The Cosmic Web
https://arxiv.org/pdf/1604.03236.pdf
Cool 3D visualization: http://cosmicweb.kimalbrecht.com
The World Wide Web is linked through the use of paths and connected components, which allows for people to effectively find what they’re looking for. Similarly, the cosmic web, “a set of discrete galaxies held together by gravity,” is also connected through paths and connected components which, with future progress, will help us understand the structure of the Universe (B. C. Coutinho et. al, 2016). In “The Network Behind the Cosmic Web” article, galaxy data obtained from a cosmological simulation is used to analyze 71 parameters that characterize each galaxy. Just how links form the nodes in the World Wide Web, the galaxies represent the nodes in the cosmic web. There were 7 different models that were tested; however, the model that worked best was one in which the length of the link depended on the position between galaxies –specifically, a galaxy was connected to its closest galaxy by a directed link. By finding correlations for all 71 parameters between all the nodes (galaxies), it was found that there were four properties which consistently displayed correlations between the galaxies. These four properties were: peculiar velocity, stellar metallicity, specific star formation rate, and color in the Blue-Vision filter band. Due to this model producing better results than its other six counterparts, it was concluded that the best way to organize the cosmic web, at least until now, is by making use of the distance between the galaxies.
Strongly connected components were also found in the simulation. For the cosmic web, a strongly connected component is the one which has the largest number of galaxies, such that there exists a directed path between every pair of galaxies. It was found that the strongly connected component began at an approximate degree of 4 for every galaxy. The term “degree” here is related to the expansion of the universe due to galaxy movement. The fact that the component began appearing at degree 4, and not degrees 1-3 could mean that this is true for every galaxy.
For comparison, the simulation results were compared to real, observational data from the Sloan Digital Sky Survey, which has detailed 3D maps of the Universe, and after running the same technique model through it, was found to match the results from the simulation data.
There are not many papers on this area yet, but hopefully this will change in the next few decades, and network theory will help us get there. Just how it was possible to get from the article on Nash equilibrium to the article on NASA in the example from the book, maybe it’s also possible to get from one galaxy to another galaxy across the universe by the “six degrees of separation,” where short paths link distant pairs of galaxies, as described in Section 13.2 (Easley).