1,591 research outputs found
Weak Gravitational Lensing by Dark Clusters
We calculate the abundance of dark-matter concentrations that are
sufficiently overdense to produce a detectable weak-gravitational-lensing
signal. Most of these overdensities are virialized halos containing
identifiable X-ray and/or optical clusters. However, a significant fraction are
nonvirialized overdensities still in the process of gravitational
collapse--these should produce significantly weaker or no X-ray emission. Our
predicted abundance of such dark clusters are consistent with the abundance
implied by the Erben et al. (2000) detection of a dark lens. Weak lensing by
these nonvirialized objects will need to be considered when determining
cosmological parameters with the lens abundance in future weak-lensing surveys.
Such weak lenses should also help shed light on the process of cluster
formation.Comment: 18 pages, 11 figures; a few sentences and a figure added, conclusions
unchanged, published in MNRA
Dirac neutrinos and anomaly-free discrete gauge symmetries
Relying on Dirac neutrinos allows an infinity of anomaly-free discrete gauge
symmetries to be imposed on the Supersymmetric Standard Model, some of which
are GUT-compatible.Comment: 24 pages, minor changes, existence of flipped discrete gauge
symmetries is pointed ou
INHIBITING HEPATITIS B VIRUS GENE EXPRESSION WITH HAMMERHEAD RIBOZYMES THAT TARGET THE HBx OPEN READING FRAME
A thesis submitted to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the
degree of
Doctor of Philosophy
Johannesburg, 2002Hepatitis B virus (HBV) infection is endemic to several populous regions and is often complicated by cirrhosis and hepatocellular carcinoma (HCC). Present treatment of chronic HBV infection is usually ineffective and novel therapeutic approaches are an important medical objective. The X open reading frame (ORF) of HBV, HBx, is a conserved sequence that overlaps with the polymerase ORF and viral c/'s-elements, and is present within all viral transcripts. In addition, the HBx ORF encodes a 17 kDa transactivator protein, HBx, which is required for the establishment of viral infection and has been implicated in HBV-associated hepatocarcinogenesis. The HBx sequence thus represents a compelling target for applying nucleic acid hybridisation-based therapeutic agents for the inhibition of HBV gene expression and replication.IT201
What is the Discrete Gauge Symmetry of the MSSM?
We systematically study the extension of the Supersymmetric Standard Model
(SSM) by an anomaly-free discrete gauge symmetry Z_N. We extend the work of
Ibanez and Ross with N=2,3 to arbitrary values of N. As new fundamental
symmetries, we find four Z_6, nine Z_9 and nine Z_18. We then place three
phenomenological demands upon the low-energy effective SSM: (i) the presence of
the mu-term in the superpotential, (ii) baryon-number conservation upto
dimension-five operators, and (iii) the presence of the see-saw neutrino mass
term LHLH. We are then left with only two anomaly-free discrete gauge
symmetries: baryon-triality, B_3, and a new Z_6, which we call proton-hexality,
P_6. Unlike B_3, P_6 prohibits the dimension-four lepton-number violating
operators. This we propose as the discrete gauge symmetry of the Minimal SSM,
instead of R-parity.Comment: Typo in item 2 below Eq.(6.9) corrected (wrong factor of "3"); 27
pages, 5 table
Emerging technologies in microguidance and control
Employing recent advances in microfabrication, the Charles Stark Draper Laboratory has developed inertial guidance instruments of very small size and low cost. Microfabrication employs the batch processing techniques of solid state electronics, such as photolithography, diffusion, and etching, to carve mechanical parts. Within a few years, microfabricated gyroscopes should perform in the 10 to 100 deg/h range. Microfabricated accelerometers have demonstrated performance in the 50 to 500 microgravity range. These instruments will result in not only the redesign of conventional military products, but also new applications that could not exist without small, inexpensive sensors and computing. Draper's microfabricated accelerometers and gyroscopes will be described and test results summarized. Associated electronics and control issues will also be addressed. Gimballed, vibrating gyroscopes and force rebalance accelerometers constructed from bulk silicon, polysilicon surface-machined tuning fork gyroscopes, and quartz resonant accelerometers and gyroscopes are examined. Draper is pursuing several types of devices for the following reasons: to address wide ranges of performance, to realize construction in a flat pack, and to lessen the risks associated with emerging technologies
Cosmological perturbations of a perfect fluid and noncommutative variables
We describe the linear cosmological perturbations of a perfect fluid at the
level of an action, providing thus an alternative to the standard approach
based only on the equations of motion. This action is suited not only to
perfect fluids with a barotropic equation of state, but also to those for which
the pressure depends on two thermodynamical variables. By quantizing the system
we find that (1) some perturbation fields exhibit a noncommutativity quite
analogous to the one observed for a charged particle moving in a strong
magnetic field, (2) local curvature and pressure perturbations cannot be
measured simultaneously, (3) ghosts appear if the null energy condition is
violated.Comment: 4 pages, uses RevTeX. Title modified, references and comments added
Phantom Energy: Dark Energy with w \u3c − 1 Causes a Cosmic Doomsday
We explore the consequences that follow if the dark energy is phantom energy, in which the sum of the pressure and energy density is negative. The positive phantom-energy density becomes infinite in finite time, overcoming all other forms of matter, such that the gravitational repulsion rapidly brings our brief epoch of cosmic structure to a close. The phantom energy rips apart the Milky Way, solar system, Earth, and ultimately the molecules, atoms, nuclei, and nucleons of which we are composed, before the death of the Universe in a “big rip.
- …