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The work presented in my thesis develops advanced Bayesian statistical methods for using astrophysical data to probe our understanding of the Universe, I cover three main areas: Should we doubt the cosmological constant? While Bayesian model selection is a useful tool to discriminate between competing cosmological models, it only gives a relative rather than an absolute measure of how good a model is. Bayesian doubt introduces an unknown benchmark model against which the known models are compared, thereby obtaining an absolute measure of model performance in a Bayesian framework. I apply this new methodology to the problem of the dark energy equation of state, comparing an absolute upper bound on the Bayesian evidence for a presently unknown dark energy model against a collection of known models including a flat Lambda cold dark matter ( CDM ) scenario. I find a strong absolute upper bound to the Bayes factor between the unknown model and CDM. The posterior probability for doubt is found to be less than 13 per cent (with a 1 per cent prior doubt) while the probability for CDM rises from an initial 25 per cent to almost 70 per cent in light of the data. I conclude that CDM remains a sufficient phenomenological description of currently available observations and that there is little statistical room for model improvement Improved constraints on cosmological parameters from supernovae type Ia data: I present a new method based on a Bayesian hierarchical model to extract constraints on cosmological parameters from SNIa data obtained with the SALT-II lightcurve fitter. I demonstrate with simulated data sets that our method delivers considerably tighter statistical constraints on the cosmological parameters and that it outperforms the usual chi-square approach 2/3 of the times. As a further benefit, a full posterior probability distribution for the dispersion of the intrinsic magnitude of SNe is obtained. I apply this method to recent SNIa data and find that it improves statistical constraints on cosmological parameters from SNIa data. From the combination of SNIa, CMB and BAO data I obtain Ωm = 0:28 ± 0:02; ΩΛ = 0:73 ± 0:01 (assuming w = -1) and Ωm = 0:28 ± 0:01, w = -0:90 ± 0:05 (assuming flatness; statistical uncertainties only). I constrain the intrinsic dispersion of the B-band magnitude of the SNIa population, obtaining σ int μ = 0:13 ± 0:01[mag]. Robustness to systematics for future dark energy probes: I extend the Figure of Merit formalism usually adopted to quantify the statistical performance of future dark energy probes to assess the robustness of a future mission to plausible systematic bias. I introduce a new robustness Figure of Merit which can be computed in the Fisher Matrix formalism given arbitrary systematic biases in the observable quantities. I argue that robustness to systematics is an important new quantity that should be taken into account when optimizing future surveys. I illustrate our formalism with toy examples, and apply it to future type Ia supernova (SNIa) and baryonic acoustic oscillation (BAO) surveys. For the simplified systematic biases that I consider, I find that SNIa are a somewhat more robust probe of dark energy parameters than the BAO. I trace this back to a geometrical alignment of systematic bias direction with statistical degeneracy directions in the dark energy parameter space.

We study the non-linear structure formation in cosmologies where the collision-less cold dark matter (CDM) is either replaced by interacting dark matter or (partly) replaced by a free-streaming non-cold dark matter component. We focus in the first case on models with a non-vanishing interaction cross-section between dark matter and radiation in the early Universe, i.e. photons (γCDM) and neutrinos (νCDM). We study the properties of the dark matter structures that form in the presence of the collisional damping using N-Body simulations. For their halo shapes, we find similar effects as for standard thermalized fermionic Warm Dark Matter (WDM). However, for the abundance of these structures, the interacting DM models are clearly distinguishable from WDM below the characteristic damping scale. We also have a closer look at dark matter halos that resemble those hosting the two main galaxies in our Local Group, the Milky Way (MW) and Andromeda (M31). By using a high-resolution zoom-simulation of Local Group-like environments, we reveal how the DM-radiation interactions help to ease certain CDM ”small scale problems”. Furthermore, the combination of these Local Group simulations with our previous cosmological simulations allows us to constrain the cross-section in our model by comparing the abundance of satellite galaxies in our Milky Way with the predictions for subhaloes. Thanks to the sensitivity of the subhalo abundance to the suppression of the primordial perturbations, even our most conservative constraints are orders of magnitude tighter than those previously obtained from CMB data. In the case of neutrinos or other non-cold dark matter, we study ways to predict numerically the evolution of this free-streaming component correctly. We identify shortcomings in all the previously proposed techniques we encountered in our studies of various models with massive neutrinos and come up with a new, adaptive Eulerian technique to treat the neutrino fluid accurately. In particular, we introduce our implementation called SEPARA. First test results for the code are presented while full cosmological simulations will be performed in the near future.

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Measurements of the diffuse Galactic emission are used for both the interpretation of sensitive cosmic microwave background data and the understanding of our Galaxy. I examine the diffuse Galactic emission at a central frequency of 4.76 GHz using data from the C-Band All-Sky Survey (C-BASS). The technical work presented here fo- cuses on microphonic oscillation mitigation, receiver noise measurements and radio frequency interference detection. Northern C-BASS is a continuous comparison ra- diometer with a system temperature of 40 K, a knee frequency of 0.10 mHz in polari- sation and a noise level of 2 mK sqrt(s). A calibration scheme was devised, using astro- nomical calibrators, to convert the data to Kelvin and correct for atmospheric opacity. This scheme is stable to 1 % over several months and accurate to better than 5 %. A major systematic in the C-BASS data is ground emission. In this work the ground emission is modelled and subtracted from the data resulting. A first scientific anal- ysis of C-BASS Northern intensity data was made to investigate the contributions of free-free and synchrotron emission within the Galactic plane. The synchrotron spectral index was determined to be −2.63±0.07 between 0.408 and 4.76 GHz and −2.71±0.14 between 1.420 and 4.76 GHz and the ratio of free-free to synchrotron emission at 4.76 GHz was found to be 53± 8 % to 47± 8 %. Bringing in higher frequency data allowed for the detection of anomalous microwave emission associated with W43, W44 and W47 at a level of 4.9σ, 6.0σ and 3.4σ, respectively and demonstrated the need for C-BASS data to constrain the spectral form of AME within certain regions.

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The volatile and noble gas inventories of comets are poorly understood relative to asteroids. Interplanetary dust particles (IDPs) and micrometeorites are thought to originate predominantly from comets, therefore they provide a means of investigating cometary volatiles which can be compared to unequilibrated meteorites and help us understand the relationship between comets and asteroids. In preparation for Xe analyses, we have determined the bulk elemental compositions, H, C, N and O isotopic compositions and carbonaceous content of 26 IDPs, in order to correlate any Xe components with particular, possibly primitive cometary, characteristics. The IDPs as a whole show great heterogeneity in their mineralogy and isotopic compositions, in accordance with other studies. We observed some extreme isotopic enrichments in D and 15N, including correlations between localised D enrichments and C/H ratios, leading to a revision of the organic endmember components devised by previous authors. The Raman parameters we report also agree with other studies on the highly disordered nature of the carbon in IDPs relative to chondrites. We observed two unusual IDPs containing previously unreported features; including a Pb-chromate and large Ti-oxide grain in a highly refractory IDP, and an extremely isotopically anomalous terminus particle in a hybrid IDP; which could be an interstellar grain impact. Perhaps the most interesting particle we analysed here is composed of 50 wt% C, is highly N-rich and contains a large abundance of presolar silicates, some of which appear to be GEMS. This C-rich “ultra-carbonaceous” IDP could provide an important link to the organic material in primitive chondrites and comets. We also report the first measurement of Xe in three individually analysed IDPs, one of which is only 8 microns in diameter, and thirteen AMMs up to 150 microns in diameter. The AMMs all yielded gas, with some enabling Xe isotopic compositions to be roughly determined. These were mostly a mixture of trapped Q, solar wind and terrestrial air, with some showing 129I-derived, presolar HL and fission-derived Xe influence, similar to the range in isotopic compositions observed in chondrites. The 132Xe concentrations in the AMMs also span the range observed in chondrites. The ultra-carbonaceous, N-rich and highly isotopically anomalous IDP has yielded a 132Xe concentration, normalised to chondritic IOM content, in the same range as IOM in primitive chondrites. This suggests a possible common Q-carrier distribution in comets and asteroids, therefore further indicating wide-scale mixing in the solar nebula.