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Polarimetry in Astrophysics and Cosmology

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Polarimetry in Astrophysics and Cosmology POLARIMETRY IN ASTROPHYSICS AND COSMOLOGY by Lingzhen Zeng A dissertation submitted to The Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy. Baltimore, Maryland June, 2012 c Lingzhen Zeng 2012 All rights reserved Abstract Astrophysicists are mostly limited to passively observing electromagnetic radia- tion from a distance, which generally shows some degree of polarization. Polariza- tion often carries a wealth of information on the physical state and geometry of the emitting object and intervening material. In the microwave part of the spectrum, polarization provides information about galactic magnetic fields and the physics of interstellar dust. The measurement of this polarized radiation is central to much modern astrophysical research. The first part of this thesis is about polarimetry in astrophysics. In Chapter 1, I review the basics of polarization and summarize the most important mechanisms that generate polarization in astrophysics. In Chapter 2, I describe the data analysis of polarization observation on M17 (a young, massive star formation region in the Galaxy) from Caltech Submillimeter Observatory (CSO) and show the physics that we learn about M17 from the polarimetry. Polarimetry also plays an important role in modern cosmology. Inflation theory predicts two types of polarization in the Cosmic Microwave Background (CMB) radi- ation, called E-modes and B-modes. Measurements to date of the E-mode signal are consistent with the predictions of anisotropic Thompson scattering, while the B-mode signal has yet to be detected. The B-mode power spectrum amplitude can be param- eterized by the relative amplitude of the tensor to scalar modes r. For the simplest inflation models, the expected deviation from scale invariance (n = 0.963 0.012) is s ± coupled to gravitational waves with r 0.1. These considerations establish a strong ≈ 32 motivation to search for this remnant from when the universe was about 10− seconds ii old. The second part of this thesis is about the Cosmology Large Angular Scale Sur- veyor (CLASS) experiment, that is designed to have an unprecedented ability to detect the B-mode polarization to the level of r 0.01. Chapter 3 is an introduction ≤ to cosmology, including the big bang theory, inflation, ΛCDM model and polariza- tion of the CMB radiation. Chapter 4 is about CLASS, including science motivation, instrument optimization and lab testing. Advisor: Prof. Charles L. Bennett Second reader: Prof. Tobias Marriage iii Acknowledgements The work described in this thesis would not have been possible without the support of many people. Foremost, I would like to express my sincere gratitude to my advisor Prof. Chuck Bennett for the continuous support of my Ph.D study and research, for his patience, motivation, enthusiasm, and immense knowledge. His guidance helped me in all the time of research and writing of this thesis. I could not have imagined having a better advisor and mentor for my Ph.D study. Besides my advisor, I would like to thank Dave Chuss. In many research projects, I have been aided for many years by him. Dave is patient and always ready to discuss whatever problems are on my mind. I would like to thank Prof. Giles Novak, who offered me much advice and insight on the millimeter/submillimeter polarimetry. I will miss the time when we worked together on Mauna Kea summit. I gratefully acknowledge Prof. Toby Marriage for his valuable advice in lab dis- cussions, supervision on lab instrument development. I would also like to thank Toby for his great help in my job application. I would like to thank David Larson and Joseph Eimer. We worked together for many years and have so many useful discussions and collaborations. My sincere thanks also goes to Ed Wollack, John Vaillancourt, George Voellmer, Gary Hinshaw, John Karakla, Karwan Rostem, Tom Essinger-Hileman and Paul Mirel for offering me help and discussions on the various research projects. I thank my fellow graduate/undergraduate students in the research group at Johns Hopkins University: Dominik Gothe, Zhilei Xu, Aamir Ali, Dave Holtz, Connor Hen- ley and Tiffany Wei for the fun and proud of working together on the CLASS project. It is a pleasure to thank my friends at JHU for making my life fun: Jiming Shi, iv Jianjun Jia, Jun Wu, Zhouhan Liang, Jian Su, Sunxiang Huang, Yuan Yuan, Longzhi Lin, Hao Chang, Di Yang, Xin Guo, Jie Chen, Xiulin Sun, Jianhua Yu, Xin Yu, Wen Wang, Hui Gao, Jinsheng Li, Jiarong Hong and Yuan Lu. I am grateful to many others for making my time at JHU enjoyable. Unfortunately, there are too many to name individually. I would also like to thank my undergraduate classmates: Huaze Ding, Xiao Hu and Jun Li for our longtime friendship. I wish all of you the best in the future. Last but not the least, I would like to thank my family: my parents Xiangxiong Zeng and Qiuying Li, for giving birth to me at the first place and supporting me spiritually throughout my life, and my sister Lingfang Zeng and brother Lingyao Zeng, for their understanding and support in so many years. v Contents Abstract ii Acknowledgements iv List of Tables ix List of Figures x I Polarimetry in Astrophysics 1 1 Introduction to Polarization in Astrophysics 2 1.1 PlaneWave ................................ 2 1.2 StokesParameters ............................ 3 1.3 Poincar´eSphere.............................. 7 1.4 Polarization in Astrophysics . 8 1.4.1 Synchrotron Emission . 8 1.4.2 Thermal Dust Emission and Absorption . 10 1.4.3 Examples of Polarization from Absorption and Scattering . 13 1.4.4 Anomalous Dust Emission . 15 2 Submillimeter Polarimetry of M17 16 2.1 Introduction to Submillimter Polarimetry . 16 2.2 Polarimetry at Caltech Submillimeter Observatory . 17 2.3 SHARP Data Pipeline . 20 2.4 IntroductiontoM17 ........................... 22 2.5 M17PolarimetryResults......................... 23 2.5.1 GeneralResults.......................... 23 2.5.2 Polarization Spectrum . 27 2.5.3 Spatial Distribution of Magnetic Field and Polarization Spectrum 30 2.5.4 Conclusion . 38 vi II Polarimetry in Cosmology 39 3 Introduction to Polarization in Cosmology 40 3.1 The Big Bang Theory . 40 3.1.1 The Expanding Universe–Hubble’s Law . 40 3.1.2 Big Bang Nucleosynthesis (BBN) . 41 3.1.3 The Cosmic Microwave Background (CMB) Radiation . 42 3.1.4 OtherEvidence .......................... 42 3.2 CosmicInflation.............................. 43 3.2.1 TheStructureProblem. .. .. 43 3.2.2 The Flatness Problem . 44 3.2.3 The Horizon Problem . 44 3.2.4 The Magnetic Monopole Problem . 44 3.3 ΛCDMCosmologicalModel ....................... 45 3.3.1 Cosmological Principles and FLRW metric . 45 3.3.2 Einstein Field Equations and Friedmann Equation . 47 3.3.3 Best-fitΛCDMModelParameters. 49 3.4 The Cosmic Microwave Background Radiation . 54 3.4.1 TheCMBAnisotropy....................... 56 3.4.2 The CMB Polarization . 57 4 The Cosmology Large Angular Scale Surveyor (CLASS) 62 4.1 Scientific Overview . 64 4.2 Sensitivity Calculation and Bandpass Optimization . 67 4.2.1 Sensitivity Calculation . 68 4.2.2 Bandpass Optimization . 71 4.3 The Variable-delay Polarization Modulator . 79 4.3.1 Polarization Transfer Function . 80 4.3.2 VPM Grid Optimization . 81 4.3.3 VPM Mirror Throw Optimization . 83 4.3.4 VPMEfficiency.......................... 89 4.3.5 CurrentStatus .......................... 91 4.4 CLASSOptics............................... 92 4.5 Smooth-walledFeedhorn ......................... 98 4.5.1 Smooth-walled Feedhorn Optimization . 98 4.5.2 Smooth-walledFeedhornforCLASS. 102 4.6 CLASSDetectors ............................. 111 4.6.1 FocalPlane ............................ 111 4.6.2 TES Bolometers . 113 4.7 LabSetupforDetectorTesting . 116 4.7.1 Cryostat.............................. 116 4.7.2 Thermometry ........................... 118 vii 4.7.3 CryostatPerformance . 118 4.7.4 DetectorReadout......................... 119 A M17 Polarization Data 125 A.1 PolarziationSpectrum: 450umvs60um . 125 A.2 PolarziationSpectrum: 450umvs100um . 127 A.3 Polarziation Spectrum: 450 um vs 350 um at RA > 18h17m30s . 129 A.4 Polarziation Spectrum: 450 um vs 350 um at RA < 18h17m30s . 130 A.5 PolarizationVectors ........................... 132 B Blackbody Radiation 136 C NEP of Photons in a Blackbody Radiation Field 138 D A Low Cross-Polarization Smooth-Walled Horn with Improved Band- width 140 D.1 Smooth-walled Feedhorn Optimization . 141 D.1.1 Beam Response Calculation . 141 D.1.2 Penalty Function . 142 D.1.3 Feedhorn Optimization . 143 D.2 FeedhornFabricationandMeasurement. 145 D.3 Conclusion................................. 148 E CLASS 40 GHz Feedhorn Profile 153 F Lab Cryostat Thermometry Codes 159 Vita 189 viii List of Tables 2.1 SHARPInstrumentSpecifications . 19 2.2 M17PolarizationSpectrumData . 30 3.1 Best-fitΛCDMModelParameters. 50 4.1 CLASSScientificOverview........................ 66 4.2 CLASSDetectorParameters. 70 4.3 CLASS VPM Mirror Throw Optimization . 88 4.4 CLASSOpticsOverview ......................... 94 4.5 CLASS40GHzFeedhornRequirements . 102 4.6 Feedhorn Profile Approximation (in Millimeters) . 104 4.7 FeedhornPerformance . .. .. 105 4.8 BeamParameters............................. 110 4.9 CryostatThermometryReadout. 122 D.1 Spline Approximation to Optimized Profile (in Millimeters) . 148 D.2 BeamParameters............................. 150 ix List of Figures 1.1 A simple plane wave. The electric (E, in x-z plane) and magnetic field (B, in y-z plane) is perpendicular to each other and to the direction of propagation(z)..............................
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