5,709 research outputs found
The Small-Scale Power Spectrum of Cold Dark Matter
One of the best motivated hypotheses in cosmology states that most of the
matter in the universe is in the form of weakly-interacting massive particles
that decoupled early in the history of the universe and cooled adiabatically to
an extremely low temperature. Nevertheless, the finite temperature and horizon
scales at which these particles decoupled imprint generic signatures on their
small scales density fluctuations. We show that the previously recognized
cut-off in the fluctuation power-spectrum due to free-streaming of particles at
the thermal speed of decoupling, is supplemented by acoustic oscillations owing
to the initial coupling between the cold dark matter (CDM) and the radiation
field. The power-spectrum oscillations appear on the scale of the horizon at
thermal decoupling which corresponds to a mass scale of
\~10^{-4}*(T_d/10MeV)^{-3} solar masses for a CDM decoupling temperature T_d.
The suppression of the power-spectrum on smaller scales by the acoustic
oscillations is physically independent from the free-streaming effect, although
the two cut-off scales are coincidentally comparable for T_d~10MeV and a
particle mass of M~100GeV. The initial conditions for recent numerical
simulations of the earliest and smallest objects to have formed in the
universe, need to be modified accordingly. The smallest dark matter clumps may
be detectable through gamma-ray production from particle annihilation, through
fluctuations in the event rate of direct detection experiments, or through
their tidal gravitational effect on wide orbits of objects near the outer edge
of the solar system.Comment: Physical Review D, in pres
The Imprint of Cosmic Reionization on Galaxy Clustering
We consider the effect of reionization on the clustering properties of galaxy
samples at intermediate redshifts (z~0.3-5.5). Current models for the
reionization of intergalactic hydrogen predict that overdense regions will be
reionized early, thus delaying the build up of stellar mass in the progenitors
of massive lower-redshift galaxies. As a result, the stellar populations
observed in intermediate redshift galaxies are somewhat younger and hence
brighter in overdense regions of the Universe. Galaxy surveys would therefore
be sensitive to galaxies with a somewhat lower dark matter mass in overdense
regions. The corresponding increase in the observed number density of galaxies
can be parameterized as a galaxy bias due to reionization. We model this
process using merger trees combined with a stellar synthesis code. Our model
demonstrates that reionization has a significant effect on the clustering
properties of galaxy samples that are selected based on their star-formation
properties. The bias correction in Lyman-break galaxies (including those in
proposed baryonic oscillation surveys at z<1) is at the level of 10-20% for a
halo mass of 10^12 solar masses, leading to corrections factors of 1.5-2 in the
halo mass inferred from measurements of clustering length. The reionization of
helium could also lead to a sharp increase in the amplitude of the galaxy
correlation function at z~3. We find that the reionization bias is
approximately independent of scale and halo mass. However since the traditional
galaxy bias is mass dependent, the reionization bias becomes relatively more
important for lower mass systems. The correction to the bias due to
reionization is very small in surveys of luminous red galaxies at z<1.Comment: 17 pages, 6 figures. Submitted to MNRA
Constraining the unexplored period between reionization and the dark ages with observations of the global 21 cm signal
Observations of the frequency dependence of the global brightness temperature
of the redshifted 21 cm line of neutral hydrogen may be possible with single
dipole experiments. In this paper, we develop a Fisher matrix formalism for
calculating the sensitivity of such instruments to the 21 cm signal from
reionization and the dark ages. We show that rapid reionization histories with
duration delta z< 2 can be constrained, provided that local foregrounds can be
well modelled by low order polynomials. It is then shown that observations in
the range nu = 50 - 100 MHz can feasibly constrain the Lyman alpha and X-ray
emissivity of the first stars forming at z = 15 - 25, provided that systematic
temperature residuals can be controlled to less than 1 mK. Finally, we
demonstrate the difficulty of detecting the 21 cm signal from the dark ages
before star formation.Comment: 11 pages, 14 figures, submitted to PR
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
Entrepreneurial Vernacular
During the 1920s, enterprising realtors, housing professionals, and builders developed the models that became the inspiration for the subdivision tract housing now commonplace in the U.S.Originally published in 2001. Suburban subdivisions of individual family homes are so familiar a part of the American landscape that it is hard to imagine a time when they were not common in the U. S. The shift to large-scale speculative subdivisions is usually attributed to the period after World War II. In Entrepreneurial Vernacular: Developers' Subdivisions in the 1920s, Carolyn S. Loeb shows that the precedents for this change in single-family home design were the result of concerted efforts by entrepreneurial realtors and other housing professionals during the 1920s. In her discussion of the historical and structural forces that propelled this change, Loeb focuses on three typical speculative subdivisions of the 1920s and on the realtors, architects, and building-craftsmen who designed and constructed them. These examples highlight the "shared set of planning and design concerns" that animated realtors (whom Loeb sees as having played the "key role" in this process) and the network of housing experts with whom they associated. Decentralized and loosely coordinated, this network promoted home ownership through flexible strategies of design, planning, financing, and construction which the author describes as a new and "entrepreneurial" vernacular
The multi-frequency angular power spectrum of the epoch of reionization 21 cm signal
Observations of redshifted 21cm radiation from HI at high redshifts is an
important future probe of reionization. We consider the Multi-frequency Angular
Power Spectrum (MAPS) to quantify the statistics of the HI signal as a joint
function of the angular multipole l and frequency separation \Delta\nu. The
signal at two different frequencies is expected to get decorrelated as
\Delta\nu is increased, and quantifying this decorrelation is particularly
important in deciding the frequency resolution for future HI observations. This
is also expected to play a very crucial role in extracting the signal from
foregrounds as the signal is expected to decorrelate much faster than the
foregrounds (which are largely continuum sources) with increasing \Delta\nu. In
this paper we develop formulae relating the MAPS to different components of the
three dimensional HI power spectrum taking into account HI peculiar velocities.
We show that the flat-sky approximation provides a very good representation
over the angular scales of interest, and a final expression which is very
simple to calculate and interpret. We present results considering two models
for the HI distribution, namely, (i) DM: where the HI traces the dark matter
and (ii) PR: where the effects of patchy reionization are incorporated through
two parameters. We find that while the DM signal is largely featureless, the PR
signal peaks at the angular scales of the individual bubbles, and the signal is
considerably enhanced for large bubble size. For most cases of interest at l
\sim 100 the signal is uncorrelated beyond \Delta\nu \sim 1 MHz or even less,
whereas it occurs around \sim 0.1 MHz at l \sim 10^3. The \Delta\nu dependence
also carries an imprint of the bubble size and the bias, and is expected to be
an important probe of the reionization scenario (abridged).Comment: Accepted for publication in MNRAS. Revised to match the accepted
versio
Cosmic Variance In the Transparency of the Intergalactic Medium After Reionization
Following the completion of cosmic reionization, the mean-free-path of
ionizing photons was set by a population of Ly-limit absorbers. As the
mean-free-path steadily grew, the intensity of the ionizing background also
grew, thus lowering the residual neutral fraction of hydrogen in ionization
equilibrium throughout the diffuse intergalactic medium (IGM). Ly-alpha photons
provide a sensitive probe for tracing the distribution of this residual
hydrogen at the end of reionization. Here we calculate the cosmic variance
among different lines-of-sight in the distribution of the mean Ly-alpha optical
depths. We find fractional variations in the effective post-reionization
optical depth that are of order unity on a scale of ~100 co-moving Mpc, in
agreement with observations towards high-redshift quasars. Significant
contributions to these variations are provided by the cosmic variance in the
density contrast on the scale of the mean-free-path for ionizing photons, and
by fluctuations in the ionizing background induced by delayed or enhanced
structure formation. Cosmic variance results in a highly asymmetric
distribution of transmission through the IGM, with fractional fluctuations in
Ly-alpha transmission that ar larger than in Ly-beta transmission.Comment: 7 pages 3 figures. Replaced with version accepted for publication in
Ap
Measuring the Direction and Angular Velocity of a Black Hole Accretion Disk via Lagged Interferometric Covariance
We show that interferometry can be applied to study irregular, rapidly
rotating structures, as are expected in the turbulent accretion flow near a
black hole. Specifically, we analyze the lagged covariance between
interferometric baselines of similar lengths but slightly different
orientations. For a flow viewed close to face-on, we demonstrate that the peak
in the lagged covariance indicates the direction and angular velocity of the
emission pattern from the flow. Even for moderately inclined flows, the
covariance robustly estimates the flow direction, although the estimated
angular velocity can be significantly biased. Importantly, measuring the
direction of the flow as clockwise or counterclockwise on the sky breaks a
degeneracy in accretion disk inclinations when analyzing time-averaged images
alone. We explore the potential efficacy of our technique using
three-dimensional, general relativistic magnetohydrodynamic (GRMHD)
simulations, and we highlight several baseline pairs for the Event Horizon
Telescope (EHT) that are well-suited to this application. These results
indicate that the EHT may be capable of estimating the direction and angular
velocity of the emitting material near Sagittarius A*, and they suggest that a
rotating flow may even be utilized to improve imaging capabilities.Comment: 8 Pages, 4 Figures, accepted for publication in Ap
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