15 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
Cosmological Parameters from Pre-Planck CMB Measurements
Recent data from the WMAP, ACT and SPT experiments provide precise
measurements of the cosmic microwave background temperature power spectrum over
a wide range of angular scales. The combination of these observations is well
fit by the standard, spatially flat LCDM cosmological model, constraining six
free parameters to within a few percent. The scalar spectral index, n_s =
0.9690 +/- 0.0089, is less than unity at the 3.6 sigma level, consistent with
simple models of inflation. The damping tail of the power spectrum at high
resolution, combined with the amplitude of gravitational lensing measured by
ACT and SPT, constrains the effective number of relativistic species to be
N_eff = 3.28 +/- 0.40, in agreement with the standard model's three species of
light neutrinos.Comment: 5 pages, 4 figure
Modeling and characterization of the SPIDER half-wave plate
Spider is a balloon-borne array of six telescopes that will observe the
Cosmic Microwave Background. The 2624 antenna-coupled bolometers in the
instrument will make a polarization map of the CMB with approximately one-half
degree resolution at 145 GHz. Polarization modulation is achieved via a
cryogenic sapphire half-wave plate (HWP) skyward of the primary optic. We have
measured millimeter-wave transmission spectra of the sapphire at room and
cryogenic temperatures. The spectra are consistent with our physical optics
model, and the data gives excellent measurements of the indices of A-cut
sapphire. We have also taken preliminary spectra of the integrated HWP, optical
system, and detectors in the prototype Spider receiver. We calculate the
variation in response of the HWP between observing the CMB and foreground
spectra, and estimate that it should not limit the Spider constraints on
inflation
Pointing control for the SPIDER balloon-borne telescope
We present the technology and control methods developed for the pointing
system of the SPIDER experiment. SPIDER is a balloon-borne polarimeter designed
to detect the imprint of primordial gravitational waves in the polarization of
the Cosmic Microwave Background radiation. We describe the two main components
of the telescope's azimuth drive: the reaction wheel and the motorized pivot. A
13 kHz PI control loop runs on a digital signal processor, with feedback from
fibre optic rate gyroscopes. This system can control azimuthal speed with <
0.02 deg/s RMS error. To control elevation, SPIDER uses stepper-motor-driven
linear actuators to rotate the cryostat, which houses the optical instruments,
relative to the outer frame. With the velocity in each axis controlled in this
way, higher-level control loops on the onboard flight computers can implement
the pointing and scanning observation modes required for the experiment. We
have accomplished the non-trivial task of scanning a 5000 lb payload
sinusoidally in azimuth at a peak acceleration of 0.8 deg/s, and a peak
speed of 6 deg/s. We can do so while reliably achieving sub-arcminute pointing
control accuracy.Comment: 20 pages, 12 figures, Presented at SPIE Ground-based and Airborne
Telescopes V, June 23, 2014. To be published in Proceedings of SPIE Volume
914
High Precision Cosmology with CMB Data
In this thesis we investigate the two cosmic epochs of inflation and recombination, through their imprints on the temperature and polarization anisotropies of the cosmic microwave background radiation. To probe the early universe we develop a map-based maximum-likelihood estimator to measure the amplitude of inflation-induced gravity waves, parametrized by , from the cosmic microwave background (CMB) polarization maps. Being optimal by construction, the estimator avoids - mixing, a possible source of contamination in the tiny -mode detection, the target of many current and near future CMB experiments.
For various observational cases, we probe the dependence of measurement on the signal from different scales of and polarization.
We make forecasts for Spider-like and Planck-like experimental specifications and to investigate the sky-coverage optimization of the Spider-like case. We also use a novel information-based framework to compare how different generations of CMB experiments reveal information about the early universe, through their measurements of .
We also probe the epoch of recombination by investigating possible fluctuations in the free electron fraction \Xe around the fiducial model of the standard recombination scenario. Though theoretically well studied, the detailed assumptions in the recombination history, based on standard atomic physics, have never been directly tested. However, for our CMB-based cosmological inferences to be reliable, the recombination scenario needs to be observationally verified. We approach this problem in a model-independent way and construct rank-ordered parameter eigen-modes with the highest power to probe \Xe. We develop an information-based criterion to truncate the eigen-mode hierarchy, which can be used in similar hierarchical model selections as well. The method is applied to simulations of Planck+ACTPol and a cosmic variance limited survey with differing simulated recombination histories and recovered \Xe trajectories are constructed. We apply the method to currently available CMB datasets, WMAP9+ACT/SPT. The first constructed eigen-mode turns out to be a direct measure of the damping envelope. Its current measurement with SPT slightly indicates a damping tail anomaly, while ACT data agree well with the standard scenario. High resolution Planck data will resolve this tension with high significance.Ph