On the Origin of the Light Elements (D, 3He, 4He and 7Li)

The abundances of the very light elements (D, 3He, 4He and 7Li) constitute indeed one of the most powerful constraints in cosmology: they are known to fix very interesting limits on the baryonic density of the Universe and on the maximum number of neutrino (lepton) families in the frame of the simplest canonical models. Given the importance of these predictions, these models should be analysed very cautiously at the light of recent developments in the observations of these elements. In order to make the simplest models consistent with the observations, it is argued that a thorough destruction of D should occur during the galactic evolution. Moreover this review deals also with some models invoking the possible existence of massive unstable neutrinos, gravitinos or photinos which would decay into high energy photons or of quark nuggets which could be created during the quark-hadron phase transitions. Such models have been designed in an attempt to overcome the limitation on the Universe density coming from these abundance determinations. Although the simple canonical models are especially attractive such models cannot be disregarded.

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From Primordial Seed Magnetic Fields to the Galactic Dynamo

Galaxies ◽

10.3390/galaxies7020047 ◽

2019 ◽

Vol 7 (2) ◽

pp. 47 ◽

Cited By ~ 16

Author(s):

Kandaswamy Subramanian

Keyword(s):

Phase Transitions ◽

Magnetic Fields ◽

Large Scale ◽

Small Scale ◽

Disk Galaxies ◽

Theoretical Understanding ◽

Hadron Phase ◽

Interesting Possibility ◽

Basic Ideas ◽

The Universe

The origin and maintenance of coherent magnetic fields in the Universe is reviewed with an emphasis on the possible challenges that arise in their theoretical understanding. We begin with the interesting possibility that magnetic fields originated at some level from the early universe. This could be during inflation, the electroweak, or the quark-hadron phase transitions. These mechanisms can give rise to fields which could be strong, but often with much smaller coherence scales than galactic scales. Their subsequent turbulent decay decreases their strength but increases their coherence. We then turn to astrophysical batteries which can generate seed magnetic fields. Here the coherence scale can be large, but the field strength is generally very small. These seed fields need to be further amplified and maintained by a dynamo to explain observed magnetic fields in galaxies. Basic ideas behind both small and large-scale turbulent dynamos are outlined. The small-scale dynamo may help to understand the first magnetization of young galaxies, while the large-scale dynamo is important for the generation of fields with scales larger than the stirring scale, as observed in nearby disk galaxies. The current theoretical challenges that turbulent dynamos encounter and their possible resolution are discussed.

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Big Bang Nucleosynthesis

Symposium - International Astronomical Union ◽

10.1017/s0074180900113695 ◽

2002 ◽

Vol 187 ◽

pp. 1-15

Author(s):

D.N. Schramm

Keyword(s):

Systematic Error ◽

Big Bang ◽

Baryon Density ◽

Galactic Evolution ◽

Big Bang Nucleosynthesis ◽

X Ray ◽

Baryonic Density ◽

The Big Bang ◽

The Universe

Big Bang Nucleosynthesis (BBN) is on the verge of undergoing a transformation now that extragalactic deuterium is being measured. Previously, the emphasis was on demonstrating the concordance of the Big Bang Nucleosynthesis model with the abundances of the light isotopes extrapolated back to their primordial values using stellar and Galactic evolution theories. Once the primordial deuterium abundance is converged upon, the nature of the field will shift to using the much more precise primordial D/H to constrain the more flexible stellar and Galactic evolution models (although the question of potential systematic error in 4He abundance determinations remains open). The remarkable success of the theory to date in establishing the concordance has led to the very robust conclusion of BBN regarding the baryon density. The BBN constraints on the cosmological baryon density are reviewed and demonstrate that the bulk of the baryons are dark and also that the bulk of the matter in the universe is non-baryonic. Comparison of baryonic density arguments from Lyman-α clouds, x-ray gas in clusters, the Sunyaev-Zeldovich effect, and the microwave anisotropy are made and shown to be consistent with the BBN value.

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The Cosmic Mystery Tour

10.1093/oso/9780198831860.001.0001 ◽

2019 ◽

Author(s):

Nicholas Mee

Keyword(s):

Science Fiction ◽

Particle Physics ◽

Cultural Context ◽

Big Bang ◽

Galactic Centre ◽

Red Giants ◽

Extraterrestrial Life ◽

Modern Physics ◽

Recent Developments ◽

The Universe

The Cosmic Mystery Tour is a brief account of modern physics and astronomy presented in a broad historical and cultural context. The book is attractively illustrated and aimed at the general reader. Part I explores the laws of physics including general relativity, the structure of matter, quantum mechanics and the Standard Model of particle physics. It discusses recent discoveries such as gravitational waves and the project to construct LISA, a space-based gravitational wave detector, as well as unresolved issues such as the nature of dark matter. Part II begins by considering cosmology, the study of the universe as a whole and how we arrived at the theory of the Big Bang and the expanding universe. It looks at the remarkable objects within the universe such as red giants, white dwarfs, neutron stars and black holes, and considers the expected discoveries from new telescopes such as the Extremely Large Telescope in Chile, and the Event Horizon Telescope, currently aiming to image the supermassive black hole at the galactic centre. Part III considers the possibility of finding extraterrestrial life, from the speculations of science fiction authors to the ongoing search for alien civilizations known as SETI. Recent developments are discussed: space probes to the satellites of Jupiter and Saturn; the discovery of planets in other star systems; the citizen science project SETI@Home; Breakthrough Starshot, the project to develop technologies to send spacecraft to the stars. It also discusses the Fermi paradox which argues that we might actually be alone in the cosmos

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The Growth of Semiconductor Thin Films Studied by RHEED

Australian Journal of Physics ◽

10.1071/ph900583 ◽

1990 ◽

Vol 43 (5) ◽

pp. 583

Author(s):

GL Price

Keyword(s):

Thin Films ◽

Surface Science ◽

High Vacuum ◽

High Energy ◽

Ultra High Vacuum ◽

Large Area ◽

Growth Modes ◽

Semiconductor Thin Films ◽

Wide Range ◽

Recent Developments

Recent developments in the growth of semiconductor thin films are reviewed. The emphasis is on growth by molecular beam epitaxy (MBE). Results obtained by reflection high energy electron diffraction (RHEED) are employed to describe the different kinds of growth processes and the types of materials which can be constructed. MBE is routinely capable of heterostructure growth to atomic precision with a wide range of materials including III-V, IV, II-VI semiconductors, metals, ceramics such as high Tc materials and organics. As the growth proceeds in ultra high vacuum, MBE can take advantage of surface science techniques such as Auger, RHEED and SIMS. RHEED is the essential in-situ probe since the final crystal quality is strongly dependent on the surface reconstruction during growth. RHEED can also be used to calibrate the growth rate, monitor growth kinetics, and distinguish between various growth modes. A major new area is lattice mismatched growth where attempts are being made to construct heterostructures between materials of different lattice constants such as GaAs on Si. Also described are the new techniques of migration enhanced epitaxy and tilted superlattice growth. Finally some comments are given On the means of preparing large area, thin samples for analysis by other techniques from MBE grown films using capping, etching and liftoff.

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Modeling Gamma-Ray SEDs and Angular Extensions of Extreme TeV Blazars from Intergalactic Proton-Initiated Cascades in Contemporary Astrophysical EGMF Models

Universe ◽

10.3390/universe7070220 ◽

2021 ◽

Vol 7 (7) ◽

pp. 220

Author(s):

Emil Khalikov

Keyword(s):

Gamma Rays ◽

Large Scale ◽

Gamma Ray ◽

High Energy ◽

Cascade Model ◽

Point Sources ◽

Spectral Energy ◽

Observation Data ◽

Cherenkov Telescopes ◽

The Universe

The intrinsic spectra of some distant blazars known as “extreme TeV blazars” have shown a hint at an anomalous hardening in the TeV energy region. Several extragalactic propagation models have been proposed to explain this possible excess transparency of the Universe to gamma-rays starting from a model which assumes the existence of so-called axion-like particles (ALPs) and the new process of gamma-ALP oscillations. Alternative models suppose that some of the observable gamma-rays are produced in the intergalactic cascades. This work focuses on investigating the spectral and angular features of one of the cascade models, the Intergalactic Hadronic Cascade Model (IHCM) in the contemporary astrophysical models of Extragalactic Magnetic Field (EGMF). For IHCM, EGMF largely determines the deflection of primary cosmic rays and electrons of intergalactic cascades and, thus, is of vital importance. Contemporary Hackstein models are considered in this paper and compared to the model of Dolag. The models assumed are based on simulations of the local part of large-scale structure of the Universe and differ in the assumptions for the seed field. This work provides spectral energy distributions (SEDs) and angular extensions of two extreme TeV blazars, 1ES 0229+200 and 1ES 0414+009. It is demonstrated that observable SEDs inside a typical point spread function of imaging atmospheric Cherenkov telescopes (IACTs) for IHCM would exhibit a characteristic high-energy attenuation compared to the ones obtained in hadronic models that do not consider EGMF, which makes it possible to distinguish among these models. At the same time, the spectra for IHCM models would have longer high energy tails than some available spectra for the ALP models and the universal spectra for the Electromagnetic Cascade Model (ECM). The analysis of the IHCM observable angular extensions shows that the sources would likely be identified by most IACTs not as point sources but rather as extended ones. These spectra could later be compared with future observation data of such instruments as Cherenkov Telescope Array (CTA) and LHAASO.

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Towards the Hydrogen Economy—A Review of the Parameters That Influence the Efficiency of Alkaline Water Electrolyzers

Energies ◽

10.3390/en14113193 ◽

2021 ◽

Vol 14 (11) ◽

pp. 3193

Author(s):

Ana L. Santos ◽

Maria-João Cebola ◽

Diogo M. F. Santos

Keyword(s):

Energy Content ◽

Water Electrolysis ◽

High Energy ◽

Operating Conditions ◽

Energy Sources ◽

Alkaline Water Electrolysis ◽

Alkaline Water ◽

New Energy ◽

Recent Developments ◽

Cell Components

Environmental issues make the quest for better and cleaner energy sources a priority. Worldwide, researchers and companies are continuously working on this matter, taking one of two approaches: either finding new energy sources or improving the efficiency of existing ones. Hydrogen is a well-known energy carrier due to its high energy content, but a somewhat elusive one for being a gas with low molecular weight. This review examines the current electrolysis processes for obtaining hydrogen, with an emphasis on alkaline water electrolysis. This process is far from being new, but research shows that there is still plenty of room for improvement. The efficiency of an electrolyzer mainly relates to the overpotential and resistances in the cell. This work shows that the path to better electrolyzer efficiency is through the optimization of the cell components and operating conditions. Following a brief introduction to the thermodynamics and kinetics of water electrolysis, the most recent developments on several parameters (e.g., electrocatalysts, electrolyte composition, separator, interelectrode distance) are highlighted.

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Phase transitions ofWcondensation for the universe with finite fermion density

Physical Review D ◽

10.1103/physrevd.40.1255 ◽

1989 ◽

Vol 40 (4) ◽

pp. 1255-1259 ◽

Cited By ~ 7

Author(s):

O. K. Kalashnikov ◽

H. Perez Rojas

Keyword(s):

Phase Transitions ◽

The Universe

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Rheed Studies of a-Axis Oriented DyBa2Cu3O7 Films Grown by All-MBE

MRS Proceedings ◽

10.1557/proc-502-221 ◽

1997 ◽

Vol 502 ◽

Author(s):

Ivan Bozovic ◽

J. N. Eckstein ◽

Natasha Bozovic ◽

J. O'Donnell

Keyword(s):

Phase Transitions ◽

Secondary Phase ◽

Atomic Layer ◽

High Energy ◽

Layer By Layer ◽

Growth Modes ◽

Flow Surface ◽

Surface Phase Transitions

ABSTRACTReal-time, in-situ surface monitoring by reflection high-energy electron diffraction (RHEED) has been the key enabling component of atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) of complex oxides. RHEED patterns contain information on crystallographic arrangements and long range order on the surface; this can be made quantitative with help of numerical simulations. The dynamics of RHEED patterns and intensities reveal a variety of phenomena such as nucleation and dissolution of secondary-phase precipitates, switching between growth modes (layer-by-layer, step-flow), surface phase transitions (surface reconstruction, roughening, and even phase transitions induced by the electron beam itself), etc. Some of these phenomena are illustrated here, using as a case study our recent growth of atomically smooth a-axis oriented DyBa2Cu3O7 films.

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