Filaments and Voids

            Typically, when one thinks about space, the idea of unfathomable expanses comes to mind. The observable universe is gigantic with an estimated radius of 46.5 billion light-years. In this immense volume lies roughly 200 billion galaxies, each with about 100 billion stars. The universe structures these masses in hierarchical patterns that are dictated by gravitational attraction. Planets orbit stars, which in turn orbit around a galactic center. Galaxies experience attraction towards each other leading to galaxy groups and clusters. Galactic clusters can merge to form superclusters and at the largest scales, cosmic filaments. These filaments structures are believed to be the largest form of order in the universe, an observation that has been called the “End of Greatness”. However, since galaxies gradually attract each other, does this mean that there are pockets with little to no mass? The creepy answer is yes, and these are referred to as voids.

 

                Voids are the opposite of filaments in that they are the “holes” in between the highly concentrated galaxy superclusters. These structures, filaments and voids, give rise to the “cosmic web” which is typically used to describe the macro scale of the universe. Contrary to the name of these structures, voids are not entirely devoid of astronomical objects or matter. One of the largest voids measured to date, the Boötes void, contains roughly 53 luminous galaxies distributed over a spherical diameter of 250 million light years. For comparison, the largest known galaxy is about 2.5 times the diameter of the Milky Way, or 250,000 light years! The density of the interstellar medium contained within the Boötes void is 1.6 x 10^ -29 g/cm^3 which is less than the Universe’s average. Because of this absurdly low density, low mass particles and photons pass through the void without any alteration from matter interaction. "The Boötes void may one day be seen as the ultimate time capsule - fire off a pattern of photons, only for the pattern to be rediscovered hundreds of millions of years later when it reaches the other side."

(The white dots in the center of the green circle are in front of the void)

                        How did such a large void form in the first place you may ask? Well the answer is not definitively known, however the prevailing theory is that it was formed from the accumulation of smaller voids. From my very limited understanding of baryonic matter, smaller voids were formed at the beginning of the Big Bang. These primordial voids would then expand in volume as the containing mass would migrate towards higher density clusters. After awhile, these smaller voids would coalesce into larger voids like the Boötes void. Although these big spheres of "nothingness" may sound boring and unimportant,  they have contributed to further understanding dark energy, cosmic inflation, and and galactic formation. In fact, voids may be the ideal environments to test theories about dark energy and dark matter.

References:

https://en.wikipedia.org/wiki/Void_(astronomy)

https://asd.gsfc.nasa.gov/blueshift/index.php/2013/07/30/jasons-blog-next-stop-voids/

http://curious.astro.cornell.edu/the-universe/stars-and-star-clusters

http://abyss.uoregon.edu/~js/ast123/lectures/lec24.html

http://www.bbc.com/earth/story/20150723-the-largest-thing-in-the-universe