|The bullet cluster (see comments for description)|
Suppose you woke up early tomorrow morning, looked at the sky, and some stars had turned pink and re-arranged themselves into the shape of an elephant. I imagine that you would think that something odd had happened. Naturally, you would probably decide that your earlier beliefs about stars were not quite complete and needed to be fixed.
Astronomers and cosmologists try to play the same game with very massive objects in the universe. The idea is the same. If we were to observe some objects large enough that they were as unlikely to exist as a pink elephant floating around in intergalactic space, then we would also be able to conclude that our standard beliefs about how large objects form in the universe is incomplete.
We do this because both the standard cosmological model and standard model of particle physics aren't particularly satisfying, but fit (almost) all the measured data really well. So we search for things to measure that might point us to something new and will give us an insight in to why these models work so well. Of course, it is also useful to simply measure as many new things as possible. History is littered with moments where we didn't bother measuring something because we knew what the result would be, only to get a big surprise whenever someone finally did measure it. So nowadays if something is measurable, then someone, somewhere, is trying to measure it. The biggest objects in the universe are nice. Because they are big, we can see them. They are measurable.
When viewed over large enough distances, everything in the universe is moving apart. This is (mostly) due to left over momentum from the big bang and is also why it is often written that “the universe is expanding”. Things are different though, when viewed over small enough distances. There, the universe is full of different structures that are not expanding, for example the Earth, the solar system and our galaxy.
The early universe was very different to this. At the start of the big bang the entire universe was expanding and was almost exactly the same. Every point in space had the same temperature, the same density and was made of the same types of matter. Compare this to now where most of space is empty and wherever there is matter it can take many different forms (planets, stars, galaxies, quasars, pulsars, black-holes). How did a very homogeneous initial universe turn into the current highly inhomogeneous universe?
Even right at the beginning, when the universe was almost exactly the same there were tiny imperfections. Most importantly, some parts of the universe were ever so slightly more dense than the average density of the universe. For ordinary matter, gravity attracts. Therefore, these ever so slightly more dense regions, which had ever so slightly more matter in them expanded ever so slightly more slowly. As the universe is expanding it is becoming less dense. Therefore, as these regions expanded ever so slightly more slowly, they also became just that little bit more dense relative to the rest of the universe than they were before. As a result, they expanded even more slowly relative to the rest of the universe and their relative density grew even larger. For any region that is now a single structure or object, this game continued until the overdensity in the region grew so large that it no longer expanded, but instead collapsed into the structure it is today. This region will still move apart from other regions, but anything inside the region is bound together by gravity.
Why are clusters interesting and what would a pink elephant cluster tell us?
One of the advantages of galaxy clusters being so big is that big things are easier to see. They are brighter and they also distort light that travels around them more. Also, light travels at a finite speed. Therefore the further away we look, the earlier in time we are seeing. Another consequence of galaxy clusters being so big is that they form very late in the universe. This means we see them nearby which also makes them easy to see. This is great because the fact that galaxy clusters are also the most extreme things in the sky makes them the best candidates for “pink elephants in the sky”. If there is something in the sky that is so big that it could not possibly have formed in the standard cosmological model, it is very likely to be a galaxy cluster.
For a region of the universe to form a bound structure, the density of matter in that region has to get big enough relative to the density of the universe. Because this happens at approximately the same threshold for every collapsed region this means that if we can see a cluster, and measure its mass all we need to do is know when it formed and we can immediately determine how big the initial fluctuation in the density of the universe was for that region. This is an incredible fact so I'll write it again. If we can measure the mass of a galaxy cluster this gives us direct information about what the region of the universe containing that cluster looked like in the earliest stages of the big bang. Therefore, if we observe enough galaxy clusters, we can determine the initial state of the most extreme regions of the universe (at least with respect to their density).
The observation of a “pink elephant cluster” would therefore tell us that our current understanding for what the initial conditions of the universe were, needs improved.
What has been seen?
Many galaxy clusters have now been observed and measured. Recently, galaxy clusters that formed quite early in time have started to be observed and measured. Recall that because these initial fluctuations in the density of the universe grow that seeing a cluster that formed earlier in the universe arose from a more extreme initial fluctuation than one seen later in the universe.
In later posts I will discuss what the implications of these extreme clusters are...