Brane Formation and Cosmological Constraint on the Number of Extra Dimensions

Special relativity is generalized to extra dimensions and quantized energy levels of particles are obtained. By calculating the probability of particles' motion in extra dimensions at high temperature of the early universe, it is proposed that the branes may have not existed since the very beginning of the universe, but formed later. Meanwhile, before the formation, particles of the universe may have filled in the whole bulk, not just on the branes. This scenario differs from that in the standard big bang cosmology in which all particles are assumed to be in the 4D spacetime. So, in brane models, whether our universe began from a 4D big bang singularity is questionable. A cosmological constraint on the number of extra dimensions is also given which favors $N\geq 7$.Comment: 11 pages, no figures. To appear in IJT

Two problems related to prescribed curvature measures

Existence of convex body with prescribed generalized curvature measures is discussed, this result is obtained by making use of Guan-Li-Li's innovative techniques. In surprise, that methods has also brought us to promote Ivochkina's $C^2$ estimates for prescribed curvature equation in \cite{I1, I}.Comment: 12 pages, Corrected typo

Dynamical Structure Factors of the S=1/2 Bond-Alternating Spin Chain with a Next-Nearest-Neighbor Interaction in Magnetic Fields

The dynamical structure factor of the S=1/2 bond-alternating spin chain with a next-nearest-neighbor interaction in magnetic field is investigated using the continued fraction method based on the Lanczos algorithm. When the plateau exists on the magnetization curve, the longitudinal dynamical structure factor shows a large intensity with a periodic dispersion relation, while the transverse one shows a large intensity with an almost dispersionless mode. The periodicity and the amplitude of the dispersion relation in the longitudinal dynamical structure factor are sensitive to the coupling constants. The dynamical structure factor of the S=1/2 two-leg ladder in magnetic field is also calculated in the strong interchain-coupling regime. The dynamical structure factor shows gapless or gapful behavior depending on the wave vector along the rung.Comment: 8 pages, 4 figures, to appear in Journal of the Physical Society of Japan, vol. 69, no. 10, (2000

Childcare, choice and social class: Caring for young children in the UK

This paper draws on the results of two qualitative research projects examining parental engagements with the childcare market in the UK. Both projects are located in the same two London localities. One project focuses on professional middle class parents, and the other on working class families, and we discuss the key importance of social class in shaping parents' differential engagement with the childcare market, and their understandings of the role childcare plays in their children's lives. We identify and discuss the different "circuits" of care (Ball et al 1995) available to and used by families living physically close to each other, but in social class terms living in different worlds. We also consider parents' relationships with carers, and their social networks. We conclude that in order to fully understand childcare policies and practices and families' experiences of care, an analysis which encompasses social class and the workings of the childcare market is needed

Structural Attributes and Photodynamics of Visible Spectrum Quantum Emitters in Hexagonal Boron Nitride

Newly discovered van der Waals materials like MoS2, WSe2, hexagonal boron nitride (h-BN), and recently C2N have sparked intensive research to unveil the quantum behavior associated with their 2D structure. Of great interest are 2D materials that host single quantum emitters. h-BN, with a band gap of 5.95 eV, has been shown to host single quantum emitters which are stable at room temperature in the UV and visible spectral range. In this paper we investigate correlations between h-BN structural features and emitter location from bulk down to the monolayer at room temperature. We demonstrate that chemical etching and ion irradiation can generate emitters in h-BN. We analyze the emitters' spectral features and show that they are dominated by the interaction of their electronic transition with a single Raman active mode of h-BN. Photodynamics analysis reveals diverse rates between the electronic states of the emitter. The emitters show excellent photo stability even under ambient conditions and in monolayers. Comparing the excitation polarization between different emitters unveils a connection between defect orientation and the h-BN hexagonal structure. The sharp spectral features, color diversity, room-temperature stability, long-lived metastable states, ease of fabrication, proximity of the emitters to the environment, outstanding chemical stability, and biocompatibility of h-BN provide a completely new class of systems that can be used for sensing and quantum photonics applications

Cosmological Measures without Volume Weighting

Many cosmologists (myself included) have advocated volume weighting for the cosmological measure problem, weighting spatial hypersurfaces by their volume. However, this often leads to the Boltzmann brain problem, that almost all observations would be by momentary Boltzmann brains that arise very briefly as quantum fluctuations in the late universe when it has expanded to a huge size, so that our observations (too ordered for Boltzmann brains) would be highly atypical and unlikely. Here it is suggested that volume weighting may be a mistake. Volume averaging is advocated as an alternative. One consequence may be a loss of the argument that eternal inflation gives a nonzero probability that our universe now has infinite volume.Comment: 15 pages, LaTeX, added references for constant-H hypersurfaces and also an idea for minimal-flux hypersurface

Automated selection and characterization of emission-line sources in ACS WFC grism data

We present complimentary techniques to find emission-line targets and measure their properties in a semi-automated fashion from grism observations obtained with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The first technique is to find all likely sources in a direct image, extract their spectra and search them for emission lines. The second method is to look for emission-line sources as compact structures in an unsharp masked version of the grism image. Using these methods we identify 46 emission-line targets in the Hubble Deep Field North using a modest (3 orbit) expenditure of HST observing time. Grism spectroscopy is a powerful tool for efficiently identifying interesting low luminosity, moderate redshift emission-line field galaxies. The sources found here have a median i band flux 1.5 mag fainter than the spectroscopic redshift catalog of Cohen et al. They have redshift z <= 1.42, high equivalent widths (typically EW > 100{\AA}), and are usually less luminous than the characteristic luminosity at the same redshift. The chief obstacle in interpreting the results is line identification, since the majority of sources have a single emission line and the spectral resolution is low. Photometric redshifts are useful for providing a first guess redshift. However, even at the depth of the state-of-the-art data used here, photometric errors can result in uncertainties in line identifications, especially for sources with i > ~24.5 ABmag. Reliable line identification for the faintest emission-line galaxies requires additional ground-based spectroscopy for confirmation. Of particular concern are the faint high EW [OII] emitters which could represent a strongly evolving galaxy population if the possibility that they are mis-identified lower redshift interlopers can be ruled out. (Slightly abridged)Comment: AJ accepted. 27 pages, 4 tables, 11 figures. Uses emulateapj.cl

Discovery of Globular Clusters in the Proto-Spiral NGC2915: Implications for Hierarchical Galaxy Evolution

We have discovered three globular clusters beyond the Holmberg radius in Hubble Space Telescope Advanced Camera for Surveys images of the gas-rich dark matter dominated blue compact dwarf galaxy NGC2915. The clusters, all of which start to resolve into stars, have M_{V606} = -8.9 to -9.8 mag, significantly brighter than the peak of the luminosity function of Milky Way globular clusters. Their colors suggest a metallicity [Fe/H] ~ -1.9 dex, typical of metal-poor Galactic globular clusters. The specific frequency of clusters is at a minimum normal, compared to spiral galaxies. However, since only a small portion of the system has been surveyed it is more likely that the luminosity and mass normalized cluster content is higher, like that seen in elliptical galaxies and galaxy clusters. This suggests that NGC2915 resembles a key phase in the early hierarchical assembly of galaxies - the epoch when much of the old stellar population has formed, but little of the stellar disk. Depending on the subsequent interaction history, such systems could go on to build-up larger elliptical galaxies, evolve into normal spirals, or in rare circumstances remain suspended in their development to become systems like NGC2915.Comment: ApJ Letters accepted; 6 pages, 2 figures, 3 table

Strong Lensing Analysis of A1689 from Deep Advanced Camera Images

We analyse deep multi-colour Advanced Camera images of the largest known gravitational lens, A1689. Radial and tangential arcs delineate the critical curves in unprecedented detail and many small counter-images are found near the center of mass. We construct a flexible light deflection field to predict the appearance and positions of counter-images. The model is refined as new counter-images are identified and incorporated to improve the model, yielding a total of 106 images of 30 multiply lensed background galaxies, spanning a wide redshift range, 1.0$<$z$<$5.5. The resulting mass map is more circular in projection than the clumpy distribution of cluster galaxies and the light is more concentrated than the mass within $r<50kpc/h$. The projected mass profile flattens steadily towards the center with a shallow mean slope of $d\log\Sigma/d\log r \simeq -0.55\pm0.1$, over the observed range, r$<250kpc/h$, matching well an NFW profile, but with a relatively high concentration, $C_{vir}=8.2^{+2.1}_{-1.8}$. A softened isothermal profile ($r_{core}=20\pm2$\arcs) is not conclusively excluded, illustrating that lensing constrains only projected quantities. Regarding cosmology, we clearly detect the purely geometric increase of bend-angles with redshift. The dependence on the cosmological parameters is weak due to the proximity of A1689, $z=0.18$, constraining the locus, $\Omega_M+\Omega_{\Lambda} \leq 1.2$. This consistency with standard cosmology provides independent support for our model, because the redshift information is not required to derive an accurate mass map. Similarly, the relative fluxes of the multiple images are reproduced well by our best fitting lens model.Comment: Accepted by ApJ. For high quality figures see http://wise-obs.tau.ac.il/~kerens/A168

Dynamical structure factors of the magnetization-plateau state in the S=1/2S=1/2 bond-alternating spin chain with a next-nearest-neighbor interaction

We calculate the dynamical structure factors of the magnetization-plateau state in the $S=1/2$ bond-alternating spin chain with a next-nearest-neighbor interaction. The results show characteristic behaviors depending on the next-nearest-neighbor interaction $\alpha$ and the bond-alternation $\delta$. We discuss the lower excited states in comparison with the exact excitation spectrums of an effective Hamiltonian. From the finite size effects, characteristics of the lowest excited states are investigated. The dispersionless mode of the lowest excitation appears in adequate sets of $\alpha$ and $\delta$, indicating that the lowest excitation is localized spatially and forms an isolated mode below the excitation continuum. We further calculate the static structure factors. The largest intensity is located at $q=\pi$ for small $\delta$ in fixed $\alpha$. With increasing $\delta$, the wavenumber of the largest intensity shifts towards $q=\pi/2$, taking the incommensurate value.Comment: to appear in Phys. Rev. B (2001