The Cosmic Web: How Galaxies Are Woven

Look up at the night sky and you see a relatively uniform sprinkling of stars. But zoom out, past our local solar system and the Milky Way, to the grandest scales of the universe, and an astonishing picture emerges.

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The cosmos isn't a random soup of galaxies; it's a colossal, three-dimensional tapestry, an intricate network of filaments and voids known as the Cosmic Web.

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This web is the universe's large-scale structure, and it holds the key to understanding how galaxies formed and how the universe evolved.

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The Bones of the Universe

The Cosmic Web is made up of three main components:

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• Filaments: These are long, thread-like structures where galaxies are densely clustered. They can span hundreds of millions of light-years and act like highways, channeling gas and galaxies towards the richest parts of the web.

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• Nodes or Clusters: Where the filaments intersect, galaxies pile up, forming massive clusters. These are the most gravitationally dominant and densely populated regions in the universe, home to thousands of galaxies. Our own Milky Way is on the outskirts of the Laniakea Supercluster, a massive concentration of galaxies.

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• Voids: These are vast, empty regions of space that contain very few or no galaxies. They are the emptiest places we know of and make up the majority of the universe's volume.

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Think of it like a sponge: the filaments and clusters are the dense, porous parts, while the voids are the holes.

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The Role of Dark Matter and Gravity

The Cosmic Web's existence is a direct consequence of the interplay between dark matter and gravity. In the very early universe, after the Big Bang, matter was distributed almost perfectly uniformly.

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However, tiny, quantum fluctuations in density meant that some regions were slightly more massive than others. Over billions of years, gravity began to pull matter into these slightly denser regions.

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Because dark matter makes up about 85% of the universe's total matter, it was the primary architect of this structure. Dark matter, which only interacts through gravity, formed the invisible scaffolds of the cosmic web first.

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Normal matter—the stuff of stars, planets, and us—was then pulled along by dark matter's gravitational influence, accumulating into the filaments, nodes, and clusters we observe today.

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A Cosmic River: The Flow of Galaxies

The Cosmic Web is not a static structure. Galaxies within the filaments are not just sitting there; they are actively being drawn along these cosmic rivers towards the great clusters at the nodes.

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This gravitational flow, sometimes called "cosmic flows," is a powerful force that drives galaxy evolution. As galaxies fall into these denser regions, they undergo dramatic transformations:

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• Mergers and Collisions: Galaxies in clusters often collide and merge, leading to bursts of star formation and the creation of larger, often elliptical, galaxies.

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• Gas Stripping: The hot, dense gas in galaxy clusters can strip away the cold gas from infalling galaxies, effectively "starving" them and halting star formation.

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• Tidal Forces: The strong gravitational pull of a cluster can distort and tear apart smaller, less massive galaxies.

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Our Place in the Web

We are not at the center of the universe, but we do have a specific address within the Cosmic Web. Our Milky Way is part of the Local Group of galaxies, which itself is part of the Virgo Supercluster.

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We are currently on the outskirts of the immense Laniakea Supercluster, which is flowing towards a gravitational anomaly known as the Great Attractor.

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This constant, invisible flow reminds us that the universe is a dynamic, interconnected system on all scales.The Cosmic Web provides cosmologists with a powerful tool to test our understanding of the universe.

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Its structure and evolution are a direct result of the laws of physics and the ingredients of the universe, including the enigmatic dark matter and dark energy.

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By mapping the web, we can trace the universe's history from its initial seeds to the magnificent structure we see today, revealing how the tapestry of galaxies was woven together over billions of years.