942 research outputs found
Non-Planck equilibrium radiation in plasma model of early Universe
Consideration of the adiabatic character of radiation expansion in early
Universe leads to the conclusion that equilibrium distribution of the
primordial radiation in the presence of charged particles could be different
from the Planck distribution in some regions of the spectrum. The equilibrium
distribution of electromagnetic radiation (the black body radiation) is
generalized for the system containing an extremely dense fully ionized plasma.
The conditions of the adiabatic expansion of radiation for the model of the
early Universe are found.Comment: 10 pages, 4 figure
Modified two-potential approach to tunneling problems
One-body quantum tunneling to continuum is treated via the two-potential
approach, dividing the tunneling potential into external and internal parts. We
show that corrections to this approach can be minimized by taking the
separation radius inside the interval determined by simple expressions. The
resulting two-potential approach reproduces the resonance energy and its width,
both for narrow and wide resonances. We also demonstrate that, without losing
its accuracy, the two-potential approach can be modified to a form resembling
the R-matrix theory, yet without any uncertainties of the latter related to the
choice of the matching radius.Comment: 7 two-column pages, 3 figures, extra-explanation added, Phys. Rev. A,
in pres
Gauged WZW models for space-time groups and gravitational actions
In this paper we investigate gauged Wess-Zumino-Witten models for space-time
groups as gravitational theories, following the trend of recent work by
Anabalon, Willison and Zanelli. We discuss the field equations in any dimension
and study in detail the simplest case of two space-time dimensions and gauge
group SO(2,1). For this model we study black hole solutions and we calculate
their mass and entropy which resulted in a null value for both.Comment: 26 pages, no figure
Archeops, mapping the CMB sky from large to small angular scales
Archeops is a balloon-borne experiment designed to measure the temperature
fluctuations of the CMB on a large region of the sky () with a high
angular resolution (10 arcminutes) and a high sensitivity ( per
pixel). Archeops will perform a measurement of the CMB anisotropies power
spectrum from large angular scales () to small angular scales
(). Archeops flew for the first time for a test flight in July
1999 from Sicily to Spain and the first scientific flight took place from
Sweden to Russia in January 2001. The data analysis is on its way and I present
here preliminary results, realistic simulations showing the expected accuracy
on the measurement of the power spectrum and perspectives for the incoming
flights (Winter 2001/2003).Comment: 6 pages, 6 figures, proceedings to TAUP2001 conference, LNGS, Italy,
Sept. 200
Real Time Relativity: exploration learning of special relativity
Real Time Relativity is a computer program that lets students fly at
relativistic speeds though a simulated world populated with planets, clocks,
and buildings. The counterintuitive and spectacular optical effects of
relativity are prominent, while systematic exploration of the simulation allows
the user to discover relativistic effects such as length contraction and the
relativity of simultaneity. We report on the physics and technology
underpinning the simulation, and our experience using it for teaching special
relativity to first year university students
Is it possible to determine the S-factor of the hep process from a laboratory experiment?
We discuss the problem of solar hep neutrinos originating from the reaction p
+ 3He -> 4He + e+ + nu and obtain a relation between the astrophysical S-factor
of the hep process and the cross section of the process e- + 4He -> 3H + n + nu
near threshold. The relation is based on the isotopic invariance of strong
interactions.
The measurement of the latter cross section would allow to obtain
experimental information on S(hep), the value of which, at the moment, is known
only from theoretical calculations.Comment: 10 pages, no figure
Energy Extraction From Gravitational Collapse to Static Black Holes
The mass--energy formula of black holes implies that up to 50% of the energy
can be extracted from a static black hole. Such a result is reexamined using
the recently established analytic formulas for the collapse of a shell and
expression for the irreducible mass of a static black hole. It is shown that
the efficiency of energy extraction process during the formation of the black
hole is linked in an essential way to the gravitational binding energy, the
formation of the horizon and the reduction of the kinetic energy of implosion.
Here a maximum efficiency of 50% in the extraction of the mass energy is shown
to be generally attainable in the collapse of a spherically symmetric shell:
surprisingly this result holds as well in the two limiting cases of the
Schwarzschild and extreme Reissner-Nordstr\"{o}m space-times. Moreover, the
analytic expression recently found for the implosion of a spherical shell onto
an already formed black hole leads to a new exact analytic expression for the
energy extraction which results in an efficiency strictly less than 100% for
any physical implementable process. There appears to be no incompatibility
between General Relativity and Thermodynamics at this classical level.Comment: 7 pages, 2 figures, to appear on Int. Journ. Mod. Phys.
Student experiences of virtual reality - a case study in learning special relativity
We present a study of student learning through the use of virtual reality. A
software package is used to introduce concepts of special relativity to
students in a game-like environment where users experience the effects of
travelling at near light speeds. From this new perspective, space and time are
significantly different to that experienced in everyday life. The study
explores how students have worked with this environment and how these students
have used this experience in their study of special relativity. A mixed method
approach has been taken to evaluate the outcomes of separate implementations of
the package at two universities. Students found the simulation to be a positive
learning experience and described the subject area as being less abstract after
its use. Also, students were more capable of correctly answering concept
questions relating to special relativity, and a small but measurable
improvement was observed in the final exam
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