“Now that the Universe has formed, what happened next?”

 

“Well, I told you before, there was a long period of darkness…”

 

“Yes, yes, but what happened after that?”

 

“Oh, then. It’s time for another story, I suppose.”

 

*Again, the narrator sits back and pauses for dramatic effect*

 

 

Story No. 2: Where Does the Structure of the Universe Come From?

 

Look above you. What do you see? Oh, yeah, get out of the house, preferably out of the city as well and wait until the night. What do you see now? OK, fine, imagine there are no clouds above. What do you see?

 

Finally you got the right answer. Stars. You see lots of stars. If you squint (or if there is no light pollution) you may also see several galaxies – they will appear as small faint blobs in the sky. You will also see an arm of our Galaxy, affectionately called the Milky Way. Note that the galaxy we inhabit is usually written with a capital G, while other galaxies are not capitalised. Talk about egocentrism! You may see some other things, such as artificial satellites and planets, but it’s stars and galaxies I want to talk about.

 

You may already know that stars, much like our own Sun, are huge balls of extremely hot matter. That matter will, in most cases, be hydrogen with some helium and tiny amounts of heavier elements added for flavour. In some cases, however, it can be mainly helium; also carbon, oxygen, neon, magnesium and other elements, up to iron. The amount of elements heavier than iron that are synthesized in stars is so small that no-one really cares about it. But no matter what they are made of, stars are rather compact and hot, ranging from 3000 to 12000 degrees Kelvin on the surface and several million degrees inside.

 

Galaxies, in turn, are composed of millions, if not billions, of stars. Some stars are packed rather close together, in groups and clusters. Some galaxies look like they are composed of spirals (the Milky Way is one of them), while others are elliptical and yet others are irregular-shaped. Galaxies themselves can also form groups, called – surprise surprise – galaxy clusters. There are suggestions that there may even be galaxy superclusters, i.e. clusters of clusters of galaxies, but this is most likely not true, because galaxies in such superclusters move so slowly that it would take them longer to move across that cluster than their current age. Anyway.

 

So you see (and have now read) that the Universe shows a lot of varied structure, i.e. it is far from uniform in density and luminosity. Observations show that some kind of structure formed only a few million years after the Big Bang (if you are wondering how we can see so far back in time, wait until some later instalment). The question is how it did that.

 

The answer is, once again, that no-one knows for sure. Such a situation comes about quite often in astrophysics, mostly because the data we have to work with is indirect and experiments are hardly possible. We have to mostly resort to theoretical models and, more often than not, there is more than one model fitting nicely with data. This is the case here as well. Very roughly, one can identify two radically different models of large scale structure formation.

 

The first model is as follows. After the recombination (when the cosmic background radiation stopped keeping matter ionised) all the matter in the Universe was of uniform density for quite some time – probably several million years. However, density was fluctuating everywhere and all the time; you can imagine it like small ripples on the calm surface of a lake. Slowly, some of these random fluctuations became large enough to attract more matter to themselves. Thus the whole Universe coalesced into some huge blobs. These blobs were each as massive as a hundred or even a thousand galaxies. As they contracted, matter started heating up and more fracturing became possible – each huge blob split into many smaller ones. The same thing happened again, and a third generation of small-ish blobs appeared. Finally, these blobs contracted to densities high enough that hydrogen started burning into helium. You can guess that these blobs are stars, while the “second generation” ones are galaxies and the largest ones are galaxy clusters. This model is sometimes called the “top-down” model.

 

The second model is very much like the first model in reverse. The primordial “soup” of matter starts fracturing after several hundred million years, then is fractured into tiny bits, which became stars. These stars were affected by gravity and so grouped into clusters, then into galaxies, then into galaxy clusters. This is called the hierarchical model. There is obviously more detail into that, but the key prediction is this simple.

There are two aspects of these models that lead to such differing conclusions. The first is the assumed evolution of what is called the Jeans mass. Not related (as far as I know) to the denim-based trousers, Jeans mass is the preferred mass to which gas clouds will collapse under their own gravity. If the cloud’s mass is larger than the Jeans Mass, it will fracture; if it is smaller, it will accrete matter from its surroundings or merge with another cloud, provided there is matter around it. The value of this mass depends on several parameters – the elemental composition of the cloud; most importantly, its temperature; the effectiveness of cooling the matter as it collapses. While the molecular composition is mostly agreed upon, the temperatures and cooling processes in the primordial Universe are a matter of considerable debate. The second aspect, closely related to the temperature problem, is the nature of dark matter. While I will not go into detail about what dark matter is right now (again, this is for another time), suffice to say that the first model requires dark matter to be “hot” (i.e. composed of extremely energetic particles), while the second requires it to be cold. So far, detections of dark matter have been very few, so its nature is not determined, and neither of these two models can be ruled out.

 

So here we go. Shorter than the first instalment, so I hope this will be easier to understand. Next time I will go through the main steps of stellar evolution.