Tuesday 27th September: Cosmology (1)
You know how some lectures make a story? This didn't. It's hard to know where to start. Because, to be honest, it started with our lectuerer having problems with the photocopier and then the projector. I made a joke about how the projector, which was constantly saying "no input", was registering a time before the Big Bang, but the lecturer (who is Russian) didn't understand my joke - rather informed me that the Big Bang is sixties to eighties cosmology and modern inflation models are much better. We did get the handout
at the end, it's worth a read
First things first, he said that the way to study cosmology is to enjoy it; the way to get through the work is to really want to know. I've got on the same line in my notebook that he remarked that some galaxies look like animals. He has obviously been on the zoo
In fact the whole faculty and most of the students have heard of the zoo, and some of been on it. This is an incredibly refreshing change from Pembrokeshire, in which a depressing majority don't even know the difference between astronomy and magazine horoscopes.
These notes will be disjointed, because it was an overview of the course we'll be doing, with various remarks about various aspects . . .
Cosmology - that is, experimental cosmology - is becoming a very fashionable field, and easier than any other to get research grants for. It was not always so. Experiments are important but they must link to theory otherwise they don't tell us anything. (This reminded me of a story that Rutherford kept telling his students, "One good experiment is worth more than all the theories in the world." Eddington once wound him up by saying he would not believe an experiment unless it was supported by a good theory.) No matter how good or precise an experiment, if it doesn't contribute to theory, it's pointless.
We'll be starting by looking at a Newtonian universe. The equations that show that an apple thrown in the air will fall back to the Earth, and the equations that show that the Universe, having expanded, should collapse again, are absolutely identical. But when can we apply Newtonian principles to the Universe? It turns out not often. They just don't describe enough. Incidentally there won't be much about relativity on this course, and he doesn't seem too happy with that!
The density of the Universe determines its destiny. Observations - counting up the matter - will let us figure the density out.
Then came the really devastating part. He asked us how long we thought cosmology could exist: forever? 3 million years? Another few silly figures? One person put their hand up for 3 million years - not me - and they were right. If the Universe continues to expand at its current rate, in only three million years' time we won't be seeing much in the way of other objects in it.
I felt shattered to hear this. Three million years is nothing
in a 13.7 billion year old universe. It's nothing even on the geological timescale. The dinosaurs were around 165 million years ago. Three million years ago there were probably animals very similar to humans. (I should know.) It seems we've come in the nick of time - just slightly slower development and we'd have been too late. I just don't want to believe the Universe will continue to expand at its current rate. I want to reach out and stop it. Pretty pointless thinking, obviously. And really in a only few generations I expect a lot of things for science and for people will have changed. But even so . . .
Anyway . . .
Someone - he didn't say who (or I didn't catch the name; the lecturer has a strong accent, which we'll probably all be used to soon) - wrote to Einstein to point out that his equations predicted either an expanding or collapsing universe. (Was it Freidmann?) This is what prompted Einstein to invent his "cosmological constant". He later retracted it and called it his "greatest blunder", but he wasn't as wrong as he thought he was, for in fact what he was predicting was dark energy.
Any instability in the Universe will be exaggerated by any prevalent force. To illustrate this, the lecturer balanced a pen on the table, upright. (It was a pen with an excellent lid so stood up on its own - he was looking for one which should fall over!) He got another pen, and pressed it down on the table. The pressure helped it remain in this unstable position - but any really tiny movement and it crashed down much faster than it would have on its own.
We looked at primordial nucleosynthesis: how hydrogen and its isotopes, and helium and a little lithium, formed in the minutes after the Big Bang, and other elements formed in stars. (I'd rather write in more detail about that elsewhere.) He asked what we needed in space to create life. Someone said hydrogen and he said absolutely not, hydrogen is poison. We need to start with dust.
He told us that stars the size of the Sun don't forge any elements heavier than carbon. You will never find oxygen in a white dwarf, he said - only in supernova explosions. The girl sitting next to me and I both opened our mouths in protest - I am sure that is not correct, I'm sure I've seen oxygen rich white dwarves in the news and here on the zoo! But his general point was that you need a big star to form heavy elements, which is generally correct. (He didn't point out that the biggest stars would have been in the Universe's early days, so the Universe is generally already seeded with heavy elements from early supernovae. But I thought I'd point that out here while it's relevant.
It was 7 or 8 years ago that the acceleration of the Universe expansion was discovered. (I wrote here
about my dilemma with that.) He explained about supernovae being standard candles: they are exactly the same brightness wherever you go, so all you need next is their luminosity and then you know how far they are. For those who don't know, the most distant supernovae were observed to be fainter - further away - than they should be. (We know that supernovae don't occur outside galaxies, so if a supernova is such-and-such a distance away, so will a galaxy be.) A supernuclear explosion is what precedes a supernova: the explosion that occurs in a heavy star's core. The flux can be compared with the luminosity, and a telescope can do this. (NB I need to find out exactly what "flux" means; I have heard it before!)
Back to the standard candles. Edwin Hubble used Cepheid variables as standard candles, to determine how far away other galaxies were. To use a standard candle, you need to know how much energy it emits - this seems to be the "flux" again. However, Hubble's estimate of the expansion of the Universe was severely wrong - and that was because he was mistaking one type of standard candle for another! His mathematics was quite right - he just needed more knowledge of what was there. At the time, nobody believed him because his measurements suggested that the Universe was younger than the Earth.
The Universe has a structure. It cannot create a structure now, nor will it be able to in the future. This structure was present in the very beginning. Early origins of the Universe seeded galaxy clusters, and galaxies themselves. This seems to suggest no new galaxies could form now; is that correct? What about irregular galaxies - have they always been there? Since the generation of galaxies, that is? It seems unlikely given their starforming rates. But I may be quite wrong.
At scales of 50-100 Mpc (megaparsecs), the Universe is homogenous and the structure is pretty much the same. Rather like a mixture of veins and holes, I think when I see the pictures.
We can estimate the Universe's age by taking two different points and working out when they were in the same place. Hubble was wrong by a factor of about 8 in 1929. Nowadays, the age of the Universe is one divided by the Hubble constant (1/H0). But there has been a lot of argument over what the Hubble constant is. Two very prestigious groups came up with two quite different Hubble constants - using the best known techniques - but it turned out they disagreed because they were using different standard candles! This was not known for some time because apparently they kept their standard candles a secret. Bring on public science in the future!
We now use supernovae as "absolute" standard candles, as not only are they similar, but they are also so huge and luminous that we can see them at the edge of the observable universe. (I take it this is type 1 supernovae? Aren't type 2 - stars simply coming to the ends of their lives - dependent on the size of the star?) Hubble could only see 3 Mpc away, we can now see 10 million Mpc away.
He also said there are 4 types of distance in cosmology: luminosity, parallax, distance between objects relative to the expansion of the Universe, and I didn't catch the last one . . .
Apparently, even if a standard candle is wrong, the linear distance is correct. Dark energy saved Hubble. Sorry - I have no clue what this means. But I'm putting it here in case anyone does know.
Finally, he reminded us of some of the amazing distances across the Universe. When we look at the centre of our own Galaxy we see it as it was before civilisation. Also, a thought experiment. Imagine two planets, some distance apart. Everything on them is identical, and all events taking place are identical, happening at the same time. BUT . . . such and such a star is nearer to one than the other. So people on one planet see it before people on the other planet see it . . . So this destroys the "identical events" thing.
Sorry this is such a mishmash. I could have made it into a story but it would have taken all day. I dare say that with the rest of the lectures, pieces will drop into place - but those of you who have read this far and are not bored yet are presumably here for the journey, not just the ending. I will do my best to make them as clear as possible, and to include the bits I didn't understand as well as the bits I did. Feel free to ask questions, interpret weird parts, add to the knowledge base (but you may not get very far if you ask "Alice, what did you mean when you wrote . . ." because in most cases I am putting something I didn't understand myself!).
Off to Research Methods next. That will be a far more practical course I think, and I don't know what I'll be writing about that, but it does look interesting!