31,050 research outputs found
The first second of the Universe
The history of the Universe after its first second is now tested by high
quality observations of light element abundances and temperature anisotropies
of the cosmic microwave background. The epoch of the first second itself has
not been tested directly yet; however, it is constrained by experiments at
particle and heavy ion accelerators. Here I attempt to describe the epoch
between the electroweak transition and the primordial nucleosynthesis.
The most dramatic event in that era is the quark--hadron transition at 10
s. Quarks and gluons condense to form a gas of nucleons and light mesons,
the latter decay subsequently. At the end of the first second, neutrinos and
neutrons decouple from the radiation fluid. The quark--hadron transition and
dissipative processes during the first second prepare the initial conditions
for the synthesis of the first nuclei.
As for the cold dark matter (CDM), WIMPs (weakly interacting massive
particles) -- the most popular candidates for the CDM -- decouple from the
presently known forms of matter, chemically (freeze-out) at 10 ns and
kinetically at 1 ms. The chemical decoupling fixes their present abundances and
dissipative processes during and after thermal decoupling set the scale for the
very first WIMP clouds.Comment: review to appear in Annalen der Physik (51 pages, 16 figures);
references added (v2); typos corrected, resembles published version (v3
Laser-to-proton energy transfer efficiency in laser-plasma interactions
It is shown that the energy of protons accelerated in laser-matter
interaction experiments may be significantly increased through the process of
splitting the incoming laser pulse into multiple interaction stages of equal
intensity. From a thermodynamic point of view, the splitting procedure can be
viewed as an effective way of increasing the efficiency of energy transfer from
the laser light to protons, which peaks for processes having the least amount
of entropy gain. It is predicted that it should be possible to achieve \apprge
100% increase in the energy efficiency in a six-stage laser proton accelerator
compared to a single laser-target interaction scheme
Thermal duality and gravitational collapse in heterotic string theories
The thermal duality of E(8) x E(8) and SO(32) heterotic string theories may
underpin a mechanism that would convert the kinetic energy of infalling matter
during gravitational collapse to form a region of a hot string phase that would
expel gravitational gradients. This phase would be the continuation of a
Ginzburg-Landau like superconductor in the Euclidean regime. In this scenario,
there would be no event horizon or singularity produced in gravitational
collapse. Solutions are presented for excitations of the string vacuum that may
form during gravitational collapse and drive the transition to the hot phase.
The proposed mechanism is developed here for the case of approximately
spherical gravitational collapse in 4 uncompactified spacetime dimensions. A
way to reconcile the large entropy apparently produced in this process with
quantum mechanics is briefly discussed. In this scenario, astrophysical objects
such as stellar or galactic cores which have undergone extreme gravitational
collapse would currently be sites of an on-going conversion process to shells
of this high temperature phase. The relationship of this proposal to the
`firewall paradox' is noted.Comment: 28 pages, 3 figures Revised estimate for the conversion time scale in
this versio
Challenges and progress on the modelling of entropy generation in porous media: a review
Depending upon the ultimate design, the use of porous media in thermal and chemical systems can provide significant operational advantages, including helping to maintain a uniform temperature distribution, increasing the heat transfer rate, controlling reaction rates, and improving heat flux absorption. For this reason, numerous experimental and numerical investigations have been performed on thermal and chemical systems that utilize various types of porous materials. Recently, previous thermal analyses of porous materials embedded in channels or cavities have been re-evaluated using a local thermal non-equilibrium (LTNE) modelling technique. Consequently, the second law analyses of these systems using the LTNE method have been a point of focus in a number of more recent investigations. This has resulted in a series of investigations in various porous systems, and comparisons of the results obtained from traditional local thermal equilibrium (LTE) and the more recent LTNE modelling approach. Moreover, the rapid development and deployment of micro-manufacturing techniques have resulted in an increase in manufacturing flexibility that has made the use of these materials much easier for many micro-thermal and chemical system applications, including emerging energy-related fields such as micro-reactors, micro-combustors, solar thermal collectors and many others. The result is a renewed interest in the thermal performance and the exergetic analysis of these porous thermochemical systems. This current investigation reviews the recent developments of the second law investigations and analyses in thermal and chemical problems in porous media. The effects of various parameters on the entropy generation in these systems are discussed, with particular attention given to the influence of local thermodynamic equilibrium and non-equilibrium upon the second law performance of these systems. This discussion is then followed by a review of the mathematical methods that have been used for simulations. Finally, conclusions and recommendations regarding the unexplored systems and the areas in the greatest need of further investigations are summarized
Massive Stars and their Supernovae
Massive stars and their supernovae are prominent sources of radioactive
isotopes, the observations of which thus can help to improve our astrophysical
models of those. Our understanding of stellar evolution and the final explosive
endpoints such as supernovae or hypernovae or gamma-ray bursts relies on the
combination of magneto-hydrodynamics, energy generation due to nuclear
reactions accompanying composition changes, radiation transport, and
thermodynamic properties (such as the equation of state of stellar matter).
Nuclear energy production includes all nuclear reactions triggered during
stellar evolution and explosive end stages, also among unstable isotopes
produced on the way. Radiation transport covers atomic physics (e.g. opacities)
for photon transport, but also nuclear physics and neutrino nucleon/nucleus
interactions in late phases and core collapse. Here we want to focus on the
astrophysical aspects, i.e. a description of the evolution of massive stars and
their endpoints, with a special emphasis on the composition of their ejecta (in
form of stellar winds during the evolution or of explosive ejecta). Low and
intermediate mass stars end their evolution as a white dwarf with an unburned C
and O composition. Massive stars evolve beyond this point and experience all
stellar burning stages from H over He, C, Ne, O and Si-burning up to core
collapse and explosive endstages. In this chapter we discuss the
nucleosynthesis processes involved and the production of radioactive nuclei in
more detail.Comment: 79 pages; Chapter of "Astronomy with Radioactivities", a book in
Springer's 'lecture notes in physics series, Vol. 812, Eds. Roland Diehl,
Dieter H. Hartmann, and Nikos Prantzos, to appear in summer 201
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