1 Acknowledgements Ha llegado el momento de la despedida, les quiero agradecer muchísimo ... realmente estoy muy emocionado. Los quiero mucho, se los digo de verdad, lo siento no sólo como persona sino como ser humano. Lo que siento se resume en una palabra: mil gracias. – Manuel Darío (Les Luthiers, Unen Canto con Humor, 1999) The path to this PhD has been a long one, surely dating back much further than September 2012, but I’m not sure I can pinpoint when it all started. I am sure however that many people have taken part in it along the way, and some of them not less for being almost 12,000 km away from where these last four years have taken place. Leiden is a wonderful, most beautiful place, and I could hardly have chosen a better location to spend these four years and carry out my PhD research. To add to this, the Sterrewacht is a lively, friendly place where I felt at all times welcome. There is a community atmosphere which always made me feel I could rely on anyone at anytime, for any matter at hand. To my working group: Andrej, Fabian, Jelte, Koen, Maciej, Marco, Margot, Martin, Remco and Ricardo, thank you for making our group meetings both useful and enjoyable. I’m glad I got to work with some of you, and hope we’ll do some more work together in the future. Marcello and Massimo, thank you for sharing your experience and wisdom with me. Most of all, thank you Henk for being a true advisor and not just a supervisor. I have learnt tremendously from my interactions with all of you, both within and outside the scientific context. I enjoyed every one of our group meetings (however hard to believe that may be), and the times we’ve shared outside of them even more. I met some amazing people in the Sterrewacht, without whom these four years could not have been nearly as enjoyable. I’m happy I got to know you all through so many breaks, lunches, evenings and weekends. Adam, Allison, Alex North, Alex South, Andra, Berenice and Jesús, Clément, David, Emanuele, Gabi, Henriette, Irene, Jeroen, Leah, María, Marissa, Matteo, Mattia and Anna, Mieke, Monica, Nicola, Renske and Thanja. Maryam, Ali, we had a great time, and some great food, together. We’re glad you got to love manjar as much as we do! Thank you for that wonderful weekend in Zurich. Tiffany, thank you for your advice throughout our three years as office mates, I really enjoyed our conversations, deep and shallow. Elena, Jarle and Huub, thank you for your advice, your help and your friendliness, academic and otherwise. I am very fortunate that such a welcoming and friendly atmosphere extends to people I have interacted with beyond the Sterrewacht itself. It is a great honour and pleasure to be part of the KiDS collaboration. The weak lensing group is just a wonderful team to work with and get advice from. Special thanks to Benjamin, Catherine, Edo, Hendrik, Peter and 2 Tom for your interest and support. Our many meetings throughout the years have been much fun (and much work!), and I look forward to working more with all of you in the future. It is also a great honour and pleasure to be part of the ACT collaboration. I start by thanking Felipe, my MSc advisor in Chile, for believing in me and for introducing me and making me part of such a great team, both in terms of the science carried out and the people involved. Arthur, Lyman, Matt, Matthew, Nick and Toby, thank you for your help and encouragement whenever it was needed, whether requested or not. Jack, Felipe, thank you for making work so much fun when I was just starting my career. From observing runs back several years ago to visits and remote discussions, thank you for helping me develop and grow both personally and scientifically, and for your enthusiasm on all matters. I look forward to seeing more of you in the coming years. I have also made good friends in the Chilean community (broadly defined) in Leiden. You have been a great support throughout these four years. Andrea, Cami, Francisco, Håvar, Javi, Nico, Pablo, Seba, Sole and Vicente. Heather, thank you for taking care of me when I arrived, I really don’t know what I would have done without you. Mariana, Mariano, we have found great friends and the most enjoyable company in you. You managed to make the best of the barbeques allowed by the variety of Dutch meat. We will miss you all. I hope our paths will cross again sometime in the near future A mis amigos de toda la vida, que sin importar la distancia, siguen estando ahí siempre, viviendo mis logros conmigo, así como yo los de ustedes. Arruga, Chasca, Chino, Claudio, Mari, Nico, Pablito, Pame, Tatán, Topo, Watón, gracias a todos. Sé que la distancia no es ningún obstáculo para contar con ustedes para lo que sea, cuando sea, y ustedes saben que es recíproco. Algún día podremos por fin repetir esas tardes de puro tenis, quién sabe dónde. A mi familia. A mis padres, gracias por creer en mí y apoyarme en este camino, y en todos los demás desde el comienzo, sin importar las condiciones. Está de más decirles que no habría podido llegar acá sin su cariño y apoyo. También agradezco a mi hermano, Matías, y a mi hermana, Bárbara, por su apoyo continuo e incondicional. Estoy muy feliz de que todos hayan podido venir a vernos. Sus visitas y los tantos viajes asociados viven en nuestra memoria como momentos maravillosos. A mis abuelos, por su apoyo y por los buenos momentos juntos. Gracias Rosita (suegra) por ser partícipe de este largo camino con nosotros. Finalmente y lo más importante, gracias Dani y Martina. Parte de esta sección está escrita en plural y no singular, porque nada de esto tendría sentido sin ustedes a mi lado. Dani, me siento tremendamente afortunado de tenerte conmigo y de tener a la Martina con nosotros. Gracias por tu amor, tus sonrisas, por acompañarme, soportarme, apoyarme, por darme ánimo y aterrizarme. Siempre sabes cuál corresponde. Martina, tu simpatía e ingenuidad llenan de alegría nuestra casa, eres lo más lindo que he visto en la vida. Las amo desde lo más profundo de mí. Cristóbal Sifón Andalaft Leiden, September 2016