Relativistic Cosmology from the linear to the non-linear regime

Cosmology is a data-driven science and in the past decades we have seen the Lambda-Cold-Dark-Matter (Lambda-CDM) model establishing itself as the standard model for cosmology. However, the standard model still presents several open problems; among them, explaining the mechanism of the current cosmic acceleration. In the near future, Large Scale Structure (LSS) surveys are expected to play a pivotal role in shedding light on this subject. Galaxy surveys will be able to map the galaxies distribution on very large volumes with greatly improved statistics, while new techniques such as the intensity mapping applied to the 21 cm neutral hydrogen emission promise to explore the evolution of the Universe until the epoch of reionization. Therefore, it is of crucial importance to combine this huge effort from the observational side with a correct model for the observable quantities. During my PhD I worked on several topics in the field of Large Scale Structure (LSS), which is considered here as a laboratory to test fundamental aspects of gravity and cosmology. The thesis is divided in three parts. In part 1 we will introduce the basics of modern cosmology. This introduction does not aim to be all-encompassing, but it will specifically address the topics that are relevant in understanding the main body of the thesis, i.e. part 2 and part 3. Part 2 of the thesis concerns the largest scales that we will be able to test with the future generation of galaxy surveys. These scales can be studied within the framework of linear perturbation theory. However, a full relativistic treatment is necessary in order to take into account all the horizon effects that are predicted by General Relativity and in general by any metric theory of gravity. The fact that photons travel from the source to the observer in an inhomogeneous universe introduces volumes and redshift correction to the observed quantities. In the linear regime they include the standard redshift-space distortions (RSD) also known as the Kaiser effect, the gravitational lensing magnification, the Doppler effect and gravitational redshift, Shapiro time-delay, Sachs-Wolfe and integrated Sachs-Wolfe effects. The standard LSS analysis includes the Kaiser effect, while the other relativistic effects are often neglected. In part 2 we investigated the relevance of the relativistic effects for future planned galaxy survey, focusing on their impact on cosmological tests and parameter estimation (chapter 4 and chapter 5) and their detectability (chapter 6). Part 3 of the thesis focuses on the highly non-linear regime of the LSS. The vector degrees of freedom are often neglected in modelling the cosmic velocities. This approximation works fairly well on large scales, i.e. where linear theory applies. However, on non-linear scales it is well-known that vorticity is generated. In chapter 7 we investigated numerically the generation of vorticity with the recently-born relativistic N-body code gevolution. Even if the generation of vorticity is a purely newtonian effect, a relativistic treatment extends the newtonian approach: for example, it allows to investigate the interplay between the vorticity and the vector degrees of freedom in the metric. Finally, in chapter 8 we summarize the results and draw possible extensions of the work presented in this manuscript

Cosmological Simulations of Number Counts

In this paper we present for the first time the angular power spectra $C_\ell(z,z')$ for number counts from relativistic N-body simulations. We use the relativistic N-body code gevolution with its exact integration of lightlike geodesics which include all relativistic scalar contributions to the number counts. We compare our non-perturbative numerical results with the results from CLASS using the HMCODE approximation for the non-linear matter power spectrum. We find that this simple description is excellent for both, the density and the convergence. On the other hand, the current implementation of redshift-space distortions in Boltzmann codes is not accurate. We also find that the largest contribution to the unequal-redshift power spectra is the cross-correlation of the density and the lensing contribution to the number counts, especially for redshift bins that are far apart. Correlating the number counts with the convergence map we find that the signal is dominated by the lensing-lensing term when the convergence field redshift is not higher than the number counts one, while it is dominated by the density-lensing term in the opposite case. In the present study, the issue of galaxy bias is deliberately left aside by considering only unbiased samples of matter particles from the simulations.Comment: 26 pages, 8 figures, version accepted for publication in JCA

Optimal galaxy survey for detecting the dipole in the cross-correlation with 21 cm Intensity Mapping

We investigate the future perspectives of the detection of the relativistic dipole by cross-correlating the 21 cm emission in Intensity Mapping (IM) and galaxy surveys at low redshift. We model the neutral hydrogen (HI) and the galaxy population by means of the halo model to relate the parameters that affect the dipole signal such as the biases of the two tracers and the Poissonian noise. We investigate the behavior of the signal-to-noise as a function of the galaxy and magnification biases, for two fixed models of the neutral hydrogen. In both cases we found that the signal-to-noise does not grow by increasing the difference between the biases of the two tracers, due to the larger shot-noise yields by highly biased tracers. We also study and provide an optimal luminosity-threshold galaxy catalogue to enhance the signal-to-noise ratio of the relativistic dipole. Interestingly, we show that the maximum magnitude provided by the survey does not lead to the maximum signal-to-noise for detecting relativistic effects and we predict the optimal value for the limiting magnitude. Our work suggests that an optimal analysis could increase the signal-to-noise ratio up to a factor five compared to a standard one

A case study for measuring the relativistic dipole of a galaxy cross-correlation with the Dark Energy Spectroscopic Instrument

ABSTRACT The data on spectroscopic galaxy clustering collected by the Dark Energy Spectroscopic Instrument (DESI) will allow the significant detection of subtle features in the galaxy two-point correlation in redshift space, beyond the ‘standard’ redshift-space distortions. Here, we present an independent assessment of the detectability of the relativistic dipole in the cross-correlation of two populations of galaxies if they would be selected from the Bright Galaxy Survey (BGS) of DESI. We build synthetic galaxy catalogues with the characteristics of the BGS using the light cone of a relativistic N-body simulation. Exploring different ways of splitting the populations of galaxies we find that with an unequal split with more bright galaxies than faint galaxies the detectability is significantly boosted, reaching 19σ in the redshift bin 0.2 ≲ z ≲ 0.3 and expected to be even higher at lower redshift. Moreover, we find that the measured dipole agrees very well with the prediction of relativistic effects from linear theory down to separations of ∼ 30 Mpc h−1

Euclid preparation -XIX. Impact of magnification on photometric galaxy clustering

Aims. We investigate the importance of lensing magnification for estimates of galaxy clustering and its cross-correlation with shear for the photometric sample of Euclid. Using updated specifications, we study the impact of lensing magnification on the constraints and the shift in the estimation of the best fitting cosmological parameters that we expect if this effect is neglected. Methods. We follow the prescriptions of the official Euclid Fisher matrix forecast for the photometric galaxy clustering analysis and the combination of photometric clustering and cosmic shear. The slope of the luminosity function (local count slope), which regulates the amplitude of the lensing magnification, and the galaxy bias have been estimated from the Euclid Flagship simulation. Results. We find that magnification significantly affects both the best-fit estimation of cosmological parameters and the constraints in the galaxy clustering analysis of the photometric sample. In particular, including magnification in the analysis reduces the 1σ errors on Ωm, 0, w0, wa at the level of 20–35%, depending on how well we will be able to independently measure the local count slope. In addition, we find that neglecting magnification in the clustering analysis leads to shifts of up to 1.6σ in the best-fit parameters. In the joint analysis of galaxy clustering, cosmic shear, and galaxy–galaxy lensing, magnification does not improve precision, but it leads to an up to 6σ bias if neglected. Therefore, for all models considered in this work, magnification has to be included in the analysis of galaxy clustering and its cross-correlation with the shear signal (3 × 2pt analysis) for an accurate parameter estimation. Key words: large-scale structure of Universe / cosmological parameters / cosmology: theor

Clinical global impression-severity score as a reliable measure for routine evaluation of remission in schizophrenia and schizoaffective disorders

Aims: This study aimed to compare the performance of Positive and Negative Syndrome Scale (PANSS) symptom severity criteria established by the Remission in Schizophrenia Working Group (RSWG) with criteria based on Clinical Global Impression (CGI) severity score. The 6-month duration criterion was not taken into consideration. Methods: A convenience sample of 112 chronic psychotic outpatients was examined. Symptomatic remission was evaluated according to RSWG severity criterion and to a severity criterion indicated by the overall score obtained at CGI-Schizophrenia (CGI-SCH) rating scale (≤3) (CGI-S). Results: Clinical remission rates of 50% and 49.1%, respectively, were given by RSWG and CGI-S, with a significant level of agreement between the two criteria in identifying remitted and non-remitted cases. Mean scores at CGI-SCH and PANSS scales were significantly higher among remitters, independent of the remission criteria adopted. Measures of cognitive functioning were largely independent of clinical remission evaluated according to both RSWG and CGI-S. When applying RSWG and CGI-S criteria, the rates of overall good functioning yielded by Personal and Social Performance scale (PSP) were 32.1% and 32.7%, respectively, while the mean scores at PSP scale differed significantly between remitted and non-remitted patients, independent of criteria adopted. The proportion of patients judged to be in a state of well-being on Social Well-Being Under Neuroleptics-Short Version scale (SWN-K) were, respectively, 66.1% and 74.5% among remitters according to RSWG and CGI-S; the mean scores at the SWN scale were significantly higher only among remitters according to CGI-S criteria. Conclusions: CGI severity criteria may represent a valid and user-friendly alternative for use in identifying patients in remission, particularly in routine clinical practic

Simple large wood structures promote hydromorphological heterogeneity and benthic macroinvertebrate diversity in low-gradient rivers

This work has been carried out within the SMART Joint Doctorate Programme ‘Science for the MAnagement of Rivers and their Tidal systems’ funded by the Erasmus Mundus programme of the European Union