cosmic structure formation

In the first moments after the Big Bang, the universe resembled a boiling cauldron. According to one hypothesis, dark matter, the invisible substance that holds galaxies together, could have been born there.

The night sky looks like a random scattering of stars. But zoom out to enormous distances, and the chaos disappears. Galaxies gather into long filaments and flat walls, with colossal voids gaping between them — cosmic voids. This structure is called the cosmic web.

The standard cosmological model ΛCDM (which includes dark energy and cold dark matter) is the score of a cosmic symphony. It describes how space expanded after the big bang. How dark matter clumped together. How galaxies ignited. For a long time, everything played flawlessly.

In a dark room, music plays. Powder is scattered on the floor — fine, almost invisible. Low frequencies make it gather into intricate patterns: filaments, clumps, voids. In much the same way, gravity over billions of years weaves galaxies into the cosmic web — except here, attraction does the work instead of sound waves.

In 1929, Edwin Hubble pointed his telescope at distant galaxies and discovered they were receding. Ninety years later, two measurements of this recession rate, from ancient light and from nearby stars, stubbornly disagree. A 9% discrepancy cannot be chalked up to error. It is as if the Universe in its youth and in its maturity runs on different clocks.

Three invisible strings oscillate in the intergalactic void. This is not poetry, but a hypothesis about the nature of dark matter — an ultralight quantum field with spin 1, carrying three polarizations.

Picture the early Universe as an unimaginably large bell. Inflation — the era of rapid expansion, dreamed up by Alan Guth, strikes it, sending a wave rippling across the fabric of spacetime.

In the first moments of the universe's life, there was not even a hint of dark energy — the very force that today pushes galaxies apart ever faster. But if you look deeper, down to the Planck scale (billions upon billions of times smaller than an atomic nucleus), spacetime ceases to be smooth.

In the search for the nature of dark matter, one of the most intriguing hypotheses traces back to the first moments after the Big Bang. Back then, quantum fluctuations of the inflation field gave rise to clumps of matter that could collapse into primordial black holes.

The story began with a missing mass. In the 1930s, Fritz Zwicky, measuring velocities in galaxy clusters, first suspected the presence of invisible matter. Later, Vera Rubin showed that the outskirts of spiral galaxies rotate too fast for the visible matter.

The early universe was a restless place. Space trembled with gravitational waves — ripples in the very fabric of reality. In this churning cauldron, dark matter is thought to have been born. But the newborn particles had no mass: it had to come from somewhere. Usually, the Higgs field is responsible for mass.

Spacetime is not empty: it shivers. After the Big Bang, the universe was filled with a hum of gravitational waves, like the low rumble of a giant bell. In this ripple, as it turns out, particles of dark matter, the invisible substance that holds galaxies together, can be born. But the previous idea had a snag.

It all began not with emptiness, but with a boiling ocean of plasma. In that primordial broth, sound waves raced through the fiery matter, colliding and compressing it. Sometimes a clump grew so dense that light could no longer escape, and right out of the scorching gas, a primordial black hole was born.

A giant cloud of invisible matter acts like a funhouse mirror — distorting the light from a distant galaxy. From these distortions, astronomers can weigh the cloud and peer into its depths.

Through the ripples on the water, pebbles at the bottom of a pool look shifted and warped. In a similar way, the gravity of invisible dark matter bends the light of distant galaxies — a phenomenon known as weak lensing.

A blacksmith forges a sword. With each hammer blow, the red-hot metal flattens, but the hammer, the anvil, and the surrounding air all warm up slightly. Part of the impact energy is irreversibly lost as heat — a trade-off familiar to anyone who has ever driven a nail, and it turns out to be the key to the biggest mysteries of the cosmos.

In a thin sandwich of semiconductors, light creates pairs, an electron and a positive hole, that stick together like a ball on a rubber band. Normally, such pairs, called excitons, quickly emit light and vanish. But there are others, dark ones, that live hundreds of times longer because their internal structure forbids them from easily giving off energy.

Drops from a leaky faucet beat a clear rhythm. A similar rhythm, only on the scale of the universe, has been found by astrophysicists: the formation of primordial black holes — tiny objects born in the first moments after the Big Bang — also proceeds not chaotically, but in a strictly measured way.

A candle flame flickers in a draft but doesn't go out — the tongue of fire briefly changes shape and then returns. Optical skyrmions behave much the same way — tiny vortices in a light beam, reminiscent of swirling eddies. Until recently, they were created only in the perfect, smooth light of a laser.

Toss a handful of glitter into a river, and some flakes will drift straight while others spin in tiny eddies. Galaxies in the universe do something similar: they don't just recede due to cosmic expansion: they also swirl in giant currents, like wood chips caught in invisible whirlpools.

No one has ever seen dark matter, but every galaxy feels its pull. Usually, it's pictured as a sea of identical, silent particles. The new work paints a livelier picture: two kinds of particles, heavy and light, constantly exchanging between them.

It all started with tiny quantum flickers on the primal canvas. Inflation — an avalanche-like expansion proposed by Alan Guth — stretched those brushstrokes to the scale of galaxies.

In the mid-20th century, astronomer Vera Rubin discovered that stars on the outskirts of galaxies were moving too fast — held by an invisible mass. Thus the mystery of dark matter was born. It doesn’t emit light but reveals itself through gravity. The big question: why is there exactly as much as we see?

The Universe looks like a giant spiderweb: galaxies aren't scattered randomly but are gathered into knots and along filaments. Inside the filaments, there's plenty of gas, and that's where new stars most often flare up. But how has this changed over time? Scientists compared data from a computer model with real observations of thousands of galaxies.

The universe began with the Big Bang and subsequent inflation — a rapid expansion. Alan Guth suggested that in some regions this growth never stopped. They remained ‘underbaked’ pockets of eternal expansion forever.

In its infancy, the universe was a hot, plasma-like mixture of particles. Microseconds after the Big Bang, as it cooled, the temperature fluctuated slightly.

The early Universe was like a construction site where, instead of slowly building floors, they used prefabricated blocks. These blocks were primordial black holes, born in the first fraction of a second after the Big Bang.

Back in the last century, Vera Rubin discovered that stars on the outskirts of galaxies rotate too fast, as if held by some invisible substance. Since then, dark matter has remained a puzzle. Usually, its origin is linked to new particles or forces. But a new study points to a simpler source — the geometry of space itself.

The night sky is full of stars, but they are only a small part of everything that exists in the universe. Astronomers have long noticed a strange thing: galaxies rotate so fast that they should fly apart, yet they don't. Something invisible holds them together, like invisible threads linking beads on a spinning wheel. This "something" was named dark matter.

In laboratories on Earth, Bose-Einstein condensates are created by cooling a cloud of atoms to billionths of a kelvin. The atoms lose their individuality and merge into a single quantum wave that behaves as one entity. But in space, such conditions are not needed: gravity itself creates deep wells where matter compresses and cools into a state of quantum fog.