841 research outputs found
An Analytic Model with Critical Behavior in Black Hole Formation
A simple analytic model is presented which exhibits a critical behavior in
black hole formation, namely, collapse of a thin shell coupled with outgoing
null fluid. It is seen that the critical behavior is caused by the
gravitational nonlinearity near the event horizon. We calculate the value of
the critical exponent analytically and find that it is very dependent on the
coupling constants of the system.Comment: 21pp., ReVTeX, 7 figures (postscript, compressed and uuencoded),
TIT/HEP-266/COSMO-4
Algebraic approach to time-delay data analysis for LISA
Cancellation of laser frequency noise in interferometers is crucial for
attaining the requisite sensitivity of the triangular 3-spacecraft LISA
configuration. Raw laser noise is several orders of magnitude above the other
noises and thus it is essential to bring it down to the level of other noises
such as shot, acceleration, etc. Since it is impossible to maintain equal
distances between spacecrafts, laser noise cancellation must be achieved by
appropriately combining the six beams with appropriate time-delays. It has been
shown in several recent papers that such combinations are possible. In this
paper, we present a rigorous and systematic formalism based on algebraic
geometrical methods involving computational commutative algebra, which
generates in principle {\it all} the data combinations cancelling the laser
frequency noise. The relevant data combinations form the first module of
syzygies, as it is called in the literature of algebraic geometry. The module
is over a polynomial ring in three variables, the three variables corresponding
to the three time-delays around the LISA triangle. Specifically, we list
several sets of generators for the module whose linear combinations with
polynomial coefficients generate the entire module. We find that this formalism
can also be extended in a straight forward way to cancel Doppler shifts due to
optical bench motions. The two modules are infact isomorphic.
We use our formalism to obtain the transfer functions for the six beams and
for the generators. We specifically investigate monochromatic gravitational
wave sources in the LISA band and carry out the maximisiation over linear
combinations of the generators of the signal-to-noise ratios with the frequency
and source direction angles as parameters.Comment: 27 Pages, 6 figure
Astrophysical Bounds on Global Strings
Global topological defects produce nonzero stress-energy throughout
spacetime, and as a result can have observable gravitational influence on
surrounding matter. Gravitational effects of global strings are used to place
bounds on their cosmic abundance. The minimum separation between global strings
is estimated by considering the defects' contribution to the cosmological
energy density. More rigorous constraints on the abundance of global strings
are constructed by examining the tidal forces such defects will have on
observable astrophysical systems. The small number of observed tidally
disrupted systems indicates there can be very few of these objects in the
observable universe.Comment: 14 pages, REVTe
Baryon inhomogeneity generation via cosmic strings at QCD scale and its effects on nucleosynthesis
We have earlier shown that cosmic strings moving through the plasma at the
time of a first order quark-hadron transition in the early universe can
generate large scale baryon inhomogeneities. In this paper, we calculate
detailed structure of these inhomogeneities at the quark-hadron transition. Our
calculations show that the inhomogeneities generated by cosmic string wakes can
strongly affect nucleosynthesis calculations. A comparison with observational
data suggests that such baryon inhomogeneities should not have existed at the
nucleosynthesis epoch. If this disagreement holds with more accurate
observations, then it will lead to the conclusions that cosmic string formation
scales above GeV may not be consistent with nucleosynthesis
and CMBR observations. Alternatively, some other input in our calculation
should be constrained, for example, if the average string velocity remains
sufficiently small so that significant density perturbations are never produced
at the QCD scale, or if strings move ultra-relativistically so that string
wakes are very thin, trapping negligible amount of baryons. Finally, if
quark-hadron transition is not of first order then our calculations do not
apply.Comment: 24 pages, 5 figures, minor changes, version to appear in Phys. Rev.
Cosmic string induced sheet like baryon inhomogeneities at quark-hadron transition
Cosmic strings moving through matter produce wakes where density is higher
than the background density. We investigate the effects of such wakes occurring
at the time of a first order quark-hadron transition in the early universe and
show that they can lead to separation of quark-gluon plasma phase in the wake
region, while the region outside the wake converts to the hadronic phase.
Moving interfaces then trap large baryon densities in sheet like regions which
can extend across the entire horizon. Typical separation between such sheets,
at formation, is of the order of a km. Regions of baryon inhomogeneity of this
nature, i.e. having a planar geometry, and separated by such large distance
scales, appear to be well suited for the recent models of inhomogeneous
nucleosynthesis to reconcile with the large baryon to photon ratio implied by
the recent measurements of the cosmic microwave background power spectrum.Comment: 8 pages, 3 figure
Second Order Dissipative Fluid Dynamics for Ultra-Relativistic Nuclear Collisions
The M\"uller-Israel-Stewart second order theory of relativistic imperfect
fluids based on Grad's moment method is used to study the expansion of hot
matter produced in ultra-relativistic heavy ion collisions. The temperature
evolution is investigated in the framework of the Bjorken boost-invariant
scaling limit. The results of these second-order theories are compared to those
of first-order theories due to Eckart and to Landau and Lifshitz and those of
zeroth order (perfect fluid) due to Euler.Comment: 5 pages, 4 figures, size of y-axis tick marks for Figs. 3 and 4 fixe
Causal Bulk Viscous Dissipative Isotropic Cosmologies with Variable Gravitational and Cosmological Constants
We consider the evolution of a flat Friedmann-Robertson-Walker Universe,
filled with a causal bulk viscous cosmological fluid, in the presence of
variable gravitational and cosmological constants. The basic equation for the
Hubble parameter, generalizing the evolution equation in the case of constant
gravitational coupling and cosmological term, is derived, under the
supplementary assumption that the total energy of the Universe is conserved. By
assuming that the cosmological constant is proportional to the square of the
Hubble parameter and a power law dependence of the bulk viscosity coefficient,
temperature and relaxation time on the energy density of the cosmological
fluid, two classes of exact solutions of the field equations are obtained. In
the first class of solutions the Universe ends in an inflationary era, while in
the second class of solutions the expansion of the Universe is non-inflationary
for all times. In both models the cosmological "constant" is a decreasing
function of time, while the gravitational "constant" increases in the early
period of evolution of the Universe, tending in the large time limit to a
constant value.Comment: 14 pages, 15 figure
Weak energy condition violation and superluminal travel
Recent solutions to the Einstein Field Equations involving negative energy
densities, i.e., matter violating the weak-energy-condition, have been
obtained, namely traversable wormholes, the Alcubierre warp drive and the
Krasnikov tube. These solutions are related to superluminal travel, although
locally the speed of light is not surpassed. It is difficult to define
faster-than-light travel in generic space-times, and one can construct metrics
which apparently allow superluminal travel, but are in fact flat Minkowski
space-times. Therefore, to avoid these difficulties it is important to provide
an appropriate definition of superluminal travel.Comment: 15 pages, 3 figures, LaTeX2e, Springer style files -included.
Contribution to the Proceedings of the Spanish Relativity Meeting-2001
(Madrid, September 2001
High-frequency monitoring of nitrogen and phosphorus response in three rural catchments to the end of the 2011–2012 drought in England
This paper uses high-frequency bankside measurements from three catchments selected as part of the UK government-funded Demonstration Test Catchments (DTC) project. We compare the hydrological and hydrochemical patterns during the water year 2011–2012 from the Wylye tributary of the River Avon with mixed land use, the Blackwater tributary of the River Wensum with arable land use and the Newby Beck tributary of the River Eden with grassland land use. The beginning of the hydrological year was unusually dry and all three catchments were in states of drought. A sudden change to a wet summer occurred in April 2012 when a heavy rainfall event affected all three catchments. The year-long time series and the individual storm responses captured by in situ nutrient measurements of nitrate and phosphorus (total phosphorus and total reactive phosphorus) concentrations at each site reveal different pollutant sources and pathways operating in each catchment. Large storm-induced nutrient transfers of nitrogen and or phosphorus to each stream were recorded at all three sites during the late April rainfall event. Hysteresis loops suggested transport-limited delivery of nitrate in the Blackwater and of total phosphorus in the Wylye and Newby Beck, which was thought to be exacerbated by the dry antecedent conditions prior to the storm. The high rate of nutrient transport in each system highlights the scale of the challenges faced by environmental managers when designing mitigation measures to reduce the flux of nutrients to rivers from diffuse agricultural sources. It also highlights the scale of the challenge in adapting to future extreme weather events under a changing climate
Bianchi Type I Universes with Causal Bulk Viscous Cosmological Fluid
We consider the dynamics of a causal bulk viscous cosmological fluid filled
constantly decelerating Bianchi type I space-time. The matter component of the
Universe is assumed to satisfy a linear barotropic equation of state and the
state equation of the small temperature Boltzmann gas. The resulting
cosmological models satisfy the condition of smallness of the viscous stress.
The time evolution of the relaxation time, temperature, bulk viscosity
coefficient and comoving entropy of the dissipative fluid is also obtained.Comment: 11 pages, 5 figures, accepted for publication in International
Journal of Modern Physics
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