The quantum creation of membranes by a totally antisymmetric tensor and gravitational fields is considered in arbitrary space-time dimensions. The creation process is described by instanton tunneling. As membranes are created, the energy density associated with the antisymmetric field decreases, reducing the effective cosmological constant to a lower value. For a collection of parameters and initial conditions, the creation mechanism goes to a halt as soon as the cosmological constant is near zero. A brief exploration of a canonical gravity representation is also considered, where the system of ADM-composition in arbitrary space-time dimensions is introduced. / Kvantprocessen av membranskapelse via en total anti-symmetrisk tensor och gravitationsfält är betänkt i arbiträra rumstidsdimensioner. Skapelseprocessen är beskriven via instantontunnling. Då membranen är skapade, reduceras värdet på energidensiteten som är associerad med det anti-symmetriska fältet. För en samling av parametrar och begynnelsevärden, stannar skapelseprocessen upp så fort den kosmologiska konstanten har ett värde nära noll. En kort utforskning av kanonisk gravitation är också betänkt, där ett system i termer av ADM-dekomposition i arbiträra rumstidsdimensioner är introducerat.

This thesis is based on research made by the intermediate Palomar Transient Factory [(i)PTF]. The focus is on stripped-envelope (SE) supernovae (SNe) discovered by (i)PTF, and it is closely tied to the research on the SE SN iPTF13bvn, that occurred in the nearby galaxy NGC 5806. This SN was initially thought to have been the explosion of a very massive Wolf-Rayet star, but we have shown that this is very likely not the case. We suggest instead that iPTF13bvn originated from a binary system where the envelope was stripped off from the SN progenitor by tidal forces from a companion (Paper I). PTF12os exploded in the same galaxy as iPTF13bvn, and our analysis shows that PTF12os and  iPTF13bvn were very similar, and that both were also remarkably similar to the Type IIb SN 2011dh, in terms of their light-curves and spectra. In Paper II, hydrodynamical models were used to constrain the explosion parameters of iPTF13bvn, PTF12os and SN 2011dh; finding 56Ni masses in the range 0.063-0.075 solar masses (Ms), ejecta masses in the range 1.85-1.91 Ms, and kinetic energies in the range 0.54-0.94 x 1051 erg. Furthermore, using nebular models and late-time spectroscopy we were able to constrain the Zero-Age Main Sequence (ZAMS) mass to ~ 12 Ms, for iPTF13bvn and ≤ 15 Ms for PTF12os. In current stellar evolution models, stars with these masses on the ZAMS cannot lose their envelopes and become SE SNe without binary interactions. In Paper III we investigate a peculiar SE SN, iPTF15dtg; this SN lacks both hydrogen and helium and shows a double-peaked LC with a broad main LC peak. Using hydrodynamical modeling we show that iPTF15dtg had a very large ejecta mass (~ 10 Ms), resulting from an explosion of a very massive star (~ 35 Ms). The initial peak in the LC can be explained by the presence of extended material around the star, likely due to an episode of strong mass-loss experienced by the progenitor prior to the explosion. In Paper IV we perform a statistical study of the spectra of all 176 SE SNe (Type IIb, Ib and Ic) discovered by (i)PTF. The spectra of Type Ic SNe show O absorption features that are both stronger and broader (indicating faster expansion velocities) compared to Type IIb and Type Ib SNe. These findings along with very weak He absorption support the traditional picture with Type Ic SNe being heavily stripped of their He envelopes prior to the explosions, and argue against alternative explanations, such as differences in explosive mixing of 56Ni among the SE SN subtypes. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.</p>

The first stars are something many scientists are curious about. How did they formand how did the universe look like at that time? These stars however probably died along time ago, or are at a distance too far away from us to be observable. If these starsexploded and formed supernovae there might be stars observable today that formed fromthe ejecta of these supernovae. Models of nucleosynthesis in the first stars may potentiallybe used to infer parameters of progenitor supernovae from chemical abundances in old,metal-poor stars. This thesis aim is to find out how precise these abundances need to beto achieve a certain precision in the supernova parameters. This is done by perturbingthe abundances for one element at a time in four different stars and see how the recoveredsupernova parameters change. The first conclusion is that it isn’t necessarily the sameelements that are important for determining the supernova parameters in all stars, andif there is one thing that decides which elements are important it would be the star’smetallicity. The stars HE1327-2326 and HE0107-5240 metallicites are close to each other,with [Fe/H] &lt; -5.0 and [Fe/H] = -5.3 respectively, and behaved similarly, the elementmost important in both stars was nitrogen. The star SMSS0313-6708, [Fe/H] &lt; -7.3, hadmore elements that changed the parameters, the most important beeing C, O and Ca.The star CS 31028-001, [Fe/H] = -2.9, also had many elements that were important, andsome of them are Na, K and Ni. / De första stjärnorna är någonting många forskare är nyfikna på. Hur formades deoch hur såg universum ut vid den tiden? Dessa stjärnor dog antagligen länge sedan,eller så är de på ett avstånd för långt bort från oss för att kunna observeras. Om dessastjärnor exploderade och bildade supernovor är det möjligt att det finns stjärnor som gåratt observera som bildades ur dessa supernovor. Modeller av nukleosyntes i dessa förstastjärnor kan potentiellt användas för att anta parametrar för de föregående supernovorur den kemiska sammansättningen för gamla, metalfattiga stjärnor. Målet för dennauppsats är att ta reda på hur exakta värden som behövs på dessa kemiska sammansättningarför att nå en viss säkerhet på supernovaparametrarna. Det görs genom att ändraämneshalten för ett ämne i taget i fyra olika stjärnor och se hur de beräknade supernovamodellsparametrarnaändras. Den första slutsatsen är att det inte nödvändigtvis ärsamma ämnen som är viktiga för att bestämma supernovaparametrarna i varje stjärna,och om det är en sak som bestämmer vilket ämne som är viktigt skulle det vara stjärnansmetallicitet. Stjärnorna HE1327-2326 och HE0107-5240 har metalliciteter som är näravarandra, med respektive [Fe/H] &lt; -5.0 och [Fe/H] = -5.3, och betedde sig snarlikt, så vardet kväve som var det viktigaste ämnet i dessa två stjärnor. Stjärnan SMSS0313-6708,[Fe/H] &lt; -7.3, hade fler ämnen som ändrade parametrarna och de viktigaste var C, O ochCa. Stjärnan CS 31028-001, [Fe/H] = -2.9, hade också många ämnen som var viktiga ochnågra av dem var Na, K och Ni.

In this project we study chameleon theories in cosmic voids. We investigate the field profile of the chameleon field as well as the fifth force in voids by numerically solving the equations of motion for chameleon fields. The possibility of an analytical approximation for voids is also studied, the well known thin and thick-shell regimes are studied and both are deemed unsuitable for use in cosmic voids. We develop an alternative approximation which is tested against the numerically solved field profile. The agreement between the numerical solution and the analytical approximation is generally poor and the disagreement is likely due in part to the fact that the chameleon field doesn't reach it's equilibrium value inside the void.

No description available.

The objective of this thesis is to retrieve Light Curves, Spectral Models, Spectral Maps, and to geta Spectral Energy Distribution (SED) plot, for the source of PKS1830-211. Finally, also to do amodelling of the whole SED coming from the source with the JetSet software. Which will then becompared to a previous modelling done by A. A. Abdo. First, a light curve is taken over the wholeFermi duration period. The Light Curve is a statistical plot of the overall flux from the source overtime. It will set every photon in a defined energy interval as a datapoint in a defined time bin width. Alight curve will first be taken over the Fermi Duration Period, to get a view of potential flares comingfrom the source. Three periods are then chosen to get a loud, an intermediate, and a loud flaringperiod. Variability is checked on all of these including the longer fermi duration period. A spectralmodel will be fitted for all of these periods. It’s a mathematical formula to the shape of the source’sflux spectrum over energy. By doing this, the electron energy distribution can be understood. SpectralMaps will then give a plot of the number of contributing photons from the source, of a given period.It also shows how well the models are adjusted to other light sources in the field of view. To get thecollected data from relevant catalogs, together with Fermi data from this analysis. A Spectral EnergyDistribution is downloaded from [Sed builder. [Online]. Available: https://tools.ssdc.asi.it/SED/] provided by the Italian space agency. The modelling is done toget an idea of the physical parameters present in the AGN. Such as the physical scales and proportionsof the structure near the core. Data from the SED will be used in the modelling with the best suitedperiod for the Fermi-LAT range.

The IceCube Neutrino Observatory at the South Pole has instrumented one cu-bic kilometer of ice by deploying digital optical modules (DOMs) in 86 drillholes, each containing a string of DOMs. So far IceCube has used the locationof the drill tower for the positions of DOMs in the transverse directions (x andy) to the vertical direction (z). Attempts to improve on this have so far failed.This thesis presents a new method for calibrating the positions of the DOMs.For a large selection of muon tracks, a maximum likelihood-based approach isused to determine the positions of DOMs. As a proof of concept, four centralstrings are studied to keep systematics uncertainties as low as possible. Themethod can find x and y (z) positions to 0.2m (0.5m) as found using simulationcorresponding to four days of data. In four days of real data we find that thereconstructed z position has a systematic offset of around 5m, which is not aphysical shift as the positions are known to 0.2m. For the x and y positions theresults are consistent with nominal positions except for string 36. As string 36is a string at the center of the IceCube array, it should be the most symmet-ric and an accurate estimate could indicate that the method has found a realdeviation.

Den här licenciatuppsatsen bygger på två vetenskapliga artiklar, varav den första är skriven som ensamförfattare och den andra är skriven tillsammans med Dr. Giacomo Monari. De är båda på temat mörk materia i Vintergatan. Den första artikeln handlar om mörk materia som fångas i solen. Förutsatt att mörk materia består av partiklar som interagerar via den svaga kraften, med en massa av storleksordningen 10--1000 GeV, så kan sådana partiklar kollidera med atomkärnor i solens inre, förlora rörelseenergi och bli gravitationellt bundna. Väl infångad så kommer en mörk materia--partikel att fortsätta kollidera och förlora rörelseenergi tills den har uppnått termisk jämvikt med solens kärna. Givet att infångade och termaliserade mörk materia--partiklar har blivit tillräckligt många till antalet så kan dessa partiklar annihiliera och producera standard modell--partiklar. Neutriner som produceras i en sådan process skulle kunna detekteras i ett neutrinoteleskop på jorden, vilket vore ett sätt att indirekt detektera mörk materia. I artikeln har jag utforskat denna termaliseringsprocess och den tid det tar för en infångad partikel att uppnå termisk jämvikt. Jag har funnit att termaliseringstiden är kort jämfört med solens ålder och kan försummas, utom i vissa finjusterade specialfall. I den andra artikeln har vi utfört en dynamisk mätning av massdensiteten i solens närområde. Astrometri från rymdteleskopet Gaia ger information om stjärnors vertikala hastighetsfördelning och hur deras antaltäthet avtar med avstånd från galaxskivan. Genom att anta jämvikt så kan man relatera dessa två fördelning till varandra genom gravitationspotentialen de rör sig genom, vilket i sin tur ger galaxskivans massfördelning. Först och främst så har vi gjort framsteg i fråga om statistisk modellering; för första gången har mätfel på alla enskilda stjärnor tagits i beaktning. Vi har funnit en massdensitet som stämmer överens med de flesta tidigare mätningar och har även kunnat dra slutsatser om solens position och hastighet i förhållande till galaxskivan.

In the last few years, there has been significant progress in the development of a new model for explaining magnetic flux concentrations, by invoking the negative effective magnetic pressure instability (NEMPI) in a highly stratified turbulent plasma. According to this model, the suppression of the turbulent pressure by a large-scale magnetic field leads to a negative contribution of turbulence to the effective magnetic pressure (the sum of non-turbulent and turbulent contributions). For large magnetic Reynolds numbers the negative turbulence contribution is large enough, so that the effective magnetic pressure is negative, which causes a large-scale instability (NEMPI). One of the potential applications of NEMPI is to explain the formation of active regions on the solar surface. On the other hand, the solar dynamo is known to be responsible for generating large-scale magnetic field in the Sun. Therefore, one step toward developing a more realistic model is to study a system where NEMPI is excited from a dynamo-generated magnetic field. In this context, the excitation of NEMPI in spherical geometry was studied here from a mean- field dynamo that generates the background magnetic field. Previous studies have shown that for NEMPI to work, the background field can neither be too weak nor too strong. To satisfy this condition for the dynamo-generated magnetic field, we adopt an “alpha squared dynamo” with an α effect proportional to the cosine of latitude and taking into account alpha quenching. We performed these mean-field simulations (MFS) using the Pencil Code. The results show that dynamo and NEMPI can work at the same time such that they become a coupled system. This coupled system has then been studied separately in more detail in plane geometry where we used both mean-field simulations and direct numerical simulations (DNS). Losada et al. (2013) showed that rotation suppresses NEMPI. However, we now find that for higher Coriolis numbers, the growth rate increase again. This implies that there is another source that provides the excitation of an instability. This mechanism acts at the same time as NEMPI or even after NEMPI was suppressed. One possibility is that for higher Coriolis numbers, an α2 dynamo is activated and causes the observed growth rate. In other words, for large values of the Coriolis numbers we again deal with the coupled system of NEMPI and mean-field dynamo. Both, MFS and DNS confirm this assumption. Using the test-field method, we also calculated the dynamo coefficients for such a system which again gave results consistent with previous studies. There was a small difference though, which is interpreted as being due to the larger scale separation that we have used in our simulations. Another important finding related to NEMPI was the result of Brandenburg et al. (2013), that in the presence of a vertical magnetic field NEMPI results in magnetic flux concentrations of equipartition field strength. This leads to the formation of a magnetic spot. This finding stimulated us to investigate properties of NEMPI for imposed vertical fields in more detail. We used MFS and DNS together with implicit large eddy simulations (ILES) to confirm that an initially uniform weak vertical magnetic field will lead to a circular magnetic spot of equipartition field strength if the plasma is highly stratified and scale separation is large enough. We determined the parameter ranges for NEMPI for a vertical imposed field. Our results show that, as we change the magnitude of the vertical imposed field, the growth rate and geometry of the flux concentrations is unchanged, but their position changes. In particular, by increasing the imposed field strength, the magnetic concentration forms deeper down in the domain.

No description available.