D-20: Large, Part One

The following was originally published on the Stellaris forums.


Hello everyone and welcome once again to what has been dubbed the Astroknowledge series. I’m your host, EJ, and today we’ll be starting a two-part series to talk about very, very large things.

(Image courtesy photography-on-the-web.com)

Picture a spiderweb coated in dew. The web itself is invisible, but the strings of droplets along it show us that there’s something there. Where the strands meet, we see more and larger droplets. Spiderwebs are small, and so are dewdrops.

Galaxies are, by any reasonable definition of the word, large. Despite this, galaxies aren’t found alone: they exist in clusters. Until recently we thought that we lived in a cluster called the Virgo Supercluster, but we have discovered that Virgo is only part of a vastly larger cluster called the Laniakea Supercluster. (If you’re wondering why we only found this out recently, it’s because it’s surprisingly difficult to study something whilst being inside it.)

We’ve known for a while now that most galaxies are far heavier than the number of stars within them would suggest, and that this can’t be explained away by assuming large numbers of black holes. The normal theory is to suppose that galaxies have a lot of extra matter in them which we can’t see. Because we can’t see it, this is known as “dark” matter.

In 2012, Jörg Dietrich’s team studied two distant superclusters, Abell 222 and Abell 223, which are near one another. Using gravitational lensing and other techniques, they found that the two clusters of galaxies were connected by… something. Something very heavy, but something which light passed through as if it wasn’t there.

Picture that spiderweb again. We see two droplets. Between them there’s a strand. We can’t see the strand but we know from its effects that it’s there. Now picture a similar strand between two clusters of galaxies, invisible but holding them together. We think that throughout the universe there’s a network of such strands, called “dark matter filaments”, and that galaxies either form on them or are attracted to them. The Laniakea Supercluster itself is, we think, the intersection of several filaments, which is why so many galaxies have collected here.

In the middle of the Laniakea Supercluster is a point which was discovered before we mapped our supercluster, a place called the Great Attractor. We noticed a long time ago that lots of things are being pulled faintly but surely towards the Great Attractor, and theorised that it might be very heavy. A lot of hypotheses were formed about it. Nowadays we think that it’s the centre of gravity of the supercluster, hence the gravitational attraction. If this is the case then it’s great because it replaces a mystery (a strange point in deep space) with an explanation we understand very well (the centre of gravity of lots of heavy stuff.)

(Image courtesy Nature.)

Dark matter sounds very mysterious, and people are often more mystical about it than they need to be. It’s not actually that difficult as a concept, however, even if the maths is annoying. If you can imagine something which you can’t see or touch, so it could pass right through you and neither of you would ever notice, then you’ve basically got a pretty good understanding of how dark matter is thought to behave.

The Milky Way is thought to have more dark matter, proportionally, than many other galaxies our size. Over 90% of the Milky Way is thought to be dark matter, and it’s concentrated out in the disk rather than in the core. This may be why we’re such a big galaxy and have attracted other, smaller galaxies and star clusters to orbit us.

Those filaments of dark matter are thought to connect throughout the universe, joining all the visible matter together into strands and sheets of light which separate vast dark voids. In a sense, we have begun to discover the skeleton of the universe. And the universe is the biggest thing we have yet discovered.


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