The so-called 'cosmic web', comprising cosmic voids and galaxy clusters, has been proven to be extremely sensitive to deviations from General Relativity. This could be further investigated by future large-scale surveys, such as with the European Space Agency satellite Euclid. In this study, the parameter |fR0| from f(R) gravity is constrained by considering the Euclid survey specications to predict the observed numbers of voids and clusters in bins of redshift, mass and, only for voids, density contrast. From these values, the Fisher matrix is computed for three values of |fR0|, 10-4, 10-6 and 10-8, by assuming a flat Universe with a component that mimics the cosmological constant. The probability density functions are obtained for |fR0| and seven other parameters from the fiducial model considered (ns, h, Ωb, Ωm, σ8, w0 and wa).

Chameleons are scalar fields coming from modied gravity theories and can be possible explanations for Dark Energy. They cause a fifth force and have a screening mechanism which allows this force to avoid solar system constraints. In this thesis, astrophysical consequences of the potential presence of the chameleon field will be studied. More precisely, the difference between the hydrostatical and the weak lensing mass of galaxy clusters due to the effect of the chameleon fifth force is discussed.

This report will first give a brief background on asymptotic giant branch (AGB) stars and the characteristics that make them interesting to study. Some methods and tools used in the field are then introduced, before the photometric variability of these stars is investigated. This is achieved by using data from dynamical models of AGB stars with differing chemical abundances. The R, J and K bands of the UBVRI system are specifcally investigated to explore whether these are good candidates for AGB photometric and spectroscopic research. Lastly, the molecular features at these wavelengths are investigated to understand the impact that they have on the photometric variability during the pulsation cycle and which molecules are most prominent in this.

Context. The Martian methane and water cycle are subject of ongoing research through simulation. Exchange with the subsurface has a potentially strong impact, but is often neglected. Aims. For methane, I determine if adsorption with an increased enthalpy can explain the observed seasonal variations and conflicting observations by the Trace Gas Orbiter and the Curiosity rover. For water, the impact of adsorption and ice formation in the subsurface on the global cycle is studied. A new way of initializing the soil, by running a decoupled subsurface model, is tested. Depths of stable subsurface ice and subsurface water distributions are studied. Methods. A General Circulation Model (GCM) is used with a purely diffusive subsurface model. For methane, different initial states, source scenarios, and decay times are tested. For water, a model without an active atmosphere is implemented to provide an initial state. The effect of the subsurface with this initial state on the full atmospheric water cycle is tested. Results. For methane, a strong influence on the global methane cycle is observed. Seasonal variations measured at Gale Crater are reproduced, but the conflicting observations cannot be explained by adsorption. For water, the new initialization can be used without completely disrupting the water cycle. It leads to a generally wetter atmosphere, in conflict with observations. Found ice table depths do not match well with observations, but ice profiles reproduce previous findings. Conclusion. Methane adsorption is able to partly explain observed variations, but cannot be the only process to influence methane abundances. The new initialization method for water works well in principle, but a more refined model is needed for more realistic results.

This thesis describes a timing calibration method for the detector array of the ALTO experiment. ALTO is a project currently at the prototype phase that aims to build a gamma-ray astronomical observatory at high-altitude in the Southern hemisphere. ALTO can be assumed as a hybrid system as each detector consists of a Water Cherenkov Detector (WCD) on top of a Scintillator Detector (SD), providing an increased signal to background discrimination compared to other WCD arrays. ALTO is planned to complement the Very-High-Energy (VHE) observations by the High Altitude Water Cherenkov (HAWC) gamma ray observatory that collects data from the Northern sky. By the time the full array of 1242 detectors is installed to the proposed site, ALTO together with HAWC and the future Cherenkov Telescope Array (CTA) will serve as a state-of-the-art detection system for VHE gamma-rays combining the WCD and the Imaging Atmospheric Cherenkov Telescope (IACT) techniques. When a VHE gamma-ray or cosmic-ray enters the Earth’s atmosphere, it initiates an Extensive Air Shower (EAS). These particles are sampled by the detector array and by checking the arrival times of nearby tanks, the method reveals whether a detector suffers from a time-offset. The data analyzed in this thesis derive from CORSIKA (COsmic Ray SImulation for KAscade) and GEANT4 (GEometry ANd Tracking) simulations of cosmic-ray events within the energy range of 1–1:6TeV, which mainly consist of protons. The high flux of this particular type of cosmic-rays, gives us a tool to statistically evaluate the results generated by the proposed timing calibration method. In the framework of this thesis, I have written code in Python programming language in order to develop the timing calibration method. The method identifies detectors that suffer from time-offsets and improves the reconstruction accuracy of the ALTO detector array. Different Python packages were used to execute different tasks: astropy to read filter-present-write large datasets, numpy (Numerical Python) to make datasets comprehensiveto functions, scipy (Scientific Python) to develop our models, sympy (Symbolic Python) to find geometrical correlations and matplotlib (Mathematical Plotting Library) to draw figures and diagrams. The current version of the method achieves sub-nanosecond accuracy. The next stepis to make the timing calibration more intelligent in order to correct itself. This self correction includes an agile adaptation to the data acquired for long periods of time, in order to make different compromises at different time intervals.

The detection of exoplanets has rapidly evolved to one of the most important frontiers of astronomical and astrophysical research. The recent decades have seen the development of various techniques for detecting exoplanets. Of these approaches the transit method has received particular interest and has lead to the largest number of discoveries to date. The Kepler K2 mission is an ongoing observational survey, which has generated light curves for thousands of stars, a large fraction of which have yet to be fully explored. To discover and characterize the transiting planets hosted by the respective stars, extensive transit screens are required. However, implementing a pipeline for transit analyses is not straight forward, considering the light curve properties of different survey, the rapid changes brought by technological advancements, and the apparent lack of a golden standard with respect to the applied methodology. The project has reviewed several aspects of exoplanet detection via the transit method. Particular focus was placed on the identification of a suitable workflow covering the relevant steps to move from raw light curve files to a final prediction and characterization of transiting planetary candidates. Adhering to the identified strategy, the major part of the project then dealt with the implementation of a pipeline that integrates and executes all the different steps in a streamlined fashion. Of note, primary focus was placed on the actual selection and implementation of methods into an operational pipeline, but due to the given time constraints extensive optimizations of each individual processing step was outside the scope of this project. Nevertheless, the pipeline was employed to predict transit candidates for K2 campaigns C7, C8, C10, C11, and C12. A comparsion of the most conservative predictions from campaigns C7 and C10 with previously reported exoplanet candidates demonstrated that the pipeline was highly capable of discovering reliable transit candidates. Since campaigns C11 and C12 have not yet been fully explored, the respective candidates predicted for those campaigns in the current project might thus harbour novel planetary transit candidates that would be suitable for follow-up confirmation runs. In summary, the current project has produced a pipeline for performing transiting exoplanet searches in K2 data, which integrates the steps from raw light curve processing to transit candidate selection and characterization. The pipeline has been demonstrated to predict credible transit candidates, but future work will have to focus on additional optimizations of individual method parameters and on the analysis of transit detection efficiencies.

The first stars in the Universe are thought to have formed in high dark matter density minihalos about 200 million years after the Big Bang. If these stars were able to contract dark matter into their stellar core while forming, some of them might have turned into dark stars (DSs) powered by the heat from dark matter annihilation. The possibilities for detection of DSs with the upcoming James Webb Space Telescope (JWST), scheduled for launch in 2021, is investigated in this work. With DS models generated in Spolyar et al. (2009) and atmosphere spectra from Gustafsson et al. (2008), spectral analysis has been carried out in MATLAB to find the unique colors of DSs compared to galaxies generated in Zackrisson et al. (2017) at z ≈ 7 − 11. It was found that lower temperature DSs (Teff ≤ 7800K) are distinguishable from galaxies and that they would be bright enough to be detected with the JWST provided a magnification factor of µ ≈ 160−1000 with the use of gravitational lensing. More recent DS models reveal that the DS of temperature Teff = 7800K is detectable even without the use of gravitational lensing. However, the probability of finding one today is really small due to DSs’ presumably short lifetime. The results of this work are hoped to give a better understanding of the properties of DSs and to increase the probability of finding one in the large imaging survey carried out by the JWST. / De första stjärnorna i universum antas ha bildats i minihalos med hög densitet av mörk materia omkring 200 miljoner år efter Big Bang. Om dessa stjärnor kunde dra till sig mörk materia under sitt bildande kan vissa av dem ha utvecklats till mörka stjärnor (s.k. dark stars) med mörk materia som energikälla. I detta arbete undersöks möjligheterna att upptäcka dem med det kommande James Webb Space Teleskopet (JWST) som planeras för uppskjutning år 2021. Med dark starmodeller genererade i Spolyar et al. (2009) och atmosfärspektra från Gustafsson et al. (2008) har spektralanalys utförts i MATLAB för att hitta vilka dark stars som går att urskilja från galaxer genererade i Zackrisson et al. (2017) vid z ≈ 7−11. Det visade sig att dark stars med låg temperatur (Teff ≤ 7800K) är urskiljbara och att de flesta av dessa dark stars, vid en förstoringsfaktor av µ ≈ 160−1000 vid användning av gravitationell linsning, är tillräckligt ljusstarka för att kunna detekteras. Jämfört med senare dark star-modeller skulle även Teff = 7800K DSs kunna detekteras utan användning av gravitaionell linsning. Sannolikheten att hitta en dark star är fortfarande väldigt liten på grund av dess förmodade korta livstid. Resultaten av detta arbete hoppas kunna ge en bättre förståelse för egenskaperna hos mörka stjärnor samt öka sannolikheten för detektion med JWST.

Syftet med denna konsumtionsuppsats var att genom en systematisk litteraturstudie, undersöka vad forskningen har att säga om problematik och lösningar med avseende på undervisning inom astronomi i skolan, med inriktning på årskurs 4–6. Vi använde oss av såväl databassökningar som manuella sökningar. Vi utgick från två frågeställningar:  1) Vilka problem kan uppstå när man undervisar inom astronomi?  2) Hur löser man dessa problem? Resultaten visade att ett stort hinder vid undervisningen inom astronomi beror på missuppfattningar och bristande kunskaper, både hos elever och lärare. Därför är det viktigt att kunna synliggöra och hantera missuppfattningar, genom olika metoder och strategier till exempel genom att gå från mindre delar till helhet. Detta är enligt forskningen något vi som lärare bör ha i åtanke när vi planerar vår undervisning i astronomi.

Comets are the most active around their perigees. The increased outgassing can lead to a coma thick enough to effectively absorb the solar EUV radiation, which engenders a self-shielding comet nucleus and inner layers of the ionosphere. This effect of self-shielding can be calculated by the attenuation of the sunlight according to the Beer-Lambert law. Here we focus on the perihelion of comet 67P/Churyumov-Gerasimenko, the target comet of the ESA Rosetta mission. We calculate attenuated photoionization frequencies and implement these into an ionospheric model constructed in a recent project work (by the same author). The ionization frequencies and ion number densities are calculated as a function of cometocentric distance and compared with the latest published peer-reviewed article by Heritier et al. (2017). Overall, the agreement is fairly good. The most significant difference is the discrepancy of number densities of O2 ions, which is higher in our model by nearly an order of magnitude. This discrepancy is attributed to the fact that Heritier et al (2017) only considered charge transfer processes for the formation of O2+, while we identify photoionization of O2 as the main production mechanism.

Extragalactic charged particles, each with energies rising up to and beyond 1 Joule, have been studied for almost a century. Yet, no precise evidence have proven to show where they might originate from as their energy levels rise above the current familiar acceleration sources in outer space. The highly energetic particles have been given the name Ultra-High Energy Cosmic Rays (UHECR) and investigations of particle properties such as primary energy, mass composition and direction can be made through indirect measurements of the interaction between the UHECR and Earth's atmosphere. The considered interaction induces an Extensive Air Shower (EAS) which emits fluorescent light in the Ultraviolet (UV) range. The probability of detecting such events is, however, as low as a few particles per km2 per century. Making observations more sufficient therefore requires larger detection volumes. By introducing the Mini-EUSO instrument, a telescope of which the main purpose is to measure the UV-light radiated from the Earth in the wavelength range of 300-400 nm, allows just for this. To be accommodating the International Space Station and targeting Earth in the nadir direction, the Mini-EUSO instrument will allow for a higher exposure to the interactions than what is currently available. The use of two Fresnel lenses provides the instrument with a large field of view (±22o) and the detections are made through multiple photomultiplier tubes. The scope of this thesis is to evaluate the main detector of the Mini-EUSO instrument (i.e. the UV-detector) through ground-based tests. The procedures involved in the evaluation have consisted of; validating the statistical distributions of the signals, implementing dark field and flat field calibrations, and radiations measurements with three kinds of radiation sources. The data from the tests were provided during two periods and the visualization was made by adapting an already existing piece of code, using Python and ROOT Cern, to perform step by step procedures such that all operations are overlooked properly. The analysis showed that the implementation of the dark field and flat field procedures improved the original image significantly. It also showed that both the lower and higher photon count values in a pixel indeed gave the expected statistical behaviours, with a Poissonian distribution for low values and a Gaussian distribution for higher values. The flat fielding screen did however show unknown fluctuations in the emitted light and further tests have to be implemented to assure its functionality. Under proper covering, almost no dark current was found, however, observation tests showed that the borders of the Multi-Anode Photomultiplier Tubes (MAPMTs) gave higher photon count values than the center part even when they were emitted with Lambertian light.