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.

The two main measurements in modern cosmology — the rate at which distant galaxies recede (the Hubble constant) and the degree of matter clumpiness in the late universe (the parameter S₈)—persistently disagree with calculations. The first number comes out noticeably larger, the second noticeably smaller. It is as if someone added energy to the void while simultaneously damping gravitational collapse. The authors of a new study show that the culprit may hide in the very process of cosmic structure formation, and it is not a new particle or unknown field, but good old-fashioned disorder.

When diffuse gas is pulled together by dark matter's gravity into dense clumps, future stars and galaxies, its particles speed up and collide. Some of the kinetic energy is converted into heat and dissipates into space. In physics terms, this means the system's order decreases and its entropy (a measure of disorder) increases. But entropy growth is not a mere formality. In a closed system, it creates pressure that tends to push boundaries outward. On cosmic scales, this pressure manifests as extra energy that nudges expansion, like warm air inflating a balloon from within.

At the same time, this heat acts like microscopic friction. Imagine a crowd trying to form a circle. If everyone moves smoothly, the circle quickly tightens. But if people start bustling, bumping into each other, and losing energy to random motion, the gathering slows down. The same goes for galaxies: dissipating energy as heat damps the coherent motion of matter toward clump centers, making the universe's final 'clumpiness' lower than pure gravitational calculations suggest. The same process, turning the mechanical energy of gravitational collapse into heat, simultaneously boosts expansion and softens structure.

This idea has a major virtue: it does not break familiar physics. Einstein's equations, Newton's law of universal gravitation, and the very concept of gravity all remain in place. No new fields, particles, or extra dimensions. Just thermodynamics, the science of heat and order, built into the description of how matter assembles into galaxies. If the calculations hold up, the two biggest cosmological puzzles get a common, surprisingly down-to-earth explanation: the universe simply heats up as it contracts, and this heating changes its fate. So the blacksmith, cooling his sword, sees the heat escape into the air, but on a cosmic scale, that heat does not vanish without a trace; it becomes a force that pushes worlds apart and stills their tremors.