1,286 research outputs found
The phase coherence of light from extragalactic sources - direct evidence against first order Planck scale fluctuations in time and space
We present a method of directly testing whether time continues to have its
usual meaning on scales of <= t_P = sqrt(hbar G/c^5) ~ 5.4E-44 s, the Planck
time. According to quantum gravity, the time t of an event cannot be determined
more accurately than a standard deviation of the form sigma_t/t = a_o
(t_P/t)^a, where a_o and a are positive constants ~1; likewise distances are
subject to an ultimate uncertainty c \sigma_t, where c is the speed of light.
As a consequence, the period and wavelength of light cannot be specified
precisely; rather, they are independently subject to the same intrinsic
limitations in our knowledge of time and space, so that even the most
monochromatic plane wave must in reality be a superposition of waves with
varying omega and {\bf k}, each having a different phase velcocity omega/k. For
the entire accessible range of the electromagnetic spectrum this effect is
extremely small, but can cumulatively lead to a complete loss of phase
information if the emitted radiation propagated a sufficiently large distance.
Since, at optical frequencies, the phase coherence of light from a distant
point source is a necessary condition for the presence of diffraction patterns
when the source is viewed through a telescope, such observations offer by far
the most sensitive and uncontroversial test. We show that the HST detection of
Airy rings from the active galaxy PKS1413+135, located at a distance of 1.2
Gpc, secures the exclusion of all first order (a=1) quantum gravity
fluctuations with an amplitude a_o > 0.003. The same result may be used to
deduce that the speed of light in vacuo is exact to a few parts in 10^32.Comment: Title change. One reference added. Final version accepted by ApJ
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Household and family in Diaspora Judaism
Abstract Any study of religiosity in the Graeco-Roman world must include an account of the Jewish experience, so often seen as precursor or shadowy reflection, of the Christian. Here the primary sources are archaeological and epigraphic, with literary texts reflecting the intentions of sometimes hostile observers. Exploring these sources affords more glimpses than might be expected of the impact of religious identity on social and domestic life, but also demonstrates the fluidity between the categories of domestic or family and institutional, provoking questions that are equally applicable to the âpaganâ and Christian contexts.</jats:p
Strong lensing time delay: a new way of measuring cosmic shear
The phenomenon of cosmic shear, or distortion of images of distant sources
unaccompanied by magnification, is an effective way of probing the content and
state of the foreground Universe, because light rays do not have to pass
through mass structures in order to be sheared. It is shown that the delay in
the arrival times between two simultaneously emitted photons that appear to be
arriving from a pair of images of a strongly lensed cosmological source
contains not only information about the Hubble constant, but also the long
range gravitational effect of galactic scale mass clumps located away from the
light paths in question. This is therefore also a method of detecting shear.
Data on time delays among a sample of strongly lensed sources can provide
crucial information about whether extra dynamics beyond gravity and dark energy
are responsible for the global flatness of space. If the standard
model is correct, there should be a large dispersion in the value of as
inferred from the delay data by (the usual procedure of) ignoring the effect of
all other mass clumps except the strong lens itself. The fact that there has
not been any report of a significant deviation from the 0.7 mark during
any of the determinations by this technique may already be pointing to
the absence of the random effect discussed here.Comment: ApJ in pres
XMM-Newton discovery of O VII emission from warm gas in clusters of galaxies
XMM-Newton recently discovered O VII line emission from ~2 million K gas near
the outer parts of several clusters of galaxies. This emission is attributed to
the Warm-Hot Intergalactic Medium. The original sample of clusters studied for
this purpose has been extended and two more clusters with a soft X-ray excess
have been found. We discuss the physical properties of the warm gas, in
particular the density, spatial extent, abundances and temperature.Comment: 8 pages, 3 figures, conference "Soft X-ray emission from clusters of
galaxies and related phenomena", ed. R. Lieu, Kluwer, in pres
Extreme Ultraviolet Emission from Clusters of Galaxies: Inverse Compton Radiation from a Relic Population of Cosmic Ray Electrons?
We suggest that the luminous extreme ultraviolet (EUV) emission which has
been detected recently from clusters of galaxies is Inverse Compton (IC)
scattering of Cosmic Microwave Background (CMB) radiation by low energy cosmic
ray electrons in the intracluster medium. The cosmic ray electrons would have
Lorentz factors of gamma ~ 300, and would lose energy primarily by emitting EUV
radiation. These particles have lifetimes comparable to the Hubble time; thus,
the electrons might represent a relic population of cosmic rays produced by
nonthermal activity over the history of the cluster. The IC model naturally
explains the observed increase in the ratio of EUV to X-ray emission with
radius in clusters. The required energy in cosmic ray electrons is typically
1--10% of the thermal energy content of the intracluster gas. We suggest that
the cosmic ray electrons might have been produced by supernovae in galaxies, by
radio galaxies, or by particle acceleration in intracluster shocks.Comment: ApJ Letters, in press, 4 pages with 1 embedded figure, Latex in
emulateapj styl
On the absence of gravitational lensing of the cosmic microwave background
The magnification of distant sources by mass clumps at lower ()
redshifts is calculated analytically. The clumps are initially assumed to be
galaxy group isothermal spheres with properties inferred from an extensive
survey. The average effect, which includes strong lensing, is exactly
counteracted by the beam divergence in between clumps (more precisely, the
average reciprocal magnification cancels the inverse Dyer-Roeder
demagnification). This conclusion is in fact independent of the matter density
function within each clump, and remains valid for arbitrary densities of matter
and dark energy. When tested against the CMB, a rather large lensing induced
{\it dispersion} in the angular size of the primary acoustic peaks of the TT
power spectrum is inconsistent with WMAP observations. The situation is
unchanged by the use of NFW profiles for the density distribution of groups.
Finally, our formulae are applied to an ensemble of NFW mass clumps or
isothermal spheres having the parameters of galaxy {\it clusters}. The acoustic
peak size dispersion remains unobservably large, and is also excluded by WMAP.
For galaxy groups, two possible ways of reconciling with the data are proposed,
both exploiting maximally the uncertainties in our knowledge of group
properties. The same escape routes are not available in the case of clusters,
however, because their properties are well understood. Here we have a more
robust conclusion: neither of the widely accepted models are good description
of clusters, or important elements of physics responsible for shaping zero
curvature space are missing from the standard cosmological model. When all the
effects are accrued, it is difficult to understand how WMAP could reveal no
evidence whatsoever of lensing by groups and clusters.Comment: ApJ v628, pp. 583-593 (August 1, 2005
A massive warm baryonic halo in the Coma cluster
Several deep PSPC observations of the Coma cluster reveal a very large-scale
halo of soft X-ray emission, substantially in excess of the well known
radiation from the hot intra-cluster medium. The excess emission, previously
reported in the central region of the cluster using lower-sensitivity EUVE and
ROSAT data, is now evident out to a radius of 2.6 Mpc, demonstrating that the
soft excess radiation from clusters is a phenomenon of cosmological
significance. The X-ray spectrum at these large radii cannot be modeled
non-thermally, but is consistent with the original scenario of thermal emission
from warm gas at ~ 10^6 K. The mass of the warm gas is on par with that of the
hot X-ray emitting plasma, and significantly more massive if the warm gas
resides in low-density filamentary structures. Thus the data lend vital support
to current theories of cosmic evolution, which predict that at low redshift
\~30-40 % of the baryons reside in warm filaments converging at clusters of
galaxies.Comment: Astrophysical Journal, in pres
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