_e/m_p (g_p is the proton g-factor and m_e/m_p is the electron/proton mass
ratio). Finally, we discuss future efforts to rule out or confirm the current
5.7 sigma optical detection.Comment: Invited review at the XXII Physics in Collision Conference (PIC02),
Stanford, CA, USA, June 2002, 11 pages, LaTeX, 7 eps figures. PSN FRA T0
Keck constraints on a varying fine-structure constant: wavelength calibration errors
The Keck telescope's HIRES spectrograph has previously provided evidence for
a smaller fine-structure constant, alpha, compared to the current laboratory
value, in a sample of 143 quasar absorption systems:
da/a=(-0.57+/-0.11)x10^{-5}. This was based on a variety of metal-ion
transitions which, if alpha varies, experience different relative velocity
shifts. This result is yet to be robustly contradicted, or confirmed, by
measurements on other telescopes and spectrographs; it remains crucial to do
so. It is also important to consider new possible instrumental systematic
effects which may explain the Keck/HIRES results. Griest et al. (2009,
arXiv:0904.4725v1) recently identified distortions in the echelle order
wavelength scales of HIRES with typical amplitudes +/-250m/s. Here we
investigate the effect such distortions may have had on the Keck/HIRES varying
alpha results. We demonstrate that they cause a random effect on da/a from
absorber to absorber because the systems are at different redshifts, placing
the relevant absorption lines at different positions in different echelle
orders. The typical magnitude of the effect on da/a is ~0.4x10^{-5} per
absorber which, compared to the median error on da/a in the sample,
~1.9x10^{-5}, is relatively small. Consequently, the weighted mean value
changes by less than 0.05x10^{-5} if the corrections we calculate are applied.
Nevertheless, we urge caution, particularly for analyses aiming to achieve high
precision da/a measurements on individual systems or small samples, that a much
more detailed understanding of such intra-order distortions and their
dependence on observational parameters is important if they are to be avoided
or modelled reliably. [Abridged]Comment: 9 pages, 2 figures, 2 tables. Invited contribution to Proc. IAU
XXVIIth General Assembly, Joint Discussion 9, "Are the fundamental constants
varying with time?". To appear in P. Molaro, E. Vangioni-Flam, eds, Memorie
della Societa Astronomica Italiana (MmSAIt), Vol. 80. Complete version of
Table 1 available at http://astronomy.swin.edu.au/~mmurphy/pub.htm
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