A plain-language summary of Disentangling the Cosmic Web toward FRB 190608 (Simha et al. 2020, The Astrophysical Journal; arXiv).
TL;DR: we worked out where the gas along one burst’s path actually sits, and most of it turned out to be in the space between galaxies along strands of what astronomers call the “cosmic web”, not around the galaxies themselves.
The context
Most of the universe consists of matter in the “plasma” state. This is the state of matter that you see everyday as a fire or in the sun. It is an extrmely energetic state of matter in the sense that it is really hot. So hot in fact, that the atoms in that state have a tough time holding onto their electrons and so the matter is essentially a loosely held soup of charged particles.
On top of this, most matter in the universe is in an extremely “diffuse” state. This means there’s a tremendous amount of space between the particles. The best vacuums we achieve on earth are still orders of magnitude denser than the average density of matter in the universe. Between galaxies, on average, you expect to see one atom in a sugar cube sized space. In contrast, the air you breathe close to sea level has about ten billion billion atoms in the same space. The combination of these two properties makes this extremely hard to see even with the most powerful telescopes.
Enter Fast radio bursts (FRBs), which are flashes of radio waves from distant galaxies that last about a thousandth of a second.
As a burst travels, the thin gas it passes through slows its low-pitched radio waves a little more than its high-pitched ones. By timing how far the low notes lag behind, we can count how much gas the burst crossed. This is what is called the “dispersion measure” (DM). But the count is a single number for the whole trip. It is like a car’s odometer: it tells you how far you drove, but not which roads you took. Was the gas around the galaxies the burst passed, or spread through the space between them?
What did we do?
We picked one burst, which travelled for about 1.5 billion years to reach us, and built a map of everything in front of it.
- We used the Keck telescope in Hawaii and an existing database, the Sloan Digital Sky Survey, to find the galaxies near the burst’s path and measure how far away each one is.
- For each galaxy, we estimated how much gas surrounds it and how much of that gas the burst went through.
- Galaxies are not scattered at random. They line up along a network of threads called the cosmic web. To trace the threads between the galaxies, we used a computer program inspired by slime mould, an organism that is very good at building efficient networks between scraps of food. Here the galaxies play the role of the food.
Our findings and what they mean
- Only a handful of galaxies sat near the burst’s path, and only one was close. The gas around galaxies added little to the total.
- Most of the gas between us and the burst’s home galaxy was in the threads of the cosmic web.
- This burst also showed signs of having passed through strongly magnetized, clumpy gas. Nothing along the path could explain that, so it must have happened close to where the burst was born.
This was the first time anyone had accounted for a burst’s entire journey using real observations of what lay in front of it. It showed that we can do more than count the hidden gas: we can say where it is. That became the blueprint for a larger survey of many bursts.
Words you’ll see in the paper
- Dispersion measure (DM): the count of how much gas a burst passed through.
- Halo: the large, invisible cloud of gas and dark matter that surrounds every galaxy.
- Intergalactic medium (IGM): the gas in the space between galaxies.
- Monte Carlo Physarum Machine: the slime-mould-inspired program.