12,429 research outputs found
The Consistent Newtonian Limit of Einstein's Gravity with a Cosmological Constant
We derive the `exact' Newtonian limit of general relativity with a positive
cosmological constant . We point out that in contrast to the case with
, the presence of a positive in Einsteins's equations
enforces, via the condition , on the potential , a range
, within which the
Newtonian limit is valid. It also leads to the existence of a maximum mass,
. As a consequence we cannot put the boundary
condition for the solution of the Poisson equation at infinity. A boundary
condition suitably chosen now at a finite range will then get reflected in the
solution of provided the mass distribution is not spherically symmetric.Comment: Latex, 15 pages, no figures, errors correcte
Current at a distance and resonant transparency in Weyl semimetals
Surface Fermi arcs are the most prominent manifestation of the topological
nature of Weyl semimetals. In the presence of a static magnetic field oriented
perpendicular to the sample surface, their existence leads to unique
inter-surface cyclotron orbits. We propose two experiments which directly probe
the Fermi arcs: a magnetic field dependent non-local DC voltage and sharp
resonances in the transmission of electromagnetic waves at frequencies
controlled by the field. We show that these experiments do not rely on quantum
mechanical phase coherence, which renders them far more robust and
experimentally accessible than quantum effects. We also comment on the
applicability of these ideas to Dirac semimetals.Comment: 10 pages, 8 figure
HST archive primer, version 4.1
This version of the HST Archive Primer provides the basic information a user needs to know to access the HST archive via StarView the new user interface to the archive. Using StarView, users can search for observations interest, find calibration reference files, and retrieve data from the archive. Both the terminal version of StarView and the X-windows version feature a name resolver which simplifies searches of the HST archive based on target name. In addition, the X-windows version of StarView allows preview of all public HST data; compressed versions of public images are displayed via SAOIMAGE, while spectra are plotted using the public plotting package, XMGR. Finally, the version of StarView described here features screens designed for observers preparing Cycle 5 HST proposals
Nonthermal Emission Associated with Strong AGN Outbursts at the Centers of Galaxy Clusters
Recently, strong AGN outbursts at the centers of galaxy clusters have been
found. Using a simple model, we study particle acceleration around a shock
excited by an outburst and estimate nonthermal emission from the accelerated
particles. We show that emission from secondary electrons is consistent with
the radio observations of the minihalo in the Perseus cluster, if there was a
strong AGN outburst >~10^8 yrs ago with an energy of ~1.8x10^62 erg. The
validity of our model depends on the frequency of the large outbursts. We also
estimate gamma-ray emission from the accelerated particles and show that it
could be detected with GLAST.Comment: Accepted for publication in ApJ
Optimal eigenvalues estimate for the Dirac operator on domains with boundary
We give a lower bound for the eigenvalues of the Dirac operator on a compact
domain of a Riemannian spin manifold under the \MIT bag boundary condition.
The limiting case is characterized by the existence of an imaginary Killing
spinor.Comment: 10 page
Improved Constraints on the Acceleration History of the Universe and the Properties of the Dark Energy
We extend and apply a model-independent analysis method developed earlier by
Daly & Djorgovski to new samples of supernova standard candles, radio galaxy
and cluster standard rulers, and use it to constrain physical properties of the
dark energy as functions of redshift. Similar results are obtained for the
radio galaxy and supernova data sets. The first and second derivatives of the
distance are compared directly with predictions in a standard model based on
General Relativity. The good agreement indicates that General Relativity
provides an accurate description of the data on look-back time scales of about
ten billion years. The first and second derivatives are combined to obtain the
acceleration parameter, assuming only the validity of the Robertson-Walker
metric, independent of a theory of gravity and of the physical nature of the
dark energy. The acceleration of the universe at the current epoch is indicated
by the analysis. The effect of non-zero space curvature on q(z) is explored. We
solve for the pressure, energy density, equation of state, and potential and
kinetic energy of the dark energy as functions of redshift assuming that
General Relativity is the correct theory of gravity, and the results indicate
that a cosmological constant in a spatially flat universe provides a good
description of each of these quantities over the redshift range from zero to
about one. We define a new function, the dark energy indicator, in terms of the
first and second derivatives of the coordinate distance and show how this can
be used to measure deviations of w from -1 and to obtain a new and independent
measure of Omega.Comment: 46 pages, submitted for publicatio
Quasars, their host galaxies, and their central black holes
We present the final results from our deep HST imaging study of the hosts of
radio-quiet quasars (RQQs), radio-loud quasars (RLQs) and radio galaxies (RGs).
We describe new WFPC2 R-band observations for 14 objects and model these images
in conjunction with the data already reported in McLure et al (1999). We find
that spheroidal hosts become more prevalent with increasing nuclear luminosity
such that, for nuclear luminosities M_V < -23.5, the hosts of both radio-loud
and radio-quiet AGN are virtually all massive ellipticals. Moreover we
demonstrate that the basic properties of these hosts are indistinguishable from
those of quiescent, evolved, low-redshift ellipticals of comparable mass. This
result kills any lingering notion that radio-loudness is determined by
host-galaxy morphology, and also sets severe constraints on evolutionary
schemes which attempt to link low-z ULIRGs with RQQs. Instead, we show that our
results are as expected given the relationship between black-hole and spheroid
mass established for nearby galaxies, and apply this relation to estimate the
mass of the black hole in each object. The results agree very well with
completely-independent estimates based on nuclear emission-line widths; all the
quasars in our sample have M(bh) > 5 x 10^8 solar masses, while the radio-loud
objects are confined to M(bh) > 10^9 solar masses. This apparent mass-threshold
difference, which provides a natural explanation for why RQQs outnumber RLQs by
a factor of 10, appears to reflect the existence of a minimum and maximum level
of black-hole radio output which is a strong function of black-hole mass.
Finally, we use our results to estimate the fraction of massive
spheroids/black-holes which produce quasar-level activity. This fraction is
\~0.1% at the present day, rising to > 10% at z = 2-3.Comment: Revised version accepted for publication in Monthly Notices of the
Royal Astronomical Society. 46 pages, the final 19 of which comprise an
Appendix. 15 figures in main text. A further 14 4-panel greyscale plots and
14 line plots which appear in the Appendix have been reproduced here with
reduced quality due to space limitations. A full resolution copy of the
manuscript can be obtained via ftp://ftp.roe.ac.uk/pub/jsd/dunlop2002.ps.g
Two centuries of farmland prices in England
The dissemination of robust asset price data can help improve
market efficiency, resource allocation and investment analysis. Land
prices influence housing affordability, food security and the carbon
infrastructure. Yet price and return histories for farmland in England
are fragmented. To provide perspective, a long farmland price series
is needed to improve transparency and bring the asset class into
line with commercial and residential real estate. After reviewing the
historical backdrop and considering methodology, this research uses
a chain-linking approach to construct a long-term farmland price
series for England. It then adjusts the series for inflation to examine
real land prices. The resulting two-century English farmland prices
series contributes to farmland market analysis. Notwithstanding
some concerns with long-run chain component heterogeneity, the
combined series helps us to understand English average farmland
price dynamics. As measured by the geometric mean, English land
price real capital returns have been positive over more than two
centuries. Farmland real price growth was 0.33 per cent annually
from 1781 to 2013 and 0.71 per cent from 1801 to 2013. The series
contributes to an understanding of land price dynamics.
Sir, – In these times, when the rental and marketable value of lan
Control of multiferroic domains by external electric fields in TbMnO3
The control of multiferroic domains through external electric fields has been
studied by dielectric measurements and by polarized neutron diffraction on
single-crystalline TbMnO. Full hysteresis cycles were recorded by varying
an external field of the order of several kV/mm and by recording the chiral
magnetic scattering as well as the charge in a sample capacitor. Both methods
yield comparable coercive fields that increase upon cooling.Comment: 12 pages, 6 figure
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