1,455 research outputs found
Semi-blind Eigen-analyses of Recombination Histories Using CMB Data
Cosmological parameter measurements from CMB experiments such as Planck,
ACTpol, SPTpol and other high resolution follow-ons fundamentally rely on the
accuracy of the assumed recombination model, or one with well prescribed
uncertainties. Deviations from the standard recombination history might suggest
new particle physics or modified atomic physics. Here we treat possible
perturbative fluctuations in the free electron fraction, \Xe(z), by a
semi-blind expansion in densely-packed modes in redshift. From these we
construct parameter eigenmodes, which we rank order so that the lowest modes
provide the most power to probe the \Xe(z) with CMB measurements. Since the
eigenmodes are effectively weighed by the fiducial \Xe history, they are
localized around the differential visibility peak, allowing for an excellent
probe of hydrogen recombination, but a weaker probe of the higher redshift
helium recombination and the lower redshift highly neutral freeze-out tail. We
use an information-based criterion to truncate the mode hierarchy, and show
that with even a few modes the method goes a long way towards morphing a
fiducial older {\sc Recfast} into the new and improved {\sc
CosmoRec} and {\sc HyRec} in the hydrogen recombination
regime, though not well in the helium regime. Without such a correction, the
derived cosmic parameters are biased. We discuss an iterative approach for
updating the eigenmodes to further hone in on if large
deviations are indeed found. We also introduce control parameters that
downweight the attention on the visibility peak structure, e.g., focusing the
eigenmode probes more strongly on the \Xe (z) freeze-out tail, as would be
appropriate when looking for the \Xe signature of annihilating or decaying
elementary particles.Comment: 28 pages, 26 Fig
CMB Anisotropy in Compact Hyperbolic Universes I: Computing Correlation Functions
CMB anisotropy measurements have brought the issue of global topology of the
universe from the realm of theoretical possibility to within the grasp of
observations. The global topology of the universe modifies the correlation
properties of cosmic fields. In particular, strong correlations are predicted
in CMB anisotropy patterns on the largest observable scales if the size of the
Universe is comparable to the distance to the CMB last scattering surface. We
describe in detail our completely general scheme using a regularized method of
images for calculating such correlation functions in models with nontrivial
topology, and apply it to the computationally challenging compact hyperbolic
spaces. Our procedure directly sums over images within a specified radius,
ideally many times the diameter of the space, effectively treats more distant
images in a continuous approximation, and uses Cesaro resummation to further
sharpen the results. At all levels of approximation the symmetries of the space
are preserved in the correlation function. This new technique eliminates the
need for the difficult task of spatial eigenmode decomposition on these spaces.
Although the eigenspectrum can be obtained by this method if desired, at a
given level of approximation the correlation functions are more accurately
determined. We use the 3-torus example to demonstrate that the method works
very well. We apply it to power spectrum as well as correlation function
evaluations in a number of compact hyperbolic (CH) spaces. Application to the
computation of CMB anisotropy correlations on CH spaces, and the observational
constraints following from them, are given in a companion paper.Comment: 27 pages, Latex, 11 figures, submitted to Phys. Rev. D, March 11,
199
CMB Anisotropy in Compact Hyperbolic Universes II: COBE Maps and Limits
We calculate the CMB anisotropy in compact hyperbolic universe models using
the regularized method of images described in paper-I, including the
'line-of-sight `integrated Sachs-Wolfe' effect, as well as the last-scattering
surface terms. We calculate the Bayesian probabilities for a selection of
models by confronting our theoretical pixel-pixel temperature correlation
functions with the COBE-DMR data. Our results demonstrate that strong
constraints on compactness arise: if the universe is small compared to the
`horizon' size, correlations appear in the maps that are irreconcilable with
the observations. This conclusion is qualitatively insensitive to the matter
content of the universe, in particular, the presence of a cosmological
constant. If the universe is of comparable size to the 'horizon', the
likelihood function is very dependent upon orientation of the manifold wrt the
sky. While most orientations may be strongly ruled out, it sometimes happens
that for a specific orientation the predicted correlation patterns are
preferred over those for the conventional infinite models. The full Bayesian
analysis we use is the most complete statistical test that can be done on the
COBE maps, taking into account all possible signals and their variances in the
theoretical skies, in particular the high degree of anisotropic correlation
that can exist. We show that standard visual measures for comparing theoretical
predictions with the data such as the isotropized power spectrum are
not so useful in small compact spaces because of enhanced cosmic variance
associated with the breakdown of statistical isotropy.Comment: 29 pages, Latex, 15 figures, submitted to Phys. Rev. D, March 11,
1999. Full resolution figures can be obtained from
ftp://ftp.cita.utoronto.ca/pogosyan/prdB
- β¦