Precision Cosmology Using Voids

Abstract

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.