With all the talk about the Higgs boson (fully justified! hooray!), another significant story runs the risk of getting lost. It’s not as triumphant, perhaps—after all, the hunt for the Higgs began decades ago—but it’s still important, especially for cosmologists like me.
One of the key pieces in the modern cosmological concordance model is dark matter, the invisible substance that comprises about 80% of all the mass in the Universe. Even though it was first proposed in the 1930s by Fritz Zwicky, the evidence of dark matter’s existence is indirect. To this day, we don’t know what kind of particles make up dark matter, only that they aren’t the same stuff as atoms, they aren’t neutrinos, and larger objects like brown dwarfs or black holes don’t have the right behavior. Despite not knowing the identity of the lurker, we see its footprints in galaxy clusters, the rotation of spiral galaxies, and the cosmic microwave background. In other words, most astronomers, cosmologists, and particle physicists are pretty certain the stuff exists, even though we can’t ID it yet.
Another major piece of evidence was published in Nature today (good timing, right?). Astronomers using archival data from the Subaru telescope in Hawaii and X-ray data from the orbiting XMM-Newton observatory have found a probable dark matter filament connecting two galaxy clusters. They used weak gravitational lensing, when a concentration of mass magnifies the light of an object far behind it, without creating a coherent image. The structure the researchers found has as much mass as a small galaxy cluster itself, but doesn’t have enough galaxies or ordinary matter. While galaxy clusters are full of hot gas, the X-ray observations of the gas in the filament show it only has about 9% of the total mass. The rest of mass must come from something else, so why not dark matter? (For more details about the discovery, see my Ars Technica article.)
By itself, this discovery is pretty exciting, but it’s important for another reason. The distribution of galaxies and galaxy clusters in the Universe is known as the large-scale structure (LSS). In LSS theory, dark matter collects (via mutual gravitational attraction) into halos, which attract the ordinary matter that becomes galaxies and so forth. However, the halos are connected to each other in a giant web, as shown in the image on the left: each dot represents a concentration of dark matter. The bigger blobs are galaxy clusters, but there are also thinner strands known as filaments. Because ordinary matter concentrates most in the halos, the filaments may contain very little to make them directly visible, even if they have a lot of dark matter mass.
Galaxy surveys, such as the 2-degree Field (2dF) survey, bear out the predictions of LSS theory: the galaxies are distributed in number and position exactly where they should be if the cosmic web is actually present. However, the filaments have proven elusive; previous observations have found them via X-ray emissions from hot gas, but nobody had measured their dark matter content—until the present study. Measurement of the dark matter mass in this filament is the Bullet Cluster for LSS: it reveals what should be there, but which has been hard to find independently.
My fellow cosmology blogger Ethan Siegel chided me for skimping in the discussion of LSS in my earlier post on the subject, “Is Cosmology in Shambles?“. In fact, I evidently equivocated too much for many people’s tastes, leading some to even think I take the side of those who think dark matter isn’t real. Rereading that post today, I understand acknowledge the validity of these criticisms, so let me state plainly for the record: no, I don’t think cosmology is in shambles, and yes, I think the balance of evidence is strongly in favor of dark matter’s existence. Things are complicated enough on the scale of galaxies and smaller structures to leave a little room for alternative ideas, but any theory replicating the success of the dark matter model has a lot of different phenomena to explain—including this new dark matter filament discovery.