Showing posts from July, 2013

London verses Sydney

This week has been horribly busy (for a number of reasons that will become apparent in the next couple of weeks), and this afternoon I am jumping on a plane to a conference on dwarf galaxies. The main reason for the conference is to give us a chance to celebrate the 60th birthday of astronomer
extraordinaire, Mike Irwin. I'll write more about this once I get back.

So, today, a quicky. I've spent the last 13 years living in Sydney, but did spend 3 years as an undergraduate in London. I was recently wondering about the relative sizes of the two, and wikipedia tells us that Sydney has a population of about 4.6 million, whereas London has a staggeringly accurate 8,308,369 residents (and bizarrely, this is a prime number). So, in terms of people, London is very roughly twice as large.

What about land area? Well, I saw this on a British Expats site recently
Possibly 20 times larger? I know London does feel large, but Sydney is no provincial town.

So, I decided to look, and found a …

Who discovered dark matter in galaxies?

Many astronomers get their history of the subject from text books, and generally discoveries are presented as neat, tied up bags with statements like "X discovered Y". However, now and again, people look more deeply and the story is always more complex.

What brought this to my mind was a recent series of papers on just who should be credited with the discovery of the expansion of the universe. This was recently summarised in a wonderful article by Virginia Trimble called Anybody but Hubble! (I wish I could write as well as Virginia).
I recommend you have a read. History is messy.

I decided to look at the question of who discovered dark matter in galaxies as the textbook give a very "X discovered Y" answer to this, and gave a brief presentation at our Astro Morning Tea. But I am not a historian, and this is not complete, but just what I found after a couple of hours of looking around (thanks Brad for his input).

OK, let's start with dark matter. The first clues…

Inferring the Andromeda Galaxy's mass from its giant southern stream with Bayesian simulation sampling

Back to research! And this week, it's a new paper from Mark Fardal. Mark is an expert in modelling galaxy collisions, and we've been looking at the big collision going on next door, the Giant Stellar Stream in Andromeda.

One of the tough things to do is measure the mass of an object. Measuring the mass of stars is possible as long as it is orbiting another star, or has a planet orbiting it. Basically, the laws of gravity and motion given to us by Isaac Newton means that we can unravel the masses involved.

However, measuring the mass of galaxies is much harder. Firstly, unlike stars, which can effectively be treated as point mass, the mass in galaxies is more complex. It is at its highest densities in the middle, but is roughly continuous, falling off as we head outwards. In this situation, using the laws of gravity and motion are messier to use.

There is another, bigger problem. That is that the majority of the mass in a galaxy is dark matter, which, while we can't see it…

Battle of the Bayes

A little Saturday morning mathematical interlude.

There has been a few posts on the interwebs about an article by Brad Efron, a professor of statistics at Stanford University. The article appeared in the prestigious journal Science and has the title "Bayes Theorem in the 21st Century".

Ted Bunn has a good discussion of the paper over on his blog, and I suggest you have a read before you read what's below. But the paper continues to smoulder what is known as the Bayesian-Frequentist debate. It's a long winded debate (that some think doesn't even exist), but it all is focused on the question of "How do by prior beliefs change in the light of new data?". As ever, xkcd explains it perfectly;
(To explain, the frequentist worked out the change of a false positive based on the role of the dice, concluding that a YES was unlikely. The Bayesian, however, also considers the prior information of the chance that the Sun had gone nova during the experiment - as the …

Tough Crowd.....

I have just finished a major round of grant and postdoc reviews, and have been wiped out for a number of weeks. Back to science at last!

But last week I took some time out from reviewing and gave a talk on "My Life as an Astronomer" to the 2nd/3rd Pennant Hills Scouts Group.
(thanks to Buffalo for the use of the picture). We spoke about a lot of things, from what life is like as a professional astronomer, from the big bang to the oldest stars, life on planets at the centre of the Milky Way to neutrino telescopes under the ice in antarctica. It was a broad snapshot of the Universe as we know it.

The great thing about talking to kids about astronomy is the constant stream of questions (so many questions that the scout leaders had a hard time keeping everyone in check), and the fact that questions were deep, really wanting to understand what is going on. When the first question is "What would happen if the Universe had corners?", you know that you are in for an inter…