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The diffeomorphism constraint operator in loop quantum gravity
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations) Also appairs in Journal of Physics : Conference Series, 2012, Vol.360, p012009We construct the smeared diffeomorphism constraint operator at finite triangulation from the basic holonomy- flux operators of Loop Quantum Gravity, evaluate its continuum limit on the Lewandowski- Marolf habitat and show that the action of the continuum operator provides an anomaly free representation of the Lie algebra of diffeomorphisms of the 3- manifold. Key features of our analysis include: (i) finite triangulation approximants to the curvature, of the Ashtekar- Barbero connection which involve not only small loop holonomies but also small surface fluxes as well as an explicit dependence on the edge labels of the spin network being acted on (ii) the dependence of the small loop underlying the holonomy on both the direction and magnitude of the shift vector field (iii) continuum constraint operators which do {\em not} have finite action on the kinematic Hilbert space, thus implementing a key lesson from recent studies of parameterised field theory by the authors.
Features (i) and (ii) provide the first hints in LQG of a conceptual similarity with the so called "mu- bar" scheme of Loop Quantum Cosmology. We expect our work to be of use in the construction of an anomaly free quantum dynamics for LQG
Erratum: Head-on infall of two compact objects: third post-Newtonian energy flux [Phys. Rev. D 82, 104005 (2010)]
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations
Detecting neutral hydrogen in emission at redshift z ≃ 1
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We use a large N-body simulation to examine the detectability of H I in emission at redshift z≃ 1, and the constraints imposed by current observations on the neutral hydrogen mass function of galaxies at this epoch. We consider three different models for populating dark matter haloes with H I, designed to encompass uncertainties at this redshift. These models are consistent with recent observations of the detection of H I in emission at z≃ 0.8. Whilst detection of 21-cm emission from individual haloes requires extremely long integrations with existing radio interferometers, such as the Giant Meter Radio Telescope (GMRT), we show that the stacked 21-cm signal from a large number of haloes can be easily detected. However, the stacking procedure requires accurate redshifts of galaxies. We show that radio observations of the field of the Deep Extragalactic Evolutionary Probe 2 (DEEP2) spectroscopic galaxy redshift survey should allow detection of the H I mass function at the 5-12σ level in the mass range 1011.4 h-1 Mȯ≤Mhalo≤ 1012.5 h-1 Mȯ, with a moderate amount of observation time. Assuming a larger noise level that corresponds to an upper bound for the expected noise for the GMRT, the detection significance for the H I mass function is still at the 1.7-3σ level. We find that optically undetected satellite galaxies enhance the H I emission profile of the parent halo, leading to broader wings as well as a higher peak signal in the stacked profile of a large number of haloes. We show that it is in principle possible to discern the contribution of undetected satellites to the total H I signal, even though cosmic variance limitation make this challenging for some of our models
Intracluster gas pressure, entropy injection and redshift evolution
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We study the effect of entropy injection in the intracluster medium (ICM) in light of
the recent observationally determined universal pressure profile of the ICM. Beginning
with a power-law entropy profile that is expected in the absence of any feedback, we
show that a simple universal prescription of entropy injection results in the final,
observed universal pressure profile. This simple prescription has two components, one
associated with an overall increase in entropy and another associated with injection in
the central parts of the cluster. Importantly, both the components of entropy injection
are needed to produce the final universal pressure profile. This is indicative of a need
of both preheating the ICM as well in situ AGN/SNe heating. We demonstrate the
usefulness of the method by extending the calculations to clusters at high redshift, and
predict redshift evolution of cluster scaling relations that can be tested against data.
We show that the self-similar evolution of the universal pressure profile is equivalent
to a negative evolution of entropy injection with redshift, with a scaling Sinj / (1 +
z)−0.8Sinj (z = 0). We also show the current observational data is indicative of the
entropy injection decreasing with redshift
Early-time particle dynamics and non-affine deformations during microstructure selection in solids
No full textRestricted Access.Solid–solid transitions are invariably associated with groups of particles whose deformations
cannot be expressed as an affine strain about a reference configuration. The dynamics of these
non-affine zones (NAZ) determine the subsequent microstructure, i.e. the mesoscale patterning
resulting from the structural transition. Here, we focus on early-time dynamics of individual
particles within an NAZ associated with a nucleation event. We show that the early-time
behavior of these particles have distinctive characteristics depending on the transition
temperature. The dynamics is heterogeneous, consisting of a few active particles exhibiting
complex intermittent jamming and flow in response to internal stresses generated during the
transformation. At low temperatures, the dynamics of these active particles is ballistic and the
structural transformation proceeds via string-like correlated movement of active particles, along
ridges in the potential energy topography set up by inactive particles. On increasing
temperature, the dynamics of active particles show an abrupt transition from ballistic to
diffusive behavior with a diffusion coefficient which appears to be independent of temperature.
This dynamical transition in the nature of the trajectories of particles is coincident with a
discontinuous transition in the microstructure of the solid. Finally, we characterize this
transition in terms of a dynamical order parameter in the space of trajectories and discuss its
connection with the glass transition and rheology of soft and granular matter
Nonlinear optical properties of nanosized rare-earth-doped strontium barium niobate ceramics
No full textRestricted Access.Nanosized, undoped and rare-earth-doped Sr0.5Ba0.5Nb2O6 (SBN) ceramic systems were synthesized by an aqueous organic gel route. The precursor gel on calcinations at 800 degrees C for 2hr produces undoped and rare-earth-doped SBN systems of pure tungsten bronze lattice structure. These systems have been characterized for their structure by XRD, SEM, and TEM. The nonlinear absorption behavior of the undoped and rare-earth-doped SBN ceramic systems was investigated by use of the open-aperture Z-scan technique using 532nm, 5ns laser pulses. The authors found that these materials are potential optical limiters at this excitation wavelength
An open aperture z-scan study of Sr2Ceo4 blue phosphor
No full textRestricted Access.Sr(2)CeO(4) blue phosphor has been prepared by the solid-state reaction method. The X-ray diffraction (XRD) study confirms the structure of the system to be orthorhombic. High resolution electron transmission microscopy reveals that Sr(2)CeO(4) prepared by the solid state reaction method is composed of elongated spherical structures of length similar to 0.2-0.6 mu m and width similar to 90-150 nm. The excitation spectrum shows a broad band which peaks at 275 nm. The emission spectrum shows a broad band which peaks at 467 nm when excited at 275 nm. The emission band is assigned to the energy transfer between the molecular orbital of the ligand and charge transfer (CT) state of the Ce(4+) ion. The Commission International de l'Eclairage (CIE) co-ordinates are x = 0.15, and y = 0.23. The nonlinear absorption behavior of Sr(2)CeO(4) has been investigated using the open aperture z-scan technique. The calculated effective two-photon absorption coefficient shows that the Sr(2)CeO(4) blue phosphor is a promising optical limiting material
Effect of an electric field on defects in a nematic liquid crystal with variable surface anchoring
No full textRestricted Access.We report experimental studies on defects in a nematic liquid crystal with negative dielectric anisotropy mounted in a cell with perfluoropolymer-coated surfaces. The sample exhibits a discontinuous anchoring transition from planar to homeotropic on cooling at zero or a small electric field, and above a cross-over voltage a continuous
‘inverse Freedericksz transition’, at which the director starts tilting in opposite directions at the two surfaces. Defects of strength ±1/2 are either annihilated or expelled when the director tilts. On the other hand, disclination lines of ±1 which end in partial point defects (boojums) at the surfaces in the planar alignment regime acquire point defects of strength ±1 at the midplane of the cell when the director tilts. At a low enough temperature, the
homeotropic anchoring becomes strong, and an electric field above the Freedericksz threshold generates the usual umbilic defects, which follow the dynamic scaling laws found in earlier studies