1,022 research outputs found
The Anisotropy in the Cosmic Microwave Background At Degree Angular Scales
We detect anisotropy in the cosmic microwave background (CMB) at degree
angular scales and confirm a previous detection reported by Wollack et al.
(1993). The root-mean-squared amplitude of the fluctuations is K. This may be expressed as the square root of the angular power spectrum
in a band of multipoles between . We find K. The measured spectral
index of the fluctuations is consistent with zero, the value expected for the
CMB. The spectral index corresponding to Galactic free-free emission, the most
likely foreground contaminant, is rejected at approximately .
The analysis is based on three independent data sets. The first, taken in
1993, spans the 26 - 36 GHz frequency range with three frequency bands; the
second was taken with the same radiometer as the first but during an
independent observing campaign in 1994; and the third, also take in 1994, spans
the 36-46 GHz range in three bands. For each telescope position and radiometer
channel, the drifts in the instrument offset are K/day over a period
of one month. The dependence of the inferred anisotropy on the calibration and
data editing is addressed.Comment: 16 pages, 2 figures. Saskatoon 1993/1994 combined analysi
Early Enrichment of the Intergalactic Medium and its Feedback on Galaxy Formation
Supernova-driven outflows from early galaxies may have had a large impact on
the kinetic and chemical structure of the intergalactic medium (IGM). We use
three-dimensional Monte Carlo cosmological realizations of a simple linear
peaks model to track the time evolution of such metal-enriched outflows and
their feedback on galaxy formation. We find that at most 30% of the IGM by
volume is enriched to values above 10^-3 solar in models that only include
objects that cool by atomic transitions. The majority of enrichment occurs
relatively early (5 < z < 12) and resulting in a mass-averaged cosmological
metallicity between 10^-3 and 10^-1.5 solar. The inclusion of Population III
objects that cool through H2 line emission has only a minor impact on these
results: increasing the mean metallicity and filling factor by at most a factor
of 1.4, and moving the dawn of the enrichment epoch to a redshift of
approximately 14 at the earliest. Thus enrichment by outflowing galaxies is
likely to have been incomplete and inhomogeneous, biased to the areas near the
starbursting galaxies themselves. Models with a 10% star formation efficiency
can satisfactorily reproduce the nearly constant (2 < z < 5, Z approximately
3.5 x 10^-4 solar) metallicity of the low column density Ly-alpha forest
derived by Songaila (2001), an effect of the decreasing efficiency of metal
loss from larger galaxies. Finally, we show that IGM enrichment is intimately
tied to the ram-pressure stripping of baryons from neighboring perturbations.
This results in the suppression of at least 20% of the dwarf galaxies in the
mass range 10^8.5 to 10^9.5 solar, in all models with filling factors greater
than 2%, and an overall suppression of approximately 50% of dwarf galaxies in
the most observationally-favored model.Comment: 8 pages, 5 figures, accepted to Ap
Mapping the CMB III: combined analysis of QMAP flights
We present results from the QMAP balloon experiment, which maps the Cosmic
Microwave Background (CMB) and probes its angular power spectrum on degree
scales. In two separate flights, data were taken in six channels at two
frequency bands between 26 to 46 GHz. We describe our method for mapmaking
(removal of 1/f-noise and scan-synchronous offsets) and power spectrum
estimation, as well as the results of a joint analysis of the data from both
flights. This produces a 527 square degree map of the CMB around the North
Celestial Pole, allowing a wide variety of systematic cross-checks. The
frequency dependence of the fluctuations is consistent with CMB and
inconsistent with Galactic foreground emission. The anisotropy is measured in
three multipole bands from l~40 to l~200, and the angular power spectrum shows
a distinct rise which is consistent with the Saskatoon results.Comment: 4 pages, with 3 figures included. Submitted to ApJL. Window functions
are available at http://pupgg.princeton.edu/~cmb/welcome.html and color
figures and links at http://www.sns.ias.edu/~angelica/skymap.html#qma
An Imprint of Molecular Cloud Magnetization in the Morphology of the Dust Polarized Emission
We describe a morphological imprint of magnetization found when considering
the relative orientation of the magnetic field direction with respect to the
density structures in simulated turbulent molecular clouds. This imprint was
found using the Histogram of Relative Orientations (HRO): a new technique that
utilizes the gradient to characterize the directionality of density and column
density structures on multiple scales. We present results of the HRO analysis
in three models of molecular clouds in which the initial magnetic field
strength is varied, but an identical initial turbulent velocity field is
introduced, which subsequently decays. The HRO analysis was applied to the
simulated data cubes and mock-observations of the simulations produced by
integrating the data cube along particular lines of sight. In the 3D analysis
we describe the relative orientation of the magnetic field with
respect to the density structures, showing that: 1.The magnetic field shows a
preferential orientation parallel to most of the density structures in the
three simulated cubes. 2.The relative orientation changes from parallel to
perpendicular in regions with density over a critical density in the
highest magnetization case. 3.The change of relative orientation is largest for
the highest magnetization and decreases in lower magnetization cases. This
change in the relative orientation is also present in the projected maps. In
conjunction with simulations HROs can be used to establish a link between the
observed morphology in polarization maps and the physics included in
simulations of molecular clouds.Comment: (16 pages, 11 figures, submitted to ApJ 05MAR2013, accepted
07JUL2013
Generalized Cardassian Expansion: Models in Which the Universe is Flat, Matter Dominated, and Accelerating
The Cardassian universe is a proposed modification to the Friedmann Robertson
Walker (FRW) equation in which the universe is flat, matter dominated, and
accelerating. Here we generalize the original Cardassian proposal to include
additional variants on the FRW equation. Specific examples are presented.
In the ordinary FRW equation, the right hand side is a linear function of the
energy density, . Here, instead, the right hand side of the FRW
equation is a different function of the energy density, .
This function returns to ordinary FRW at early times, but modifies the
expansion at a late epoch of the universe. The only ingredients in this
universe are matter and radiation: in particular, there is {\it no} vacuum
contribution. Currently the modification of the FRW equation is such that the
universe accelerates. The universe can be flat and yet consist of only matter
and radiation, and still be compatible with observations. The energy density
required to close the universe is much smaller than in a standard cosmology, so
that matter can be sufficient to provide a flat geometry. The modifications may
arise, e.g., as a consequence of our observable universe living as a
3-dimensional brane in a higher dimensional universe. The Cardassian model
survives several observational tests, including the cosmic background
radiation, the age of the universe, the cluster baryon fraction, and structure
formation. As will be shown in future work, the predictions for observational
tests of the generalized Cardassian models can be very different from generic
quintessence models, whether the equation of state is constant or time
dependent.Comment: 5 pages, Conference Proceeding, Meeting on Sources and Detection of
Dark Matter and Dark Energy in the Universe, Marina del Rey, CA, February
200
The cosmological constant and the paradigm of adiabaticity
We discuss the value of the cosmological constant as recovered from CMB and
LSS data and the robustness of the results when general isocurvature initial
conditions are allowed for, as opposed to purely adiabatic perturbations. The
Bayesian and frequentist statistical approaches are compared. It is shown that
pre-WMAP CMB and LSS data tend to be incompatible with a non-zero cosmological
constant, regardless of the type of initial conditions and of the statistical
approach. The non-adiabatic contribution is constrained to be < 40% (2sigma
c.l.).Comment: 9 pages, 5 figures, to appear in New Astronomy Reviews, Proceedings
of the 2nd CMBNET Meeting, 20-21 February 2003, Oxford, U
Model Independent Primordial Power Spectrum from Maxima, Boomerang, and DASI Data
A model-independent determination of the primordial power spectrum of matter
density fluctuations could uniquely probe physics of the very early universe,
and provide powerful constraints on inflationary models. We parametrize the
primordial power spectrum as an arbitrary function, and deduce its
binned amplitude from the cosmic microwave background radiation anisotropy
(CMB) measurements of Maxima, Boomerang, and DASI. We find that for a flat
universe with (scale-invariant) for scales h/Mpc, the
primordial power spectrum is marginally consistent with a scale-invariant
Harrison-Zeldovich spectrum. However, we deduce a rise in power compared to a
scale-invariant power spectrum for 0.001 h/{Mpc} \la k \la 0.01 h/{Mpc}. Our
results are consistent with large-scale structure data, and seem to suggest
that the current observational data allow for the possibility of unusual
physics in the very early universe.Comment: substantially revised and final version, accepted by Ap
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