How the humble slime mold helped physicists map the cosmic web

Reconstructing the cosmic web - the vast network of filamentous structures of matter that stretches across the universe - modeled on the growth patterns of slime mold.
Zoom in / Reconstructing the cosmic web – the vast network of filamentous structures of matter that stretches across the universe – modeled on the growth patterns of slime mold.

Joseph N. Borchett et al / AJL

There is rarely time to write about every great science story that comes our way. So this year, we’re showing again a “Twelve Days of Christmas” flyer series, highlighting one science story that fell through cracks in 2020, every day from December 25 to January 5. Today: How physicists used modest slime mold growth patterns to model the dark matter that makes up the filaments of the cosmic web.

The backbone of our universe is a vast network of interconnected gigantic filaments, with huge gaps between them. Most of the materials that make it upCosmic network“Is dark matter, along with diffuse and distant gases. Earlier this year, a team of scientists at the University of California, Santa Cruz (UCSC) devised a unique approach to modeling this cosmic web, based on slime mold growth patterns, and found some phases. The similarities are striking and they describe their approach March paper Published in The Astrophysical Journal Letters.

“We don’t think the universe was created by a giant clay mold,” co-author Joseph Burchett told Lars, dashing our hopes of forming a new myth of creation. “This is really due to the similarities between the products of these two very different processes. At the end of the day, the similarities emerge in the optimization of nature.”

Slime molds are sometimes confused with fungi, but they are technically a type of amoeba: Single-celled organisms Which lack neurons, let alone brains. However, they are capable of some amazing heroic feats Some scholars meditate Whether they are nonetheless capable of some kind of rudimentary perception. There are many types of sticky mold (more than 900, in fact); In this case we are talking about Polysephalum poly, Yellow, spongy oozing mud mold that usually grows on a forest floor, or on decaying tree trunks.

Previous studies have shown that slime molds are capable of this Solve mazes And the Escape from trapsFor example, in addition to making simple decisions. And the In 2010Scientists left a sticky mold in a petri dish containing bits of oatmeal (a favorite food) that represent Tokyo’s railway stations, and they found that they were able to recreate this transportation network – basically finding optimal roads.

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When researchers place food in cities on the map, the slime template cooperates, diffuses to map many possible configurations and then vanishes to highlight the shortest paths between cities and the most efficient overall system map.

At first glance, the slime molds seem very far from the cosmic web. Like Matthew Francis Explained in Ars Technica Back in 2014:

Early in its history, the universe had no stars or galaxies, and the density of all matter was remarkably uniform. However, small fluctuations in this intensity – as noted in Cosmic microwave backgroundMove to small areas where the amount of dark matter is slightly higher than anywhere else. This slight increased density in turn attracted more matter, producing a slow streak: Some places collected a lot of dark matter and gas, while others were largely emptied.

According to sophisticated supercomputer simulations, the result is the cosmic web: nodes of dark matter tied by thinner strings, with wide spaces between them. Galaxies and galaxy clusters formed in the denser regions of gas attracted by the gravity of dark matter. The resulting network is called the large-scale structure of the universe, and much observational cosmology includes the process of mapping this cosmic network in three dimensions … the strings that tie everything together are fundamentally difficult to notice. This is because it contains a lot of dark matter, relatively little gas, and few or no stars. Astronomers discovered some filaments by measuring the light absorbed by the gas. Others used dark matter gravitational lenses.

Per Burchett, the slime molds began working as an archival data analysis project funded by the Hubble Space Telescope to study the gas distribution within the cosmic grid – meaning “the storage of gas and fuel from which galaxies form,” he said. While astronomers can identify the nodes of this network in the form of dark matter halos surrounding galaxies, identifying the connecting filaments has proven more difficult. Burchett’s plans Use of pseudo-stars background Essentially to “illuminate” the foreground gas, but I had difficulty designing a reliable algorithm to find those strings within large sets of observational data.

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“The problem you’re definitely facing is, where do I draw the cosmic web?” Because “the true backbone of the cosmic web is dark matter. The diffuse gas that permeates the structure of dark matter is really the only way we have to discover matter,” he said. Burchett found the most sensitive approach was to use quasars in the background. Until then, he said, “you have to observe the gas as a shadow you can see in the background light of the quasar.”

Borchett mentioned this type of beer research with his UCSD Postdoctoral colleague Oscar Ilic. More inspiration came from an unexpected source. A teacher to you, Angus Forbes (another co-author) told him about the work of a Berlin-based media artist named Sage Jenson, who was using computer simulations to grow slime molds to create art, based on 2010 paper By Geoff Jones in The Journal of Artificial Life.

I was struck by the growth patterns of sticky mold in Jenson’s Creations (you can see examples On its website), Which included the same kind of filamentary properties of the cosmic web. He and a programmer’s friend created a 3D version of a slime mold growth simulation – called the Monte Carlo Physarum Machine (MCPM) – and plugged in a dataset of 37,000 galaxies from the Sloan Digital Sky Survey (SDSS), just to see what would happen.

Durability assessment of the MCPM model. Some of the fine details are lost, but the overall structure is well preserved.
Zoom in / Durability assessment of the MCPM model. Some of the fine details are lost, but the overall structure is well preserved.

Joseph N. Borchett et al / AJL

Burchett admits he was a bit skeptical when Ilek first touched upon this possibility, given the already large work-group using traditional modeling methods. But the results worked surprisingly well. “Visually, it’s really amazing,” said Burchett. “You can determine where the filaments should be if you look at a map of galaxies in the sky, and the sticky-mold model fits that intuition well.” Just as a slime mold creates an optimized transportation network, and finding more efficient pathways for the delivery of food sources, the growth of structure in the cosmic web produces similar perfect networks. The basic operations differ for each, but they produce similar mathematical structures.

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However, Elk Lars said: “It’s not that someone has demonstrated that there is a difficult mathematical reason that makes the cosmic web a perfect transportation network.” “It just got close to it. It was a more intuitive feeling that got these things close enough to actually make them work.”

To verify results from MCPM, Burchett Et al. He built an index of dark matter halos based on data from classic cosmological simulations, then ran the algorithm to reconstruct the filamentous network linking those auras. The results correlated strongly with the original cosmic simulation. This enabled the team to improve their slime mold model. As another “rational examination”, the team also compared the slime-mold prediction of the gas density in the intergalactic medium to the star formation activity of the galaxies listed in SDSS.

Finally, they tested their model predictions of the cosmic network’s structure against UV data from 350 quasars from the Cosmic Origins Spectrometer of the Hubble Space Telescope. “We knew where the strings of the cosmic web should be thanks to the slime mold, so we could go to the archived Hubble spectra looking for quasars that probe this space and look for gas fingerprints,” said Burchett. “Wherever we saw a filament in our model, the Hubble spectra showed a gas signal, and the signal got stronger toward the middle of the filaments where the gas should be denser.” As expected, the signal decreased in the denser regions, as the gas became very hot and ionized, canceling the absorption effect.

There is still more interesting research to follow based on the success of MCPM. Birchit turned his attention from mapping gas in strings to the cosmic network to galaxies themselves, and determining how their properties relate to where they are within the cosmic network. As for Elek, he views the slime mold model as a “structure discoverer” and explores how it applies to bioprinting, among other uses.

“We see this as the tip of the iceberg of what we can do scientifically,” said Burchett. “This separates all kinds of cool applications inside and outside of astrophysics.”

DOI: The Astrophysical Journal Letters, 2020. 10.3847 / 2041-8213 / ab700c (About DOIs).

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