15,607 research outputs found
British Bryological Society expedition to Mulanje Mountain, Malawi : 13., new and other unpublished records
Further results are provided of the 1991 British Bryological Society Expedition to Mulanje Mt., Malawi including 168 taxa of bryophyte, comprising 72 taxa of liverwort (38 new to Malawi) and 96 taxa of mosses (45 new to Malawi)
Bryophytes of Uganda : 2., new and interesting records
51 hepatics and 46 mosses are reported new to Uganda, including one moss new to Africa, one hepatic and two mosses new to mainland Africa, and 2 hepatics that are otherwise known only from their type collection
Density Perturbations and the Cosmological Constant from Inflationary Landscapes
An anthropic understanding of the cosmological constant requires that the
vacuum energy at late time scans from one patch of the universe to another. If
the vacuum energy during inflation also scans, the various patches of the
universe acquire exponentially differing volumes. In a generic landscape with
slow-roll inflation, we find that this gives a steeply varying probability
distribution for the normalization of the primordial density perturbations,
resulting in an exponentially small fraction of observers measuring the COBE
value of 10^-5. Inflationary landscapes should avoid this "\sigma problem", and
we explore features that can allow them to do that. One possibility is that,
prior to slow-roll inflation, the probability distribution for vacua is
extremely sharply peaked, selecting essentially a single anthropically allowed
vacuum. Such a selection could occur in theories of eternal inflation. A second
possibility is that the inflationary landscape has a special property: although
scanning leads to patches with volumes that differ exponentially, the value of
the density perturbation does not vary under this scanning. This second case is
preferred over the first, partly because a flat inflaton potential can result
from anthropic selection, and partly because the anthropic selection of a small
cosmological constant is more successful.Comment: 23 page
A Simple Model for r-Process Scatter and Halo Evolution
Recent observations of heavy elements produced by rapid neutron capture
(r-process) in the halo have shown a striking and unexpected behavior: within a
single star, the relative abundances of r-process elements heavier than Eu are
the same as the same as those of solar system matter, while across stars with
similar metallicity Fe/H, the r/Fe ratio varies over two orders of magnitude.
In this paper we present a simple analytic model which describes a star's
abundances in terms of its ``ancestry,'' i.e., the number of nucleosynthesis
events (e.g., supernova explosions) which contributed to the star's
composition. This model leads to a very simple analytic expression for the
abundance scatter versus Fe/H, which is in good agreement with the data and
with more sophisticated numerical models. We investigate two classes of
scenarios for r-process nucleosynthesis, one in which r-process synthesis
events occur in only \sim 4% of supernovae but iron synthesis is ubiquitous,
and one in which iron nucleosynthesis occurs in only about 9% of supernovae.
(the Wasserburg- Qian model). We find that the predictions in these scenarios
are similar for [Fe/H] \ga -2.5, but that these models can be readily
distinguished observationally by measuring the dispersion in r/Fe at [Fe/H] \la
-3.Comment: AASTeX, 21 pages, includes 4 figure
The Rate of Type Ia Supernovae at High Redshift
We derive the rates of Type Ia supernovae (SNIa) over a wide range of
redshifts using a complete sample from the IfA Deep Survey. This sample of more
than 100 SNIa is the largest set ever collected from a single survey, and
therefore uniquely powerful for a detailed supernova rate (SNR) calculation.
Measurements of the SNR as a function of cosmological time offer a glimpse into
the relationship between the star formation rate (SFR) and Type Ia SNR, and may
provide evidence for the progenitor pathway. We observe a progressively
increasing Type Ia SNR between redshifts z~0.3-0.8. The Type Ia SNR
measurements are consistent with a short time delay (t~1 Gyr) with respect to
the SFR, indicating a fairly prompt evolution of SNIa progenitor systems. We
derive a best-fit value of SFR/SNR 580 h_70^(-2) M_solar/SNIa for the
conversion factor between star formation and SNIa rates, as determined for a
delay time of t~1 Gyr between the SFR and the Type Ia SNR. More complete
measurements of the Type Ia SNR at z>1 are necessary to conclusively determine
the SFR--SNR relationship and constrain SNIa evolutionary pathways.Comment: 37 pages, 9 figures, accepted for publication in Astrophysical
Journal. Figures 7-9 correcte
Classical 5D fields generated by a uniformly accelerated point source
Gauge fields associated with the manifestly covariant dynamics of particles
in spacetime are five-dimensional. In this paper we explore the old
problem of fields generated by a source undergoing hyperbolic motion in this
framework. The 5D fields are computed numerically using absolute time
-retarded Green-functions, and qualitatively compared with Maxwell fields
generated by the same motion. We find that although the zero mode of all fields
coincides with the corresponding Maxwell problem, the non-zero mode should
affect, through the Lorentz force, the observed motion of test particles.Comment: 36 pages, 8 figure
Revealing Cosmic Rotation
Cosmological Birefringence (CB), a rotation of the polarization plane of
radiation coming to us from distant astrophysical sources, may reveal parity
violation in either the electromagnetic or gravitational sectors of the
fundamental interactions in nature. Until only recently this phenomenon could
be probed with only radio observations or observations at UV wavelengths.
Recently, there is a substantial effort to constrain such non-standard models
using observations of the rotation of the polarization plane of cosmic
microwave background (CMB) radiation. This can be done via measurements of the
-modes of the CMB or by measuring its TB and EB correlations which vanish in
the standard model. In this paper we show that correlations-based
estimator is the best for upcoming polarization experiments. The based
estimator surpasses other estimators because it has the smallest noise and of
all the estimators is least affected by systematics. Current polarimeters are
optimized for the detection of -mode polarization from either primordial
gravitational waves or by large scale structure via gravitational lensing. In
the paper we also study optimization of CMB experiments for the detection of
cosmological birefringence, in the presence of instrumental systematics, which
by themselves are capable of producing correlations; potentially mimicking
CB.Comment: 10 pages, 3 figures, 2 table
The Polarization of the Cosmic Microwave Background Due to Primordial Gravitational Waves
We review current observational constraints on the polarization of the Cosmic
Microwave Background (CMB), with a particular emphasis on detecting the
signature of primordial gravitational waves. We present an analytic solution to
the Polanarev approximation for CMB polarization produced by primordial
gravitational waves. This simplifies the calculation of the curl, or B-mode
power spectrum associated with gravitational waves during the epoch of
cosmological inflation. We compare our analytic method to existing numerical
methods and also make predictions for the sensitivity of upcoming CMB
polarization observations to the inflationary gravitational wave background. We
show that upcoming experiments should be able either detect the relic
gravitational wave background or completely rule out whole classes of
inflationary models.Comment: 25 pages, 4 figures, review published in IJMP
Double Distribution of Dark Matter Halos with respect to Mass and Local Overdensity
We present a double distribution function of dark matter halos, with respect
to both object mass and local over- (or under-) density. This analytical tool
provides a statistical treatment of the properties of matter surrounding
collapsed objects, and can be used to study environmental effects on
hierarchical structure formation. The size of the "local environment" of a
collapsed object is defined to depend on the mass of the object. The
Press-Schechter mass function is recovered by integration of our double
distribution over the density contrast. We also present a detailed treatment of
the evolution of overdensities and underdensities in Einstein-deSitter and flat
LCDM universes, according to the spherical evolution model. We explicitly
distinguish between true and linearly extrapolated overdensities and provide
conversion relations between the two quantities.Comment: 25 pages, 10 figures, comments welcom
Synoptic Sky Surveys and the Diffuse Supernova Neutrino Background: Removing Astrophysical Uncertainties and Revealing Invisible Supernovae
The cumulative (anti)neutrino production from all core-collapse supernovae
within our cosmic horizon gives rise to the diffuse supernova neutrino
background (DSNB), which is on the verge of detectability. The observed flux
depends on supernova physics, but also on the cosmic history of supernova
explosions; currently, the cosmic supernova rate introduces a substantial
(+/-40%) uncertainty, largely through its absolute normalization. However, a
new class of wide-field, repeated-scan (synoptic) optical sky surveys is coming
online, and will map the sky in the time domain with unprecedented depth,
completeness, and dynamic range. We show that these surveys will obtain the
cosmic supernova rate by direct counting, in an unbiased way and with high
statistics, and thus will allow for precise predictions of the DSNB. Upcoming
sky surveys will substantially reduce the uncertainties in the DSNB source
history to an anticipated +/-5% that is dominated by systematics, so that the
observed high-energy flux thus will test supernova neutrino physics. The
portion of the universe (z < 1) accessible to upcoming sky surveys includes the
progenitors of a large fraction (~ 87%) of the expected 10-26 MeV DSNB event
rate. We show that precision determination of the (optically detected) cosmic
supernova history will also make the DSNB into a strong probe of an extra flux
of neutrinos from optically invisible supernovae, which may be unseen either
due to unexpected large dust obscuration in host galaxies, or because some
core-collapse events proceed directly to black hole formation and fail to give
an optical outburst.Comment: 11 pages, 6 figure
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