545 research outputs found
Microlensing By a Prolate All-Macho Halo
It is widely believed that dark matter halos are flattened, that is closer to
oblate than prolate. The evidence cited is based largely on observations of
galaxies which do not look anything like our own and on numerical simulations
which use ad hoc initial conditions. Given what we believe to be a ``reasonable
doubt'' concerning the shape of dark Galactic halo we calculate the optical
depth and event rate for microlensing of stars in the LMC assuming a wide range
of models that include both prolate and oblate halos. We find, in agreement
with previous analysis, that the optical depth for a spherical (E0) halo and
for an oblate (E6) halo are roughly the same, essentially because two competing
effects cancel approximately. However the optical depth for an E6 prolate halo
is reduced by ~35%. This means that an all-Macho prolate halo with reasonable
parameters for the Galaxy is consistent with the published microlensing event
rate.Comment: 7 pages (24K), LaTeX; 2 Postscript figure
The CMB Quadrupole in a Polarized Light
The low quadrupole of the cosmic microwave background (CMB), measured by COBE
and confirmed by WMAP, has generated much discussion recently. We point out
that the well-known correlation between temperature and polarization
anisotropies of the CMB further constrains the low multipole anisotropy data.
This correlation originates from the fact that the low-multipole polarization
signal is sourced by the CMB quadrupole as seen by free electrons during the
relatively recent cosmic history. Consequently, the large-angle temperature
anisotropy data make restrictive predictions for the large-angle polarization
anisotropy, which depend primarily on the optical depth for electron scattering
after cosmological recombination, tau. We show that if current cosmological
models for the generation of large angle anisotropy are correct and the
COBE/WMAP data are not significantly contaminated by non-CMB signals, then the
observed C_te amplitude on the largest scales is discrepant at the 99.8% level
with the observed C_tt for the concordance LCDM model with tau=0.10. Using
tau=0.17, the preferred WMAP model-independent value, the discrepancy is at the
level of 98.5%.Comment: 6 pages, 6 figures, ApJ in pres
CMB-Normalized Predictions for Sunyaev-Zel'dovich effect fluctuations
We predict the level of small-scale anisotropy in the cosmic microwave
background (CMB) due to the Sunyaev--Zel'dovich (SZ) effect for the ensemble of
cosmological models that are consistent with current measurements of
large-scale CMB anisotropy. We argue that the recently reported detections of
the small-scale (arcminutes) CMB anisotropy are only marginally consistent with
being the SZ effect when cosmological models are calibrated to the existing
primary CMB data on large scales. The discrepancy is at more than 2-2.5 sigma,
and is mainly due to a lower sigma_8 <0.8 favored by the primary CMB and a
higher sigma_8 > 1 favored by the SZ effect. A degeneracy between the optical
depth to Thomson scattering and the CMB-derived value of sigma_8 suggests that
the discrepancy is reduced if the universe was reionized very early, at a
redshift of about 25.Comment: 4 pages, 2 figures, submitted to ApJ
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