Particle universe simulator

What you can do

• Watch about 45 seconds of growth: tiny density ripples become sheets, then filaments, then clusters.
• Drag to turn the box, pinch or scroll to zoom, WASD to fly round it; double-tap resets the view.
• Tap Demo beat or Your music: the growth breathes with the bass and kicks light up the knots.
• Colors follow density: void blue, filament cyan and knot gold, with brightness carrying the same information.

How did the cosmic web form?

Just after the Big Bang, matter was spread almost perfectly evenly. The cosmic microwave background, light released 380,000 years later, shows the leftover ripples: differences in density of about one part in 100,000. Everything since grew from those ripples, because a region slightly denser than average pulls in a little more matter, which makes it denser still.

In 1970 the Soviet physicist Yakov Zel'dovich worked out what that growth looks like. Each bit of matter moves in a straight line set by the ripples around it, and collapse happens along one direction first. Matter flattens into vast sheets, which he called pancakes; where sheets cross they make filaments, and where filaments meet they make dense knots. Galaxy surveys such as the Sloan Digital Sky Survey later mapped exactly this: galaxies strung along filaments around huge, nearly empty voids.

This simulator uses his approximation. A random field of ripples with statistics like those of cold dark matter was computed in advance; each of the 262,144 particles then moves along its own displacement as the growth factor rises, and its density comes straight from his formula. The box wraps around at its edges, like a small piece of an endless universe.

The shortcut: the approximation is exact only until streams of matter cross, so it never forms the tight, round halos a full gravity simulation makes. The bright knots mark where those halos would be.

Questions

Is this particle simulator free?

Yes. It runs free in your web browser on WebGL 2, with no download, no account and no ads. The particle field is about 6 MB, loaded once; on a slower device it renders at a lower resolution to keep the motion smooth.

How many particles does it simulate?

262,144, a 64 by 64 by 64 grid. Each one moves along its own path every frame on your graphics chip.

Is it real physics?

Yes, with one simplification. It is the Zel'dovich approximation, a standard starting point that cosmologists still use to set up their big simulations. It is exact until streams of matter cross; after that a full N-body simulation, where every particle pulls on every other, is needed to form the dense halos where galaxies sit.

What is the cosmic web made of?

Mostly dark matter, which outweighs ordinary matter about five to one. Galaxies form in the densest knots and along the filaments, and the voids between them can be hundreds of millions of light-years across.

Is this the Everdust app?

It is one of its worlds. The Everdust app, a particle universe simulator, is free on the App Store for iPhone, iPad, Apple Vision Pro and Apple TV, on Google Play for Android, and on the Meta Horizon Store for Meta Quest 3 and 3S, with galaxy collisions, black holes and space science games.

Related

• Strange attractor simulator 200,000 particles on the Lorenz, Aizawa, Thomas and Halvorsen attractors.
• Galaxy collision simulator Two galaxies of about 200,000 stars pass and merge.
• Universe simulator How the Everdust app simulates the universe, from the Big Bang on.
• Physics sandbox apps for Android An honest guide to particle and gravity sandboxes on Android.