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Future astrophysical surveys will provide unprecedented datasets in both scale and precision for testing our most concordant theories of dark matter and dark energy. For the first time, the confidence of our understanding of the components of the Universe will be limited by systematic uncertainties rather than the statistical precision we can achieve. Improved modeling of measurement biases in survey data is necessary to achieve the theory-constraining power of ambitious future astrophysical surveys. Recent in-lab, on-sky, and simulated experiments offer opportunities to investigate coherence between systematic and phenomenological observables and invent new techniques to differentiate them. This thesis details my contribution to measuring the Dark Universe, and it is structured in two parts. The first part of this thesis improves template modeling of the foreground gamma-ray emission in the Galactic plane in 12 years of Fermi-LAT data. This has implications for the observed excess \gray{} emission near the Galactic center, which was previously associated with dark matter. We compare the likelihood of observing gamma-ray emission from cold hydrogen gas clouds using models derived from spectral line emission maps of abundant ($^{12}\mathrm{CO}$) and rare ($^{13}\mathrm{CO}$) tracer molecules. Using this approach, the rare tracer predicts previously unmodeled gamma-ray emission at $47\sigma$ significance, localized to the pixels around the densest regions, where emission is undercounted due to reabsorption along the line-of-sight. We also identify unassociated point-like sources in the Galactic plane which were previously smoothed out by re-absorption consistent with peaked gas structure as well as point-like gas structure which could have been mistaken for millisecond pulsars by earlier works. We further utilize a convolutional neural network (CNN) to try to predict the $^{13}\mathrm{CO}$ distribution in the same region, which could prove useful to predict the $^{13}

The Legacy Survey of Space and Time (LSST) carried out by the Vera C. Rubin Observatory is one of the most ambitious optical surveys planned in the near future. It will generate $\sim 20$~TB of data every night during its 10 year survey duration. Simulated data sets (``Data Challenges'') have been generated by the LSST Dark Energy Science Collaboration (DESC) to allow development of efficient analysis algorithms, evaluation of their performance and scaling capabilities, and to study the impact of systematic uncertainties. In this work we focus on the analysis of Large Scale Structure using galaxies selected from the DESC Data Challenge 2 (DC2) corresponding to a survey area of 300 square degrees. We develop and perform an end-to-end analysis of the galaxy Large Scale Structure with the DC2 data, validating the extraction of galaxy power spectra and galaxy bias in the presence of complex systematics. We provide a range of strong consistency checks of both the simulated data and the analysis techniques.Based on matching selected galaxies with the DC2 truth catalog, we optimize selection to obtain a clean sample of galaxies. After generating and applying survey masks, we produce galaxy density maps binned in redshift, with six bins from $z = 0.2 - 1.4$. We generate the power spectra measurements from the processed data and fit the data using theoretical predictions to estimate the galaxy bias as a function of redshift. Finally, we study the effect of systematic uncertainties on estimation of the galaxy bias.We find that the relative uncertainty $\sigma_\ell / C_\ell$ in the measured angular power spectra is smaller than 10\% in most cases, and smaller than 4\% at multipoles above 1500.% Changes in the angular power spectra due to systematic uncertainties are found to be on average of order 0.5\%, i.e. $\Delta C_\ell / C_\ell \approx 0.5\%$, with a maximum deviation of 1.5\%. % Systematic effects are small compared to the statistical uncertainty of the measured angular

This report is the result of a joint discussion between the Rubin and Euclid scientific communities. The work presented in this report was focused on designing and recommending an initial set of Derived Data products (DDPs) that could realize the science goals enabled by joint processing. All interested Rubin and Euclid data rights holders were invited to contribute via an online discussion forum and a series of virtual meetings. Strong interest in enhancing science with joint DDPs emerged from across a wide range of astrophysical domains: Solar System, the Galaxy, the Local Volume, from the nearby to the primaeval Universe, and cosmology.

Survey astronomy has become a powerful tool for discoveries in astrophysics and cosmology. In the coming years this approach will be taken even further with the start of the ten year survey of the Large Synoptic Survey Telescope. This instrument, with its unique capabilities in temporal sampling, single-image depth, and covered sky-area, will explore wholly new parts of parameter space of known systems and the Universe. The time at which this instrument is coming online also presents a unique opportunity, given the recent discovery of multimessenger transients emitting both gravitational and electromagnetic signals, to study the population of binary neutron star mergers in the Universe. This will be scientifically beneficial, not only for studies of the astrophysics of these sources, but also for determination of fundamental cosmological parameters. Given the reach of the LSST, it is expected that this instrument will detect these binary neutron star mergers to greater distances than detectable by current and near-term gravitational wave detectors. This presents further scientific opportunity to study the selection effects for detection of these sources in gravitational waves, and also potentially to recover the undetected gravitational wave signals counterpart to the detection their associated electromagnetic emission. In this thesis I give a brief summary of survey astronomy, the LSST instrument and observing strategy, multimessenger astronomy and the use of binary neutron star mergers as cosmological standard sirens. I then outline the work I have undertaken to optimise the observing strategy of the LSST to detect binary neutron star mergers, and the determination that indeed a significant portion of these detected objects will be subthreshold to detection of their gravitational wave emission. Then I outline the current work to produce self-consistent simulations of a population of these events which will be useful for studying the combined selection function of

The scientific development of astronomy today mainly comes from the information that can be obtained via electromagnetic radiation. Today, the Gaia optical telescope is one of the most ambitious space projects man has ever completed. Another optical telescope LSST is designed to complement Gaia. In about 15 years it will also be possible to measure gravitational waves with the telescope LISA, which will detect binary stars, especially white dwarfs. Using gravitational waves will give completely different types of measurements. It has also been found that a combination of the data from the two methods will give the possibility of a synergy effect, which means that it provides more information about the Universe than the sum of the information from each telescope. This is called multi-messenger astronomy, which in many areas has resulted in a remarkable development of opportunities in recent years (Egberts (2020)). Synergy effects will make it easier to identify objects and reduce uncertainty in distance. It gives the possibility to calculate extra values such as the mass of the respective stars in binaries. Multi-messenger astronomy will also help understand the historical evolution of stars, galaxies and dark matter. Different reviews are presented on the possibility of performing multi-messenger detection by Gaia, LSST and LISA. They have been discussed in many different papers. The purpose of this thesis is to review these observations and find out what their synergies together give which could not be achieved alone., Astronomins vetenskapliga utveckling kommer idag främst från den information som kan erhållas via ljusstrålning från teleskop. Det viktigaste är teleskopet Gaia, som är ett av de mest ambitiösa rymdprojekt som människan någonsin har genomfört. Det kartlägger över en miljard stjärnor med position, avstånd och rörelse. Genom Gaias observationer skapas den första realistiska bilden av hur vår galax, Vintergatan, ser ut. Gaia lanserades av Europeiska rymdorganisationen och började skanningen av himlen 2014. Ett annat optiskt teleskop LSST, Large Synoptic Survey Telescope, är utformat för att komplettera Gaia genom att detektera ännu svagare objekt längre bort. Det är ett markbaserat teleskop som för närvarande byggs i Chile i ett samarbete mellan offentliga och privata organisationer. Ett än mer avancerat projekt, som förväntas vara klart år 2034, ska ge möjlighet att mäta gravitationsvågor med teleskopet LISA, LaserInterferometer Space Antenna. Det kommer att upptäcka dubbelstjärnor, som kretsar runt samma tyngdpunkt och kommer mestadels att vara vita dvärgar. Även neutronstjärnor och svarta hål kommer att detekteras då de också är små och kan rotera snabbt. Att använda gravitationsvågor ger helt olika typer av mätningar. LISA kommer att kunna kartlägga och studera utvecklingen av dubbelstjärnor i Vintergatan, som utgör hälften av alla stjärnor från det vi vet hittills. Projekteringen leds av Europeiska rymdorganisationen med stöd från NASA. Gaia, LSST och LISA mäter delvis samma föremål. För dessa kan en kombination av informationen från olika teleskop möjliggöra en synergieffekt. Det innebär att det ger mer information än summan av informationen från respektive teleskop. Detta kallas multi-messenger astronomi, som i många sammanhang har haft en enastående utveckling under de senaste åren. Olika undersökningar har utförts om möjligheten att uppnå synergieffekter av dessa tre teleskop. De har diskuterats i många olika artiklar. I detta arbetet, har jag granskat dessa o

The scientific development of astronomy today mainly comes from the information that can be obtained via electromagnetic radiation. Today, the Gaia optical telescope is one of the most ambitious space projects man has ever completed. Another optical telescope LSST is designed to complement Gaia. In about 15 years it will also be possible to measure gravitational waves with the telescope LISA, which will detect binary stars, especially white dwarfs. Using gravitational waves will give completely different types of measurements. It has also been found that a combination of the data from the two methods will give the possibility of a synergy effect, which means that it provides more information about the Universe than the sum of the information from each telescope. This is called multi-messenger astronomy, which in many areas has resulted in a remarkable development of opportunities in recent years (Egberts (2020)). Synergy effects will make it easier to identify objects and reduce uncertainty in distance. It gives the possibility to calculate extra values such as the mass of the respective stars in binaries. Multi-messenger astronomy will also help understand the historical evolution of stars, galaxies and dark matter. Different reviews are presented on the possibility of performing multi-messenger detection by Gaia, LSST and LISA. They have been discussed in many different papers. The purpose of this thesis is to review these observations and find out what their synergies together give which could not be achieved alone., Astronomins vetenskapliga utveckling kommer idag främst från den information som kan erhållas via ljusstrålning från teleskop. Det viktigaste är teleskopet Gaia, som är ett av de mest ambitiösa rymdprojekt som människan någonsin har genomfört. Det kartlägger över en miljard stjärnor med position, avstånd och rörelse. Genom Gaias observationer skapas den första realistiska bilden av hur vår galax, Vintergatan, ser ut. Gaia lanserades av Europeiska rymdorganisationen och började skanningen av himlen 2014. Ett annat optiskt teleskop LSST, Large Synoptic Survey Telescope, är utformat för att komplettera Gaia genom att detektera ännu svagare objekt längre bort. Det är ett markbaserat teleskop som för närvarande byggs i Chile i ett samarbete mellan offentliga och privata organisationer. Ett än mer avancerat projekt, som förväntas vara klart år 2034, ska ge möjlighet att mäta gravitationsvågor med teleskopet LISA, LaserInterferometer Space Antenna. Det kommer att upptäcka dubbelstjärnor, som kretsar runt samma tyngdpunkt och kommer mestadels att vara vita dvärgar. Även neutronstjärnor och svarta hål kommer att detekteras då de också är små och kan rotera snabbt. Att använda gravitationsvågor ger helt olika typer av mätningar. LISA kommer att kunna kartlägga och studera utvecklingen av dubbelstjärnor i Vintergatan, som utgör hälften av alla stjärnor från det vi vet hittills. Projekteringen leds av Europeiska rymdorganisationen med stöd från NASA. Gaia, LSST och LISA mäter delvis samma föremål. För dessa kan en kombination av informationen från olika teleskop möjliggöra en synergieffekt. Det innebär att det ger mer information än summan av informationen från respektive teleskop. Detta kallas multi-messenger astronomi, som i många sammanhang har haft en enastående utveckling under de senaste åren. Olika undersökningar har utförts om möjligheten att uppnå synergieffekter av dessa tre teleskop. De har diskuterats i många olika artiklar. I detta arbetet, har jag granskat dessa o