6,732 research outputs found
Nonlinear Shear-free Radiative Collapse
We study realistic models of relativistic radiating stars undergoing
gravitational collapse which have vanishing Weyl tensor components. Previous
investigations are generalised by retaining the inherent nonlinearity at the
boundary. We transform the boundary condition to an Abel equation of the first
kind. A variety of nonlinear solutions are generated all of which can be
written explicitly. Several classes of infinite solutions exist.Comment: 13 pages, To appear in Math. Meth. Appl. Sc
Self-assembly and DNA binding of the blocking factor in X chromosome inactivation
X chromosome inactivation (XCI) is the phenomenon occurring in female mammals whereby dosage compensation of
X-linked genes is obtained by transcriptional silencing of one of their two X chromosomes, randomly chosen during
early embryo development. The earliest steps of random X-inactivation, involving counting of the X chromosomes and
choice of the active and inactive X, are still not understood. To explain "counting and choice," the longstanding
hypothesis is that a molecular complex, a "blocking factor" (BF), exists. The BF is present in a single copy and can
randomly bind to just one X per cell which is protected from inactivation, as the second X is inactivated by default. In
such a picture, the missing crucial step is to explain how the molecular complex is self-assembled, why only one is
formed, and how it binds only one X. We answer these questions within the framework of a schematic Statistical
Physics model, investigated by Monte Carlo computer simulations. We show that a single complex is assembled as a
result of a thermodynamic process relying on a phase transition occurring in the system which spontaneously breaks
the symmetry between the X’s. We discuss, then, the BF interaction with X chromosomes. The thermodynamics of the
mechanism that directs the two chromosomes to opposite fates could be, thus, clarified. The insights on the selfassembling
and X binding properties of the BF are used to derive a quantitative scenario of biological implications
describing current experimental evidences on "counting and choice.
Fluid-solid transition in unsteady, homogeneous, granular shear flows
Discrete element numerical simulations of unsteady, homogeneous shear flows
have been performed by instantly applying a constant shear rate to a random,
static, isotropic assembly of identical, soft, frictional spheres at either
zero or finite pressure by keeping constant the solid volume fraction until the
steady state is reached. If the system is slowly sheared, or, equivalently, if
the particles are sufficiently rigid, the granular material exhibits either
large or small fluctuations in the evolving pressure, depending whether the
average number of contacts per particle (coordination number) is less or larger
than a critical value. The amplitude of the pressure fluctuations is
rate-dependent when the coordination number is less than the critical and
rate-independent otherwise, signatures of fluid-like and solid-like behaviour,
respectively. The same critical coordination number has been previously found
to represent the minimum value at which rate-independent components of the
stresses develop in steady, simple shearing and the jamming transition in
isotropic random packings. The observed complex behaviour of the measured
pressure in the fluid-solid transition clearly suggests the need for
incorporating in a nontrivial way the coordination number, the solid volume
fraction, the particle stiffness and the intensity of the particle agitation in
constitutive models for the onset and the arrest of granular flows.Comment: 20 pages, 14 figures, submitted to Granular Matte
All static spherically symmetric anisotropic solutions of Einstein's equations
An algorithm recently presented by Lake to obtain all static spherically
symmetric perfect fluid solutions, is extended to the case of locally
anisotropic fluids (principal stresses unequal). As expected, the new formalism
requires the knowledge of two functions (instead of one) to generate all
possible solutions. To illustrate the method some known cases are recovered.Comment: 8 pages Latex. To appear in Phys. Rev. D. New reference added. Some
references correcte
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