A census that only records the distance between houses misses a lot. You can tell where the cities are and where the villages are, but you can't distinguish a skyscraper from a single-family home. That's roughly how astronomers study the universe with the standard two-point correlation function: they count how many pairs of galaxies are separated by a given distance. It's a powerful tool, but it's blind to the fact that galaxies come in different flavors. Some live in densely populated clusters, others in cosmic "villages."
The authors of the new study added "color-coded tags" to this method — marked correlation functions (MCFs). Each galaxy is assigned a weight that depends on the density of its surroundings. If there are many neighbors nearby, the galaxy becomes "heavier" and has a stronger influence on the statistics. A similar principle works when analyzing a city, where you consider not only the distances between buildings but also their heights. The map gains depth: you can see where matter is more clumped and where it's more diffuse. Such "tags" help capture the influence of dark matter and dark energy, which drive the growth of cosmic structures.
The scientists tested the idea on 129 computer simulations of the universe in cubes a billion light-years on a side. They examined how well different combinations of "tags" recover two key numbers: the matter density fraction and the amplitude of density fluctuations. Even three weight schemes give a factor of 1.7–2.5 improvement. Five — up to a factor of 2.4. The method turned out to be surprisingly robust: it didn't break when they changed the size of the analyzed region or the type of galaxies selected. Particularly useful is angular statistics: it looks not just at the distance between galaxies but at their relative positions at different angles. Different types of "tags" complement each other, like two views of the same sculpture. And then the census becomes truly three-dimensional: we see not only distances but also the weight of each patch of the cosmic fabric.
If we enlarged an atom to the size of a football field, the nucleus at its center would be the size of a pea — everything else is empty space. The universe is similarly empty on the scale of galaxies, but it's precisely this "emptiness" that helps us see how dark matter pulls matter into filaments and knots.
In Carl Sagan's novel "Contact," aliens transmit to humans the blueprints for a machine that allows travel through wormholes — bridges in spacetime. To find such bridges in reality, we need ultra-precise maps of the distribution of mass in the universe — exactly what "color-coded tags" promise to improve.