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  • Pulkkinen, Petri (Helsingin yliopisto, 2014)
    Nanoparticles of gold, copper and copper sulfide with different kinds of protective ligands were successfully synthesized and characterized. Of special interest were particles protected with calixarenes, and their complexation with guest molecules. The copper sulfide nanocrystals were protected with benzyl thiol moieties that were generated by reducing RAFT chain transfer agent in the particle synthesis. The nanocrystals were characterized in detail and their sintering on paper was studied. The as-prepared crystals were in Cu2S (chalcocite) form, but thermal sintering resulted in the consolidation of the crystallites as well as the transformation of the material into a semiconductive Cu1.8S (digenite) film. The metallic copper nanoparticles were obtained by reducing copper salts in the presence of polyethylene imine (PEI) or analogous small organic compounds (tetraethylene pentamine, TEPA). In the case of the polymer, the strong complexation of the copper ions into the polymer matrix retarded the reduction reaction enabling the preparation of crystallites with less than 10 nm diameter. The particles were coated with a surface oxide layer that catalyzed the thermal decomposition of the PEI protecting layer, resulting in low sintering temperatures for the nanoparticles. The sacrificial decomposition of PEI resulted in reduction of the oxide layer back into metallic copper. The particles were sintered on paper and the layers were found to be semiconductive. Calix[4]arene protected gold nanoparticles were prepared and the interactions of pyridinium with the calixarene cavity was examined in great detail. It was found that the calixarenes bound to the gold surface are able to complex pyridinium cations, and that the complexation induced the aggregation of the nanoparticles. Further, the complexing ability of the calixarene could be tuned by preparing mixed monolayers of calixarenes and alkanethiols: key factors were the amount of calixarene in the mixed monolayer, the calixarene type and the length of the calixarene spacer, as well as the length of the alkanethiol chain. Finally, alkanethiol protected gold nanoparticles were prepared, inkjetted and sintered to form an electrode on paper. The results indicated the electrode was of comparable electronic quality to gold electrodes printed onto glass substrates. In this way, gold nanoparticles can be used in inexpensive roll-to-roll printed paper-based electronic platforms.
  • Häme (née Häkkinen), Silja (Helsingin yliopisto, 2015)
    Aerosol particles are important atmospheric constituents. They exist in both polluted and remote areas but the sizes and concentrations of these particles vary greatly depending on location. Aerosol particles damage human health via inhalation, reduce visibility with high mass loadings, and among all, contribute to climate change. Particles directly scatter and absorb solar radiation. In addition, particles that are large enough can participate in cloud formation and affect cloud properties by acting as cloud condensation nuclei (CCN). A notable fraction of submicron atmospheric aerosol mass consists of organic compounds, and a large fraction of this material has been formed through condensation of organic vapors onto aerosol particles (secondary organic aerosol, SOA). Most of the global SOA mass is deemed to be biogenic in origin, but recent studies suggest that a significant fraction of it may be controlled by anthropogenic pollution. However, due to poor understanding of this anthropogenic enhancement in biogenic SOA formation, it is not systematically accounted for in current atmospheric models. Due to these kind of uncertainties in global SOA mass burden and lack of detailed knowledge of chemical, physical and optical properties of SOA, estimates of organic aerosol effect on the climate are highly uncertain. To decrease the uncertainty in the climate effects of the organic aerosol, an improved understanding of the formation mechanisms and properties of SOA is needed. In addition, nanoparticle growth to CCN-sizes by condensation of secondary organic matter needs to be accurately described in atmospheric models. In this thesis the formation of SOA is investigated in the presence of both biogenic and anthropogenic compounds. The chemical and physical properties volatility and hygroscopicity of SOA are examined via field and laboratory experiments combined with process modeling. The thesis introduces improvements for the treatment of SOA related to nanoparticle growth in atmospheric models and evaluates their performance. The thesis shows that interactions between atmospheric biogenic and anthropogenic aerosol components can form aerosol material of low-volatility. For instance organic salt formation via chemical reactions between organic acids and inorganic salts can lower aerosol volatility. Particulate-phase processing may also alter aerosol hygroscopic properties. Description of nanoparticle growth by low-volatility secondary organics is important in improving the estimates of particle and CCN numbers. The thesis highlights the significance of biogenic organic matter formed under anthropogenic influence in the nanoparticle growth. This warrants future studies focusing on the formation mechanisms and properties of anthropogenically driven biogenic organic aerosol.
  • Pirkkalainen, Kari (Helsingin yliopisto, 2011)
    X-ray synchrotron radiation was used to study the nanostructure of cellulose in Norway spruce stem wood and powders of cobalt nanoparticles in cellulose support. Furthermore, the growth of metallic clusters was modelled and simulated in the mesoscopic size scale. Norway spruce was characterized with x-ray microanalysis at beamline ID18F of the European Synchrotron Radiation Facility in Grenoble. The average dimensions and the orientation of cellulose crystallites was determined using x-ray microdiffraction. In addition, the nutrient element content was determined using x-ray fluorescence spectroscopy. Diffraction patterns and fluorescence spectra were simultaneously acquired. Cobalt nanoparticles in cellulose support were characterized with x-ray absorption spectroscopy at beamline X1 of the Deutsches Elektronen-Synchrotron in Hamburg, complemented by home lab experiments including x-ray diffraction, electron microscopy and measurement of magnetic properties with a vibrating sample magnetometer. Extended x-ray absorption fine structure spectroscopy (EXAFS) and x-ray diffraction were used to solve the atomic arrangement of the cobalt nanoparticles. Scanning- and transmission electron microscopy were used to image the surfaces of the cellulose fibrils, where the growth of nanoparticles takes place. The EXAFS experiment was complemented by computational coordination number calculations on ideal spherical nanocrystals. The growth process of metallic nanoclusters on cellulose matrix is assumed to be rather complicated, affected not only by the properties of the clusters themselves, but essentially depending on the cluster-fiber interfaces as well as the morphology of the fiber surfaces. The final favored average size for nanoclusters, if such exists, is most probably a consequence of these two competing tendencies towards size selection, one governed by pore sizes, the other by the cluster properties. In this thesis, a mesoscopic model for the growth of metallic nanoclusters on porous cellulose fiber (or inorganic) surfaces is developed. The first step in modelling was to evaluate the special case of how the growth proceeds on flat or wedged surfaces.
  • Laherto, Antti (Helsingin yliopisto, 2012)
    The rapid development and growing societal importance of nanoscience and nanotechnology (NST) have evoked educational concerns throughout the world. A mounting need for education in this emerging field has been recognized not only at the academic level but also in terms of citizens abilities to deal with personal, social and global issues related to NST. Some understanding of NST has been postulated to be relevant in up-to-date scientific literacy for all. This doctoral dissertation addresses such concerns and lays the research-based groundwork for the future development of learning environments on NST. The aim was to map the educational needs, possibilities and challenges of bringing the topics of NST to secondary schools and out-of-school settings. To this end, the methodological framework of the Model of Educational Reconstruction was employed. The model combines analytical and empirical research in order to analyse a field s educational significance, identify its essential features, investigate both learners and teachers perspectives and develop approaches for teaching and learning. Accordingly, the research presented here adopted a pragmatist multi-method approach to scrutinize NST from diverse educational viewpoints. The role of NST in scientific literacy was first explored through a theoretical-analytical study on the content structure, the nature and the implications of NST. Next, a group of secondary school teachers who had attended a course on NST was invited to evaluate the educational significance of the field s contents and their appropriateness for the curriculum. Another survey addressed Finnish science teachers views on barriers that hinder incorporating NST into the curriculum, and facilitators for overcoming these barriers. Specific challenges in learning and communicating NST were investigated through a literature review that was subsequently complemented with an interview study on science centre visitors perspectives on NST. On the basis of all these findings, research-based suggestions were put forth for the planning of NST education both in classrooms and through visits to science exhibitions and industry sites. Both theoretical and empirical analyses identified several content areas as well as social and epistemological aspects of NST that render the field educationally interesting and relevant to scientific literacy. The results imply that, by addressing NST, science education could stimulate dialogue on important contemporary issues in the intersection of science, technology and society, and provide up-to-date views on the nature of science. However, the teachers also pointed out a number of difficulties in arranging instruction on NST in practice. Many of the indicated barriers are extrinsic to teachers and related to curricular constraints in particular. It is concluded that NST would be best incorporated in the curriculum as a transdisciplinary theme. The field has, in addition, a potential to integrate traditional science subjects and approaches by shifting the focus to the scale of natural phenomena. In any case, including NST in science classes also requires in-service teacher training and new resources for materials and equipment. This dissertation highlights the research outcomes that should be taken into account when planning any learning environments on NST. Prior research has identified several challenges in learning and communicating NST, but also effective strategies for supporting the understanding of the nanoscale and its phenomena. The results of the interview study carried out here confirmed earlier findings. For instance, they implied that scanning tunnelling microscope (STM) images, powerful and thus used extensively in nanoscience communication, are liable to cause epistemological misunderstandings. Some of the identified barriers for teaching NST may be circumvented by out-of-school methods. This dissertation suggests research-based models for the development of two specific learning environments: exhibitions in science museums and school group visits to industrial sites. The models strive to bridge the notorious gap between academic research and the development of educational practice. Their application to NST education as well as their broader implications are discussed. Furthermore, some methodological issues are raised because this research also explored the potential of the Model of Educational Reconstruction in informal and out-of-school contexts.
  • Asmi, Ari (Helsingin yliopisto, 2012)
    Aerosol particles are everywhere in the atmosphere. They are a key factor in many important processes in the atmosphere, including cloud formation, scattering of incoming solar radiation and air chemistry. The aerosol particles have relatively short lifetimes in lower atmosphere, typically from days to weeks, and thus they have a high spatial and temporal variability. This thesis concentrates on the extent and reasons of sub-micron aerosol particle variability in the lower atmosphere, using both global atmospheric models and analysis of observational data. Aerosol number size distributions in the lower atmosphere are affected strongly by the new particle formation. Perhaps more importantly, a strong influence new particle formation is also evident in the cloud condensation nuclei (CCN) concentrations, suggesting a major role of the sulphuric acid driven new particle formation in the climate system. In this thesis, the sub-micron aerosol number size distributions in the European regional background air were characterized for the first time from consistent, homogenized and comparable datasets. Some recent studies have suggested that differences in aerosol emissions between weekdays could also affect the weather via aerosol-cloud interactions. In this thesis, the weekday-to-weekday variation of CCN sized aerosol number concentrations in Europe were found to be much smaller than expected from earlier studies, based on particle mass measurements. This result suggests that a lack of week-day variability in meteorology is not necessarily a sign of weak aerosol-cloud interactions. An analysis of statistically significant trends in past decades of measured aerosol number concentrations from Europe, North America, Pacific islands and Antarctica generally show decreases in concentrations. The analysis of these changes show that a potential explanation for the decreasing trends is the general reduction of anthropogenic emissions, especially SO2, although a combination of several drivers for these changes in the number concentrations are likely. The IPCC representative emission pathways prognose radical reductions of anthropogenic emissions in the next decades, especially of sulphur dioxide, that will most likely cause strong reduction in the present-day cooling effect of the atmospheric aerosols. The model simulations of this thesis show that effect will cause strong additional positive forcing on the atmosphere, possibly causing further increase in the near-surface mean temperatures. The effect was further magnified when new particle formation in atmosphere was also considered in the model calculations. Strong reductions in primary aerosol emissions and especially secondary aerosol precursors should be thus considered with caution.
  • Suksi, Juhani (Helsingin yliopisto, 2001)
  • Vesterinen, Veli-Matti (Helsingin yliopisto, 2012)
    Nature of science (NOS) describes what science is, how it works, how scientists operate, and the interaction between science and society. As a crucial element of scientific literacy, knowledge about NOS is widely recognized as one of the key aims of chemistry education. To enhance students understanding of NOS, teachers need adequate understanding of NOS as well as sufficient pedagogical content knowledge related to NOS for translating their understanding of NOS into classroom practice. This thesis reports an educational design research project on the design and development of a pre-service chemistry teacher education course on NOS instruction. Educational design research is the systematic study of the design and development of educational interventions for addressing complex educational problems. It advances the knowledge about the characteristics of designed interventions and the processes of design and development. The thesis consists of four interconnected studies and documents two iterative design research cycles of problem analysis, design, implementation, and evaluation. The first two studies describe how NOS is presented in the national frame curricula and upper secondary school chemistry textbooks. These studies provide a quantitative method for analysis of representations of NOS in chemistry textbooks and curricula, as well as describe the components of domain-specific NOS for chemistry education. The other two studies document the design, development, and evaluation of the goals and instructional practices used on the course. Four design solutions were produced: (i) description of central dimensions of domain-specific NOS for chemistry education, (ii) research group visits to prevent the diluting of relevance to science content and research, (iii) a teaching cycle for explicit and structured opportunities for reflection and discussion, and (iv) collaborative design assignments for translating NOS understanding into classroom practice. The evaluations of the practicality and effectiveness of the design solutions are based on the reflective essays and interviews of the pre-service teachers, which were collected during the course, as well as on the four in-depth interviews of selected participants, collected a year after they had graduated as qualified teachers. The results suggest that one critical factor influencing pre-service chemistry teachers commitment to teach NOS was the possibility to implement NOS instruction during the course. Thus, the use of collaborative peer teaching and integrating student teaching on NOS instruction courses is suggested as a strategy to support the development of the attitudes, beliefs, and skills necessary for teaching NOS. And even though the outside forces of school culture (e.g. school community, curriculum, textbooks) tend to constrain rather than support novice teachers efforts to implement new practices, the results also demonstrate that a pre-service teacher education course can be successful in producing innovators or early adopters of NOS instruction. Thus it might be one of the first steps in the challenging task of injecting NOS instruction into the chemistry curriculum for enhancing students understanding of NOS and strengthening their scientific literacy.
  • Heilimo, Esa (Helsingin yliopisto, 2011)
    Sanukitoid series intrusions can be found throughout the Archean Karelian Province of the Fennoscandian shield. All sanukitoids share the same controversial elemental characteristics: they have high content of incompatible elements such as K, Ba, and Sr as well as high content of the compatible elements Mg, Cr, and Ni, and high Mg#. This composition is explained by an enriched mantle wedge origin in a Neoarchean subduction setting. This study concentrates on sanukitoid intrusions and tonalite-trondhjemite-granodiorite series (TTGs) from Finnish part of the Karelian Province. The collected rock samples have been studied in the field and under microscope as well as for their whole-rock (including isotopes) and mineral compositions. The new data together with previously published analyses help us to better understand the petrogenesis, tectonic setting and reworking of the Archean rock units. TTGs from the Karelian Province form a voluminous series of granitoids and reworked migmatites. This study divides TTG series into two subgroups based on their elemental composition: low-HREE (heavy rare earth element) TTGs and high-HREE TTGs indicating pressure differences in their source. Sanukitoid series is a minor, divergent group of intrusions. These intrusions are variable sized, and the texture varies from even-grained to K-feldspar porphyritic. The elemental composition differentiates sanukitoids from more voluminous TTG groups, the SiO2 in sanukitoids varies to include series of gabbro, diorite, and granodiorite. U Pb age determinations from sanukitoid series show temporally limited emplacement between ~ 2745 2715 Ma after the main crust forming period in the area. Hafnium, neodymium, common lead, and oxygene isotopes indicate well homogenized characteristics. Recycled crust has made a variable, yet minor, contribution to sanukitoids, as evidenced by oxygene isotopes and inherited zircon cores. A proposed tectonic setting for the formation of the sanukitoid series is slab breakoff of oceanic lithosphere in subduction setting, with sanukitoids deriving from an enriched mantle wedge. The proposed setting explains some of the peculiar features of sanukitoids, such as their temporally limited occurrence and controversial elemental composition. Sanukitoids would occur after cessation of the regional growth of Archean crust, and they could be derived from mantle wedge previously enriched by melts and fluids from oceanic crust and sediments. A subsequent event during the Paleoproterozoic Svecofennian orogeny at ~1.9 Ga affected the appearance and microstructures of the rocks as well as caused redistribution of lead between minerals and whole rock. However, the deformation was not able to obliterate the original geochemical characteristics of these sanukitoids.
  • Sakko, Arto (Helsingin yliopisto, 2011)
    Inelastic x-ray scattering spectroscopy is a versatile experimental technique for probing the electronic structure of materials. It provides a wealth of information on the sample's atomic-scale structure, but extracting this information from the experimental data can be challenging because there is no direct relation between the structure and the measured spectrum. Theoretical calculations can bridge this gap by explaining the structural origins of the spectral features. Reliable methods for modeling inelastic x-ray scattering require accurate electronic structure calculations. This work presents the development and implementation of new schemes for modeling the inelastic scattering of x-rays from non-periodic systems. The methods are based on density functional theory and are applicable for a wide variety of molecular materials. Applications are presented in this work for amorphous silicon monoxide and several gas phase systems. Valuable new information on their structure and properties could be extracted with the combination of experimental and computational methods.
  • Aalto, Juha (Helsingin yliopisto, 2015)
    Climate, Earth surface processes and soil thermal hydrological conditions drive landscape development, ecosystem functioning and human activities in high latitude regions. These systems are at the focal point of concurrent global change studies as the ongoing shifts in climate regimes has already changed the dynamics of fragile and highly specialized environments across pan Arctic. This thesis aimed to 1) analyze and model extreme air temperatures, soil thermal and hydrological conditions, and the main Earth surface processes (ESP) (cryoturbation, solifluction, nivation and palsa mires) controlling the functioning of high latitude systems in current and future climate conditions; 2) identify the key environmental factors driving the spatial variation of the studied phenomena; and 3) develop methodology for producing novel high quality datasets. To accomplish these objectives, spatial analyses were conducted throughout geographical scales by utilizing multiple statistical modelling approaches, such as regression, machine learning techniques and ensemble forecasting. This thesis was based on unique datasets from the northern Fennoscandia; climate station records from Finland, Sweden and Norway, state of the art climate model simulations, fine scale field measurements collected in arctic alpine tundra and remotely sensed geospatial data. In paper I, accurate extreme air temperature maps were produced, which were notably improved after incorporating the influence of local factors such as topography and water bodies into the spatial models. In paper II, the results showed extreme variation in soil temperature and moisture over very short distances, while revealing the factors controlling the heterogeneity of ground thermal and hydrological conditions. Finally, the modelling outputs in papers III and IV provided new insights into the determination of geomorphic activity patterns across arctic alpine landscapes, while stressing the need for accurate climate data for predictive geomorphological distribution mapping. Importantly, Earth surface processes were found to be extremely climatic sensitivity, and drastic changes in geomorphic systems towards the end of 21st century can be expected. The increase of current temperature conditions by 2 ˚C was projected to cause a near complete loss of active ESPs in the high latitude study area. This thesis demonstrated the applicability of spatial modelling techniques as a useful framework in multiple key challenges of contemporary physical geography. Moreover, with the utilized model ensemble approach, the modelling uncertainty can be reduced while presenting the local trends in response variables more robustly. In future Earth system studies, it is essential to further assess the dynamics of arctic alpine landscapes under changing climatic conditions and identify potential tipping points of these sensitive systems.
  • Linkola, Hannu (Helsingin yliopisto, 2013)
    Finnish geography has been tightly linked to the construction of the Finnish nation state. The discipline was formed as a part of the governmentalisation of the modern state, and its development has followed changes in the spatial regulation of the Finnish society. However, geography has not only reflected societal structures, but geographers have also actively taken part in constructing Finland and Finnishness by developing research and representational methods, which have turned the spatial complexity of the state into governable, controllable, and hierarchical regionalities. Consequently, geographical research and its documents have positioned themselves in their social contexts by supporting the state ideas that have characterized different epochs. I understand landscapes as social ways of seeing. It follows that the forms of representing landscapes are statements in favour of certain social realities and aspirations. This study scrutinizes the landscape photography of Finnish geography from this perspective by defining the ways in which landscape photography has been used to represent and produce a desired statehood and citizenship. I position the examined photographs at the intersection of discourses about scientific content and social role of geography in order to clarify how geographers have negotiated their position in the Finnish nation state while creating ideas of Finnish regions and landscapes. My study draws from the methodologies of visual studies and sociology of science. I use content and discourse analyses to study the landscape photographs published in Terra (the journal of The Geographical Society of Finland), Finnish geography schoolbooks, and the non-fiction book series Suomenmaa ( The land of Finland ). I concentrate on the 1920s, 1940s and 1960s, which are three decades during which Finland turned from an aerial nation state into a modern welfare state. Geographical landscape imagery captured this change well. In the 1920s, representations of landscapes were linked to the constitutional consensus politics of the state, whereas in the 1940s landscape photography was used to legitimate expansive Finnish wartime politics and to unify the post-war state. In turn, the photographs from the 1960s depicted a state that had become a producer of civic services and a distributor of welfare. At the same time, within the field of geography, the landscape photographs turned from sensory observation material into sources of chorological analysis and further into documents that had a lesser role in research but which were nevertheless crucial in popular communication. My study represents the geographical landscape imagery as a political but conventional totality, which is characterized by many temporal layers. The imagery draws upon several traditions, which include, for example, the chorological and visual traditions that stem from the era preceding the institutionalisation of geography and the nineteenth century national and nature romantic traditions. Most importantly, however, the imagery is characterized by a twentieth century tradition, which represents the spatial and functional structure of the modern nation state. The importance of this tradition shows that the Finnish geography has been very loyal to the state institution. My study thus shows that the landscape photographs have been intertwined with the processes of social spatialization of the Finnish nation state and reveals that the geographical ways of representing landscapes have been highly dependent on societal spatial interests of knowledge.
  • Koskela, Outi (Helsingin yliopisto, 2009)
    Proteins are complex biomacromolecules playing fundamental roles in the physiological processes of all living organisms. They function as structural units, enzymes, transporters, process regulators, and signal transducers. Defects in protein functions often derive from genetic mutations altering the protein structure, and impairment of essential protein functions manifests itself as pathological conditions. Proteins operate through interactions, and all protein functions depend on protein structure. In order to understand biological mechanisms at the molecular level, one has to know the structures of the proteins involved. This thesis covers structural and functional characterization of human filamins. Filamins are actin-binding and -bundling proteins that have numerous interaction partners. In addition to their actin-organizing functions, filamins are also known to have roles in cell adhesion and locomotion, and to participate in the logistics of cell membrane receptors, and in the coordination of intracellular signaling pathways. Filamin mutations in humans induce severe pathological conditions affecting the brain, bones, limbs, and the cardiovascular system. Filamins are large modular proteins composed of an N-terminal actin-binding domain and 24 consecutive immunoglobulin-like domains (IgFLNs). Nuclear magnetic resonance (NMR) spectroscopy is a versatile method of gaining insight into protein structure, dynamics and interactions. NMR spectroscopy was employed in this thesis to study the atomic structure and interaction mechanisms of C-terminal IgFLNs, which are known to house the majority of the filamin interaction sites. The structures of IgFLN single-domains 17 and 23 and IgFLN domain pairs 16-17 and 18-19 were determined using NMR spectroscopy. The structures of domain pairs 16 17 and 18 19 both revealed novel domain domain interaction modes of IgFLNs. NMR titrations were employed to characterize the interactions of filamins with glycoprotein Ibα, FilGAP, integrin β7 and dopamine receptors. Domain packing of IgFLN domain sextet 16 21 was further characterized using residual dipolar couplings and NMR relaxation analysis. This thesis demonstrates the versatility and potential of NMR spectroscopy in structural and functional studies of multi-domain proteins.
  • Salminen, Tapio (Helsingin yliopisto, 2011)
    In this thesis the current status and some open problems of noncommutative quantum field theory are reviewed. The introduction aims to put these theories in their proper context as a part of the larger program to model the properties of quantized space-time. Throughout the thesis, special focus is put on the role of noncommutative time and how its nonlocal nature presents us with problems. Applications in scalar field theories as well as in gauge field theories are presented. The infinite nonlocality of space-time introduced by the noncommutative coordinate operators leads to interesting structure and new physics. High energy and low energy scales are mixed, causality and unitarity are threatened and in gauge theory the tools for model building are drastically reduced. As a case study in noncommutative gauge theory, the Dirac quantization condition of magnetic monopoles is examined with the conclusion that, at least in perturbation theory, it cannot be fulfilled in noncommutative space.
  • Väihkönen, Antti (Helsingin yliopisto, 2006)
    This thesis consists of four research papers and an introduction providing some background. The structure in the universe is generally considered to originate from quantum fluctuations in the very early universe. The standard lore of cosmology states that the primordial perturbations are almost scale-invariant, adiabatic, and Gaussian. A snapshot of the structure from the time when the universe became transparent can be seen in the cosmic microwave background (CMB). For a long time mainly the power spectrum of the CMB temperature fluctuations has been used to obtain observational constraints, especially on deviations from scale-invariance and pure adiabacity. Non-Gaussian perturbations provide a novel and very promising way to test theoretical predictions. They probe beyond the power spectrum, or two point correlator, since non-Gaussianity involves higher order statistics. The thesis concentrates on the non-Gaussian perturbations arising in several situations involving two scalar fields, namely, hybrid inflation and various forms of preheating. First we go through some basic concepts -- such as the cosmological inflation, reheating and preheating, and the role of scalar fields during inflation -- which are necessary for the understanding of the research papers. We also review the standard linear cosmological perturbation theory. The second order perturbation theory formalism for two scalar fields is developed. We explain what is meant by non-Gaussian perturbations, and discuss some difficulties in parametrisation and observation. In particular, we concentrate on the nonlinearity parameter. The prospects of observing non-Gaussianity are briefly discussed. We apply the formalism and calculate the evolution of the second order curvature perturbation during hybrid inflation. We estimate the amount of non-Gaussianity in the model and find that there is a possibility for an observational effect. The non-Gaussianity arising in preheating is also studied. We find that the level produced by the simplest model of instant preheating is insignificant, whereas standard preheating with parametric resonance as well as tachyonic preheating are prone to easily saturate and even exceed the observational limits. We also mention other approaches to the study of primordial non-Gaussianities, which differ from the perturbation theory method chosen in the thesis work.
  • Battefeld, Diana (Helsingin yliopisto, 2008)
    In this thesis we examine multi-field inflationary models of the early Universe. Since non-Gaussianities may allow for the possibility to discriminate between models of inflation, we compute deviations from a Gaussian spectrum of primordial perturbations by extending the delta-N formalism. We use N-flation as a concrete model; our findings show that these models are generically indistinguishable as long as the slow roll approximation is still valid. Besides computing non-Guassinities, we also investigate Preheating after multi-field inflation. Within the framework of N-flation, we find that preheating via parametric resonance is suppressed, an indication that it is the old theory of preheating that is applicable. In addition to studying non-Gaussianities and preheatng in multi-field inflationary models, we study magnetogenesis in the early universe. To this aim, we propose a mechanism to generate primordial magnetic fields via rotating cosmic string loops. Magnetic fields in the micro-Gauss range have been observed in galaxies and clusters, but their origin has remained elusive. We consider a network of strings and find that rotating cosmic string loops, which are continuously produced in such networks, are viable candidates for magnetogenesis with relevant strength and length scales, provided we use a high string tension and an efficient dynamo.
  • Eriksson, Stefan (Helsingin yliopisto, 2002)
  • Kaliva, Kasimir (Helsingin yliopisto, 2011)
    The aim of this dissertation is to model economic variables by a mixture autoregressive (MAR) model. The MAR model is a generalization of linear autoregressive (AR) model. The MAR -model consists of K linear autoregressive components. At any given point of time one of these autoregressive components is randomly selected to generate a new observation for the time series. The mixture probability can be constant over time or a direct function of a some observable variable. Many economic time series contain properties which cannot be described by linear and stationary time series models. A nonlinear autoregressive model such as MAR model can a plausible alternative in the case of these time series. In this dissertation the MAR model is used to model stock market bubbles and a relationship between inflation and the interest rate. In the case of the inflation rate we arrived at the MAR model where inflation process is less mean reverting in the case of high inflation than in the case of normal inflation. The interest rate move one-for-one with expected inflation. We use the data from the Livingston survey as a proxy for inflation expectations. We have found that survey inflation expectations are not perfectly rational. According to our results information stickiness play an important role in the expectation formation. We also found that survey participants have a tendency to underestimate inflation. A MAR model has also used to model stock market bubbles and crashes. This model has two regimes: the bubble regime and the error correction regime. In the error correction regime price depends on a fundamental factor, the price-dividend ratio, and in the bubble regime, price is independent of fundamentals. In this model a stock market crash is usually caused by a regime switch from a bubble regime to an error-correction regime. According to our empirical results bubbles are related to a low inflation. Our model also imply that bubbles have influences investment return distribution in both short and long run.