37,368 research outputs found
Einstein's Real "Biggest Blunder"
Albert Einstein's real "biggest blunder" was not the 1917 introduction into
his gravitational field equations of a cosmological constant term \Lambda,
rather was his failure in 1916 to distinguish between the entirely different
concepts of active gravitational mass and passive gravitational mass. Had he
made the distinction, and followed David Hilbert's lead in deriving field
equations from a variational principle, he might have discovered a true (not a
cut and paste) Einstein-Rosen bridge and a cosmological model that would have
allowed him to predict, long before such phenomena were imagined by others,
inflation, a big bounce (not a big bang), an accelerating expansion of the
universe, dark matter, and the existence of cosmic voids, walls, filaments, and
nodes.Comment: 4 pages, LaTeX, 11 references, Honorable Mention in 2012 Gravity
Research Foundation Essay Award
The exterior degree of a pair of finite groups
The exterior degree of a pair of finite groups , which is a
generalization of the exterior degree of finite groups, is the probability for
two elements in such that . In the present paper,
we state some relations between this concept and the relative commutatively
degree, capability and the Schur multiplier of a pair of groups.Comment: To appear in Mediterr. J. Mat
First-principles thermodynamic modeling of lanthanum chromate perovskites
Tendencies toward local atomic ordering in (A,A′)(B,B′)O_(3−δ) mixed composition perovskites are modeled to explore their influence on thermodynamic, transport, and electronic properties. In particular, dopants and defects within lanthanum chromate perovskites are studied under various simulated redox environments. (La_(1−x),Sr_x)(Cr_(1−y),Fe_y)O_(3−δ) (LSCF) and (La_(1−x),Sr_x)(Cr_(1−y),Ru_y)O_(3−δ) (LSCR) are modeled using a cluster expansion statistical thermodynamics method built upon a density functional theory database of structural energies. The cluster expansions are utilized in lattice Monte Carlo simulations to compute the ordering of Sr and Fe(Ru) dopant and oxygen vacancies (Vac). Reduction processes are modeled via the introduction of oxygen vacancies, effectively forcing excess electronic charge onto remaining atoms. LSCR shows increasingly extended Ru-Vac associates and short-range Ru-Ru and Ru-Vac interactions upon reduction; LSCF shows long-range Fe-Fe and Fe-Vac interaction ordering, inhibiting mobility. First principles density functional calculations suggest that Ru-Vac associates significantly decrease the activation energy of Ru-Cr swaps in reduced LSCR. These results are discussed in view of experimentally observed extrusion of metallic Ru from LSCR nanoparticles under reducing conditions at elevated temperature
First-principles thermodynamic modeling of atomic ordering in yttria-stabilized zirconia
Yttria-stabilized zirconia YSZ is modeled using a cluster expansion statistical thermodynamics method
built upon a density-functional theory database. The reliability of cluster expansions in predicting atomic
ordering is explored by comparing with the extensive experimental database. The cluster expansion of YSZ is
utilized in lattice Monte Carlo simulations to compute the ordering of dopant and oxygen vacancies as a
function of concentration. Cation dopants show a strong tendency to aggregate and vacate significantly sized
domains below 9 mol % Y_2O_3, which is likely important for YSZ aging processes in ionic conductivity.
Evolution of vibrational and underlying electronic properties as a function of Y doping is explored
Line Emitting Galaxies Beyond a Redshift of 7: An Improved Method for Estimating the Evolving Neutrality of the Intergalactic Medium
The redshift-dependent fraction of color-selected galaxies revealing Lyman
alpha emission has become the most valuable constraint on the evolving
neutrality of the early intergalactic medium. However, in addition to resonant
scattering by neutral gas, the visibility of Lyman alpha is also dependent on
the intrinsic properties of the host galaxy, including its stellar population,
dust content and the nature of outflowing gas. Taking advantage of significant
progress we have made in determining the line emitting properties of galaxies, we propose an improved method, based on using the measured
slopes of the rest-frame ultraviolet continua of galaxies, to interpret the
growing body of near-infrared spectra of galaxies in order to take into
account these host galaxy dependencies. In a first application of our new
method, we demonstrate its potential via a new spectroscopic survey of
galaxies undertaken with the Keck MOSFIRE spectrograph. Together with earlier
published data our data provides improved estimates of the evolving visibility
of Lyman alpha, particularly at redshift . As a byproduct, we also
present a new line emitting galaxy at a redshift which supersedes an
earlier redshift record. We discuss the improving constraints on the evolving
neutral fraction over and the implications for cosmic reionization.Comment: To be submitted to Ap
The status of traditional Scottish animal breeds and plant varieties and the implications for biodiversity
The aim of this scoping study was to evaluate the effects on Scottish biodiversity of
changes in the use of traditional breeds and varieties. The overall objectives were:
a) The evaluation of the importance of genetic loss from the reduction in use of these
breeds and varieties, for example, the loss of unusual characteristics that might have
been of particular local use.
b) An assessment of the impacts of reduction in the ability to conduct further breeding or
research on rare and traditional varieties and breeds.
c) Identification of the loss of certain farming techniques associated with particular
varieties and breeds.
d) An assessment of possible losses of biodiversity associated with reduction in the use of
these breeds and varieties and the farming systems associated with them
Shift in the LHC Higgs diphoton mass peak from interference with background
The Higgs diphoton amplitude from gluon fusion at the LHC interferes with the
continuum background induced by quark loops. I investigate the effect of this
interference on the position of the diphoton invariant mass peak used to help
determine the Higgs mass. At leading order, the interference shifts the peak
towards lower mass by an amount of order 150 MeV or more, with the precise
value dependent on the methods used to analyze and fit the data.Comment: 10 pages. v2: comments on scale variation added, reference adde
Observation of the Purcell effect in high-index-contrast micropillar
We have fabricated pillar microcavity samples with Bragg mirrors consisting
of alternate layers of GaAs and Aluminium Oxide. Compared to the more widely
studied GaAs/AlAs micropillars these mirrors can achieve higher reflectivities
with fewer layer repeats and reduce the mode volume. We have studied a number
of samples containing a low density of InGaAs/GaAs self assembled quantum dots
in a cavity and here report observation of a three fold enhancement in the
radiative lifetime of a quantum dot exciton state due to the Purcell effect
Oxide-apertured microcavity single-photon emitting diode
We have developed a microcavity single-photon source based on a single
quantum dot within a planar cavity in which wet-oxidation of a high-aluminium
content layer provides lateral confinement of both the photonic mode and the
injection current. Lateral confinement of the optical mode in optically pumped
structures produces a strong enhancement of the radiative decay rate. Using
microcavity structures with doped contact layers, we demonstrate a
single-photon emitting diode where current may be injected into a single dot
Note on Varying Speed of Light Cosmologies
The various requirements on a consistent varying speed of light (`VSL')
theory are surveyed, giving a short check-list of issues that should be
satisfactorily handled by such theories.Comment: 6 pages; to appear in the GRG Journa
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