Quasar illuminates a cosmic filament—and a mystery

Dark matter binds the galaxy together. According to detailed computer simulations, it also binds galaxies to each other, creating a vast structure often called the "cosmic web" due to its appearance. In these models, the filaments that connect this web are traced by a combination of dark matter and atoms in the form of very low-density gas. While astronomers can identify the nodes of this web in the form of dark matter halos surrounding galaxies, the connecting threads have proven a little more challenging to spot.

Now, a group of astronomers identified one such filament close to a very distant galaxy. Sebastiano Cantapulo and colleagues observed the light emitted by the filament's gas as it glowed under bombardment from a quasar, a powerful jet of particles propelled from a massive black hole. However, the researchers also found at least ten times more gas than expected from cosmological simulations, which suggests that there may be more gas between galaxies than models predict.

Early in its history, the Universe had no stars or galaxies, and the density of all matter was remarkably uniform. However, tiny fluctuations in this density—as observed in the cosmic microwave background—led to small regions where the amount of dark matter was slightly higher than elsewhere. Those slight overdensities in turn attracted more matter, producing a slow cascade: some places collected a lot of dark matter and gas, while others were largely emptied out.

According to sophisticated supercomputer simulations, the result was the cosmic web: nodes of dark matter linked by thinner filaments, with vast voids between. (Computer models are necessary because the calculations are far too involved to perform by hand from first principles.) Galaxies and clusters of galaxies formed in the denser regions from gas attracted by the gravitational pull of dark matter. The resulting web is termed the large-scale structure of the Universe, and much of observational cosmology involves the process of mapping this cosmic web in three dimensions. (We see the sky as a two-dimensional surface and must use sophisticated techniques to infer the third dimension: distance from Earth.)