The universe is structured on a vast scale, with matter clumped into immense structures separated by enormous regions of seemingly empty space. These sprawling, underdense areas are known as cosmic voids, representing the largest structures in the cosmos. Far from dense galaxy clusters, a cosmic void contains remarkably little matter. Voids are defined by what they lack, offering a unique laboratory for cosmologists to study the underlying forces that shape the universe.
Defining Cosmic Voids in the Universe’s Structure
Cosmic voids are integral components of the universe’s large-scale architecture, often described as the “Cosmic Web.” This web is a network where matter—including galaxies, gas, and dark matter—is distributed in a foam-like structure. The dense sections of this structure are filaments, walls, and superclusters, which act as the boundaries surrounding and connecting the voids.
Voids occupy the vast majority of the universe’s volume, acting as the spacious cells within the Cosmic Web. Visible matter aggregates into high-density strands, leaving the intervening spaces hollow. Galaxies and clusters flow along the filaments and walls, pulled by gravity toward the densest knots, which further empties the void regions.
This structural arrangement demonstrates that the universe is far from uniform on the largest scales. The distribution of galaxies is highly anisotropic, meaning it is not the same in all directions. Voids are the clearest manifestation of this unevenness, allowing astronomers to trace the outlines of these immense structures by mapping millions of galaxies.
The Physics of Near-Emptiness
Cosmic voids are not perfect vacuums but possess an extremely low average density. The density of matter inside a typical void is less than one-tenth of the overall average density of the universe. Consequently, a void contains only a small fraction of the ordinary matter and dark matter found in the surrounding filaments.
The minuscule amount of ordinary matter that exists within a void is primarily residual gas and dust at extremely low temperatures. On average, the density can be so low that a cubic meter of space may contain only one or two hydrogen atoms. Any isolated galaxies that form in this sparse environment are often small and relatively pristine, having experienced fewer gravitational interactions than those in denser regions.
The most significant physical property of voids is the dominance of dark energy within their boundaries. In dense areas, the attractive force of gravity is the primary influence. In the near-emptiness of a void, the repulsive force of dark energy becomes the most influential factor, driving the expansion of space more effectively in these underdense zones.
Scale and Significant Examples
The sheer size of cosmic voids reflects the large-scale structure of the cosmos. Standard voids typically range from 30 million to over 300 million light-years in diameter. The largest of these, referred to as supervoids, span even greater distances, making them some of the largest single structures known.
One famous example is the Boötes Void, estimated to be 250 million to 330 million light-years across. Due to this immense size, if the Milky Way were at its center, it would be extremely difficult to detect other galaxies until reaching the void’s distant edges.
Another notable example is the Eridanus Supervoid, associated with an unusually cold area in the Cosmic Microwave Background (CMB) radiation. This supervoid is estimated to be 500 million to a billion light-years wide and contains a matter density about 30% lower than the surrounding region. Studying these colossal spaces helps cosmologists test models of the universe’s evolution.
The Formation and Evolution of Voids
The origin of cosmic voids traces back to minute quantum fluctuations in the primordial plasma during the universe’s early moments. These fluctuations created tiny, initial variations in density. Regions with slightly less matter became the seeds for voids, while regions with slightly more matter eventually formed galaxy clusters.
Over billions of years, gravitational attraction amplified these initial density differences. Gravity pulled matter away from the less dense regions toward the denser areas, effectively draining the voids. This process of gravitational collapse and outflow sculpted the large-scale structure, causing matter to aggregate into filaments and walls, and the intervening spaces to become increasingly empty.
The continued growth of voids is strongly influenced by dark energy, which drives the accelerated expansion of the universe. Since voids contain little matter, the gravitational pull inward is minimal. This allows dark energy’s expansive force to dominate and stretch these regions more quickly than dense clusters. As the universe expands, voids are expected to grow larger and more numerous, further defining the Cosmic Web’s spacious structure.