Why Computer Galaxies Don't Look Like Real Ones ✛
A census by body shape: that's one way to describe what astronomers did with 6348 massive galaxies. Instead of height and weight, they measured how concentrated the light is toward the center, how flattened the galaxy is, and how asymmetric its shape is. Then they performed the same measurements on galaxies from three computer models of the universe and compared the results.
Each model turned out to be like an artist drawing a portrait from memory: it gets the general features right but muddles the details. One model produces too many galaxies with bright, dense cores; another puffs up disks to unrealistic sizes; and the third rarely creates the very flat galaxies that are common in the real sky.
Previously, it was thought that if a model correctly counts the total number of galaxies, their shapes would also be correct. Now it's clear that's not the case. For future models, this is a tough exam: they'll have to pass not only the count but also the shape test.
The Sérsic index of galaxies typically lies between 1 (a pure exponential disk) and 4 (a classical de Vaucouleurs elliptical profile), but in simulations values up to 8 are found, which is almost never observed in reality.
In Liu Cixin's novel 'The Three-Body Problem', the Trisolaran civilization tries to predict the structure of their chaotic star system but faces fundamental limitations in modeling. Similarly, modern galaxy formation simulations face limitations in reproducing structural diversity, despite impressive computational power.