Universit`adegli Studi di Trieste FACOLTA` DI SCIENZE MATEMATICHE FISICHE E NATURALI Corso di Laurea Specialistica in Astrofisica e Fisica Spaziale Analysis and characterization of thermal systematic e↵ects on the PLANCK LFI data Analisi e caratterizzazione degli e↵etti sistematici di tipo termico sui dati di PLANCK - LFI Tesi di Laurea di: Relatore: Tanja Petruˇsevska Dott.ssa Anna Gregorio Correlatore: Dott. Aniello Mennella Sessione III – Anno Accademico 2009 - 2010 Contents Abstract 1 1 Cosmic Microwave Background radiation 5 1.1 Introduction . 5 1.2 Friedmann-Robertson-Walker Cosmological Model . 6 1.3 Cosmic microwave background radiation . 7 1.4 CMB anisotropies . 8 1.5 Measuring the cosmic background radiation . 16 2CMBexperiments 21 2.1 Introduction . 21 2.2 Ground based experiments . 21 2.3 Balloon-borne experiments . 24 2.4 Satellite experiments . 25 2.4.1 COBE . 25 2.4.2 WMAP ........................... 27 3 The Planck mission 33 3.1 Introduction . 33 3.2 The Planck satellite . 34 3.2.1 Planck cryogenic system . 35 3.2.2 HighFrequencyInstrument . 38 3.2.3 Low Frequency Instrument . 38 3.2.4 Systematic e↵ects . 42 3.3 Data processing . 43 4 Assessment of thermal systematic e↵ects on maps and power spectra 49 4.1 Characteristics of thermal systematic e↵ects . 51 i ii CONTENTS 4.1.1 Front-end temperature fluctuations . 51 4.1.2 Back-end temperature fluctuations . 51 4.1.3 4 K reference load temperature fluctuations . 52 4.2 Time-ordered data generation . 52 4.3 Destriping and map generation . 54 5 Results and discussion 57 5.1 Back-end temperature fluctuations . 57 5.1.1 Analysis of the period from OD 91 to OD 389 . 57 5.1.2 Analysis in the periods OD 91-115 and OD 275-389 . 65 5.2 Front-endtemperaturefluctuations . 72 5.2.1 Analysis of the period from OD 91 to OD 389 . 72 5.2.2 Analysis in the periods OD 91-115 and OD 275-389 . 80 5.3 4 K reference load temperature fluctuations . 85 5.3.1 Analysis of the period from OD 91 to OD 389 . 85 6 Conclusions and perspectives 97 Appendix A 100 Bibliography 106 Abstract La radiazione cosmica di fondo nelle microonde scoperta nel 1965 da Penzias e Wilson, `econsiderata una delle pi`uimportanti prove sperimentali a favore del modello cosmologico standard noto come Big Bang caldo. Questa radiazione, fornisce un’immagine dell’Universo quando aveva circa 380.000 anni ed ha una distribuzione spettrale di corpo nero alla temperatura T = 2.725 0.002 K, ± indice dell’equilibrio termodinamico con la materia nell’epoca quando `estata rilasciata. La radiazione cosmica di fondo presenta anisotropie a livello di 5 10− che forniscono delle informazioni preziose riguardo l’origine e l’evoluzione dell’Universo. Dopo la scoperta della radiazione di fondo, sono stati condotti decine di esperimenti per misurare la radiazione e le sue anisotropie. Nel 1992, il satellite COBE ha rivoluzionato la cosmologia rilevando queste anisotropie per la prima volta. Lanciata il 14 maggio 2009, Planck `euna missione dell’Agenzia Spaziale Europea progettata per misurare le anisotropie della radiazione cosmica di fondo con una precisione senza precedenti. Planck osserva tutto il cielo con . 6 sensibilit`a ∆T/T 2 10− , risoluzione angolare fino a 5’ e copertura in 9 bande ⇠ di frequenza da 30 a 857 GHz. Per raggiungere questi ambiziosi requisiti, Planck utilizza un sistema criogenico che ra↵redda gli strumenti fino a 0.1 K. L’elevata sensibilit`adello strumento e il complesso sistema criogenico rende lo studio degli e↵etti termici sistematici (argomento principale di questa tesi) di importanza fondamentale per il successo scientifico della missione. La tesi si articola in sei capitoli: 1. nel Capitolo 1 si presentano le propriet`adella radiazione cosmica di fondo elesueanisotropie; 2. nel Capitolo 2 si fa una breve panoramica dei vari esperimenti dedicati allo studio della radiazione cosmica di fondo; 3. nel Capitolo 3 si descrive la missione Planck, i suoi strumenti ed i suoi 1 2 Abstract scopi, in particolare lo strumento di bassa frequenza (Low Frequency Instrument, LFI) e la sua struttura termica; 4. nel Capitolo 4 si descrive l’analisi dei dati sulla stabilit`a termica dello strumento LFI di Planck. Il lavoro `estato e↵ettuato presso il dipartimento di fisica dell’Universit`adi Trieste e presso il Data Processing Centre di LFI situato presso INAF (Instituto Nazionale di AstroFisica) - OATS (Osservatorio Astronomico di Trieste). 5. nel Capitolo 5 si presentano i risultati di questa analisi; 6. nel Capitolo 6 si traggono le conclusioni e si mostrano le proposte per il futuro proseguimento del lavoro. Abstract The cosmic microwave background radiation discovered by Penzias and Wilson in 1965, is considered one of the most important experimental evidences in favour of the Hot Big Bang standard cosmological model. This radiation provides an image of the Universe when it was about 380,000 years old and has a blackbody spectral distribution at temperature T=2.725 0.002 K, ± index of thermodynamic equilibrium with the mater in epoch when it was released. The cosmic background radiation presents anisotropies at level of 5 10− which provide valuable information about the origin and the evolution of the Universe. After the discovery of background radiation, tens of experiments have been performed to measure this radiation and its anisotropies. In 1992 the COBE satellite revolutionized cosmology by detecting temperature anisotropies for the first time. Launched on May 14, 2009, Planck is a European Space Agency mission designed to measure the CMB anisotropies with an accuracy set by fundamental astrophysical limits. To do this, Planck is imaging the whole sky . 6 with an unprecedented combination of sensitivity (∆T/T 2 10− ), angular ⇠ resolution (to 5’), and 9 frequency coverage (30 857 GHz). To reach these − ambitious requirements, Planck uses an active cryogenic thermal system which cools the instruments to 0.1 K. The high sensitivity of the instrument and the cryogenic system makes the thermal systematic e↵ects study of crucial importance to the scientific success. The thesis is divided into six chapters: 1. Chapter 1 explains the properties of the cosmic background radiation and its anisotropies; 2. Chapter 2 presents a short overview of the various experiments dedicated to the study of cosmic background radiation; 3. Chapter 3 describes the Planck mission, its instruments and its goals, in 3 4 Abstract particular the Low Frequency Instrument (LFI) and its thermal structure; 4. Chapter 4 discusses the analysis of the LFI thermal stability. This work was carried out in the Physics department at the University of Trieste, at the LFI Data Processing Centre located at INAF (Instituto Nazionale di AstroFisica) - OATS (Astronomical Observatory of Trieste); 5. Chapter 5 presents the results of this analysis; 6. in Chapter 6 conclusions are drawn and proposals for future work are discussed. Chapter 1 Cosmic Microwave Background radiation 1.1 Introduction In the past few decades, the field of cosmology has entered the era of the so called precision cosmology, going from a science in need for data in which often highly speculative theories went unconstrained to a research driven by experiments where many models have been ruled out leaving space for the standard cosmology. The Cosmic Microwave Background (CMB) is at the centre of this revolution as the one of the main instruments of the modern cosmology. The standard cosmological model Hot Big Bang currently represents the best model to describe the origin and the evolution of the Universe. It is based on three observational evidences: the expansion of the Universe, the primordial nucleosynthesis and the cosmic microwave background. In this model the Universe began in a very hot, dense state from which it expanded and cooled. When the Universe was one millionth of its present size, the temperature would have been about 3 000 000 K: a temperature high enough that the typical energy of a photon in the thermal distribution was considerably more than the ionization energy of hydrogen atom. The Universe at that time was therefore a sea of free nuclei and electrons, and because photons interact strongly with free electrons via Thomson scattering, the mean free path of the photons was short. As the Universe expanded and cooled, the photons lost energy and became less and less able to ionize the forming atoms. Eventually all the electrons found their way into the ground state and the photons were no longer able to interact at all. When the Universe was 380000 years old, 5 6 Chapter 1. Cosmic Microwave Background radiation it switched from being opaque to being completely transparent. Then, the photons were able to travel undisturbed for the rest of the Universe evolution. These photons are what today is observed as cosmic microwave background radiation. The CMB is present today as a 2.7 K thermal background and it represents the most distant direct image of the Universe one can see. The microwave background visible today was once in thermal equilibrium with the primordial plasma of the Universe, and the Universe at that time was highly uniform. At the last-scattering redshift, gravitational instability theory says that fractional density perturbations must have existed in order for galaxies and clusters to form. The study of small temperature and polarisation fluctuations in the microwave background, which are only a few parts in 105 over the sky, are reflecting small variations in density and velocity in the early Universe, and have the potential to provide precise constraints on the overall properties of the Universe. Table 1.1: History of the early Universe evolution with the redshifts and times of the most relevant events for the CMB.