Comment on ``Theorem for nonrotating singularity-free universes''

We show that Raychaudhuri's recently proposed theorem on nonrotating universes cannot be used to rule out realistic singularity-free descriptions of the universe, as suggested by him in PRL 80, 654 (1998).Comment: 1 page, to appear in Phys.Rev.Let

Singularity: Raychaudhuri Equation once again

I first recount Raychaudhuri's deep involvement with the singularity problem in general relativity. I then argue that precisely the same situation has arisen today in loop quantum cosmology as obtained when Raychaudhuri discovered his celebrated equation. We thus need a new analogue of the Raychaudhuri equation in quantum gravity.Comment: 11 pages, Contribution to Special issue of Pramana on Raychaudhuri Equation at Cross-roads, edited by Naresh Dadhich, Pankaj Joshi and Probir Ro

Structure and optical properties of Cd substituted ZnO (Zn1-xCdxO) nanostructures synthesized by high pressure solution route

We report synthesis of Cd substituted ZnO nanostructures (Zn1-xCdxO with x upto \approx .09) by high pressure solution growth method. The synthesized nanostructures comprise of nanocrystals that are both particles (~ 10-15 nm) and rods which grow along (002) direction as established by Transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. Rietveld analysis of the XRD data shows monotonous increase of the unit cell volume with the increase of Cd concentration. The optical absorption as well as the photoluminescence (PL) shows red shift on Cd substitution. The line width of the PL spectrum is related to the strain inhomogenity and it peaks in the region where the CdO phase separates from the Zn1-xCdxO nanostructures. The time resolved photoemission showed a long lived (~10ns) component. We propose that the PL behavior of the Zn1-xCdxO is dominated by strain in the sample with the redshift of the PL linked to the expansion of the unit cell volume on Cd substitution

Electric field induced reversible control of visible photoluminescence from ZnO nanoparticles

Reversible control of the photoluminescence of ZnO occurring in the visible range, has been achieved by application of a few volts (< 5V) to a device consisting of nanostructured ZnO film sandwiched between Indium Tin Oxide electrode and polyethylene oxide-lithium perchlorate, a solid polymer electrolyte. The photoluminescence intensity shows nearly 100% modulation with a response time less than 30 seconds, when the bias is applied at the electrolyte-electrode. A model is proposed for the observed effect that is based on defect states of ZnO and the band bending at the ZnO-electrolyte interface that can be changed by the applied bias

Oscillating non-singular relativistic spherical model

A particular choice of the time function in the recently presented spherical solution by Dadhich [1] leads to a singularity free cosmological model which oscillates between two regular states. The energy-stress tensor involves anisotropic pressure and a heat flux term but is consistent with the usual energy conditions (strong, weak and dominant). By choosing the parameters suitably one can make the model consistent with observational data. An interesting feature of the model is that it involves blue shifts as in the quasi steady state model [2] but without violating general relativity.Comment: 6 pages, REVTEX versio