Browsing by Subject "Cosmic Microwave Background"

Now showing 1 - 6 of 6
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    Bicep Array: Searching for Signals of Inflation From The South Pole
    (2022-01) Crumrine, Michael
    The $\Lambda$CDM cosmological model posits a universe that began with a big bang - like singularity, which contains mostly cold dark matter, and which is experiencing an accelerating expansion due to a dark energy component. This model has experienced resounding success and is consistently upheld by experiments across the globe. The model is incomplete however, it cannot describe how the specific initial conditions required to create the universe we see today came about. Inflation is an extension to the $\Lambda$CDM model which hypothesizes that the universe underwent a period of exponential expansion just after the big bang, sufficient to set up the required initial conditions. Most inflationary theories predict a stochastic gravitational wave background generated as a result of this expansion which would have imprinted a characteristic B-mode signal into the polarization pattern of the Cosmic Microwave Background. Detecting this primordial gravitational wave signal would provide direct evidence for inflation The \textsc{Bicep}/{\it Keck} program constitutes a series of polarization sensitive microwave telescopes situated at the geographic South Pole targeting the degree-angular scale $B$-modes and searching for a primordial signal. Over the last two decades this program has consistently reported the tightest constraints on this signal, with the most recent analysis of data through $2018$ providing an upper limit on the tensor-to-scalar ratio $r<0.036$ at $95\%$ confidence. {\sc Bicep} Array is the latest experiment in the series and replaces the {\it Keck Array}, expanding the frequency coverage to two new low-frequency bands and, once fully operational, increasing the detector count by over an order of magnitude. {\sc Bicep} Array is expected to achieve $\sigma(r) = 0.002 - 0.004$ depending on foreground complexity and the degree of lensing removal. In this dissertation I cover the design of this new experiment -- with a focus on the design and performance of the cryogenics down to $4$\,K -- and the first year's observations. I analyze the first year of new low frequency data in combination with the recently release BK18 results and find that the new data provides no improvement on $\sigma(r)$. However, it provides significant constraining power on galactic synchrotron radiation resulting in a factor of two decrease in the uncertainty on the amplitude of this foreground signal.
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    Constraining Inflation Models with the BICEP/Keck B-mode Experiments
    (2023-08) Lau, Kenny
    Modern observational Cosmology is highly developed and the observable Universe on a large scale is currently well described by the standard LCDM model. The model has a small number of free parameters and describes how the Universe evolved from a hot, dense and homogeneous state with primordial perturbations that are Gaussian, adiabatic and close to scale-invariant. The inflation paradigm extends the LCDM model and interprets the initial conditions as natural consequences of a hypothesized exponential expansion. However, generic inflationary models make a prediction that has not yet been observed: the existence of a background of primordial gravitational waves (PGWs), which would leave an imprint of B-mode polarization in the Cosmic Microwave Background (CMB). Measuring the degree-scale B-mode polarization therefore emerges as one of the most promising methods to detect or set limits on PGWs. For the past decade, the BICEP/Keck collaboration has been operating a series of telescopes at the Amundsen-Scott South Pole Station optimized for B-mode observation. These telescopes are compact refracting polarimeters mapping about 2% of the sky under the exceptionally stable and transparent atmosphere of the Geographical South Pole. They observe at a broad range of frequencies to separate the cosmological signals from polarized Galactic synchrotron and thermal dust emission which dominate at the two ends of the microwave regime. In this dissertation, we discuss the BICEP/Keck experimental progress in two major areas. We first present the "BK18 analysis" utilizing data collected up to the 2018 observing season, in conjunction with selected WMAP and Planck polarization maps. The analysis particularly exploits new data from (1) the 3-year BICEP3 map, the current deepest CMB polarization measurement at the foreground-minimum 95 GHz; and (2) the Keck 220 GHz map which has a higher signal-to-noise ratio on the dust foreground than the Planck 353 GHz map. The likelihood analysis of the BB auto- and cross-spectra of the maps reduces the experimental uncertainty on the tensor-to-scalar ratio r to sigma(r)=0.009, and the inference of r from our baseline model is tightened to r=0.014+0.010-0.011 and r<0.036 at 95% confidence, the most powerful constraints on PGWs to date. These B-mode spectra hence provide unprecedented power to discriminate among popular classes of inflation theories in the r-ns plane — the natural inflation models and the monomial power law inflation models are now strongly disfavored by the data. We subsequently discuss the instrument development of BICEP Array, a BICEP3-style multiple-frequency (30/40/95/150/220/270 GHz) telescope succeeding Keck from 2020. We focus on the construction of a novel mount and the corresponding pointing model for the telescope. The performance of both are validated by the first robust detection of cosmological polarization signals at 40 GHz in our observation field. Preliminary forecasts show that the constraint can be improved to sigma(r) < 0.003 using the upcoming BICEP3, BICEP Array and SPT-3G data up to 2027.
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    Development of Novel Anti-reflection Coatings for Millimeter/Submillimeter-wave Telescopes and Optimization of Cosmic Microwave Background Instruments
    (2021-07) Wen, Qi
    I report the development of the anti-reflection coatings (ARC) using laser ablated sub-wavelength structures (SWS) in millimeter and sub-millimeter (MSM) wavelengths. This technology provides a promising solution for broadband, cryogenically robust ARC on high-index materials (HIM) - alumina, sapphire and silicon - for a broad range of MSM telescopes. The effective behaviors of SWS are studied using numerical simulations by finite element method. We observe complex behaviors of SWS when the pitch of SWS is not negligibly small compared to the electromagnetic wavelength. Based on the study, a practical guide to design optimal SWS ARC is provided. Ultrashort pulsed lasers are used to fabricate SWS on HIM, majority of which are difficult to be modified by other traditional fabrication methods such as dicing and chemical etching. We have successfully ablated structures with height from a few hundred  m to around 2.5 mm. Excellent anti-reflection performances have been demonstrated by experimental measurements of transmittance/reflectance as well as by numerical simulations based on the measured structure shapes. Higher than 20 mm^3/min average ablation rates have been experimentally verified on alumina and sapphire through an optimization effort using a high-power picosecond laser. The demonstrated high rates strongly support the feasibility of laser ablated SWS ARC on large (>=30 cm) optical elements. A novel ablation model that relates the structure height and laser cumulative fluence is presented. Using a best-fit procedure with experimental data, for both alumina and sapphire, we find threshold fluence \theta_{th} \approx 2 J/cm^3 and average absorption length \bar{\delta} \approx 650 nm for peak fluence values between 30 and 70 J/cm^2. With the best-fit values, the model and data values for cumulative fluence agree to within 10%. The model is used to predict average ablation rate as a function of SWS height and average laser power. I also report the results of several projects that aim to optimize cosmic microwave background instruments. These projects include (1) an optical design study of cross Dragone system for PICO, the Probe of Inflation and Cosmic Origins, a next-generation space telescope; (2) a mechanical design of the focal plane for PICO; (3) a trade study on the aperture size for Tau Surveyor, a balloon-borne instrument aiming to measure the optical depth to reionization \tau; (4) development of low-conductance, lenslet coupled, multichroic bolometers for balloon-borne platforms.
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    The EBEX cryostat and supporting electronics.
    (2011-05) Sagiv, Ilan Shai
    EBEX is a balloon-borne polarimeter designed to measure the B-mode polarization of the cosmic microwave background radiation. The instrument includes a 1.5 meter Gregorian-type telescope and a cryogenic receiver housing 1432 bolometric transition edge sensor detectors operating at 0.3◦ K. In this thesis I describe my work on the development and characterization of the EBEX cryogenic system and of several electronics sub-systems. I developed CANbus-based software to monitor temperatures inside the receiver and to control the operation of two sub-Kelvin adsorption refrigerators. I commissioned and tested an experiment-wide timing system that tags data from all subsystems with an accuracy that is a factor of 10 better than required. I constructed and tested two pressure vessels that store data on board. Data collected during the EBEX test flight in June 2009 show that all these subsystems performed according to predictions. The temperatures of the cryostat were stable. An analysis of the temperature data finds no scan synchronous signal in the cryostat temperatures. The timing system and pressure vessels operated as expected. A calibrator was installed inside the receiver to monitor detector responsivity variations. I analyzed the data from the test flight and show that in its current configuration the calibrator is inadequate.
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    Foreground Cleaning for Cosmic Microwave Background Polarimeters in the Presence of Instrumental Effects
    (2015-06) Bao, Chaoyun
    The Cosmic Microwave Background (CMB) B-mode polarization signal offers a direct probe of inflation, a period of exponential expansion in the extreme early universe. The inflationary CMB B-mode polarization signal, however, is subject to the contamination of polarized galactic thermal dust foreground emission. A robust foreground cleaning method is essential for CMB polarimeters targeting the inflationary B-mode signal. In this thesis I present my work on developing foreground cleaning algorithms particularly in the presence of instrumental effects. One of the instrumental effects I focus on in this work is the frequency dependent polarization rotation effect such as the one caused by an achromatic half-wave plate (AHWP). As an example, I use the AHWP of the E and B Experiment (EBEX) in this work and study the relation between the frequency dependent rotation effect and the characteristic parameters of the AHWP. To address the effect of an AHWP while removing galactic dust foreground contamination, I developed two foreground cleaning algorithms: a simple method that assumes perfect knowledge of the AHWP and a few simplifying assumptions, and a more sophisticated algorithm based on maximum likelihood method. Based on simulation results, the maximum likelihood foreground cleaning algorithm can recover CMB B-mode signal without any bias in the presence of band shape uncertainty, frequency dependent rotation effect and instrumental noise with realistic measurement accuracy of instrumental parameters. In this thesis I also present my work on calculating the atmospheric loading in the millimeter wave regime for sub-orbital CMB experiments such as EBEX. Having a proper prediction of the atmospheric loading is an important input to detector designs for CMB experiments.
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    Gauge Field Amplification during Axion Inflation
    (2018-08) UNAL, CANER
    This thesis studies the interaction of different fields during inflation and resultant phenomenology at different scales. It particularly focuses on one of the most well motivated inflationary models, called Axion inflation. Axions are pseudo-scalars that possess the shift symmetry at least at the approximate level, which protects their potential from quantum corrections and elevates them as a strong inflaton candidate. However, in the particle inventory of UV complete theories, axion particles are abundant, which motivates studying axions as inflaton or spectator field during inflation. In this work, we study a chiral shift symmetric dimension-five operator arising naturally in any axion theory. Due to this coupling, the gauge field'Äôs dispersion relation is modified and one helicity of the gauge field is produced abundantly as a function of a dimensionless parameter proportional to speed of the axion. This breaks the parity conservation. Furthermore, this amplified gauge quanta inversely decays (ie. sources back) to scalar and tensor degrees of freedom via two-to-one way; hence, the sourced perturbations obey non-Gaussian statistics. These sourced modes leave unique imprints on cosmological observables such as : Chiral gravitational wave (GW) background, large tensor non-Gaussianity, non-zero TB and EB correlators, detectable GW background at interferometer scales and the production of primordial black holes.