D-19: Large, Part Two

The following was originally published on the Stellaris forums.


Hello once again everyone, and welcome to the second part of our series on large things. Last time we discussed things which are very large: galaxy clusters and dark matter filaments. Now let’s discuss something that’s even larger.

We live in the Laniakea supercluster. On every side this supercluster is surrounded by colossal voids, empty spaces as large as the supercluster itself in which there are almost no stars, let alone galaxies. Beyond one of them in the direction we arbitrarily call “north” is the Hercules supercluster, one of our neighbouring clusters. The Hercules supercluster sits close to several others in a web that’s referred to as the Great Wall. Beyond them…

…is nothing.

Welcome to the Boötes Void, also called simply the Great Void. This is a roughly spherical empty space which is so large as to defy description. There are no words for how large it is. There are numbers, but they are almost meaningless to humans. Its diameter is more than one-four-hundredth of the diameter of the universe. Is that meaningful to you? No. It’s not meaningful to me either, and I study this stuff. It’s colossal. The only suggestion of scale I can give is to say that entire web of superclusters called the Great Wall forms only part of one side of it, but even that doesn’t get it across.

Here’s a map. There are some nearby stars in the way but you can see the Great Void past them.
(Image courtesy Atlas of the Universe.)

The Boötes Void is not, intriguingly, entirely empty. In the 1980s we started to discover a few isolated galaxies in the middle of it. By the late 1990s we had concluded that there were 60 galaxies there, strung out in a rough line, possibly along a fragmentary dark matter filament. If someone lived on one of those galaxies, they would need mid 20th century astronomy tech to be able to tell that there was anything else in the universe except them.

In the 1920s, before the term “galaxy” became popularised, they were sometimes called “island universes.” That’s such a fantastically evocative name that it’s a shame we no longer use it (even if it’s a terrible name from an astronomical point of view) and it perfectly encapsulates what it must be like to live in a galaxy in the middle of the Boötes Void.

Even worse would be an isolated star. The sky around it would simply be black, even with an optical telescope. There’s a novel called Against A Dark Background which takes place on planets orbiting such a star. It could indeed happen, and here’s where it could happen.

Out beyond the Boötes Void, five times as far away as it is, there’s another big void. It’s even bigger in fact: it’s 3-4 times the diameter. We call this the Giant Void, because once you have a Great Void there’s really not many words you can use to one-up it. This is… look, it’s big, okay? Big and empty. There isn’t a great deal I can say about it. I study stars, and a void is a place where there are few if any stars. It’s not my field. It’s a Giant Void. It says so on its name plate. Done. Next.

We think that the structure that consists of voids, dark matter filaments, and galaxy clusters in places where the dark matter meets, is the basic structure of the universe. Picture it, if you will, as a sponge. In fact, you don’t have to picture it because here’s a picture I found on Wikipedia (of all places) of what it looks like.

(Image courtesy NASA.)

Here’s another one of how we think it formed.

(Image courtesy University of Chicago.)

See that? The mixture of energy in the early universe gathers around the strands and walls, abandoning the areas which become voids. The Boötes Void and the Laniakea Supercluster, and other places like them, were formed by the way energy blobbed around in the early universe.

Would you like to see a picture of the early universe? Good, because we can give you one.

Astronomers don’t just look out into the sky, they also look back into the past. When we see the Moon we are seeing something slightly more than one light second away; this means we’re seeing slightly more than one second into the past. The giant star WR104 which I covered before is 7500 light years away. The Boötes Void is hundreds of millions of light years across. When we look at them, we’re looking into the distant past.

If we look far enough, we can see far back enough to see this. You’ve seen it before. Now, hopefully, you might be able to make more sense of it.

(Image courtesy European Space Agency.)

This picture is too early for galaxies to have developed yet. What we’re seeing is patterns of energy, high and low, which will one day form the strands of galaxies and superclusters we live in. As the universe aged it shrank around its dark matter structure. Matter began to accumulate and would one day gather into stars and other things. The places between that matter became the space between them: the great emptiness between stars, galaxies, galactic clusters and galactic superclusters.

We will never travel across this great emptiness. Heartbreakingly, faster-than-light exploration is probably not possible, which means we can only gaze longingly at the void and know in our hearts that we are part of something vastly greater than ourselves. But we can dream, and we can learn, and we can teach, and we can tell stories.

A story told to another is a story that never dies, and that is truly the largest thing I know of.


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