118 research outputs found
Application of Ultrasonic Technique for Measurement of Instantaneous Burn Rate of Solid Propellants .
The ultrasonic pulse-echo technique has been applied for the measurement of instantaneous burnrate of aluminised composite solid propellants. The tests have been carried out on end-burning 30 mmthick propellant specimens at nearly constant pressure of about 1.9 MPa. Necessary software forpost-test data processing and instantaneous burn rate computations have been developed. The burnrates measured by the ultrasonic technique have been compared with those obtained from ballisticevaluation motor tests on propellant from the same mix. An accuracy of about +- 1 per cent ininstantaneous burn rate measurements and reproducibility of results have been demonstrated byapplying ultrasonic technique
Trace anomaly of the conformal gauge field
The proposed by Bastianelli and van Nieuwenhuizen new method of calculations
of trace anomalies is applied in the conformal gauge field case. The result is
then reproduced by the heat equation method. An error in previous calculation
is corrected. It is pointed out that the introducing gauge symmetries into a
given system by a field-enlarging transformation can result in unexpected
quantum effects even for trivial configurations.Comment: 9 pages, LaTeX file, BI-TP 93/3
Tachyonization of the \LaCDM cosmological model
In this work a tachyonization of the CDM model for a spatially flat
Friedmann-Robertson-Walker space-time is proposed. A tachyon field and a
cosmological constant are considered as the sources of the gravitational field.
Starting from a stability analysis and from the exact solutions for a standard
tachyon field driven by a given potential, the search for a large set of
cosmological models which contain the CDM model is investigated. By
the use of internal transformations two new kinds of tachyon fields are derived
from the standard tachyon field, namely, a complementary and a phantom tachyon
fields. Numerical solutions for the three kinds of tachyon fields are
determined and it is shown that the standard and complementary tachyon fields
reproduces the CDM model as a limiting case. The standard tachyon
field can also describe a transition from an accelerated to a decelerated
regime, behaving as an inflaton field at early times and as a matter field at
late times. The complementary tachyon field always behaves as a matter field.
The phantom tachyon field is characterized by a rapid expansion where its
energy density increases with time.Comment: Version accepted for publication in GR
Cosmic microwave background: polarization and temperature anisotropies from symmetric structures
I consider the case of anisotropies in the Cosmic Microwave Background (CMB)
from one single ordered perturbation source, or seed, existing well before
decoupling between matter and radiation. Such structures could have been left
by high energy symmetries breaking in the early universe.
I focus on the cases of spherical and cylindrical symmetry of the seed. I
give general analytic expressions for the polarization and temperature linear
perturbations, factoring out of the Fourier integral the dependence on the
photon propagation direction and on the geometric coordinates describing the
seed. I show how the CMB perturbations manifestly reflect the symmetries of
their seeds. CMB anisotropies are obtained with a line of sight integration.
This treatment highlights the undulatory properties of the CMB. I show with
numerical examples how the polarization and temperature perturbations propagate
beyond the size of their seeds, reaching the CMB sound horizon at the time
considered. Just like the waves from a pebble thrown in a pond, CMB anisotropy
from a seed intersecting the last scattering surface appears as a series of
temperature and polarization waves surrounding the seed, extending on the scale
of the CMB sound horizon at decoupling, roughly in the sky. Each wave
is characterized by its own value of the CMB perturbation, with the same mean
amplitude of the signal coming from the seed interior.
These waves could allow to distinguish relics from high energy processes of
the early universe from point-like astrophysical sources, because of their
angular extension and amplitude. Also, the marked analogy between polarization
and temperature signals offers cross correlation possibilities for the future
Planck Surveyor observations.Comment: 21 pages, seven postscript figures, final version accepted for
publication in Phys.Rev.
Stress effects in structure formation
Residual velocity dispersion in cold dark matter induces stresses which lead
to effects that are absent in the idealized dust model. A previous Newtonian
analysis showed how this approach can provide a theoretical foundation for the
phenomenological adhesion model. We develop a relativistic kinetic theory
generalization which also incorporates the anisotropic velocity dispersion that
will typically be present. In addition to density perturbations, we consider
the rotational and shape distortion properties of clustering. These quantities
together characterize the linear development of density inhomogeneity, and we
find exact solutions for their evolution. As expected, the corrections are
small and arise only in the decaying modes, but their effect is interesting.
One of the modes for density perturbations decays less rapidly than the
standard decaying mode. The new rotational mode generates precession of the
axis of rotation. The new shape modes produce additional distortion that
remains frozen in during the subsequent (linear) evolution, despite the rapid
decay of the terms that caused it.Comment: significantly improved discussion of kinetic theory of CDM velocity
dispersion; to appear Phys. Rev.
Kaluza-Klein Type Robertson Walker Cosmological Model With Dynamical Cosmological Term
In this paper we have analyzed the Kaluza-Klein type Robertson Walker (RW)
cosmological models by considering three different forms of variable :
, and
. It is found that, the connecting free parameters of the
models with cosmic matter and vacuum energy density parameters are equivalent,
in the context of higher dimensional space time. The expression for the look
back time, luminosity distance and angular diameter distance are also derived.
This work has thus generalized to higher dimensions the well-known results in
four dimensional space time. It is found that there may be significant
difference in principle at least, from the analogous situation in four
dimensional space time.Comment: 16 pages, no figur
Bianchi Type III Anisotropic Dark Energy Models with Constant Deceleration Parameter
The Bianchi type III dark energy models with constant deceleration parameter
are investigated. The equation of state parameter is found to be time
dependent and its existing range for this model is consistent with the recent
observations of SN Ia data, SN Ia data (with CMBR anisotropy) and galaxy
clustering statistics. The physical aspect of the dark energy models are
discussed.Comment: 12 pages, 2 figures, Accepted version of IJT
Chaotic scalar fields as models for dark energy
We consider stochastically quantized self-interacting scalar fields as
suitable models to generate dark energy in the universe. Second quantization
effects lead to new and unexpected phenomena is the self interaction strength
is strong. The stochastically quantized dynamics can degenerate to a chaotic
dynamics conjugated to a Bernoulli shift in fictitious time, and the right
amount of vacuum energy density can be generated without fine tuning. It is
numerically observed that the scalar field dynamics distinguishes fundamental
parameters such as the electroweak and strong coupling constants as
corresponding to local minima in the dark energy landscape. Chaotic fields can
offer possible solutions to the cosmological coincidence problem, as well as to
the problem of uniqueness of vacua.Comment: 30 pages, 3 figures. Replaced by final version accepted by Phys. Rev.
Reconstructing the Equation of State of Tachyon
Recent progress in theoretical physics suggests that the dark energy in the
universe might be resulted from the rolling tachyon field of string theory.
Measurements to SNe Ia can be helpful to reconstruct the equation of state of
the rolling tachyon which is a possible candidate of dark energy. We present a
numerical analysis for the evolution of the equation of state of the rolling
tachyon and derive the reconstruction equations for the equation of state as
well as the potential.Comment: 6 pages, 3 figures, to appear Phys. Rev.
Boundary Term in Metric f(R) Gravity: Field Equations in the Metric Formalism
The main goal of this paper is to get in a straightforward form the field
equations in metric f(R) gravity, using elementary variational principles and
adding a boundary term in the action, instead of the usual treatment in an
equivalent scalar-tensor approach. We start with a brief review of the
Einstein-Hilbert action, together with the Gibbons-York-Hawking boundary term,
which is mentioned in some literature, but is generally missing. Next we
present in detail the field equations in metric f(R) gravity, including the
discussion about boundaries, and we compare with the Gibbons-York-Hawking term
in General Relativity. We notice that this boundary term is necessary in order
to have a well defined extremal action principle under metric variation.Comment: 12 pages, title changes by referee recommendation. Accepted for
publication in General Relativity and Gravitation. Matches with the accepted
versio
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