249 research outputs found
Singularity: Raychaudhuri Equation once again
I first recount Raychaudhuri's deep involvement with the singularity problem
in general relativity. I then argue that precisely the same situation has
arisen today in loop quantum cosmology as obtained when Raychaudhuri discovered
his celebrated equation. We thus need a new analogue of the Raychaudhuri
equation in quantum gravity.Comment: 11 pages, Contribution to Special issue of Pramana on Raychaudhuri
Equation at Cross-roads, edited by Naresh Dadhich, Pankaj Joshi and Probir
Ro
A quantum mechanical relation connecting time, temperature, and cosmological constant of the universe: Gamow's relation revisited as a special case
Considering our expanding universe as made up of gravitationally interacting
particles which describe particles of luminous matter and dark matter and dark
energy which is described by a repulsive harmonic potential among the points in
the flat 3-space, we derive a quantum mechanical relation connecting,
temperature of the cosmic microwave background radiation, age, and cosmological
constant of the universe. When the cosmological constant is zero, we get back
the Gamow's relation with a much better coefficient. Otherwise, our theory
predicts a value of the cosmological constant
when the present values of cosmic microwave background temperature of 2.728 K
and age of the universe 14 billion years are taken as input.Comment: 4 pages, 1 figure, Study of the Universe from a condensed matter
point of view, section III corrected with a single body potentia
The Circumstellar Environment of High-Mass Protostellar Objects: IV. C17O Observations and Depletion
We observe 84 candidate young high-mass sources in the rare isotopologues
C17O and C18O to investigate whether there is evidence for depletion
(freeze-out) towards these objects. Observations of the J=2-1 transitions of
C18O and C17O are used to derive the column densities of gas towards the
sources and these are compared with those derived from submillimetre continuum
observations. The derived fractional abundance suggests that the CO species
show a range of degrees of depletion towards the objects. We then use the
radiative transfer code RATRAN to model a selection of the sources to confirm
that the spread of abundances is not a result of assumptions made when
calculating the column densities. We find a range of abundances of C17O that
cannot be accounted for by global variations in either the temperature or dust
properties and so must reflect source to source variations. The most likely
explanation is that different sources show different degrees of depletion of
the CO. Comparison of the C17O linewidths of our sources with those of CS
presented by other authors reveal a division of the sources into two groups.
Sources with a CS linewidth >3 km/s have low abundances of C17O while sources
with narrower CS lines have typically higher C17O abundances. We suggest that
this represents an evolutionary trend. Depletion towards these objects shows
that the gas remains cold and dense for long enough for the trace species to
deplete. The range of depletion measured suggests that these objects have
lifetimes of 2-4x10^5 years.Comment: 18 pages. Accepted for publication in Astronomy & Astrophysic
The Chemical Evolution of the Milky Way
The field of chemical evolution modeling of the Galaxy is experiencing in the
last years a phase of high activity and important achievements. There are,
however, several open questions which still need to be answered. In this review
I summarize what have been the most important achievements and what are some of
the most urgent questions to be answered.Comment: 10 pages including 3 figs, to appear in "The Chemical Evolution of
the Milky Way. Stars vs Clusters", Proceedings of the Sept.1999 Vulcano
Workshop, F.Giovannelli and F.Matteucci eds (Kluwer, Dordrecht) in pres
Skewness in the Cosmic Microwave Background Anisotropy from Inflationary Gravity Wave Background
In the context of inflationary scenarios, the observed large angle anisotropy
of the Cosmic Microwave Background (CMB) temperature is believed to probe the
primordial metric perturbations from inflation. Although the perturbations from
inflation are expected to be gaussian random fields, there remains the
possibility that nonlinear processes at later epochs induce ``secondary''
non-gaussian features in the corresponding CMB anisotropy maps. The
non-gaussianity induced by nonlinear gravitational instability of scalar
(density) perturbations has been investigated in existing literature. In this
paper, we highlight another source of non-gaussianity arising out of higher
order scattering of CMB photons off the metric perturbations. We provide a
simple and elegant formalism for deriving the CMB temperature fluctuations
arising due to the Sachs-Wolfe effect beyond the linear order. In particular,
we derive the expression for the second order CMB temperature fluctuations. The
multiple scattering effect pointed out in this paper leads to the possibility
that tensor metric perturbation, i.e., gravity waves (GW) which do not exhibit
gravitational instability can still contribute to the skewness in the CMB
anisotropy maps. We find that in a flat universe, the skewness in
CMB contributed by gravity waves via multiple scattering effect is comparable
to that from the gravitational instability of scalar perturbations for equal
contribution of the gravity waves and scalar perturbations to the total rms CMB
anisotropy. The secondary skewness is found to be smaller than the cosmic
variance leading to the conclusion that inflationary scenarios do predict that
the observed CMB anisotropy should be statistically consistent with a gaussian
random distribution.Comment: 10 pages, Latex (uses revtex), 1 postscript figure included. Accepted
for publication in Physical Review
Structure formation and CMBR anisotropy spectrum in the inflessence model
The inflessence model has recently been proposed in an attempt to explain
both early inflation and present day accelerated expansion within a single
mechanism. The model has been successfully tested against the Hubble diagram of
Type Ia Supernovae, the shift parameter, and the acoustic peak parameter. As a
further mandatory test, we investigate here structure formation in the
inflessence model determining the evolution of matter density contrast in the linear regime. We compare the growth factor
and the growth index
to these same quantities for the successful concordance CDM model with
a particular emphasis on the role of the inflessence parameters . We also evaluate the anisotropy spectrum of the cosmic microwave
background radiation (CMBR) to check whether the inflessence model may be in
agreement with the observations. We find that, for large values of , structure formation proceeds in a similar way to that in the
CDM scenario, and it is also possible to nicely fit the CMBR spectrum.Comment: 10 pages, 9 figures, accepted for publication on Astronomy &
Astrophysics. A bug in a code has been corrected so that some of the figures
on the growth factor and growth index have been changed, but the main results
are unchange
Use of a global model to understand speciated atmospheric mercury observations at five high-elevation sites
© 2015 Author(s). Atmospheric mercury (Hg) measurements using the TekranÂź analytical system from five high-elevation sites (1400-3200 m elevation), one in Asia and four in the western US, were compiled over multiple seasons and years, and these data were compared with the GEOS-Chem global model. Mercury data consisted of gaseous elemental Hg (GEM) and "reactive Hg" (RM), which is a combination of the gaseous oxidized (GOM) and particulate bound ( < 2.5 ÎŒm) (PBM) fractions as measured by the TekranÂź system. We used a subset of the observations by defining a "free tropospheric" (FT) data set by screening using measured water vapor mixing ratios. The oxidation scheme used by the GEOS-Chem model was varied between the standard run with Br oxidation and an alternative run with OH-O 3 oxidation. We used this model-measurement comparison to help interpret the spatio-temporal trends in, and relationships among, the Hg species and ancillary parameters, to understand better the sources and fate of atmospheric RM. The most salient feature of the data across sites, seen more in summer relative to spring, was that RM was negatively correlated with GEM and water vapor mixing ratios (WV) and positively correlated with ozone (O 3 ), both in the standard model and the observations, indicating that RM was formed in dry upper altitude air from the photo-oxidation of GEM. During a free tropospheric transport high RM event observed sequentially at three sites from Oregon to Nevada, the slope of the RM/GEM relationship at the westernmost site was-1020 ± 209 pg ng -1 , indicating near-quantitative GEM-to-RM photochemical conversion. An improved correlation between the observations and the model was seen when the model was run with the OH-O3 oxidation scheme instead of the Br oxidation scheme. This simulation produced higher concentrations of RM and lower concentrations of GEM, especially at the desert sites in northwestern Nevada. This suggests that future work should investigate the effect of Br-and O 3 -initiated gas-phase oxidation occurring simultaneously in the atmosphere, as well as aqueous and heterogeneous reactions to understand whether there are multiple global oxidants for GEM and hence multiple forms of RM in the atmosphere. If the chemical forms of RM were known, then the collection efficiency of the analytical method could be evaluated better.Taiwan. Environmental Protection Administratio
Total and Monomethyl Mercury in Fog Water from the Central California Coast
[1] Total mercury (HgT) and monomethyl mercury (MMHg) concentrations in fog collected from 4 locations in and around Monterey Bay, California during June-August of 2011 were 10.7 ± 6.8 and 3.4 ± 3.8 ng Lâ1respectively. In contrast, mean HgT and MMHg concentrations in rain water from March-June, 2011 were 1.8 ± 0.9 and 0.1 ± 0.04 ng Lâ1 respectively. Using estimates of fog water deposition from 6 sites in the region using a standard fog water collector (SFC), depositions of HgT and MMHg via fog were found to range from 42â4600 and 14â1500 ng mâ2 yâ1, which accounted for 7â42% of HgT and 61â99% of MMHg in total atmospheric deposition (fog, rain, and dry deposition), estimated for the coastal area. These initial measurements suggest that fog precipitation may constitute an important but previously overlooked input of MMHg to coastal environments. Preliminary comparisons of these data with associated chemical, meteorological and oceanic data suggest that biotically formed MMHg from coastal upwelling may contribute to the MMHg in fog water
Vertical distribution of mercury, CO, ozone, and aerosol scattering coefficient in the Pacific Northwest during the spring 2006 INTEXâB campaign
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94827/1/jgrd14532.pd
Foreground removal from WMAP 7yr polarization maps using an MLP neural network
One of the fundamental problems in extracting the cosmic microwave background
signal (CMB) from millimeter/submillimeter observations is the pollution by
emission from the Milky Way: synchrotron, free-free, and thermal dust emission.
To extract the fundamental cosmological parameters from CMB signal, it is
mandatory to minimize this pollution since it will create systematic errors in
the CMB power spectra. In previous investigations, it has been demonstrated
that the neural network method provide high quality CMB maps from temperature
data. Here the analysis is extended to polarization maps. As a concrete
example, the WMAP 7-year polarization data, the most reliable determination of
the polarization properties of the CMB, has been analysed. The analysis has
adopted the frequency maps, noise models, window functions and the foreground
models as provided by the WMAP Team, and no auxiliary data is included. Within
this framework it is demonstrated that the network can extract the CMB
polarization signal with no sign of pollution by the polarized foregrounds. The
errors in the derived polarization power spectra are improved compared to the
errors derived by the WMAP Team.Comment: Accepted for publication in Astrophysics & Space Scienc
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