5,415 research outputs found
How Massless Neutrinos Affect the Cosmic Microwave Background Damping Tail
We explore the physical origin and robustness of constraints on the energy
density in relativistic species prior to and during recombination, often
expressed as constraints on an effective number of neutrino species, Neff.
Constraints from current data combination of Wilkinson Microwave Anisotropy
Probe (WMAP) and South Pole Telescope (SPT) are almost entirely due to the
impact of the neutrinos on the expansion rate, and how those changes to the
expansion rate alter the ratio of the photon diffusion scale to the sound
horizon scale at recombination. We demonstrate that very little of the
constraining power comes from the early Integrated Sachs-Wolfe (ISW) effect,
and also provide a first determination of the amplitude of the early ISW
effect. Varying the fraction of baryonic mass in Helium, Yp, also changes the
ratio of damping to sound-horizon scales. We discuss the physical effects that
prevent the resulting near-degeneracy between Neff and Yp from being a complete
one. Examining light element abundance measurements, we see no significant
evidence for evolution of Neff and the baryon-to-photon ratio from the epoch of
big bang nucleosynthesis to decoupling. Finally, we consider measurements of
the distance-redshift relation at low to intermediate redshifts and their
implications for the value of Neff.Comment: 11 pages. Replaced version extends our discussion of origin of
constraints and updates for current data, submitted to PR
Sensitization of Human Cancer Cells to Gemcitabine by the Chk1 Inhibitor MK-8776: Cell Cycle Perturbation and Impact of Administration Schedule in Vitro and in Vivo
Chk1 inhibitors have emerged as promising anticancer therapeutic agents particularly when combined with antimetabolites such as gemcitabine, cytarabine or hydroxyurea. Here, we address the importance of appropriate drug scheduling when gemcitabine is combined with the Chk1 inhibitor MK-8776, and the mechanisms involved in the schedule dependence
Worcester Chamber of Commerce: Recruiting Minority Business Owners
Our capstone project was to help the Worcester Regional Chamber of Commerce identify how to re-frame their marketing so it would be appealing to immigrant and minority owned businesses. Based on interviews and external research, our group was able to create a tangible and resourceful data set that provided justified recommendations and ideas on how the Chamber could make adjustments to their marketing plan to attract more businesses of this particular demographic in the city of Worcester. By implementing these recommendations, we believe the Chamber has the opportunity to create a more diverse group of Chamber members, add value to a previously underserved community, and support minority and immigrant owned small businesses through Worcester’s growth in the coming years
Some explicit constructions of Dirac-harmonic maps
We construct explicit examples of Dirac-harmonic maps between
Riemannian manifolds and which are non-trivial in the sense
that is not harmonic. When , we also produce examples where
is harmonic, but not conformal, and is non-trivial.Comment: to appear in J. Geom. Phy
CTCF-mediated transcriptional regulation through cell type-specific chromosome organization in the {\beta}-globin locus
The principles underlying the architectural landscape of chromatin beyond the
nucleosome level in living cells remains largely unknown despite its potential
to play a role in mammalian gene regulation. We investigated the 3-dimensional
folding of a 1 Mbp region of human chromosome 11 containing the {\beta}-globin
genes by integrating looping interactions of the insulator protein CTCF
determined comprehensively by chromosome conformation capture (3C) into a
polymer model of chromatin. We find that CTCF-mediated cell type specific
interactions in erythroid cells are organized to favor contacts known to occur
in vivo between the {\beta}-globin locus control region (LCR) and genes. In
these cells, the modeled {\beta}-globin domain folds into a globule with the
LCR and the active globin genes on the periphery. By contrast, in non-erythroid
cells, the globule is less compact with few but dominant CTCF interactions
driving the genes away from the LCR. This leads to a decrease in contact
frequencies that can exceed 1000-fold depending on the stiffness of the
chromatin and the exact positioning of the genes. Our findings show that an
ensemble of CTCF contacts functionally affects spatial distances between
control elements and target genes contributing to chromosomal organization
required for transcription.Comment: Full article, including Supp. Mat., is available at Nucleic Acids
Research, doi: 10.1093/nar/gks53
Amino acid-dependent stability of the acyl linkage in aminoacyl-tRNA.
Aminoacyl-tRNAs are the biologically active substrates for peptide bond formation in protein synthesis. The stability of the acyl linkage in each aminoacyl-tRNA, formed through an ester bond that connects the amino acid carboxyl group with the tRNA terminal 3\u27-OH group, is thus important. While the ester linkage is the same for all aminoacyl-tRNAs, the stability of each is not well characterized, thus limiting insight into the fundamental process of peptide bond formation. Here, we show, by analysis of the half-lives of 12 of the 22 natural aminoacyl-tRNAs used in peptide bond formation, that the stability of the acyl linkage is effectively determined only by the chemical nature of the amino acid side chain. Even the chirality of the side chain exhibits little influence. Proline confers the lowest stability to the linkage, while isoleucine and valine confer the highest, whereas the nucleotide sequence in the tRNA provides negligible contribution to the stability. We find that, among the variables tested, the protein translation factor EF-Tu is the only one that can protect a weak acyl linkage from hydrolysis. These results suggest that each amino acid plays an active role in determining its own stability in the acyl linkage to tRNA, but that EF-Tu overrides this individuality and protects the acyl linkage stability for protein synthesis on the ribosome
Elastocaloric Cooling of Additive Manufactured Shape Memory Alloys with Large Latent Heat
The stress-induced martensitic phase transformation of shape memory alloys (SMAs) is the basis for elastocaloric cooling. Here we employ additive manufacturing to fabricate TiNi SMAs, and demonstrate compressive elastocaloric cooling in the TiNi rods with transformation latent heat as large as 20 J g−1. Adiabatic compression on as-fabricated TiNi displays cooling DT as high as −7.5 °C with recoverable superelastic strain up to 5 %. Unlike conventional SMAs, additive manufactured TiNi SMAs exhibit linear superelasticity with narrow hysteresis in stress-strain curves under both adiabatic and isothermal conditions. Microstructurally, we find that there are Ti2Ni precipitates typically one micron in size with a large aspect ratio enclosing the TiNi matrix. A stress transfer mechanism between reversible phase transformation in the TiNi matrix and mechanical deformation in Ti2Ni precipitates is believed to be the origin of the unique superelasticity behavior
A generative flow for conditional sampling via optimal transport
Sampling conditional distributions is a fundamental task for Bayesian
inference and density estimation. Generative models, such as normalizing flows
and generative adversarial networks, characterize conditional distributions by
learning a transport map that pushes forward a simple reference (e.g., a
standard Gaussian) to a target distribution. While these approaches
successfully describe many non-Gaussian problems, their performance is often
limited by parametric bias and the reliability of gradient-based (adversarial)
optimizers to learn these transformations. This work proposes a non-parametric
generative model that iteratively maps reference samples to the target. The
model uses block-triangular transport maps, whose components are shown to
characterize conditionals of the target distribution. These maps arise from
solving an optimal transport problem with a weighted cost function,
thereby extending the data-driven approach in [Trigila and Tabak, 2016] for
conditional sampling. The proposed approach is demonstrated on a two
dimensional example and on a parameter inference problem involving nonlinear
ODEs.Comment: 18 pages, 5 figure
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