56 research outputs found
Bayesian analysis of an anisotropic universe model: systematics and polarization
We revisit the anisotropic universe model previously developed by Ackerman,
Carroll and Wise (ACW), and generalize both the theoretical and computational
framework to include polarization and various forms of systematic effects. We
apply our new tools to simulated WMAP data in order to understand the potential
impact of asymmetric beams, noise mis-estimation and potential Zodiacal light
emission. We find that neither has any significant impact on the results. We
next show that the previously reported ACW signal is also present in the 1-year
WMAP temperature sky map presented by Liu & Li, where data cuts are more
aggressive. Finally, we reanalyze the 5-year WMAP data taking into account a
previously neglected (-i)^{l-l'}-term in the signal covariance matrix. We still
find a strong detection of a preferred direction in the temperature map.
Including multipoles up to l=400, the anisotropy amplitude for the W-band is
found to be g = 0.29 +- 0.031, nonzero at 9 sigma. However, the corresponding
preferred direction is also shifted very close to the ecliptic poles at (l,b)=
(96,30), in agreement with the analysis of Hanson & Lewis, indicating that the
signal is aligned along the plane of the solar system. This strongly suggests
that the signal is not of cosmological origin, but most likely is a product of
an unknown systematic effect. Determining the nature of the systematic effect
is of vital importance, as it might affect other cosmological conclusions from
the WMAP experiment. Finally, we provide a forecast for the Planck experiment
including polarization.Comment: 9 pages, 8 figure
Cosmological Parameters from CMB Maps without Likelihood Approximation
We propose an efficient Bayesian MCMC algorithm for estimating cosmological
parameters from CMB data without use of likelihood approximations. It builds on
a previously developed Gibbs sampling framework that allows for exploration of
the joint CMB sky signal and power spectrum posterior, P(s,Cl|d), and addresses
a long-standing problem of efficient parameter estimation simultaneously in
high and low signal-to-noise regimes. To achieve this, our new algorithm
introduces a joint Markov Chain move in which both the signal map and power
spectrum are synchronously modified, by rescaling the map according to the
proposed power spectrum before evaluating the Metropolis-Hastings accept
probability. Such a move was already introduced by Jewell et al. (2009), who
used it to explore low signal-to-noise posteriors. However, they also found
that the same algorithm is inefficient in the high signal-to-noise regime,
since a brute-force rescaling operation does not account for phase information.
This problem is mitigated in the new algorithm by subtracting the Wiener filter
mean field from the proposed map prior to rescaling, leaving high
signal-to-noise information invariant in the joint step, and effectively only
rescaling the low signal-to-noise component. To explore the full posterior, the
new joint move is then interleaved with a standard conditional Gibbs sky map
move. We apply our new algorithm to simplified simulations for which we can
evaluate the exact posterior to study both its accuracy and performance, and
find good agreement with the exact posterior; marginal means agree to less than
0.006 sigma, and standard deviations to better than 3%. The Markov Chain
correlation length is of the same order of magnitude as those obtained by other
standard samplers in the field.Comment: 9 pages, 3 figures, Published in Ap
Lorentz Violation in Goldstone Gravity
We consider a theory of gravity in which a symmetric two-index tensor in
Minkowski spacetime acquires a vacuum expectation value (vev) via a potential,
thereby breaking Lorentz invariance spontaneously. When the vev breaks all the
generators of the Lorentz group, six Goldstone modes emerge, two linear
combinations of which have properties that are identical to those of the
graviton in general relativity. Integrating out massive modes yields an
infinite number of Lorentz-violating radiative-correction terms in the
low-energy effective Lagrangian. We examine a representative subset of these
terms and show that they modify the dispersion relation of the two propagating
graviton modes such that their phase velocity is direction-dependent. If the
phase velocity of the Goldstone gravitons is subluminal, cosmic rays can emit
gravi-Cherenkov radiation, and the detection of high-energy cosmic rays can be
used to constrain these radiative correction terms. Test particles in the
vicinity of the Goldstone gravitons undergo longitudinal oscillations in
addition to the usual transverse oscillations as predicted by general
relativity. Finally, we discuss the possibility of having vevs that do not
break all six generators and examine in detail one such theory.Comment: 14 page
Demonstration of magnetic field tomography with starlight polarization towards a diffuse sightline of the ISM
The availability of large datasets with stellar distance and polarization
information will enable a tomographic reconstruction of the
(plane-of-the-sky-projected) interstellar magnetic field in the near future. We
demonstrate the feasibility of such a decomposition within a small region of
the diffuse ISM. We combine measurements of starlight (R-band) linear
polarization obtained using the RoboPol polarimeter with stellar distances from
the second Gaia data release. The stellar sample is brighter than 17 mag in the
R band and reaches out to several kpc from the Sun. HI emission spectra reveal
the existence of two distinct clouds along the line of sight. We decompose the
line-of-sight-integrated stellar polarizations to obtain the mean polarization
properties of the two clouds. The two clouds exhibit significant differences in
terms of column density and polarization properties. Their mean
plane-of-the-sky magnetic field orientation differs by 60 degrees. We show how
our tomographic decomposition can be used to constrain our estimates of the
polarizing efficiency of the clouds as well as the frequency dependence of the
polarization angle of polarized dust emission. We also demonstrate a new method
to constrain cloud distances based on this decomposition. Our results represent
a preview of the wealth of information that can be obtained from a tomographic
map of the ISM magnetic field.Comment: 25 pages, 14 figures, published in ApJ, data appear in journa
Cross-correlating Carbon Monoxide Line-intensity Maps with Spectroscopic and Photometric Galaxy Surveys
Line-intensity mapping (LIM or IM) is an emerging field of observational
work, with strong potential to fit into a larger effort to probe large-scale
structure and small-scale astrophysical phenomena using multiple complementary
tracers. Taking full advantage of such complementarity means, in part,
undertaking line-intensity surveys with galaxy surveys in mind. We consider the
potential for detection of a cross-correlation signal between COMAP and blind
surveys based on photometric redshifts (as in COSMOS) or based on spectroscopic
data (as with the HETDEX survey of Lyman- emitters). We find that
obtaining accuracy in redshifts and
sources per Mpc with spectroscopic redshift determination
should enable a CO-galaxy cross spectrum detection significance at least twice
that of the CO auto spectrum. Either a future targeted spectroscopic survey or
a blind survey like HETDEX may be able to meet both of these requirements.Comment: 19 pages + appendix (31 pages total), 16 figures, 6 tables; accepted
for publication in Ap
Foreground Separation and Constraints on Primordial Gravitational Waves with the PICO Space Mission
PICO is a concept for a NASA probe-scale mission aiming to detect or
constrain the tensor to scalar ratio , a parameter that quantifies the
amplitude of inflationary gravity waves. We carry out map-based component
separation on simulations with five foreground models and input values
and . We forecast determinations using a
Gaussian likelihood assuming either no delensing or a residual lensing factor
= 27%. By implementing the first full-sky, post
component-separation, map-domain delensing, we show that PICO should be able to
achieve = 22% - 24%. For four of the five foreground models we
find that PICO would be able to set the constraints r < 1.3 \times 10^{-4}
\,\, \mbox{to} \,\, r <2.7 \times 10^{-4}\, (95\%) if , the
strongest constraints of any foreseeable instrument. For these models,
is recovered with confidence levels between and
. We find weaker, and in some cases significantly biased, upper
limits when removing few low or high frequency bands. The fifth model gives a
detection when and a bias with .
However, by correlating determinations from many small 2.5% sky areas with
the mission's 555 GHz data we identify and mitigate the bias. This analysis
underscores the importance of large sky coverage. We show that when only low
multipoles are used, the non-Gaussian shape of the true
likelihood gives uncertainties that are on average 30% larger than a Gaussian
approximation.Comment: 34 pages, 13 figures, published in JCA
Joint power spectrum and voxel intensity distribution forecast on the CO luminosity function with COMAP
We develop a framework for joint constraints on the CO luminosity function
based on power spectra (PS) and voxel intensity distributions (VID), and apply
this to simulations of COMAP, a CO intensity mapping experiment. This Bayesian
framework is based on a Markov chain Monte Carlo (MCMC) sampler coupled to a
Gaussian likelihood with a joint PS + VID covariance matrix computed from a
large number of fiducial simulations, and re-calibrated with a small number of
simulations per MCMC step. The simulations are based on dark matter halos from
fast peak patch simulations combined with the
model of Li et al. (2016). We find that the relative power to constrain the CO
luminosity function depends on the luminosity range of interest. In particular,
the VID is more sensitive at both small and large luminosities, while the PS is
more sensitive at intermediate luminosities. The joint analysis is superior to
using either observable separately. When averaging over CO luminosities ranging
between , and over 10 cosmological realizations
of COMAP Phase 2, the uncertainties (in dex) are larger by 58 % and 30 % for
the PS and VID, respectively, when compared to the joint analysis (PS + VID).
This method is generally applicable to any other random field, with a
complicated likelihood, as long a fast simulation procedure is available.Comment: 13 pages, 5 figures. As accepted to Ap
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