>Magister Scientiae - MSc / In the post-reionisation universe, intensity mapping (IM) with the 21 cm line of neutral hydrogen (HI) provides a potential means of probing the large-scale structure of the universe. With such a probe, a wide variety of interesting phenomena such as the Baryon Acoustic Oscillations (BAO) and Redshift Space Distortions (RSD) can be studied. The MeerKAT telescope has the potential to make full use of this technique, especially in the single-dish mode, which will probe the scales relevant to BAO and RSD. A useful complementary of this is HI IM with MeerKAT in interferometer-mode, which will enable the extraction of cosmological information on semi-linear and small scales. In this study, full end-to-end simulations of interferometric observations with MeerKAT for HI IM were developed. With this, the power spectrum extraction was analysed using the foreground avoidance technique. This took into account the foreground wedge from point source contamination extracted from real MIGHTEE COSMOS data, as well as RFI flagging. The errors on the power spectrum estimator were then calculated through a Monte Carlo process using 1000s of realisations of both the thermal noise and HI signal. In doing so, precision constraints on the HI power spectrum are found at z = 0:27 on scales 0:4 < k < 10 Mpc-1 for mock visibility data sets which contain the HI signal contaminated by noise, mimicking the MIGHTEE COSMOS field for total observation times & 20 hours. These results illustrate the potential of doing precision cosmology with MeerKAT’s MIGHTEE survey and interferometer-mode HI IM.

>Magister Scientiae - MSc / In the post-reionisation universe, intensity mapping (IM) with the 21 cm line of neutral hydrogen (HI) provides a potential means of probing the large-scale structure of the universe. With such a probe, a wide variety of interesting phenomena such as the Baryon Acoustic Oscillations (BAO) and Redshift Space Distortions (RSD) can be studied. The MeerKAT telescope has the potential to make full use of this technique, especially in the single-dish mode, which will probe the scales relevant to BAO and RSD. A useful complementary of this is HI IM with MeerKAT in interferometer-mode, which will enable the extraction of cosmological information on semi-linear and small scales. In this study, full end-to-end simulations of interferometric observations with MeerKAT for HI IM were developed. With this, the power spectrum extraction was analysed using the foreground avoidance technique. This took into account the foreground wedge from point source contamination extracted from real MIGHTEE COSMOS data, as well as RFI flagging. The errors on the power spectrum estimator were then calculated through a Monte Carlo process using 1000s of realisations of both the thermal noise and HI signal. In doing so, precision constraints on the HI power spectrum are found at z = 0:27 on scales 0:4 < k < 10 Mpc-1 for mock visibility data sets which contain the HI signal contaminated by noise, mimicking the MIGHTEE COSMOS field for total observation times & 20 hours. These results illustrate the potential of doing precision cosmology with MeerKAT’s MIGHTEE survey and interferometer-mode HI IM.

This thesis presents instrumentation with which to measure the abundance of neutral hydrogen gas in the Universe. Measuring where the Universe’s hydrogen is, and tracing how its distribution evolves with time, holds the key to understanding how galaxies evolve, the nature of dark energy, and how the first cosmic structures formed. In particular, this thesis looks at instrumentation for 21-cm intensity mapping telescopes. In 21-cm intensity mapping, the collective emission of many galaxies is measured, without individual detections. This technique promises to allow detection of the baryonic acoustic oscillation peaks in the power spectrum of the Universe’s matter distribution. Such a detection would increase constraints on cosmological parameters. There are two main approaches to designing a 21-cm intensity mapping instruments: using a filled aperture instrument such as a single-dish telescope, or using a sparse aperture instrument such as an interferometric array of dipoles. This thesis investigates analogue components for a sparse aperture instrument operating at 1.0-1.5 GHz. As part of this work, a 16-element sparse aperture array was designed and constructed. To test the array’s performance, field testing was conducted; the results of which are presented here. In addition to this, I have designed a new digital spectrometer for redshifted hydrogen line science, named HISPEC. A copy of this spectrometer has been installed on the Parkes 64 m telescope, as a digital signal processor for the 21-cm multibeam receiver. HISPEC has increased instantaneous bandwidth, higher interchannel isolation, and improved quantization efficiency as compared to the existing backend, MBCORR. The HISPEC equipped multibeam receiver is an ideal instrument for 21-cm intensity mapping at redshifts z<0.2.

Intensity mapping (IM) is a new observational technique to survey the large-scale structure of matter using emission lines, such as the 21 cm emission line of atomic hydrogen (HI) and the rotational lines of the carbon monoxide molecule (CO). Sensitive radio surveys have the potential to detect the HI power spectrum at low redshifts (z &lt;1) in order to constrain the properties of dark energy and massive neutrinos. Observations of the HI signal will be contaminated by instrumental noise and, more significantly, by astrophysical foregrounds, such as the Galactic synchrotron emission, which is at least four orders of magnitude brighter than the HI signal. In this thesis, we study the ability of the Generalized Needlet Internal Linear Combination (GNILC) method to subtract radio foregrounds and to recover the cosmological HI signal for HI IM experiments. The GNILC method is a new technique that uses both frequency and spatial information to separate the components of the observed data. For simulated radio observations including HI emission, Galactic synchrotron, Galactic free-free, extragalactic point sources and thermal noise, we find that it can reconstruct the HI plus noise power spectrum with 7.0% accuracy for 0.13 &lt;z &lt;0.48 (960 - 1260 MHz) and l &lt;400. In this work, GNILC is also applied to a particular CO IM experiment: the CO Mapping Array Pathfinder (COMAP). In this case, the simulated radio observations include CO emission, Galactic synchrotron, Galactic free-free, Galactic anomalous microwave emission, extragalactic point sources and thermal noise. We find that GNILC can reconstruct the CO plus noise power spectra with 7.3% accuracy for COMAP phase 1 (l &lt;1800) and 6.3% for phase 2 (l &lt;3000). In both cases, we have 2.4 &lt;z &lt;3.4 (26 - 34 GHz). In this work, we also forecast the uncertainties on cosmological parameters for the upcoming HI IM experiments BINGO (BAO from Integrated Neutral Gas Observations) and SKA (Square Kilometre Array) phase-1 dish array operating in auto-correlation mode. For the optimal case of BINGO with no foregrounds, the combination of the HI angular power spectra with Planck results allows w to be measured with a precision of 4%, while the combination of the BAO acoustic scale with Planck gives a precision of 7%. We consider a number of potentially complicating effects, including foregrounds and redshift dependent bias, which increase the uncertainty on w but not dramatically; in all cases the final uncertainty is found to be less than 8% for BINGO. For the combination of SKA-MID in auto-correlation mode (total-power) with Planck, we find that, in ideal conditions, w can be measured with a precision of 4% for the redshift range 0.35 &lt;z &lt;3 (350 - 1050 MHz) and 2% for 0 &lt;z &lt;0.49 (950 - 1421 MHz). Extending the model to include the sum of neutrino masses yields a 95% upper limit of less than 0.30 eV for BINGO and less than 0.12 eV for SKA phase 1, competitive with the current best constraints in the case of BINGO and significantly better in the case of SKA.

Philosophiae Doctor - PhD / The next generation of radio telescopes will revolutionize cosmology by providing large three-dimensional surveys of the universe. This work presents forecasts using the technique 21cm intensity mapping (IM) combined with results from the cosmic microwave background, or mock data of galaxy surveys. First, we discuss prospects of constraining curvature independently of the dark energy (DE) model, finding that the radio instrument HIRAX will reach percent-level accuracy even when an arbitrary DE equation of state is assumed. This is followed by a study of the potential of the multi-tracer technique to surpass the cosmic variance limit, a crucial method to probe primordial non-Gaussianity and large scale general relativistic e↵ects. Using full sky simulations for the Square Kilometre Array phase 1 (SKA 1 MID) and the Large Synoptic Survey Telescope (LSST), including foregrounds, we demonstrate that the cosmic variance contaminated scenario can be beaten even in the noise free case. Finally, we derive the signal to noise ratio for the cosmic magnification signal from foreground HI intensity maps combined with background galaxy count maps. Instruments like SKA1 MID and HIRAX are highly complementary and well suited for this measurement. Thanks to the powerful design of the planned radio instruments, all results confirm their potential and promise an exciting future for cosmology.

L'analyse des propriétés statistiques de la distribution de la matière dans le cosmos (Grandes Structures, LSS or Large Scale Structure) est l'une des principales sondes cosmologiques qui permettent l'étude du modèle standard cosmologique, en particulier les paramètres caractérisant la matière noire et l'énergie noire. Les Oscillations Acoustiques Baryoniques (BAO's) sont l'une des mesures qui peuvent être extraites de l'étude de la distribution de matière à grande échelle (LSS).L'observation de la distribution cosmique de la matière à partir de l'émission à 21 cm de l'hydrogène atomique neutre (HI) est une nouvelle méthode, complémentaire des relevés optiques pour cartographier la distribution de la matière dans le cosmos. La méthode de cartographie d'intensité (Intensity Mapping) a été proposée depuis moins d'une dizaine d'années comme une méthode efficace pour cartographier en trois dimensions l'émission radio à 21 cm. Elle n'implique en particulier pas la détection des objets individuels (galaxies), et peut donc être effectué avec des instruments plus modestes en taille que ceux comme SKA ou FAST qui sont conçus pour détecter les galaxies à 21 cm à des distances cosmologiques. Des interféromètres radio utilisant un ensemble de réflecteurs cylindriques ou paraboliques fixes, observant le ciel en mode transit sont adaptés à la cartographie d'intensité. Le mode d'observation spécifique de ce type de radio télescope en cartographie d'intensité est étudié dans le cadre de ce travail de thèse. On montre en particulier qu'une méthode spécifique de reconstruction des cartes du ciel à partir des visibilités peut être appliquée aux observations de ces interféromètres fonctionnant en mode transit. Cette méthode correspond à la décomposition en modes m des harmoniques sphériques et est très performante pour la reconstruction de grandes zones du ciel observées en mode transit. Un code de reconstruction fondé sur ce principe a été développé, ainsi que différents critères de comparaison des performances instrumentales, comme le lobe d'antenne synthétisé, le spectre de bruit sur les cartes reconstruites et la réponse globale de l'instrument dans le plan (l,m) des harmoniques sphériques. La méthode a été appliquée à différentes configurations des interféromètres composés de réflecteurs paraboliques ou cylindriques dans le cadre des projets PAON-4 et Tianlai. Outre l'optimisation des configurations des interféromètres Tianlai et PAON-4, le travail présenté inclut une première application de la méthode aux données PAON-4. / The analysis of the statistical properties of the distribution of matter in the cosmos (LSS or Large Scale Structure) is one of the main cosmological probes that allow the study of the cosmological standard model, in particular the parameters characterizing dark matter and dark energy. Baryonic Acoustic Oscillations (BAO's) are one of the measurements that can be extracted from the study of matter distribution in large-scale structure (LSS).The observation of the cosmic distribution of the matter from neutral atomic hydrogen (HI) 21 cm emission is a new method, complementary to the optical observation to map the distribution of matter in the cosmos. In the last decade, the Intensity Mapping method has been proposed as an effective method for mapping the 21cm radio emission in three dimensions. In particular, it does not require the detection of individual objects (galaxies), and can therefore be performed with instruments smaller in size than those such as SKA or FAST, which are designed to detect 21 cm galaxies at cosmological distances. A radio interferometer using a set of fixed cylindrical or parabolic reflectors observing the sky in transit mode are suitable instruments for intensity mapping surveys. The specific observational mode from this type of radio telescope by intensity mapping is studied in the context of this thesis. We show in particular that a specific sky maps reconstruction method from the visibilities can be applied to the observations of these interferometers operating in transit mode. This method corresponds to the m-modes decomposition of the spherical harmonics and is very efficient for the reconstruction of large sky areas observed in transit mode. A reconstruction code based on this principle has been developed, as well as different criteria for the comparison of instrumental performances, such as the synthesized antenna lobe, the noise spectrum of the reconstructed maps and the overall instrument response in the spherical harmonics (l,m) plane. The method has then been applied to different configurations of interferometers composed of parabolic or cylindrical reflectors in the PAON-4 and Tianlai projects. In addition to optimizing the Tianlai and PAON-4 interferometer configurations, the work presented here includes a first application of the method to the PAON-4 data.