Showing posts from March, 2014

The PAndAS Field of Streams: stellar structures in the Milky Way halo toward Andromeda and Triangulum

A cool paper this week, written by long-time collaborator,  Nicolas F. Martin . Again, it's a  PAndAS paper  but the focus is not Andromeda but our very own Milky Way. How, I hear you say. The starting point is the  Field of Streams , a truly wonderful image of a massive area of sky taken as part of the  Sloan Digital Sky Survey . Here it is The Sloan Survey had many goals, but one of those was to look at the stars in our own Galaxy, especially in the  Galactic Halo . If our picture of how galaxies form and grow are correct, basically by eating smaller systems, we should expect the halo to be full of the debris from things being consumed at the present time. And this is what we see in the Field of Streams. The big swoop across the image is the tidal debris from the  Sagittarius Dwarf Galaxy , which, even though it was only discovered in 1994, completely wraps the Galaxy. There's lots of other debris in there too, including the  Monoceros Rin g which wraps the outer part

Bad Astronomy: The Expanding Universe

I'm not going to join the chatter on the  BICEP2 result  as there's already a lot of commentary out there (although  ABC News 24  seem to have missed it, so no chance to mess it up :) So, a tiny post today. Here's a picture from the BBC story. This is supposed to be the Big Bang, with galaxies rushing from a  central explosion. I've written about this before, but this is not what the Big Bang was. It was not an explosion in preexisting space. If we take the cosmological parameters as we currently know then (and assume a simple  topology ) then the Universe is now infinite in extent. And it was a billion years ago, and it was 13.5 billion years ago, and it was throughout the history of the Universe. When the Universe was a 100 million billion degrees celsius, it was this temperature everywhere throughout the infinite Universe. As it cooled down, it cooled down everywhere in the infinite Universe. When it got to a few hundred thousand years after the Big Ban

Stupid Americans: 1 in 4 think the Sun goes around the Earth

I am going to avoid the breathless headlines doing the rounds about  an upcoming press release about the detection of cosmological gravitational waves.  Firstly, a press release about an upcoming press release is a seriously poor way to do science. And I am already groaning internally about how poorly any result will be presented in the media (looking at you  ABC News 24 , especially  Michael Rowland , who still calls the  Higgs boson  the Higgs  bosun ). So, back to something we all enjoy, America bashing (every year I eagerly await the letters in the newspapers complaining about  halloween  in Australia, telling us that we don't want this American rubbish here while chewing down on a McDonalds, sipping coke and driving a ford, stopping at the American stop sign before rushing home to watch the latest American cop show on TV). And before any American readers get riled up, the preceding text is an example of  sarcasm. Apparently,  1 in 4 Americans  think that the Sun goes arou

A thousand shadows of Andromeda: rotating planes of satellites in the Millennium-II cosmological simulation

Before we start, well done to the  Running Astronomer  on her most recent race. Papers are like buses. Well, their not really, but hot on the heels of yesterdays cosmology paper, another paper on the arxiv, this time with the friendly title of  A thousand shadows of Andromeda: rotating planes of satellites in the Millennium-II cosmological simulation . This is a cool paper and harks back to our discovery last year of the  discovery of a strange corotating plane of satellites  in the  Andromeda Galaxy . Well, here we are more than a year later, and there is still no satisfactory answer to what it is doing there (or the equally strange, and badly named,  Vast Polar Structure of the Milky Way ). Over the last few decades, we've built up a complex model for the formation and evolution of structure in the Universe, and the favourite picture, know as  ΛCDM , works extremely well on large scales. The Λ part refers to the influence of  dark energy , where as the CDM = Cold Dark Mat

Non-linear Chaplygin Gas Cosmologies

Ultra-quick post today, but a new paper on  The Arxiv . The title is " Non-linear Chaplygin Gas Cosmologies ", catchy eh! But what does it mean? Essentially, we have two big dark mysteries in the universe,  dark matter  and  dark energy , and what we want to do is try and reduce the number of mysteries by a factor of two. We can do this with a  Chaplygin gas , modifying its usual properties so that on small scales it looks like dark matter, while on large scales it acts as dark energy. In this first paper, we basically lay out our new model, led by  Pedro Avelino  who was visiting the  Sydney Institute for Astronomy  from Portugal. It's a bit mathematical, but in coming papers we will detail a number of observational tests we compare the model  to. That is yet to come, and I will write a lot more detail then, but I have to run. Before I go, I'll note that this is my first paper with  Krzysztof Bolejko , and so my  Erdos Number  has significantly shrunk. More on