Two of the most captivating and complex networks in nature are the network of neural cells in the human brain and the cosmic web of galaxies.
The human brain is an intricate structure composed of many cellular, molecular and neuronal wonders. It can be modelled as a wiring diagram called ‘the human connectome’, in which neurones cluster into circuits, columns, and different interconnected functional areas. The structure of the neuronal network and its interconnections forms the physical and biological basis of cognition. Amongst other things, some of the major challenges of modern-day neuroscience include successfully constructing the connectome (i.e. the complete map of neural connections in a brain) and understanding how this structure can produce complex cognitive functions.
The Universe seems to be reasonably well described by a “consensus” physical model called the ΛCDM model (Lambda Cold Dark Matter), which accounts for gravity from ordinary and dark matter (i.e. very weakly interacting particles), for the expanding space-time described by General Relativity, and for the anti-gravitational energy associated to the empty space, called the “dark energy”. Amongst other things, some of the major challenges that cosmology still faces include the physical nature of dark energy, the composition of dark matter (or the realm of alternative scenarios for it), and the apparent tension between different measurements of the expansion rate of the universe.
Despite the substantial difference in scale between these two networks (more than 27 orders of magnitude), their quantitative analysis and observation through microscopic and telescopic techniques have captured a fascinating similar morphology, to the point that it has often been noted that the cosmic web and the web of neurones look alike. In this paper, an astrophysicist and a neurosurgeon applied methods of cosmology, neuroscience, and network analysis to explore this thought-provoking question for the first time, to their knowledge. They have presented a detailed comparison between the neuronal network and the cosmic web, with the goal of assessing the level of similarity between these two physical systems in an objective way.
The human brain functions thanks to its wide neuronal network that is believed to contain approximately 69 billion neurones. On the other hand, the observable universe is composed of a cosmic web of at least 100 billion galaxies. Within both systems, only 30% of their masses are composed of galaxies and neurones. Within both systems, galaxies and neurones arrange themselves in long filaments or nodes between the filaments. Finally, within both systems, 70% of the distribution of mass or energy is composed of components playing an apparently passive role and has only an indirect role in their internal structure: water in the brain and dark energy in the Universe.
Despite these immediate similarities, the scientists wanted to take a more quantitative look at the two systems. So they used a method called ‘power spectrum analysis’, which is a technique often used in astrophysics to study the large-scale distribution of galaxies. They measured the strength of tiny fluctuations throughout a range of spatial scales of both a simulation of galaxies and sections of the cerebellum and cerebral cortex of a brain.
The results of the analysis shows that the distribution of the fluctuation within the cerebellum neuronal network on a scale from 1 micrometre to 0.1 millimetres follows the same progression of the distribution of matter in the cosmic web but, of course, on a larger scale that goes from 5 million to 500 million light-years.
The two researchers also calculated some structural parameters characterising both the neuronal network and the cosmic web: the average number of connections in each node and how these nodes clustered together. The results of the structural parameters suggest that the connectivity within the two networks evolves following similar physical principles, despite the striking and obvious difference between the physical powers regulating galaxies and neurones.
These two complex networks show more similarities than those shared between the cosmic web and a galaxy or a neuronal network and the inside of a neuronal body. However, these similarities only arise when researchers compare a specific scale of each system. This is particularly important when comparing something infinite like the universe, to our very finite brain. However, the results of this paper suggest that vastly different physical processes can lead to very similar complex and organised structures.
Original Source: F. Vazza and A. Feletti (2020) The Quantitative Comparison Between the Neuronal Network and the Cosmic Web. Frontiers in Physics. 8:525731. https://doi.org/10.3389/fphy.2020.525731