When observing the Universe from our scaled down perspective, the distribution of galaxies seems to be random and sporadic with no clear pattern or structure. Its only when we zoom out and look at the Universe from a larger scale that the structure of the Universe begins to reveal itself to us. This structure, just like the structure of stars and planets, arises primarily from gravitational force. Once galaxies form, they clump up into clusters or even superclusters. This arrangement of the Universe mimics that of a spider web or a foam like composition, also known as ‘the cosmic web’, and is comprised of filaments and voids.
The branches of galactic density in the cosmic web are known as galactic filaments and are the largest known structures in the Universe known to man. They are comprised of walls of superclusters and can be as large as 80 Mpc. Filaments create borders between voids. The vast open spaces between these filaments are called cosmic voids. Voids were initially discovered by scientists in the 1970’s by means of redshift surveys of galaxies. Their sizes can vary from 10 to 100 Mpc and make up most of the volume of the Universe, roughly 80%. Voids are defined as areas in space with a very low numbers of galaxies that are distributed far from one another. If voids are large enough, they can even be dubbed as super voids. The largest known void is the Boötes void, discovered by Robert Kirshner et al, and has a diameter of 0.27% of the observable Universe.
Figure 1: simulation of the cosmic web.
In this figure, the blue threads represent filaments, and the vacant spaces represent voids.
The dominant theory of void formation is that they were created by means of baryon acoustic oscillations, BAO, in the early Universe. BAO can be described as quantum fluctuations in the densities of baryonic matter, also known as visible matter. It is believed that in the early Universe, fluctuations in the density of baryonic matter resulted in increased concentrations of dark matter being formed. Baryonic matter was then attracted to it, by means of gravitational attraction, and formed stars and galaxies. This resulted in areas of high density becoming denser and areas of low density becoming even less dense. Thus, filaments indicate areas of high dark matter density while voids are areas of low dark matter density. From this we can postulate that dark matter dictates the structure of the universe at the largest scale.
Voids are often overlooked as being areas of empty space in the Universe, but they are a key component in understanding the expansion of the universe and dark energy. Due to the existence of super voids, 70% of the energy in the Universe must consist of dark energy. This number is consistent with the current estimates of 68.3% obtained in 2013 from observations made by the Planck spacecraft and thus, consistent with the Lambda-CDM model. Voids are extremely sensitive to cosmological alterations. This indicates that the shape of a void is indicative of the expansion of the Universe and somewhat governed by dark energy. By studying the shape of voids over time, we can become one step closer to modelling an equation of state for dark energy.
- NASA, ESA, and E. Hallman (University of Colorado, Boulder)