Seeing Strands in the Cosmic Web

Science writing is something I enjoy immensely — hence this blog — so when I found out that the AAAS runs a Mass Media Fellowship for science students and recent graduates, I jumped on it. As part of the application, I wrote a 750-word news story about a scientific paper that came out in July. This story will never appear in any news outlet, but I figured you all might find it interesting to read. (Science news outlets did cover this paper back in July.) So, hot off the presses, I give you a story about the hidden structure of our own universe:

For the first time, a part of the dark matter “skeleton” of the universe has revealed itself. The discovery strengthens our understanding of the universe’s history and tells us more about the formation of galaxies like our own, billions of years ago.

Current theories about the largest structures in the universe predict the existence of giant structures made of dark matter — the unseen substance that comprises over 80% of the matter in the universe — between most galaxy clusters. Now, for the first time, a team of cosmologists led by Jörg Dietrich at University Observatory Munich has found hard evidence that the long-sought-after strands of dark matter actually exist.

“This fills in a piece of the puzzle in our picture of the universe…under the direction of Newton’s gravity, matter has coalesced into clumps, streams and filaments,” says Fermilab cosmologist Brian Nord, who was not involved in the research.

Image Credit: Jörg Dietrich, University Observatory Munich
The dark matter filament between galaxy clusters Abell 222 and Abell 223. The blue area within the contours shows where the background images of galaxies have been lensed, indicating the presence of dark matter.
Image Credit: Jörg Dietrich, University Observatory Munich

Evidence for the existence of dark matter — which, as the name suggests, does not interact with light — has been around for decades. But over the last twenty years, a flood of new data and ever-increasing computational power has given cosmologists a more detailed picture of how dark matter affected the formation of galaxies and galaxy clusters in the first few billion years after the Big Bang. By running computer simulations of a universe inhabited by dark matter and a small amount of regular matter, cosmologists can see what kinds of structures are formed by gravity in their simulated universes. These simulations strongly suggest that most of the dark matter in the universe arranges itself into a three-dimensional web. “This web consists of voids that are surrounded by walls, and filaments tend to form at locations where two walls intersect, just as edges of a cube form at the intersection of its faces,” says August Evrard, a cosmologist at the University of Michigan also not involved in the research. These filaments are strands of dark matter millions of light-years in length. Their incredible mass — hundreds of times greater than that of our entire Milky Way galaxy — means that huge numbers of atoms were gravitationally pulled to them earlier in the universe’s history, collapsing and forming into clusters of galaxies along the filament. This implies that galaxy clusters roughly trace the distribution of dark matter — shiny beads along the dark cosmic threads. “We’ve seen linear structures in large-scale maps of galaxies for decades, and computer simulations of cosmic structure also exhibit this type of feature.” says Evrard. “[But this is] the first time that a filament has been identified by dark matter rather than galaxies.”

Dietrich and his team found the filament by looking at how nearby light was affected by the huge mass of dark matter. Dark matter must bend light, because Einstein’s theory of general relativity dictates that mass — all mass, be it dark matter or atoms — warps the fabric of space-time. “This bending of space-time produces a lens that magnifies and distorts the shape of light coming from a source in the distance. The amount of bending of the [light] on the opposite side of the lens…tells us the size and shape of the lens,” says Nord. Dietrich and his team took advantage of this lensing effect, inferring the existence of the dark matter filament by studying the images of over 40,000 distant galaxies in the background, behind the two galaxy clusters connected by the filament. “Gravitational lensing causes a subtle change in the observed shape of these galaxies. By measuring these distortions of the faint background galaxies, we were able to map the mass distribution of…the [dark matter] filament,” says Dietrich. “We have seen dark matter in galaxy clusters for several decades now, but this is the first time we can map the distribution of dark matter in filaments.”

With this discovery, we are finally getting a glimpse of the largest structures in existence — structures that made our own existence possible. “[T]his is a great observation that brings together the worlds that make up us, humans, and that of the…dark matter,” says Nord. We may not be able to directly detect dark matter yet, but it seems likely our galaxy — and we, its inhabitants — are a direct consequence of the gravitational pull of a web of dark matter.

Leave a Reply