Type Ia Supernova Cosmology : Quantitative Spectral Analysis

Type Ia supernovae have been successfully used as standardized candles to study the expansion history of the Universe. In the past few years, these studies led to the exciting result of an accelerated expansion caused by the repelling action of some sort of dark energy. This result has been confirmed by measurements of cosmic microwave background radiation, the large-scale structure, and the dynamics of galaxy clusters. The combination of all these experiments points to a “concordance model” of the Universe with flat large-scale geometry and a dominant component of dark energy. However, there are several points related to supernova measurements which need careful analysis in order to doubtlessly establish the validity of the concordance model. As the amount and quality of data increases, the need of controlling possible systematic effects which may bias the results becomes crucial. Also important is the improvement of our knowledge of the physics of supernovae events to assure and possibly refine their calibration as standardized candle. This thesis addresses some of those issues through the quantitative analysis of supernova spectra. The stress is put on a careful treatment of the data and on the definition of spectral measurement methods. The comparison of measurements for a large set of spectra from nearby supernovae is used to study the homogeneity and to search for spectral parameters which may further refine the calibration of the standardized candle. One such parameter is found to reduce the dispersion in the distance estimation of a sample of supernovae to below 6%, a precision which is comparable with the current lightcurve-based calibration, and is obtained in an independent manner. Finally, the comparison of spectral measurements from nearby and distant objects is used to test the possibility of evolution with cosmic time of the intrinsic brightness of type Ia supernovae.

Barshalder 2 : Studies of late Iron Age Gotland

The prehistoric cemetery of Barshalder is located along the main road on the boundary between Grötlingbo and Fide parishes, near the southern end of the island of Gotland in the Baltic Sea. The ceme-tery was used from c. AD 1-1100. The level of publication in Swedish archaeology of the first millennium AD is low compared to, for instance, the British and German examples. Gotland’s rich Iron Age cemeteries have long been intensively excavated, but few have received monographic treatment. This publication is intended to begin filling this gap and to raise the empirical level of the field. It also aims to make explicit and test the often somewhat intuitively conceived re-sults of much previous research. The analyses deal mainly with the Migration (AD 375–540), Vendel (AD 520–790) and Late Viking (AD 1000–1150) Periods. The following lines of inquiry have been prioritised. 1. Landscape history, i.e. placing the cemetery in a landscape-historical context. (Vol. 1, section 2.2.6) 2. Migration Period typochronology, i.e. the study of change in the grave goods. (Vol. 2, chapter 2) 3. Social roles: gender, age and status. (Vol. 2, chapter 3) 4. Religious identity in the 11th century, i.e. the study of religious indicators in mortuary cus-toms and grave goods, with particular emphasis on the relationship between Scandinavian paganism and Christianity. (Vol. 2, chapter 4) Barshalder is found to have functioned as a central cemetery for the surrounding area, located on pe-ripheral land far away from contemporary settle-ment, yet placed on a main road along the coast for maximum visibility and possibly near a harbour. Computer supported correspondence analysis and seriation are used to study the gender attributes among the grave goods and the chronology of the burials. New methodology is developed to distin-guish gender-neutral attributes from transgressed gender attributes. Sub-gender grouping due to age and status is explored. An independent modern chronology system with rigorous type definitions is established for the Migration Period of Gotland. Recently published chronology systems for the Vendel and Viking Periods are critically reviewed, tested and modified to produce more solid models. Social stratification is studied through burial wealth with a quantitative method, and the results are tested through juxtaposition with several other data types. The Late Viking Period graves of the late 10th and 11th centuries are studied in relation to the contemporary Christian graves at the churchyards. They are found to be symbolically soft-spoken and unobtrusive, with all pagan attributes kept apart from the body in a space between the feet of the deceased and the end of the over-long inhumation trench. A small number of pagan reactionary graves with more forceful symbolism are however also identified. The distribution of different 11th cen-tury cemetery types across the island is used to in-terpret the period’s confessional geography, the scale of social organisation and the degree of alle-giance to western and eastern Christianity. 11th century society on Gotland is found to have been characterised by religious tolerance, by an absence of central organisation and by slow piecemeal Christianisation.

Type Ia Supernovae : Homogeneity and Diversity

The use of type Ia supernovae as distance estimators has shown that about 75% of the energy content of the universe has a negative equation of state parameter and thus, drives the acceleration of the universe. Constraining the exact nature of this energy is one of the main goals in cosmology. As the statistics of observed high-redshift supernovae increases, systematic effects become the limiting factor to pursue such investigations, thus deeper understanding of the physical properties of SNe is of great importance. In this thesis we investigate spectral homogeneity and diversity of local and high redshift supernovae. Special emphasis has been given to the analysis of optical spectra of local peculiar supernovae 1999aa and 1999ac. The study of the spectra of SN 1999aa pointed out that this SN could be a link between the extreme peculiar SN 1991T and normal SNe. Moreover, the identification of a high velocity component of Ca II and possibly of a low velocity component of C III suggests some degree of asphericity in the ejecta of this supernova. Evidence for a deflagration of a C+O white dwarf was found in the early spectra of SN 1999ac. The spectral proprieties of a vast sample of local SNe are also studied by means of newly introduced spectral indicators. These were used to possibly improve the intrinsic spread of SN peak magnitudes to 0.15 mag, independently of light curve parameters. The first quantitative comparison between local and high redshift supernova is carried out. No evidence for extreme peculiar sub-luminous SNe was found in our data set including 13 SNe with redshift range z=0.279-0.912. Furthermore, SN2002fd (z=0.279) was found to show spectral characteristics similar to SN 1991T/SN 1999aa-like supernovae. We also present a feasibility study of the Hubble diagram in rest frame I-band up to z~0.5, and show the possibility to probe the presence of intergalactic dust, which could possibly mimic the effect of dark energy in the Hubble diagram.