Precision Cosmology Using Voids

In the late 1990s, the discovery that the expansion rate of the Universe was accelerating was a decisive moment for cosmology. The last 25 years have seen the consolidation of this component, called dark energy, which dominates the total energy of the Universe at the present day. Cosmologists have developed many techniques to measure the properties of dark energy and attempt to reveal insights into the physics behind this mysterious component. Many explanations for dark energy exist, the simplest being that it is a form of energy permeating all of space (a cosmological constant), and alternatives include modifications to theories of general relativity and scalar fields. The modern era of precision cosmology has been dedicated to the measurement of cosmological parameters that describe and distinguish different models. Despite decades of work in this area, little insight has been found into the nature of dark energy. More accurate measurements from next-generation cosmological surveys are needed to uncover the underlying physics behind this fundamental component of the Universe. Cosmic voids are patches of the Universe that are less dense than the cosmic average. These large-scale underdensities are a natural consequence of structure growth. Voids are special places in the Universe where the physics of their growth can be easily modelled. Although the density is non-linear (the density in the centre of voids is close to zero), the motions of galaxies still track their primordial form making it possible to extract cosmological information. This information primarily comes from two physical processes - the Alcock-Paczynski (AP) effect and Redshift-Space Distortions (RSDs). The AP effect is a geometrical consequence where an object's shape becomes distorted if measured using a wrong cosmological model. Stacking voids will produce a spherical averaged shape only if the AP parameter, D_M/D_H, is correct (where D_M is the transverse comoving distance that is a measure perpendicular to the line of sight and D_H is the Hubble distance which is a measure parallel to the line of sight). RSDs are the distortions of measured distances due to the Doppler effect of a galaxy's peculiar velocity. On large scales, the growth rate of cosmological structure is the dominant source of RSDs. Using the linear motions of galaxies around voids, RSDs are used to measure the growth rate of structure parameterised by f(z) sigma_8(z) (where f(z) relates to the growth rate of structure and sigma_8 relates to the redshift space galaxy power spectrum). Measurements of voids within the large-scale structure of the Universe can be made using galaxy spectroscopic surveys. These surveys use the positions of galaxies as tracers of the underlying matter distribution. Information in these surveys has primarily been extracted using two techniques: Baryonic Acoustic Oscillations (BAO) and RSD. BAO provide a standard ruler through which the expansion rate of the Universe can be measured, while RSD allows for a measurement of the growth rate of structure. The use of voids has emerged as another technique to extract even more information from these surveys. This thesis presents the background and modelling that can be used to extract and analyze this information. After all necessary background is summarised, measurements of the anisotropic cross-correlation of galaxies and cosmic voids in data from the Sloan Digital Sky Survey Main Galaxy Sample (MGS), Baryon Oscillation Spectroscopic Survey (BOSS) and extended BOSS (eBOSS) luminous red galaxy catalogues from SDSS Data Releases 7, 12 and 16, covering the redshift range 0.07<z<1.0 are presented. This uses the clustering of galaxies around voids to extract information and is the first time that a consistent analysis method has been applied to extract information from voids in this full redshift range. A reconstruction method is applied to the galaxy data before void-finding to remove selection biases when constructing the void samples. Results of a joint fit to the multipole moments of the measured cross-correlation for the growth rate of structure and the ratio D_M/D_H are reported in six redshift bins. For D_M/D_H, voids are able to achieve significantly higher precision than that obtained from analyses of BAO and RSD in the same datasets. Our growth rate measurements are of lower precision but still comparable with galaxy clustering results. For both quantities, the results agree well with the expectations for a LambdaCDM model. The degeneracy directions obtained for the study of voids in galaxy spectroscopic surveys are consistent with and complementary to those from other cosmological probes and result in a significant gain of information. These results consolidate void-galaxy cross-correlation measurements as a pillar of modern observational cosmology. Also presented are cosmological models fits to voids and the combination of voids with other probes. A standard LambdaCDM cosmological model is fit to measurements from voids as well as various extensions including a constant dark energy equation of state not equal to -1, a time-varying dark energy equation of state, and these same models allowing for spatial curvature. Results on key parameters of these models are reported for void-galaxy and galaxy-galaxy clustering alone, both of these combined, and all these combined with measurements from the cosmic microwave background (CMB) and supernovae (SN). The results show a remarkable agreement with a flat LambdaCDM cosmology for all cosmological models tested. The gain of information from void measurements made at multiple redshifts, compared to compressing all information into one measurement at a single effective redshift, is also demonstrated. Finally, a forward look to the future of voids as cosmological probes is presented. This thesis uses the best public galaxy redshift survey data available to date; however, this will soon be surpassed once DESI and Euclid results are released within the next few years. Forecast constraints from applying a consistent analysis method to that presented in this thesis on a mock catalogue expected to match data from Euclid are shown. Cosmic voids provide another analysis method that can extract independent cosmological constraints with complementary parameter degeneracies that, combined with information from BAO/RSD, increase the precision of information extracted from galaxy spectroscopic surveys. Future surveys will need to continue to build on the current modelling of voids to reduce systematic errors and provide valuable hints towards the fundamental nature of our Universe

Measurements of cosmic expansion and growth rate of structure from voids in the Sloan Digital Sky Survey between redshift 0.07 and 1.0

We present measurements of the anisotropic cross-correlation of galaxies and cosmic voids in data from the Sloan Digital Sky Survey Main Galaxy Sample (MGS), Baryon Oscillation Spectroscopic Survey (BOSS) and extended BOSS (eBOSS) luminous red galaxy catalogues from SDSS Data Releases 7, 12 and 16, covering the redshift range $0.07<z<1.0$. As in our previous work analysing voids in subsets of these data, we use a reconstruction method applied to the galaxy data before void-finding in order to remove selection biases when constructing the void samples. We report results of a joint fit to the multipole moments of the measured cross-correlation for the growth rate of structure, $f\sigma_8(z)$, and the ratio $D_\mathrm{M}(z)/D_\mathrm{H}(z)$ of the comoving angular diameter distance to the Hubble distance, in six redshift bins. For $D_\mathrm{M}/D_\mathrm{H}$, we are able to achieve a significantly higher precision than that obtained from analyses of the baryon acoustic oscillations (BAO) and galaxy clustering in the same datasets. Our growth rate measurements are of lower precision but still comparable with galaxy clustering results. For both quantities, the results agree well with the expectations for a $\Lambda$CDM model. Assuming a flat Universe, our results correspond to a measurement of the matter density parameter $\Omega_\mathrm{m}=0.337^{+0.026}_{-0.029}$. For more general models the degeneracy directions obtained are consistent with and complementary to those from other cosmological probes. These results consolidate void-galaxy cross-correlation measurements as a pillar of modern observational cosmology.Comment: Author's accepted manuscript. 17 Pages, 8 Figures. MNRAS (2022

Cosmological measurements from void-galaxy and galaxy-galaxy clustering in the Sloan Digital Sky Survey

We present the cosmological implications of measurements of void-galaxy and galaxy-galaxy clustering from the Sloan Digital Sky Survey (SDSS) Main Galaxy Sample (MGS), Baryon Oscillation Spectroscopic Survey (BOSS), and extended BOSS (eBOSS) luminous red galaxy catalogues from SDSS Data Release 7, 12, and 16, covering the redshift range $0.07 < z < 1.0$. We fit a standard $\Lambda$CDM cosmological model as well as various extensions including a constant dark energy equation of state not equal to $-1$, a time-varying dark energy equation of state, and these same models allowing for spatial curvature. Results on key parameters of these models are reported for void-galaxy and galaxy-galaxy clustering alone, both of these combined, and all these combined with measurements from the cosmic microwave background (CMB) and supernovae (SN). For the combination of void-galaxy and galaxy-galaxy clustering plus CMB and SN, we find tight constraints of $\Omega_\mathrm{m} = 0.3127\pm 0.0055$ for a base $\Lambda$CDM cosmology, $\Omega_\mathrm{m} = 0.3172\pm 0.0061, w = -0.930\pm 0.039$ additionally allowing the dark energy equation of state $w$ to vary, and $\Omega_\mathrm{m} = 0.3239\pm 0.0085, w = -0.889\pm 0.052, \mathrm{and}\ \Omega_\mathrm{k} = -0.0031\pm 0.0028$ further extending to non-flat models.Comment: 11 pages, 9 figures. Submitted to MNRA

Helicity in the large-scale Galactic magnetic field

17 pages, 11 figures, Accepted to MNRAS 28 September 2020International audienceWe search for observational signatures of magnetic helicity in data from all-sky radio polarization surveys of the Milky Way Galaxy. Such a detection would help confirm the dynamo origin of the field and may provide new observational constraints for its shape. We compare our observational results to simulated observations for both a simple helical field, and for a more complex field that comes from a solution to the dynamo equation. Our simulated observations show that the large-scale helicity of a magnetic field is reflected in the large-scale structure of the fractional polarization derived from the observed synchrotron radiation and Faraday depth of the diffuse Galactic synchrotron emission. Comparing the models with the observations provides evidence for the presence of a quadrupolar magnetic field with a vertical component that is pointing away from the observer in both hemispheres of the Milky Way Galaxy. Since there is no reason to believe that the Galactic magnetic field is unusual when compared to other galaxies, this result provides further support for the dynamo origin of large-scale magnetic fields in galaxies

The completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: geometry and growth from the anisotropic void-galaxy correlation function in the luminous red galaxy sample

19 pages, 14 figures, 5 tables including appendix. Matches MNRAS accepted version. v2: slightly expanded appendix, minor changes to text elsewhere. Data and code to reproduce likelihood analysis available from https://github.com/seshnadathur/victor; doi:10.1093/mnras/staa3074International audienceWe present an analysis of the anisotropic redshift-space void-galaxy correlation in configuration space using the Sloan Digital Sky Survey extended Baryon Oscillation Spectroscopic Survey (eBOSS) Data Release 16 luminous red galaxy sample in combination with the high redshift tail of the Baryon Oscillation Spectroscopic Survey Data Release 12 CMASS sample between redshifts 0.6 and 1.0. We use a reconstruction method to undo redshift-space distortion (RSD) effects from the galaxy field before applying a watershed void-finding algorithm to remove bias from the void selection. We then perform a joint fit to the multipole moments of the correlation function for the growth rate $f\sigma_8$ and the geometrical distance ratio $D_M/D_H$, finding $f\sigma_8(z_\mathrm{eff})=0.356\pm0.079$ and $D_M/D_H(z_\mathrm{eff})=0.868\pm0.017$ at the effective redshift $z_\mathrm{eff}=0.69$ of the sample. The posterior parameter degeneracies are orthogonal to those from galaxy clustering analyses applied to the same data, and the constraint achieved on $D_M/D_H$ is significantly tighter. In combination with the consensus galaxy BAO and full-shape analyses of the same sample, we obtain $f\sigma_8=0.447\pm0.039$, $D_M/r_d=17.48\pm0.23$ and $D_H/r_d=20.10\pm0.34$. These values are in good agreement with the $\Lambda$CDM model predictions and represent reductions in the uncertainties of 13%, 23% and 28% respectively compared to the combined results from galaxy clustering, or an overall reduction of 55% in the allowed volume of parameter space

CHANG-ES. XX. High-resolution Radio Continuum Images of Edge-on Galaxies and Their AGNs: Data Release 3

International audienc

CHANG-ES. XXII. Coherent magnetic fields in the halos of spiral galaxies

International audienceContext. The magnetic field in spiral galaxies is known to have a large-scale spiral structure along the galactic disk and is observed as X-shaped in the halo of some galaxies. While the disk field can be well explained by dynamo action, the three-dimensional structure of the halo field and its physical nature are still unclear.Aims: As first steps towards understanding the halo fields, we want to clarify whether or not the observed X-shaped field is a wide-spread pattern in the halos of spiral galaxies. We also aim to investigate whether these halo fields are simply turbulent fields ordered by compression or shear (anisotropic turbulent fields), or have a large-scale regular structure.Methods: Analysis of the Faraday rotation in the halo is used as a tool to distinguish anisotropic turbulent fields from large-scale magnetic fields. However, this has been challenging until recently because of the faint halo emission in linear polarization. Our sensitive VLA broadband observations in C-band and L-band of 35 spiral galaxies seen edge-on (called CHANG-ES) allowed us to perform rotation measure synthesis (RM synthesis) in their halos and to analyze the results. We further accomplished a stacking of the observed polarization maps of 28 CHANG-ES galaxies in C-band.Results: Though the stacked edge-on galaxies were of different Hubble type, and had differing star formation activity and interaction activity, the stacked image clearly reveals an X-shaped structure of the apparent magnetic field. We detected a large-scale (coherent) halo field in all 16 galaxies that have extended polarized emission in their halos. We detected large-scale field reversals in all of their halos. In six galaxies, these are along lines that are approximately perpendicular to the galactic midplane (vertical RMTL) with about 2 kpc separation. Only in NGC 3044 and possibly in NGC 3448 did we observe vertical giant magnetic ropes (GMR) similar to those detected recently in NGC 4631.Conclusions: The observed X-shaped structure of the halo field seems to be an underlying feature of spiral galaxies. It can be regarded as the two-dimensional projection of the regular magnetic field which we found to have scales of typically 1 kpc or larger observed over several kiloparsecs. The ordered magnetic field extends far out in the halo and beyond. We detected large-scale magnetic field reversals in the halo that may indicate that GMR are more or less tightly wound. With these discoveries, we hope to stimulate model simulations for the halo magnetic field that should also explain the determined asymmetry of the polarized intensity (PI)