A Cosmic Web of Galaxies: How Scientists Map the Universe

Since the middle of the twentieth century we have known that the universe is around 14 billion years old and that it is expanding, having started as a tiny fraction of its present size. But the expansion does not scatter galaxies at random. Over the 1980s, observations with powerful telescopes showed that galaxies are strung along filaments of a vast “cosmic web” with near-empty voids in between, rather like an enormous cobweb.

The filaments string together dozens or even hundreds of galaxies. Each galaxy is around 10,000 times smaller than the filament itself, so appears as a single bright dot on this scale, and yet that dot contains hundreds of billions of stars, each of which may have multiple planets. So the structure I am talking about is traced through specks of light like dew glistening on a cobweb of dizzyingly gargantuan proportions.

One of the first projects to reveal the bizarre cosmic web structure was led by the astronomer Marc Davis. Comfortable with technology—Davis had funded his college studies by working for a software company—he built an automated, digitalized system for mapping the locations of galaxies in our universe. Not unlike Holmberg decades before, Davis realized that the existing catalogs of galaxies had been assembled haphazardly, and decided to automate the process of scanning the sky with the aid of computers. Inside the telescope dome, “there were wires running all over… I didn’t do the neatest job in the world, but it did work,” he later recalled.

Yet the results seemed a major puzzle: How and why had the galaxies been arranged in this way? Davis turned his attention to finding an explanation, assembling three young researchers to look into the problem using simulations. First was rising star Simon White and his PhD student Carlos Frenk, who had just written a thesis arguing for the existence of dark matter in our own galaxy. Today, on the verge of retirement, Frenk carries an irrepressible, boyish enthusiasm for cosmology. “I can’t quite believe it but somehow I’ve ended up with the best job in the universe,” he said in a 2022 lecture.

The team was completed by George Efstathiou, then finalizing his thesis at Durham University and the author of the only computer code in the world that could perform simulations of the necessary scale and sophistication. Efstathiou was in charge of Cambridge’s Institute of Astronomy when I arrived to start work on my own thesis in 2005, and to me he was a faintly terrifying figure of authority. But in the 1980s, Efstathiou drove a loud motorbike and wore a leather jacket. The young tearaways became known to the broader community as the “gang of four,” a reference to the Chinese Communist Party radicals.

To appreciate one advantage of Efstathiou’s code over its predecessors, consider that the universe does not appear to have any edge, as far as we know. When cosmologists talk about the universe expanding, we do not mean there is a bubble of material expanding into an empty abyss; instead, the entirety of space that our telescopes observe is already filled with the cosmic web of galaxies, and yet all of the galaxies are gradually receding from the others. This is very hard to picture mentally, and also a practical conundrum for simulations. How can any finite computer represent a boundless universe?

The solution is to use mathematical tricks to make a small simulated universe look infinite. The closest analogy is the classic arcade game Asteroids, in which you pilot a 2D spacecraft around a computer-screen-sized universe, attempting to shoot space rocks before they hit you. If a rock, or your spacecraft, flies through the right-hand edge of the screen, it reappears on the left-hand side, and vice versa.

Similarly, flying off the top teleports you to the bottom. Rather beautifully, this provides a gaming universe that has no edge but that is limited in extent and therefore tractable in terms of computational demand. Efstathiou’s code implemented this idea, taming the demands of simulating space, placing it inside a miraculous box without walls.

The gang of four combined this universe-in-a-box with the standard kick-drift-through-time approach to simulations and showed how dark matter, with its enormous gravitational influence, would gradually construct a web of material over billions of years. Wherever there is extra dark matter, gravitational attraction sucks in more; conversely, where there’s less dark matter, gravity is weak and material will easily be pulled out.

This results in a runaway effect: a small patch of dense material will rapidly hoover up everything around it, and over time will form giant structures like galaxies. As these galaxies start to attract each other, some collide and merge, just like Holmberg showed. Others are not close enough to coalesce, but instead line up in a web of galaxies strikingly similar to Davis’s maps of the universe.

Like climate scientists, cosmologists can tweak the assumptions within simulations to discover how these different structures respond and whether they match reality. In the 1980s, the burning question centered around neutrinos: Could these mysterious particles account for all the hidden extra mass that the universe seems to harbor? On the face of it, neutrinos were perfect—being completely invisible, abundant throughout the universe, and (unlike any other possible candidate for dark matter) confirmed to exist through laboratory experiments here on Earth.

Those experiments had also showed that neutrinos must be exceptionally light—at most, something like a hundred-millionth of a hydrogen atom. In itself, this doesn’t prevent neutrinos acting as dark matter; there are expected to be so many of them in our universe that their total gravitational effect could still be enormous. But the Nobel Prize-winning cosmologist Jim Peebles cautioned that such incredibly lightweight particles move fast; just as a cricket ball is easier to throw at speed than a cannonball, the opening moments of cosmic history knock light neutrinos into a frenzy of movement.

Once tweaked to include the resulting rapid motions, the team’s simulations confirmed that it was impossible to form the kind of dense, knotty web that had been seen in reality. Neutrinos moved so fast that they shot across the universe instead of creating the required structures.

The discovery was dramatic, because it confirmed that no particle known to physics could account for dark matter: something totally new was required, and it would come to be known, a little cryptically, as “cold” dark matter. The terminology arises from the idea that fast-moving particles like neutrinos are “hot”: what we experience as heat is actually rapid motion, albeit usually on microscopic scales.

Conversely, cold dark matter refers to heavy, slow-moving invisible particles. These form structures much more like the real thing. I think of it a little like fondue: if the universe is made of material that’s too hot, it becomes thin and runny, but when it’s cold dark matter, it lumps everything together into the gloopy webs of structure that telescopes had discovered.

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Adapted from The Universe in a Box: Simulations and the Quest to Code the Cosmos by Andrew Pontzen. Copyright © 2023. Published by Riverhead Books, an imprint of Penguin Publishing Group, a division of Penguin Random House LLC.