How do we find cosmic voids?

To answer that question of what’s inside a void, we have to first decide what a void…is. I know it’s easy enough to describe in big, broad, vague terms. Voids are the empty places. Voids are the things that aren’t. If you zoom out to truly enormous scales, well beyond the sizes of mere galaxies, where you take such a huge portrait of the universe that individual galaxies appear as nothing more than tiny points of light, then a) welcome to cosmology, and b) holy crap the voids really stand out. In fact, we got our first taste of voids all the way back in the late 1970’s, right when we started to build our first deep surveys of the universe. Once we started making maps, we noticed places where the maps were empty. And two different groups found the voids around the same time, although only one group called them voids. The other group called them “big holes” for one I’m glad they didn’t win that particular jargon war.

It took a few years for the rest of the cosmological community to accept that voids were real, because if you see a gap in your data it’s easy enough to argue that, hey, maybe you didn’t do all that great of a job when you were collecting your data, or maybe it was just a trick of the eye, like we’re seeing a pattern that isn’t really there. But survey after survey kept having these…big holes…and voids were here to stay.

So voids are the places where there ain’t a lot of galaxies, and they’re easy enough to pick out by eye. You make a map of the universe and you find the…big holes…and you can point to them and say hey, that’s a void right there. And we have a very solid idea of where they come from. When you zoom out to those same ultra-gigantic scales, you see lots of cool stuff: galaxies, filaments, walls, and clusters.

We call this the cosmic web, and it’s like a glittering city that stands out from the darkness. But just like any city, it had to be built, brick by brick. The matter to build the cosmic web had to come from somewhere, and it came from the voids. My favorite analogy is Paris, the city of light. Its unique white limestone is dug out from underneath it. If you’ve ever visited the catacombs, you’re walking through the underground quarry that made Paris. The city of lights is sitting on top of a void of darkness.

But if we’re going to be specific – and this is science so being specific is a big plus – how do we define the edge of a void? Its boundary? Its center? How can we categorically point to one place in the universe and say THAT’S A VOID and another and say NOPE NOT THAT?

Well, do we have an answer for you. it’s called VIDE, which comes from the French word for “empty” (and our team developed this in paris, which is how I thought of the name). OF COURSE it’s a lame acronym, for void identification and examination toolkit, and it’s just the sort of nerdy thing that us scientists get excited about – it’s also free and available to use if you ever want to go void hunting yourself, but sadly no I will NOT be devoting any portion of this episode to the technical details of how to use it, you’re just going to have to read the manual like everybody else

There are a lot of ways you could POTENTIALLY define voids. We chose one particular method because it seemed like a good idea at the time, and as the years have gone on it has proven to be a rather good idea indeed (part dedicated effort, part luck). And in fact today it is the number 1 void finder in the world. Chances are that if you catch a new research paper about voids (and I know you subscribe to all the hot cosmology gossip) the research used VIDE – our methods and tools – as its backbone. Which is pretty neat

So here’s how we decided to define a void:

Start with a collection of galaxies (or map of dark matter, or just the clusters) – it doesn’t matter, as long as you are tracing where the STUFF is Imagine that this map is a topographic landscape (like mountains and valleys, where mountains = high density clusters, filaments, walls) Imagine dropping rain onto the map, and following where the rain collects The basins where rain collects are the voids (low-density valleys) and the places that split the direction of raindrops are the ridgelines that separate voids YES, this is the same method used to find “watersheds” Method is nice because: it just automatically works for any kind of input, doesn’t have any tunable parameters, easily picks up voids within voids within voids etc

This is what VIDE does: it takes in a collection of matter and spits out where all the voids are, how deep they are, how far they extend, and so on. With this tool, we’ve found thousands of voids in our largest galaxy surveys.