Cosmic voids look empty but still hold some matter
Between the great filaments of galaxies lie voids up to 300 million light-years across that seem almost bare. They are not truly empty: even their sparsest parts contain more than about 15 percent of the universe's average matter density. What is missing is the big stuff, because conditions inside voids suppress the growth of massive galaxies and clusters.
Voids were first identified in 1978, when Stephen Gregory and Laird Thompson at Kitt Peak National Observatory, along with a second team, found gaps in front of the Coma and A1367 clusters. The discovery depended on redshift surveys, which added depth to flat sky maps by using each galaxy's redshift to estimate its distance, producing the first three-dimensional views of the universe. In 1981 astronomers found an enormous void in Boötes.
Surveys through the following decades showed voids are everywhere. The Center for Astrophysics survey in 1989 revealed large voids, sharp filaments and walls dominating the structure; the Las Campanas survey confirmed it in 1991; and by 2009 the Sloan Digital Sky Survey offered the most detailed picture yet. Researchers usually define a void as a region with less than a tenth of the average density, often counted by galaxies per volume rather than total mass, although no single definition is agreed.
They grew from tiny irregularities seeded by quantum fluctuations in the early universe. Denser patches collapsed faster under gravity, draining matter from their surroundings and leaving a cosmic web of walls, filaments and clusters around vast underdense bubbles. Voids in crowded neighbourhoods end up smaller than those in sparse regions. Inside them, a long phase dominated by the curvature term holds back the formation of big structures.
Voids also offer evidence for dark energy. Through the Sachs-Wolfe effect, cold patches in the cosmic microwave background line up with voids and warm patches with filaments, and that effect matters only when radiation or dark energy dominates the universe. Modern void-finding algorithms search for low galaxy density, trace geometry in the dark matter distribution, or locate gravitationally unstable points.
Source: Void (astronomy)
