The word “astronomy” means the arrangement of the stars but we apply it to the arrangement of the entire Universe, not just stars. It is an observational science, since experiments are difficult to perform on solar systems and galaxies – let alone the entire Universe. Cosmology, on the other hand, is more philosophical and is concerned with the evolution, structure and nature of the Universe. It takes astronomy’s evidence then extrapolates it both forwards and backwards in time. In terms of GCSE command words, astronomy is the “describe” aspect of space studies whereas cosmology is the “explain”.
Edwin Hubble’s famous correlation of galaxy distances (based on apparent size) with the red-shift of their spectra was pure observational astronomy but its use to support the Big Bang theory took it into the realm of cosmology. At that time (1929) the same evidence was also explained by Fritz Zwicky using his “tired light” theory so why do we now refer to Hubble’s work as being conclusive evidence for the Big Bang?
There is a great historical account of how Hubble’s work was received at the time, written by Helge Kragh in 2017, which is available as a free download from Harvard’s ADS (Astrophysics Data System) here. There is also a short, mathematical summary of the modern evidence against the “tired light” theory, by Edward Wright, on the UCLA website, here.
The most important fact to grasp is that Hubble’s work did not provide conclusive evidence for the Big Bang. In school, we talk about running time backwards so all the separated galaxies that we see today start to converge on one point in space – and when time runs forwards from that one point we get the infamous “big bang” event that scattered the Universe into an expanding volume. But that’s an over-simplified story.
The Universe hasn’t simply “expanded”: it has evolved into the structure that we see today. Putting marks on a flaccid balloon then inflating it to observe that every dot moves further away from every other dot is a common classroom demonstration but it’s not right. Worse than that, this powerful model will need to be forgotten (or at least put to one side) if you want to go on and develop your learning further.
There is a fantastic introduction to cosmology that I highly recommend: it’s superbly written and is available as a free download. You might find that you need to read it twice to grasp the details but it’s really worth the effort. The source, written by the same Edward Wright who summarised the evidence against “tired light” mentioned above, can be downloaded from the World Scientific website, here.
As you will realise if you follow the link above, this source is actually the first chapter of a newly updated book, Origin and Evolution of the Universe. This is one of three space-themed books that I have reviewed for School Science Review over the last year. You can download my review of Origin and Evolution of the Universe, together with Paul Murdin’s Secrets of the Universe (which I also recommend) by clicking on the link in the caption for the image below.
Finally, if you would rather read information online (instead of using books) then there is probably no better place to start than Cornell University’s Ask an Astronomer website, where you can find answers to almost every imaginable question about astronomy and cosmology. Best of all, the questions (and their answers) are graded to indicate the level of complexity; Beginner, Intermediate and Advanced. The link below will take you straight to a page related to questions about the Big Bang and has a further link to Hubble’s original red-shift experiment. Enjoy!
http://curious.astro.cornell.edu/the-universe/cosmology-and-the-big-bang
Mr Tarrant's physbang 'blog 