Small-Scale Anisotropies of the Cosmic Microwave Background: Experimental and Theoretical Perspectives Eric R. Switzer A DISSERTATION PRESENTED TO THE FACULTY OF PRINCETON UNIVERSITY IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY RECOMMENDED FOR ACCEPTANCE BY THE DEPARTMENT OF PHYSICS [Adviser: Lyman Page] November 2008 c Copyright by Eric R. Switzer, 2008. All rights reserved. Abstract In this thesis, we consider both theoretical and experimental aspects of the cosmic microwave background (CMB) anisotropy for ℓ > 500. Part one addresses the process by which the universe first became neutral, its recombination history. The work described here moves closer to achiev- ing the precision needed for upcoming small-scale anisotropy experiments. Part two describes experimental work with the Atacama Cosmology Telescope (ACT), designed to measure these anisotropies, and focuses on its electronics and software, on the site stability, and on calibration and diagnostics. Cosmological recombination occurs when the universe has cooled sufficiently for neutral atomic species to form. The atomic processes in this era determine the evolution of the free electron abundance, which in turn determines the optical depth to Thomson scattering. The Thomson optical depth drops rapidly (cosmologically) as the electrons are captured. The radiation is then decoupled from the matter, and so travels almost unimpeded to us today as the CMB. Studies of the CMB provide a pristine view of this early stage of the universe (at around 300,000 years old), and the statistics of the CMB anisotropy inform a model of the universe which is precise and consistent with cosmological studies of the more recent universe from optical astronomy. The recombination history is the largest known uncertainty in the standard cosmological model for CMB anisotropy formation. Here we investigate the formation of neutral helium during cos- mological recombination and find that it is significantly different than was previously understood. We show that several new effects produce a modest ( 0.5% at ℓ = 3000) change in the CMB anisotropy that should be included in the analysis of data∼ from small-scale anisotropy experiments. These small scales contain further information from the primary CMB and from interactions of the CMB with intervening matter as it travels from the recombination era to us today. This information will improve constraints on the spectral slope of primordial perturbations, the baryon density, and possibly dark energy. The methods described in the second half of this thesis are part of the ACT analysis pipeline. iii Acknowledgments The Princeton cosmology community has historically supported a wide range of research, from experimental investigations of the cosmic microwave background to theoretical studies of the ear- liest moments of the universe. This outlook was established by the “gravity group” of Dicke and others in the 1960s. The work on cosmological recombination in Chapter 2 is an extension of the theory described by P. J. E. Peebles at Princeton in 1968, and the largest correction described in this thesis was also proposed by Peebles in the 1990s. The Atacama Cosmology Telescope, de- scribed in the second half of this thesis, is also part of the “gravity group.” The research described here has been possible only because of this diverse community. I am greatly indebted to Lyman Page and Chris Hirata for bringing me on board with the gravity group. I appreciate their guidance, patience, and instruction over the last few years. The goal of the chapters describing ACT is to document work by the collaboration, and work I did for the collaboration. The construction of ACT required the effort, insight, planning, and pa- tience of many extraordinary people. Chapter 3 describes the ACT telescope systems. Develop- ment of these systems has involved nearly everyone in the experimental part of the collaboration. The telescope motion control took great effort to achieve and I would like to specially acknowledge everyone who has spent time uncovering the riddles of “the pendant”: M. Cozza, M. Devlin, R. Dunner,¨ J. Funke, J. Fowler, A. Hincks, J. Klein, and M. Limon. Chapter 3 focuses on the software systems, conceived and coordinated by J. Fowler. J. Fowler and A. Hincks integrated the telescope robotic systems, encoders, synchronization pulses, and raw detector data into a tightly interlocking system that produces the data that are read directly into the mapmaking pipeline. A. Hincks, T. Marriage, and M. Nolta developed the system described in Chapter 3 that schedules all telescope activities. M. Nolta developed the system to track files from when they are acquired to when they are archived at Princeton. M. Amiri, B. Burger, M. Halpern, and M. Hasselfield were the primary developers of the firmware and software for the camera data acquisition systems described here. My role in the software systems-level effort was to develop a message passing system that relays scheduled commands or operator requests to the data acquisition and housekeeping systems, where they are interpreted and executed. The housekeeping systems (described in Chapter 3) have involved a large number of people, both for the work with the prototype camera and the three-band ACT camera that is now deployed in the field. The work described here was completed in collaboration with A. Aboobaker, M. Devlin, T. Devlin, S. Fletcher, A. Hincks, L. Page, M. Kaul, J. Klein, J. Lau, D. Marsden, B. Netterfield, A. Sederberg, and D. Swetz. We were fortunate to inherit much of the ACT housekeeping from BLAST, in particular through M. Devlin, A. Hincks, J. Klein, and B. Netterfield. I have worked closely with this group to develop the thermal control and cryogenic cycling systems (electronics and software) used both in prototype and final ACT cameras that allow the camera to operate autonomously under remote control. I would like to thank and acknowledge J. Appel, R. Fisher, J. Fowler, A. Hincks, T. Marriage, M. Niemack, B. Reid, A. Sederberg, J. Sievers, S. Staggs, and D. Swetz for their collaboration with the work described in Chapter 5 describing the calibration of the 145 GHz camera. M. Niemack laid the groundwork for understanding the time constants and efficiency in the lab and in the field. J. Appel, R. Fisher, J. Fowler, A. Hincks, T. Marriage, B. Reid, T. Marriage and I then built up a set of standard calibration tools for the analysis pipeline. A. Hincks has developed an independent pipeline specifically to understand the beams, pointing, and planet amplitudes. The calibration work described here draws on all of these efforts. Finally, mapmaking (which is described in the last chapter of this thesis) has been a large collaborative effort. R. Dunner,¨ R. Fisher, A. Hincks, T. Marriage, J. Sievers and I have worked together to understand the different types of data patholo- iv gies, and to find ways that they can be identified and treated. The core map estimation has been led by R. Lupton, T. Marriage, and D. Spergel, and is broken into subgroups for the atmosphere, pointing and calibration. I would like to thank D. Spergel for discussions and methods described in the mapmaking section and appendix. The core analysis work described here concerns calibra- tion. It has been a pleasure to work with the collaboration as a whole. We’ve dealt with a num- ber of surprises, and even in the hardest, most frustrating times at 5190 m where several difficult systems needed attention, a strong sense of dedication to solving the problem and an adven- turous spirit prevailed. In addition to Lyman Page, my advisor, we have a tight-knit group, and I’ve learned much from and enjoyed working with Mark Devlin, Joe Fowler, Mark Halpern, Norm Jarosik, Michele Limon, Jeff Klein, David Spergel and Suzanne Staggs. Aside from work, I’ve had some great officemates, coworkers and mentors in the lab: Asad Aboobaker, John Appel, Adam Hincks, Tom Essinger-Hileman, Ryan Fisher, Lewis Hyatt, Judy Lau, Jeff McMahon, Glen Nixon, and Yue Zhao. I would also like to thank and acknowledge the administrative staff, machinists, and information technologies staff of the physics department. This work has made considerable use of the department computing cluster. I would like to thank C. Champagne especially for his work (and patience) with large lists of obscure components that are now part of the telescope. The hallway is an unappreciated space in departments. Jeff McMahon and Chris Hirata con- vinced me (indirectly) to join ACT and work on recombination during conversations we had in the hallway. It’s a great experience not only to interact with people who are passionate about their work, but also to discover new work directions to be passionate about. I would like to thank Jo Dunkley, Ryan Fisher, Adam Hincks, Chris Hirata, Toby Marriage, Michael Niemack, and Sergio Verdu for critically reading different parts of this thesis. The primary readers, Lyman Page and Suzanne Staggs, provided many comments that were both insightful and detailed, not only in one draft, but through several revisions. I would like to thank Andrew Bazarko, Peter Meyers, and Urosˇ Seljak for their guidance early in the graduate program and in teaching. My graduate work was primary conducted from Princeton, but I would like to thank Scott Do- delson (Fermilab), Salman Habib and Katrin Heitmann (Los Alamos), the organizers of the Trieste numerical cosmology workshop (Urosˇ Seljak) and the Max Planck recombination workshop (J. Chluba, C. Hernandez-Monteagudo, J. A. Rubino-Martin, R. Sunyaev) for guidance and discus- sions. I would also like to thank the community in San Pedro de Atacama (Chile) and Astro Norte for their hospitality. Much of the work with housekeeping was done at Penn. I’d like to thank Dan Swetz for hosting me a few days there during some critical parts of the setup. I would also like to thank Lyman and Lisa for their hospitality for the last part of the thesis work.