4,305 research outputs found
What will anisotropies in the clustering pattern in redshifted 21 cm maps tell us?
The clustering pattern in high redshift HI maps is expected to be anisotropic
due to two distinct reasons, the
Alcock-Paczynski effect and the peculiar velocities, both of which are
sensitive to the cosmological parameters. The signal is also expected to be
sensitive to the details of the HI distribution at the epoch when the radiation
originated. We use simple models for the HI distribution at the epoch of
reionizaation and the post-reionization era to investigate exactly what we hope
to learn from future observations of the anisotropy pattern in HI maps. We find
that such observations will probably tell us more about the HI distribution
than about the background cosmological model. Assuming that reionization can be
described by spherical, ionized bubbles all of the same size with their centers
possibly being biased with respect to the dark matter, we find that the
anisotropy pattern at small angles is expected to have a bump at the
characteristic angular size of the individual bubbles whereas the large scale
anisotropy pattern will reflect the size and the bias of the bubbles. The
anisotropy also depends on the background cosmological parameters, but the
dependence is much weaker. Under the assumption that the HI in the
post-reionization era traces the dark matter with a possible bias, we find that
changing the bias and changing the background cosmology has similar effects on
the anisotropy pattern. Combining observations of the anisotropy with
independent estimates of the bias, possibly from the bi-spectrum, may allow
these observations to constrain cosmological parameters.Comment: Minor changes, Accepted to MNRA
Spontaneously Localized Photonic Modes Due to Disorder in the Dielectric Constant
We present the first experimental evidence for the existence of strongly
localized photonic modes due to random two dimensional fluctuations in the
dielectric constant. In one direction, the modes are trapped by ordered Bragg
reflecting mirrors of a planar, one wavelength long, microcavity. In the cavity
plane, they are localized by disorder, which is due to randomness in the
position, composition and sizes of quantum dots located in the anti-node of the
cavity. We extend the theory of disorder induced strong localization of
electron states to optical modes and obtain quantitative agreement with the
main experimental observations.Comment: 6 page
Galaxy Selection and Clustering and Lyman alpha Absorber Identification
The effects of galaxy selection on our ability to constrain the nature of
weak Ly\alpha absorbers at low redshift are explored. Current observations
indicate the existence of a population of gas-rich, low surface brightness
(LSB) galaxies, and these galaxies may have large cross sections for Ly\alpha
absorption. Absorption arising in LSB galaxies may be attributed to HSB
galaxies at larger impact parameters from quasar lines of sight, so that the
observed absorption cross sections of galaxies may seem unreasonably large.
Thus it is not possible to rule out scenarios where LSB galaxies make
substantial contributions to Ly\alpha absorption using direct observations.
Less direct tests, where observational selection effects are taken into account
using simulations, should make it possible to determine the nature of Ly\alpha
absorbers by observing a sample of ~100 galaxies around quasar lines of sight
with well-defined selection criteria. Such tests, which involve comparing
simulated and observed plots of the unidentified absorber fractions and
absorbing galaxy fractions versus impact parameter, can distinguish between
scenarios where absorbers arise in particular galaxies and those where
absorbers arise in gas tracing the large scale galaxy distribution. Care must
be taken to minimize selection effects even when using these tests. Results
from such tests are likely to depend upon the limiting neutral hydrogen column
density. While not enough data are currently available to make a strong
conclusion about the nature of moderately weak absorbers, evidence is seen that
such absorbers arise in gas that is around or between galaxies that are often
not detected in surveys.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journa
Exploiting Cross Correlations and Joint Analyses
In this report, we present a wide variety of ways in which information from
multiple probes of dark energy may be combined to obtain additional information
not accessible when they are considered separately. Fundamentally, because all
major probes are affected by the underlying distribution of matter in the
regions studied, there exist covariances between them that can provide
information on cosmology. Combining multiple probes allows for more accurate
(less contaminated by systematics) and more precise (since there is
cosmological information encoded in cross-correlation statistics) measurements
of dark energy. The potential of cross-correlation methods is only beginning to
be realized. By bringing in information from other wavelengths, the
capabilities of the existing probes of dark energy can be enhanced and
systematic effects can be mitigated further. We present a mixture of work in
progress and suggestions for future scientific efforts. Given the scope of
future dark energy experiments, the greatest gains may only be realized with
more coordination and cooperation between multiple project teams; we recommend
that this interchange should begin sooner, rather than later, to maximize
scientific gains.Comment: Report from the "Dark Energy and CMB" working group for the American
Physical Society's Division of Particles and Fields long-term planning
exercise ("Snowmass"
Feasibility of a Small, Rapid Optical-to-IR Response, Next Generation Gamma Ray Burst Mission
We present motivations for and study feasibility of a small, rapid optical to
IR response gamma ray burst (GRB) space observatory. By analyzing existing GRB
data, we give realistic detection rates for X-ray and optical/IR instruments of
modest size under actual flight conditions. Given new capabilities of fast
optical/IR response (about 1 s to target) and simultaneous multi-band imaging,
such an observatory can have a reasonable event rate, likely leading to new
science. Requiring a Swift-like orbit, duty cycle, and observing constraints, a
Swift-BAT scaled down to 190 square cm of detector area would still detect and
locate about 27 GRB per yr. for a trigger threshold of 6.5 sigma. About 23
percent of X-ray located GRB would be detected optically for a 10 cm diameter
instrument (about 6 per yr. for the 6.5 sigma X-ray trigger).Comment: Elaborated text version of a poster presented at 2012 Malaga/Marbella
symposiu
Clustering, Angular Size and Dark Energy
The influence of dark matter inhomogeneities on the angular size-redshift
test is investigated for a large class of flat cosmological models driven by
dark energy plus a cold dark matter component (XCDM model). The results are
presented in two steps. First, the mass inhomogeneities are modeled by a
generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is
characterized by a smoothness parameter and a power index ,
and, second, we provide a statistical analysis to angular size data for a large
sample of milliarcsecond compact radio sources. As a general result, we have
found that the parameter is totally unconstrained by this sample of
angular diameter data.Comment: 9 pages, 7 figures, accepted in Physical Review
Effect of in-plane magnetic field on the photoluminescence spectrum of modulation-doped quantum wells and heterojunctions
The photoluminescence (PL) spectrum of modulation-doped GaAs/AlGaAs quantum
wells (MDQW) and heterojunctions (HJ) is studied under a magnetic field
() applied parallel to the two-dimensional electron gas (2DEG) layer.
The effect of strongly depends on the electron-hole separation
(), and we revealed remarkable -induced modifications of the PL
spectra in both types of heterostructures. A model considering the direct
optical transitions between the conduction and valence subband that are shifted
in k-space under , accounts qualitatively for the observed spectral
modifications. In the HJs, the PL intensity of the bulk excitons is strongly
reduced relatively to that of the 2DEG with increasing . This means
that the distance between the photoholes and the 2DEG decreases with increased
, and that free holes are responsible for the hole-2DEG PL.Comment: 6pages, 5figure
Testing the Cosmological Constant as a Candidate for Dark Energy
It may be difficult to single out the best model of dark energy on the basis
of the existing and planned cosmological observations, because many different
models can lead to similar observational consequences. However, each particular
model can be studied and either found consistent with observations or ruled
out. In this paper, we concentrate on the possibility to test and rule out the
simplest and by far the most popular of the models of dark energy, the theory
described by general relativity with positive vacuum energy (the cosmological
constant). We evaluate the conditions under which this model could be ruled out
by the future observations made by the Supernova/Acceleration Probe SNAP (both
for supernovae and weak lensing) and by the Planck Surveyor cosmic microwave
background satellite.Comment: 6 pages, 2 figures, revtex
Observational Bounds on Cosmic Doomsday
Recently it was found, in a broad class of models, that the dark energy
density may change its sign during the evolution of the universe. This may lead
to a global collapse of the universe within the time t_c ~ 10^{10}-10^{11}
years. Our goal is to find what bounds on the future lifetime of the universe
can be placed by the next generation of cosmological observations. As an
example, we investigate the simplest model of dark energy with a linear
potential V(\phi) =V_0(1+\alpha\phi). This model can describe the present stage
of acceleration of the universe if \alpha is small enough. However, eventually
the field \phi rolls down, V(\phi) becomes negative, and the universe
collapses. The existing observational data indicate that the universe described
by this model will collapse not earlier than t_c > 10 billion years from the
present moment. We show that the data from SNAP and Planck satellites may
extend the bound on the "doomsday" time to t_c > 40 billion years at the 95%
confidence level.Comment: 11 pages, 6 figures, revtex
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