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Cosmology on the Edge of Lambda-Cold Dark Matter

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Universitat de Barcelona. Facultat de Física

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[eng] Cosmology is the science that studies the Universe as whole, aiming to understand its origin, composition and evolution. During the last decades, cosmology has transitioned from a “data staved” to a “data driven” science, inaugurating what is known as precision cosmology. This huge observational effort has confirmed and fostered theoretical research, and established the standard model of cosmology: Lambda-Cold Dark Matter (LCDM). 

This model successfully reproduces most of the observations. However, there are some persistent tensions between experiments that might be smoking guns of new physics beyond this model. Anyways, there is a difference between modeling and understanding, and LCDM is a phenomenological model that, for instance, does not describe the nature of the dark matter or dark energy. This thesis collects part of my research focused on pushing the limits of the standard cosmological model and its assumptions, regarding also existing tensions between experiments. New strategies to optimize the performance of future experiments are also proposed and discussed.

The largest existing tension is between the direct measurements of the Hubble constant using the distance ladder in the local Universe and the inferred value obtained from observations of the Cosmic Microwave Background when LCDM is assumed. A model independent reconstruction of the late-time expansion history of the Universe is carried out, which allows us to identify possible sources and solutions of the tension. We also introduce the concept of the low redshift standard ruler, and measure it in a model independent way. Finally, we introduce a statistical methodology to analyze several data sets in a conservative way, no matter the level of discrepancy between them, accounting for the potential presence of systematic errors. 

The role of primordial black holes as candidates for dark matter is addressed in this thesis, too. Concretely, the impact of an abundant population of primordial black holes in the rest of cosmological parameters is discussed, considering also populations with extended mass distributions. In addition, massive primordial black holes might be the seeds that are needed to explain the origin of the supermassive black holes located in the center of the galaxies. We predict the contribution of a population of massive primordial black holes to the 21 cm radiation from the dark ages. This way, observations of the 21 cm intensity mapping observations of the dark ages could be used to ascertain if the seeds of the supermassive black holes are primordial.

Finally, we estimate the potential of radio-continuum galaxy surveys to constrain LCDM. These kind of experiments can survey the sky quicker than spectroscopic and optical photometric surveys and cover much larger volumes. Therefore, they will be specially powerful to constrain physics which has impact on the largest observable scales, such as primordial non Gaussianity. On the other hand, intensity mapping experiments can reach higher redshifts than galaxy surveys, but the cosmological information of this signal is coupled with astrophysics. We propose a methodology to disentangle astrophysics and optimally extract cosmological information from the intensity mapping spectrum. Thanks to this methodology, intensity mapping will constrain the expansion history of the Universe up to reionization, as shown in this thesis.

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[spa] El modelo estándar de cosmología, LCDM, se apoya en una cantidad ingente de observaciones extremadamente precisas, que es capaz de reproducir con gran exactitud. Sin embargo, este es un modelo fenomenológico que no es capaz de responder algunas de las preguntas fundamentales sobre el Universo, como la naturaleza de la materia oscura o la energía oscura. Además, cuando este modelo se utiliza para interpretar las observaciones, aparecen tensiones entre experimentos independientes. Estas tensiones, en el caso de no estar producidas por errores sistemáticos no tenidos en cuenta, necesitarían un modelo cosmológico diferente para ser resueltas.

Esta tesis recoge trabajos publicados en revistas científicas investigando estos problemas de LCDM. Concretamente, se cubren tres temas principales: la tensión en la constante de Hubble entre las medidas directas usando la escalera de distancias y los valores inferidos a partir de las observaciones de la colaboración Planck asumiendo LCDM; el rol de los agujeros negros primordiales como semillas de los agujeros negros supermasivos, o como candidato para conformar una parte significativa de la materia oscura; y el potencial y las estrategias óptimas a aplicar en experimentos que mapean la estructura a gran escala del Universo para examinar LCDM y medir posibles desviaciones del modelo.

De este modo, el trabajo aquí recogido tiene como objetivo investigar las tensiones presentes en LCDM, así como las preguntas que deja sin responder de una manera crítica y desde un punto de vista agnóstico. Además, pretende sentar las bases para futuras investigaciones en estas líneas, cuando estén disponibles nuevas y mejores observaciones, e indicar el camino para poder poner a prueba el modelo estándar de cosmología en los años venideros en regímenes en los que aún no se ha hecho ninguna medida.

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Cosmology on the Edge of Lambda-Cold Dark Matter