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The first billion years from the beginning of the Universe is the focus of multiple astronomical facilities in the upcoming decade. This unique era is marked by the formation of the first stars and galaxies, which release ionising radiation into the intergalactic medium(IGM). As a result, these sources initiate a period during which the cold and dense IGM, primarily consisting of neutral hydrogen (HI ), is heated and ionised. We refer to this era as the Epoch of Reionisation (EoR). How the EoR transpired hence depends on the properties of these ionising sources, and this forms a vital piece to the puzzle of understanding the early Universe. This licentiate thesis aims to educate the reader on the power spectrum (PS) statistic of the 21-cm signal from HI during the EoR. The PS is a prospective observable by radio interferometers, such as the Low-Frequency Array (LOFAR) and the future Square Kilometre Array (SKA). It is an essential stepping stone in comprehending the dominant physical processes affecting the IGM at a given length scale during the EoR. In Paper I, we analyse the decomposition of the 21-cm PS from Lidz et al. (2007) (eq. 2)and study the evolution of its terms. We conduct our investigation for a set of C 2 -Ray and 21cmFAST simulations with volumes of (714 cMpc)3, concentrating on large-scales (k < 0.3 Mpc−1 ) as the signal-to-noise ratio of observing these scales will be high. We find that the 21-cm PS tracks the PS of neutral hydrogen fluctuations, which itself past a certain scale tracks the matter PS after a global ionisation fraction of x̄HII∼ 0.1. Hence, the 21-cm PS possesses a two-regime form for which the large-scale PS is a biased version of the cosmological density field and the small-scale PS depends on the astrophysics of the EoR. We construct a bias parameter to explore whether the 21-cm PS can be used as a probe of cosmology on large k-scales. We discover a transition feature for both simulations, following the ktrans ≈ 2/λMFP empirical formula. The transition scale between the scale-independent and scale-dependent bias regimes is directly related to the value of the mean free path of ionising photons (λMFP ).

This paper uses the Pantheon+ data set that consists of 1701 light curves of 1550 unique type IaSupernova to find constraints on various cosmological models and compare them with a varying darkenergy model proposed by Chevallier, Polarski and Linder in the early 2000s and analyze it deeper.The results show a tipping point from a decelerating universe to an accelerating one at a redshift ofz = 0.35, and a second tipping point back to a decelerating universe in the future at z = −0.19. The flatChevallier-Polarski-Linder (CPL) model leads to a matter dominated universe with Ωm = 0.52 ± 0.08while the relative dark energy density Ωde = 0.48±0.08. Finally, taking all these models and comparingthem with the results that are found from Taylor expanding the distance relation shows that they aremostly consistent with a deceleration parameter around q0 = −0.28.

This work is a preliminary study of the background of a search for dark, long-lived particles in the IceCube detector. The high flux of atmospheric muons in IceCube is considered background to the detector's primary science goal, which is to detect astrophysical neutrinos through the emission of Cherenkov radiation. However, high muon rates may break fresh ground for the detection of hypothetical dark particles. These could be created during the Bremsstrahlung-like interaction of an energetic muon and decay into a muon and an electron. Such an event is expected to produce a characteristic signal in the detector: A track-like signal produced by the Cherenkov-photon emitting muon, followed by a gap due to the electric neutrality of the dark particle. The subsequent decay of the dark particle into a muon produces a further track-like signal. In order to probe the success rate of this endeavor, two precursory statistical analyses are made using simulated data provided by CORSIKA. In order to obtain a clear track-gap signature that is not diluted by other particles, atmospheric muons should preferably travel in a bundle of few to no other muons. The muon multiplicity in incident bundles is estimated. The study reveals that 50% of all muons are single muons at the point of production, while their relative number increases to 70% by the time they reach the detector boundary. A possible background to this search is assumed to be produced by a collinear neutrino interacting after a stop of the muon. The muon may stop due to energy loss or spontaneous decay. In that case, the resulting signal is expected to be identical to the desired track-gap signature. A preliminary background rate is obtained by performing an event selection. The resulting background rate after the selection process is 5e-5 Hz, which is a reasonable result given a neutrino flux of mHz. A more thorough selection taking into account the particle energies as well as IceCube's limited energy resolution is expected to lower the rate.

Aims. The aim of this work is to investigate any presence of transit timing variations (TTVs) in a sample of observed targets that has light curves in both K2 mission data and TESS mission data.  Methods. The original sample utilised here was one from the doctoral thesis of D. Soto (2020) with candidates from K2 data. Cross-referencing for corresponding light curves in TESS was done with a customised Python script created for the purposes of this work, automating the process of obtaining light curves using only one mission ID. A transit search was performed on the light curves of each mission separately with the Python software package OpenTS. The candidates with transits in both light curves were subjected to a TTV search using the Python software package PyTTV. The PyTTV software utilises both mission light curves in creating a joint light curve.  Results. Orbital periods, Porb, and transit center times, t0, for 30 targets were updated using joint light curves from the K2 and TESS missions. Seventeen of these systems are found to have non-linear trends in their transit times. These also have constraints for the periods and amplitudes of the TTVs.  The disposition distribution of the 30 systems is that 19 candidates are unregistered candidates, ten are registered planetary candidates or TOIs and eight are registered known planets.  Conclusions. Out of the reference sample by D. Soto (2020) consisting of 564 targets, 257 targets had a corresponding TESS light curve. Out of the 257 targets, a new sample of 45 targets was contrived through a transit search where they all show distinct transits in both missions light curves. Out of these, fifteen targets were not suitable for a TTV search. A final sample of 30 targets are presented, with seventeen targets showing signs of TTVs and thirteen targets showing a linear trend. The parameters of orbital period and transit centre times are updated for all 30 final candidates. Seventeen candidates are shown to have variations in the transit times and are presented with diagnostics. The candidates with transit timing variations should be further investigated for potential validation or follow-up observations. The unregistered candidates as well as the planetary candidates and TOIs should be considered for follow-up observations or similar validation, to confirm or discard a planetary status.

The determination of the solar oxygen abundance is a topic that has long beenunder debate, where the exact value has varied between researchers. These variationsin the oxygen abundance have implications on our understanding of the Sun and itsinterior as well as our understanding of other stars. The aim of this report is to explorethe di erences between three observations; the Liège, Hamburg and SST, and their effecton the solar oxygen abundance calculated from the 777 nm triplet. In the reportit was found that the abundance from the Liège and SST data was lower and in betteragreement (A(O) ≈ 8.709 dex) compared to the Hamburg data (A(O) ≈ 8.732 dex).One of the potential reasons for these di erences explored in the thesis were the e ectsof magnetic  elds. After comparing the di erences it was argued that the SST dataset as well as the Liège could be considered more reliable compared to the Hamburg.The argued abundance was therefore of a somewhat lower value which does not helplessen the inconsistency between solar models and helioseismology. / Bestämningen av mängden syre i solen har länge varit ett debatterat område där det exakta numeriska värdet varierat bland forskare. Dessa variationer av mängden syre ger konsekvenser för vår förståelse av solen och dess inre samt för vår förståelse av andra stjärnor. Syftet med denna rapport är att utforska skillnaderna mellan tre observationer; från Liège, Hamburg och SST, samt deras påverkan på mängden syre i solen beräknad från 777 nm tripletten. Rapporten fann att mängden syre beräknad från Liège- och SST-datan både var lägre samt stämde bättre överens (A(O) ≈ 8.709 dex) i jämförelse med mängden från Hamburg observationen (A(O) ≈ 8.732 dex). En potentiell anledning till skillnaden i resultat som utforskades i denna uppsats var påverkan av magnetiska fält. Efter jämförelse av skillnaderna kunde det konstateras att datan från SST så väl som från Liège kan anses mer pålitliga i förhållande till Hamburg-datan. Den förespråkade mängden syre var åt det lägre hållet vilket inte bidrar till minskningen av avvikelserna mellan modeller av solen och mätningar via helioseismologi.

Detecting neutrinos, especially ultra-high-energy (UHE) neutrinos, is inherently challenging. Highly sensitive detection devices are required to effectively capture these rare particles, which often results in significant noise in the data. This project focuses on enhancing the detection sensitivity of UHE neutrinos interacting with glacier ice by employing deep learning and transformer models. These models are trained on simulated data that mimics the radio signals produced by neutrino interactions in ice. The developed models have demonstrated improved performance compared to current hardware implementations, offering a promising advancement in neutrino detection technology.

40 light years from Earth an Earth sized exoplanet called Trappist 1d orbits the M-dwarf star called Trappist 1. Trappist 1d is located in the habitable zone where liquid water could exist on the surface of the planet which raises the question: Could Trappist 1d be habitable? Since it is not known what the planet looks like, several simulations of potentially habitable climates were made including different water levels and atmospheric pressures with Earth-like atmospheres. Real observations with JWST and VLT are currently being made for the light passing through Trappist 1d’s potential atmosphere. In order to interpret the data and make any conclusions about the habitability of Trappist 1d, simulated spectra need to be made for the different scenarios. The goal of this project was to produce radiance spectrum of how observations viewed through different instruments would look like for the different planetary scenarios. The result of the project gave spectra that were quite similar, but differed specifically in the depths of the lines, meaning that in theory it could be possible to distinguish between the planetary scenarios via observations. In reality, because of uncertainties in the observations, it is probably not possible to distinguish between the different planetary models, but it might be enough to conclude if the planet has an Earth like atmosphere containing CO2 and H2O or not. / 40 ljusår bort från jorden kretsar en jordlik planet vid namn Trappist 1d runt en röd dvärgstjärna. Trappist 1d ligger i den så kallade beboeliga zonen där det är möjligt för flytande vatten att existera på planetens yta. Detta medför frågan: Finns det förutsättningar för liv på Trappist 1d? Eftersom det inte är känt hur det ser ut på planeten har flera potentiellt beboeliga klimat simulerats för olika vattennivåer och atmosfärstryck med en jordlik atmosfär. Olika instrument på teleskopen JWST och VLT samlar för tillfället in data för observationer genom Trappist 1d:s potentiella atmosfär. För att kunna tolka datan och dra slutsatser om förutsättningarna för liv på Trappist 1d behövs simulerade spektrum att jämföra med. Målet med det här projektet är att producera simulerade radians spektrum för hur observationer med de olika instrumenten hade sett ut för de olika scenarierna. Resultatet gav spektrum som främst skiljde sig i djupet av linjerna i graferna, vilket betyder att i teorin är det möjligt att skilja mellan de olika scenariona för en observation. På grund av osäkerheter i observationen, är det troligtvis inte möjligt i praktiken att se exakt vilket scenario det tillhör, men det kan vara tillräckligt för att säga ifall planeten har en jordlik atmosfär som innehåller vatten och koldioxid eller ej.

In this project we search for a planet in a triple-stellar system that could be habitable and a potential host to complex, human-like life. The first step in the search for potentially habitable planets involved examining catalogs of triple-stellar systems with known exoplanets and quadruple-star systems in which one of the stars could be swapped for a planet. Then, for all the potential planets, we estimated whether they lay within the habitable zone. For all systems with planets in the habitable zone, we used previously published climate model simulations of similar systems to gain a better understanding of the potential climate of these planets. Furthermore, the simulation program VPlanet was used to check the dynamical stability of systems in which one of the stars was swapped with a planet. In total, 10 planets were found to be within the habitable zone and were closer examined, with 6 of them being already existing planets and 4 of them being fabricated. Despite all of the planets lying within the habitable zone, they showed varying degrees of suitability for hosting life, with most planets being substantially cooler than Earth. None of the existing exoplanets had a suitable climate for human-like life, and none of the fabricated systems proved to be dynamically stable. However, the fabricated system that demonstrated the highest amount of stability in simulations was the one in which the planet and the stars were most similar in size. This leads us to conclude that optimal dynamical stability is achieved when the system components are of comparable size.

Analyzing stellar spectra plays a big role in understanding the evolution of our galaxy. Having good data for spectral line properties is very important when analyzing these spectra. One part of the Gaia-ESO public spectroscopic survey (GES) is to gather data for spectral line properties from stellar spectra. The scope of this project is to study one of these properties, the spectral line width caused by collisional broadening by hydrogen. Collisional broadening by hydrogen occurs when a hydrogen atom collides with a particle. The goal of this project is to successfully calculate the collisional spectral line broadening of iron lines where new data is missing from the GES using modern quantum mechanical calculations. These calculations are done using the ABO theory, which is more advanced than previously established theory.A table of Fe-I (Neutral iron) spectral lines without collisional broadening data in the GES has been provided. Using the ABO theory and the accompanying ABO cross section calculator code, estimates of collisional broadening by hydrogen have been calculated for these lines. The new calculations predict that the line width of the spectral lines are typically twice as large compared to older estimates calculated using simpler theory. This new data can be expected to improve stellar spectrum analysis in the Gaia-ESO survey spectra. / Analys av stjärnspektran spelar en stor roll i vår förståelse av vintergatans utveckling. Att ha bra data för spektrallinjers egenskaper är oerhört viktigt vid analys av dessa spektran. En del i Gaia-ESO public spectroscopic survey (GES) är att samla in data för dessa spektrallinjers egenskaper ur stjärnspektran. Omfattningen av detta projekt innefattar att titta närmare på en av dessa egenskaper, spektrallinjebreddning orsakad av kollisionsbreddning av väte. Kollisionsbreddning av väte uppstår när en väteatom kolliderar med en annan partikel. Målet med projektet är att med framgång beräkna kollisionsbreddningen av spektrallinjer från järn där ny data saknas ur GES genom att använda moderna kvantmekaniska beräkningar. Dessa beräkningar är gjorda med den så kallade ABO-teorin, vilken är mer avancerad än tidigare etablerade teorier. En tabell med Fe-I (neutralt järn) spektrallinjer utan kollisionsbreddningsdata i GES har tillhandahållits. Med hjälp av ABO-teorin och den medföljande ABO-tvärsnittsräknar-koden har beräkningar av kollisionsbreddning med väte utförts för dessa linjer. De nya beräkningarna förutser att spektrallinjernas bredd blir typiskt två gånger så stor jämfört med de äldre beräkningarna gjorda med enklare teori. Denna nya data kan förväntas att förbättra analysen av stjärnspektrum ur Gaia-ESO kartläggningen.