How cosmology can help and benefit from other areas of physics

[eng] CONTEXT: The current ΛCDM cosmological paradigm has seen remarkable success in recent decades (whether it is the discovery of the CMB, the formation of large-scale structures, the expansion of the universe etc.). Like any paradigm, there comes a point when the theory reaches its limits and the question arises as to whether it is simply due to the incompleteness of the theory or whether it should be questioned. In this thesis, I focused particularly on two aspects of the ΛCDM theory: how to effectively include massive neutrinos in the formation of structures and find a methodology to bring a new perspective in the tension surrounding the expansion of the universe. AIMS: Our objective for this thesis is to show how cosmology can help and benefit from other areas of physics because there is a mutual interest in tackling problems at the interface between cosmology and other areas of physics. METHODS: In the first part of the thesis, I devoted myself to the development of an emulator in order to be able to quickly and easily implement the effect of massive neutrinos on the clustering of dark matter halos. For that I used the HADES numerical simulations to calibrate the bias (a very useful quantity in cosmology which describes the relation between the clustering of dark matter and the different observables). By combining the result of these calibrations with a software which makes it possible to calculate the perturbative terms of the power spectrum, I was able to develop a competitive emulator up to non-linear scales. Secondly, I devoted myself to the study of globular clusters in a cosmological con- text. I used a catalog of clusters provided by the HST telescope in the F606W and F814 filters. After removing the clusters with poor photometry, I performed a Bayesian analysis which allowed me to constrain the various parameters common to stars in the same cluster. Models that describe stellar evolution, like the ones I used for the analysis, are often defined for a given set of parameters that vary from model to model. Therefore to reduce the systematic errors due to the use of a specific stellar model, I studied the influence of the parameters governing the different evolutionary phases of stars with the MESA software. RESULTS: The work carried out within the framework of the realization of an emulator, allowed us to highlight important results such as: even if the shape of the bias must be calibrated using numerical simulations, its amplitude can be simply rescaled by a factor proportional to the mass of the neutrinos or the combination of pre-computed coefficients with a software calculating the perturbative terms can be fast enough to be implemented in Monte Carlo sampling softwares. On the other hand, the estimate of the age of the globular clusters that we obtained, coupled with some hypotheses on the time of galaxy formation makes it possible to put a lower limit on the age of the universe. After the reduction of the systematic errors, the age of the universe is thus compared to those obtained from the inference of cosmological parameters of various surveys. With our current precision it is not yet possible to discard any of the measurements at odds (namely Planck or SHOES) in the H0 tension but the results are very promising. CONCLUSION: Through this thesis I wanted to show that physics and more particularly cosmology could benefit from its interplay with other fields. With a better modeling of the effect of massive neutrinos it will be possible to both improve the inference of cosmological parameters and precisely constrain the mass of neutrinos (for the moment inaccessible to particle physics). A rigorous analysis of globular clusters also offers an estimate of the age of the universe almost independent of a cosmological model which is very valuable in this period of tension when the cosmological paradigm is questioned and challenged.[spa] El paradigma cosmológico actual LCDM ha tenido un éxito notable en las últimas décadas. Como cualquier paradigma, llega un momento en que la teoría alcanza sus límites y surge la pregunta de si se debe simplemente a que la teoría está incompleta o si debe ser cuestionada. En esta tesis, me enfoqué particularmente en dos aspectos de la teoría LCDM: cómo incluir efectivamente neutrinos masivos en la formación de estructuras y encontrar una metodología para traer una nueva perspectiva en la tensión que rodea la expansión del universo. Nuestro objetivo para esta tesis es mostrar cómo la cosmología puede ayudar y beneficiarse de otras áreas de la física porque existe un interés mutuo en abordar problemas en la interfaz entre la cosmología y otras áreas de la física. En la primera parte de la tesis me dediqué al desarrollo de un emulador para poder implementar rápida y fácilmente el efecto de neutrinos masivos sobre la agrupación de halos de materia oscura. Para eso utilicé simulaciones numéricas para calibrar el “bias”. Combinando el resultado de estas calibraciones con un software que permite calcular los términos perturbativos del espectro de potencia, pude desarrollar un emulador competitivo hasta escalas no lineales. En segundo lugar, me dediqué al estudio de los cúmulos globulares en un contexto cosmológico. Usé un catálogo de grupos proporcionado por el telescopio HST. Después de eliminar los cúmulos con mala fotometría, realicé un análisis bayesiano que me permitió restringir los diversos parámetros comunes a las estrellas en el mismo cúmulo. Los modelos que describen la evolución estelar, como los que utilicé para el análisis, a menudo se definen para un conjunto dado de parámetros que varían de un modelo a otro. Por tanto, para reducir los errores sistemáticos debidos al uso de un modelo estelar específico, estudié la influencia de los parámetros que gobiernan las diferentes fases evolutivas de las estrellas. El trabajo realizado en el marco de la realización de un emulador, permitió resaltar resultados importantes como: aunque la forma del bias deba calibrarse mediante simulaciones numéricas, su amplitud puede ser simplemente reescalada por un factor proporcional a la masa. de los neutrinos o la combinación de coeficientes precalculados con un software que calcula los términos perturbativos puede ser lo suficientemente rápido como para ser implementado en el software de “sampling” Monte Carlo. Por otro lado, la estimación de la edad de los cúmulos globulares que obtuvimos, junto con algunas hipótesis sobre el tiempo de formación de las galaxias, permite poner un límite inferior a la edad del universo. Después de la reducción de los errores sistemáticos, se compara la edad del universo con las obtenidas a partir de la inferencia de parámetros cosmológicos de varios estudios. Con nuestra precisión actual, todavía no es posible descartar ninguna de las mediciones en desacuerdo en la tensión H0, pero los resultados son muy prometedores. Un análisis riguroso de los cúmulos globulares también ofrece una estimación de la edad del universo casi independiente de un modelo cosmológico que es muy valioso en este período de tensión cuando el paradigma cosmológico es cuestionado y desafiado

BE-HaPPY: Bias Emulator for Halo Power Spectrum including massive neutrinos

We study the clustering properties of dark matter halos in real- and redshift-space in cosmologies with massless and massive neutrinos through a large set of state-of-the-art N-body simulations. We provide quick and easy-to-use prescriptions for the halo bias on linear and mildly non-linear scales, both in real and redshift space, which are valid also for massive neutrinos cosmologies. Finally we present a halo bias emulator,$\textbf{BE-HaPPY}$, calibrated on the N-body simulations, which is fast enough to be used in the standard Markov Chain Monte Carlo approach to cosmological inference. For a fiducial standard $\Lambda$CDM cosmology $\textbf{BE-HaPPY}$ provides percent or sub-percent accuracy on the scales of interest (linear and well into the mildly non-linear regime), meeting therefore for the halo-bias the accuracy requirements for the analysis of next-generation large--scale structure surveys.Comment: 23 pages, (33 with references and appendices), 14 figures. Changes to match accepted version by JCA

Validation of semi-analytical, semi-empirical covariance matrices for two-point correlation function for Early DESI data

We present an extended validation of semi-analytical, semi-empirical covariance matrices for the two-point correlation function (2PCF) on simulated catalogs representative of Luminous Red Galaxies (LRG) data collected during the initial two months of operations of the Stage-IV ground-based Dark Energy Spectroscopic Instrument (DESI). We run the pipeline on multiple extended Zel'dovich (EZ) mock galaxy catalogs with the corresponding cuts applied and compare the results with the mock sample covariance to assess the accuracy and its fluctuations. We propose an extension of the previously developed formalism for catalogs processed with standard reconstruction algorithms. We consider methods for comparing covariance matrices in detail, highlighting their interpretation and statistical properties caused by sample variance, in particular, nontrivial expectation values of certain metrics even when the external covariance estimate is perfect. With improved mocks and validation techniques, we confirm a good agreement between our predictions and sample covariance. This allows one to generate covariance matrices for comparable datasets without the need to create numerous mock galaxy catalogs with matching clustering, only requiring 2PCF measurements from the data itself. The code used in this paper is publicly available at https://github.com/oliverphilcox/RascalC.Comment: 19 pages, 1 figure. Code available at https://github.com/oliverphilcox/RascalC, table and figure data available at https://dx.doi.org/10.5281/zenodo.775063

The age of the Universe with globular clusters: reducing systematic uncertainties

International audienceThe dominant systematic uncertainty in the age determination of galactic globular clusters is the depth of the convection envelope of the stars. This parameter is partially degenerate with metallicity which is in turn degenerate with age. However, if the metal content, distance and extinction are known, the position and morphology of the red giant branch in a color-magnitude diagram are mostly sensitive to the value of the depth of the convective envelope. Therefore, using external, precise metallicity determinations this degeneracy and thus the systematic error in age, can be reduced. Alternatively, the morphology of the red giant branch of globular clusters color magnitude diagram can also be used to achieve the same. We demonstrate that globular cluster red giant branches are well fitted by values of the depth of the convection envelope consistent with those obtained for the Sun and this finding is robust to the adopted treatment of the stellar physics. With these findings, the uncertainty in the depth of the convection envelope is no longer the dominant contribution to the systematic error in the age determination of the oldest globular clusters, reducing it from 0.5 to 0.23 or 0.33 Gyr, depending on the methodology adopted: i.e., whether resorting to external data (spectroscopic metallicity determinations) or relying solely on the morphology of the clusters's color-magnitude diagrams. This results in an age of the Universe t_ U=13.5+0.16 -0.14 (stat.)± 0.23(0.33) ( sys.) at 68% confidence level, accounting for the formation time of globular clusters and its uncertainty. An uncertainty of 0.27(0.36) Gyr if added in quadrature. This agrees well with 13.8 ± 0.02 Gyr, the cosmological model-dependent value inferred by the Planck mission assuming the ΛCDM model

Inferring the Age of the Universe with Globular Clusters

International audienceWe present an estimate of the absolute age of 68 galactic globular clusters obtained by exploiting the distribution of stars in the full color-magnitude diagram. In particular, we jointly estimate the absolute age, distance, reddening, metallicity ([Fe/H]) and [α/Fe] of each cluster, imposing priors motivated by independent observations; we also estimate possible systematics from stellar modeling. Our derived distances for the globular cluster sample are in agreement with those obtained from GAIA using main-sequence dwarf stars (where available), and the inferred ages are in good agreement with those previously published. The novelty of our approach is that, with the adopted priors, we are able to estimate robustly these parameters from the globular cluster color-magnitude diagram. We find that the average age of the oldest globular clusters is tGC=13.32  0.1  (stat.)  0.5  (sys.), at 68% confidence level, including systematic uncertainties from stellar modeling. These measurements can be used to infer the age of the Universe, largely independently of the cosmological parameters: we find an age of the Universe tU=13.5+0.16−0.14  (stat.)  0.5 (sys.) at 68% confidence level, accounting for the formation time of globular clusters and its uncertainty. This value is compatible with 13.8  0.02 Gyr, the cosmological model-dependent value inferred by the Planck mission assuming the ΛCDM model

The trouble beyond H0H_0 and the new cosmic triangles

International audienceThe distance ladder using supernovae yields higher values of the Hubble constant H0 than those inferred from measurements of the cosmic microwave background (CMB) and galaxy surveys, a discrepancy that has come to be known as the “Hubble tension”. This has motivated the exploration of extensions to the standard cosmological model in which higher values of H0 can be obtained from CMB measurements and galaxy surveys. The trouble, however, goes beyond H0; such modifications affect other quantities, too. In particular, their effects on cosmic times are usually neglected. We explore here the implications that measurements of the age tU of the Universe, such as a recent inference from the age of the oldest globular clusters, can have for potential solutions to the H0 tension. The value of H0 inferred from the CMB and galaxy surveys is related to the sound horizon at CMB decoupling (or at radiation drag), but it is also related to the matter density and to tU. Given this observation, we show how model-independent measurements may support or disfavor proposed new-physics solutions to the Hubble tension. Finally, we argue that cosmological measurements today provide constraints that, within a given cosmological model, represent an overconstrained system, offering a powerful diagnostic tool of consistency. We propose the use of ternary plots to simultaneously visualize independent constraints on key quantities related to H0 like tU, the sound horizon at radiation drag, and the matter density parameter. We envision that this representation will help find a solution to the trouble of and beyond H0

Validation of semi-analytical, semi-empirical covariance matrices for two-point correlation function for early DESI data

We present an extended validation of semi-analytical, semi-empirical covariance matrices for the two-point correlation function (2PCF) on simulated catalogs representative of luminous red galaxies (LRGs) data collected during the initial 2 months of operations of the Stage-IV ground-based Dark Energy Spectroscopic Instrument (DESI). We run the pipeline on multiple effective Zel'dovich (EZ) mock galaxy catalogs with the corresponding cuts applied and compare the results with the mock sample covariance to assess the accuracy and its fluctuations. We propose an extension of the previously developed formalism for catalogs processed with standard reconstruction algorithms. We consider methods for comparing covariance matrices in detail, highlighting their interpretation and statistical properties caused by sample variance, in particular, non-trivial expectation values of certain metrics even when the external covariance estimate is perfect. With improved mocks and validation techniques, we confirm a good agreement between our predictions and sample covariance. This allows one to generate covariance matrices for comparable data sets without the need to create numerous mock galaxy catalogs with matching clustering, only requiring 2PCF measurements from the data itself. The code used in this paper is publicly available at https://github.com/oliverphilcox/RascalC