Cosmic Evolution and Voids By Jesse Ashworth

What’s to Come What are cosmic voids? Overview of the universe’s structure, and how cosmic voids fit into the picture How the major structural elements of the universe, including cosmic voids, were formed as of the Big Bang Experimental evidence for the universe’s expansion provided by superstructures How researchers find cosmic voids; some prominent examples

What are Cosmic Voids? Large, sparsely populated regions of space containing hardly any galaxies Typically have one tenth the average matter density of the observable universe or less Voids separate galactic clusters and filaments from each other in the cosmic web Occupy ~95% of the universe’s volume Typically around 10 to 150 megaparsecs in diameter (~3.3×10 7 to 4.9×10 8 light years)

The Hierarchy of the Universe Two primary superstructures: Walls and voids Walls contain average matter density Walls can be broken down even further: Clusters/superclusters Filaments Sloan Great Wall: 500 mil light years by 200 mil light years by 15 mil light years Image source:

Origins of the Universe’s Superstructures: BAOs & Gravitational Accretion Baryon acoustic oscillations (BAOs): Perturbations to the Hamiltonian of the early universe Net gravitational force toward certain regions of matter Net accumulation of matter takes place Inflation results in the expansion of the perturbations Fast forward in time: Galactic walls with cosmic voids in-between

BAO Model: The Zel’dovich Approximation

BAO Model: The Adhesion Model

The Cosmic Microwave Background Thermal background leftover from when neutral atoms formed several hundred thousand years after the Big Bang Nearly isotropic ~3 Kelvin temperature distribution Slight anisotropies present primarily due to the Non-integrated Sachs-Wolfe Effect Regions of higher mass density result in photons leaving a slightly deeper gravitational potential well and thus getting redshifted Image source:

Evidence for Expansion: The Integrated Sachs-Wolfe Effect Photons from the surface of last scattering gain energy and are blueshifted as they enter a galactic supercluster The photons are redshifted as they leave the supercluster, but the photons ultimately loose less energy than they gained Opposite effect occurs for voids: Photons loose energy entering voids, then gain less energy then they lost upon exit Net energy losses and gains correspond to temperature fluctuations which show up as slight anisotropies in the CMB

Evidence for Expansion: The Integrated Sachs-Wolfe Effect Indication of cosmic expansion stretching the gravitational potential well/hill created by the cluster/void Image source:

Evidence for Expansion: The Integrated Sachs-Wolfe Effect Image source:

Evidence for Expansion: The Integrated Sachs-Wolfe Effect GIF source:

Finding Cosmic Voids: VoidFinder Algorithm Searches for voids purely based on local galaxy number density Selects a galaxy and forms a sphere about it whose radius is the distance to the third-closest galaxy Gradually increases the radius of the sphere until the density immediately outside is the average wall density Results in very distinct boundaries, with a mean density of 10%  20% at the edges  100% outside

ZOBOV (Zone Bordering on Voidness) Algorithm Uses advanced geometric techniques to define void boundaries based on galaxy number density More accurate than VoidFinder in determining void shapes and sizes Does not vary any free parameters when determining void boundaries, and thus typically finds relatively small voids Labels each void with the ratio of its minimum density to its average density to place to give the void a statistical significance

DIVA (Dynamical Void Analysis) Algorithm

Reliability of Void-Finding Algorithms Each algorithm undergoes a process called robustness testing For a given algorithm, aspects of calculated voids including shape, size, and number are compared to results from a simulation Simulation models the evolution of the universe’s superstructures, particularly voids (e.g., the Adhesion Model) Algorithm is more “robust” the more its calculations agree with varying reliable simulations

Example: Boötes Void Center is located about 215 Mpc from the Milky Way Roughly spherical in shape and 75 to 100 Mpc in diameter Contains only 60 galaxies Compare: The distance between the Milky Way and Andromeda is ~1% the diameter of the Boötes Void Image source:

Example: Taurus Void Roughly 30 Mpc in diameter Only a few galaxies have been found inside, including two of which are about 185 million light years away: UGC 2627 and UGC 2629 Image source:

Example: Eridanus Supervoid (The Great Void) Currently unconfirmed to exist, but is a possible explanation for an unusually large “cold spot” in the CMB Estimated to be roughly 150 Mpc (500 million light years) across, possibly twice as much Somewhere between 6 to 10 billion light years away

Conclusions Most modern structural astrophysics research focuses on the galactic walls of the universe Not as much is known about cosmic voids, but their baron landscape potentially allows researchers to more precisely determine the nature of dark energy and the evolution of the universe Studying voids thus could provide vital cosmological information which is unobtainable from galaxy clusters and filaments

Bibliography 1. (What is the Sachs-Wolfe effect?) 2. (Formation of Structure in the Universe) 3. (The Sticky Geometry of the Cosmic Web); corresponding paper at (Voids) 5. (Supervoids and Superclusters) 6. (The most direct signal of dark energy?) 7. (Voids and Supervoids in the Universe) 8. (Precision cosmology with voids: definition, methods, and dynamics)