632 research outputs found

### Can Cosmic Shear Shed Light on Low Cosmic Microwave Background Multipoles?

The lowest multipole moments of the cosmic microwave background (CMB) are
smaller than expected for a scale-invariant power spectrum. One possible
explanation is a cutoff in the primordial power spectrum below a comoving scale
of $k_c \simeq 5.0 \times 10^{-4}$ Mpc$^{-1}$. This would affect not only the
CMB but also the cosmic-shear (CS) distortion of the CMB. Such a cutoff
increases significantly the cross-correlation between the large-angle CMB and
cosmic-shear patterns. The cross-correlation may be detectable at $> 2\sigma$
which, when combined with the low CMB moments, may tilt the balance between a
$2\sigma$ result and a firm detection of a large-scale power-spectrum cutoff.
As an aside, we also note that the cutoff increases the large-angle
cross-correlation between the CMB and low-redshift tracers of the mass
distribution.Comment: 5 pages, 3 figures, revised statistical analysis, submitted to PR

### A new cosmic microwave background constraint to primordial gravitational waves

Primordial gravitational waves (GWs) with frequencies > 10^{-15} Hz
contribute to the radiation density of the Universe at the time of decoupling
of the cosmic microwave background (CMB). The effects of this GW background on
the CMB and matter power spectra are identical to those due to massless
neutrinos, unless the initial density-perturbation amplitude for the
gravitational-wave gas is non-adiabatic, as may occur if such GWs are produced
during inflation or some post-inflation phase transition. In either case,
current observations provide a constraint to the GW amplitude that competes
with that from big-bang nucleosynthesis (BBN), although it extends to much
lower frequencies (~10^{-15} Hz rather than the ~10^{-10} Hz lower limit from
BBN): at 95% confidence-level, Omega_gw h^2 < 6.9 x 10^{-6} for homogeneous
(i.e., non-adiabatic) initial conditions. Future CMB experiments, like Planck
and CMBPol, should allow sensitivities to Omega_gw h^2 < 1.4 x 10^{-6} and
Omega_gw h^2 < 5 x 10^{-7}, respectively.Comment: 5 pages, 2 figures, submitted to Phys. Rev. Let

### Galactic Substructure and Dark Matter Annihilation in the Milky Way Halo

We study the effects of substructure on the rate of dark-matter annihilation
in the Galactic halo. We use an analytic model for substructure that can extend
numerical simulation results to scales too small to be resolved by the
simulations. We first calibrate the analytic model to numerical simulations,
and then determine the annihilation boost factor, for standard WIMP models as
well as those with Sommerfeld (or other) enhancements, as a function of
Galactocentric radius in the Milky Way. We provide an estimate of the
dependence of the gamma-ray intensity of WIMP annihilation as a function of
angular distance from the Galactic center. This methodology, coupled with
future numerical simulation results can be a powerful tool that can be used to
constrain WIMP properties using Fermi all-sky data.Comment: 10 pages, 7 figures, submitted to Phys. Rev. D; added a few
reference

### Galactic Substructure and Energetic Neutrinos from the Sun and the Earth

We consider the effects of Galactic substructure on energetic neutrinos from
annihilation of weakly-interacting massive particles (WIMPs) that have been
captured by the Sun and Earth. Substructure gives rise to a time-varying
capture rate and thus to time variation in the annihilation rate and resulting
energetic-neutrino flux. However, there may be a time lag between the capture
and annihilation rates. The energetic-neutrino flux may then be determined by
the density of dark matter in the Solar System's past trajectory, rather than
the local density. The signature of such an effect may be sought in the ratio
of the direct- to indirect-detection rates.Comment: 4 pages, 4 figures. Replaced with version accepted for publicatio

### The Contribution of the First Stars to the Cosmic Infrared Background

We calculate the contribution to the cosmic infrared background from very
massive metal-free stars at high redshift. We explore two plausible
star-formation models and two limiting cases for the reprocessing of the
ionizing stellar emission. We find that Population III stars may contribute
significantly to the cosmic near-infrared background if the following
conditions are met: (i) The first stars were massive, with M > ~100 M_sun. (ii)
Molecular hydrogen can cool baryons in low-mass haloes. (iii) Pop III star
formation is ongoing, and not shut off through negative feedback effects. (iv)
Virialized haloes form stars at about 40 per cent efficiency up to the redshift
of reionization, z~7. (v) The escape fraction of the ionizing radiation into
the intergalactic medium is small. (vi) Nearly all of the stars end up in
massive black holes without contributing to the metal enrichment of the
Universe.Comment: 11 pages, 6 figures, expanded discussion, added mid-IR to Fig 6,
MNRAS in pres

### Pure pseudo-C_l estimators for CMB B-modes

Fast heuristically weighted, or pseudo-C_l, estimators are a frequently used
method for estimating power spectra in CMB surveys with large numbers of
pixels. Recently, Challinor & Chon showed that the E-B mixing in these
estimators can become a dominant contaminant at low noise levels, ultimately
limiting the gravity wave signal which can be detected on a finite patch of
sky. We define a modified version of the estimators which eliminates E-B mixing
and is near-optimal at all noise levels.Comment: 9 pages and 3 figures; Proceedings of the Fundamental Physics With
CMB workshop, UC Irvine, March 23-25, 2006, to be published in New Astronomy
Review

### Geometry and Statistics of Cosmic Microwave Polarization

Geometrical and statistical properties of polarization of CMB are analyzed.
Singular points of the vector field which describes CMB polarization are found
and classified. Statistical distribution of the singularities is studied. A
possible signature of tensor perturbations in CMB polarization is discussed.
For a further analysis of CMB statistics Minkowski functionals are used, which
present a technically simple method to search for deviations from a Gaussian
distribution.Comment: 37 pages, 5 figures, submitted in Int.J.Mod.Phys.

### Metals at the surface of last scatter

Standard big-bang nucleosynthesis (BBN) predicts only a trace abundance of lithium and no heavier elements, but some alternatives predict a nonzero primordial metallicity. Here we explore whether CMB measurements may set useful constraints to the primordial metallicity and/or whether the standard CMB calculations are robust, within the tolerance of forthcoming CMB maps, to the possibility of primordial metals. Metals would affect the recombination history (and thus CMB power spectra) in three ways: (1) LyÎ± photons can be removed (and recombination thus accelerated) by photoionizing metals; (2) The Bowen resonance-fluorescence mechanism may degrade LyÎ² photons and thus enhance the LyÎ² escape probability and speed up recombination; (3) Metals could affect the low-redshift tail of the CMB visibility function by providing additional free electrons. The last two of these provide the strongest CMB signal. However, the effects are detectable in the Planck satellite only if the primordial metal abundance is at least a few hundredths of solar for (2) and a few tenths of solar for (3). We thus conclude that Planck will not be able to improve upon current constraints to primordial metallicity, at the level of a thousandth of solar, from the Lyman-Î± forest and ultra-metal-poor halo stars, and that the CMB power-spectrum predictions for Planck suffer no uncertainty arising from the possibility that there may be primordial metals

### First CMB Constraints on Direction-Dependent Cosmological Birefringence from WMAP-7

A Chern-Simons coupling of a new scalar field to electromagnetism may give
rise to cosmological birefringence, a rotation of the linear polarization of
electromagnetic waves as they propagate over cosmological distances. Prior work
has sought this rotation, assuming the rotation angle to be uniform across the
sky, by looking for the parity-violating TB and EB correlations a uniform
rotation produces in the CMB temperature/polarization. However, if the scalar
field that gives rise to cosmological birefringence has spatial fluctuations,
then the rotation angle may vary across the sky. Here we search for
direction-dependent cosmological birefringence in the WMAP-7 data. We report
the first CMB constraint on the rotation-angle power spectrum for multipoles
between L = 0 and L = 512. We also obtain a 68% confidence-level upper limit of
1 degree on the square root of the quadrupole of a scale-invariant
rotation-angle power spectrum.Comment: 14 pages, 12 figures, 4 tables; accepted to PR

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