Friday, 24 August 2012

‘Melbourne researchers rewrite Big Bang theory’ … or not

It's been a little while, but I have written a new article for The Conversation in response to some screaming headlines earlier in the week. Here's the claim from the Sydney Morning Herald.


Heady stuff! The Big Bang, the mass of evidence and interpretation gathered over the last century, is about to be rewritten! This is important stuff (well, to me at least), and headlines everywhere breathlessly trumpeted that we are entering a new era in cosmological understanding.

So, I read the paper which appears in a very reputable, Phys Rev D. Here it is

Huh? I don't see the words cosmology, big bang, rewritten etc anywhere there.

Alas, we have another case of notion that science can only be sold if its significance is overblown. Press releases must challenge the orthodox, shift the paradigm and "rewrite the textbooks". If we went by press-releases, serious researchers would be constantly burning their textbooks and giving Amazon a roaring trade!  This couldn't be further from the truth (I still use Pauli's book on relativity and that was written in 1921)

Anyway, you can read my take on it at The Conversation in ‘Melbourne researchers rewrite Big Bang theory’ … or not. I also suggest that you take a look at the more pithy comments of my ex-student, and now professional cosmologist, Luke Barnes, in his blog-post How to overhype a science press release.

If you know any science journalists, ask them to have a read also.

Added after the initial post:

What is the secret of good comedy? Timing.

This press release came out very recently: Spacetime: A smoother brew than we knew
Or so an intergalactic photo finish would suggest. Physicist Robert Nemiroff of Michigan Technological University reached this heady conclusion after studying the tracings of three photons of differing wavelengths that had been recorded by NASA's Fermi Gamma-ray Space Telescope in May 2009. The photons originated about 7 billion light years away from Earth in one of three pulses from a gamma-ray burst and arrived at the orbiting telescope just one millisecond apart, in a virtual tie. Gamma-ray bursts are short-lived bursts of gamma-ray photons, the most energetic form of light. They can originate far across the universe, and astronomers believe many are caused by giant stars collapsing, often billions of years before the Earth was formed. "Gamma-ray bursts can tell us some very interesting things about the universe," Nemiroff said. In this case, those three photons recorded by the Fermi telescope suggest that spacetime may not be not as bubbly as some scientists think.

Read more at: http://phys.org/news/2012-08-spacetime-smoother-brew-knew.html#jCp
Or so an intergalactic photo finish would suggest. Physicist Robert Nemiroff of Michigan Technological University reached this heady conclusion after studying the tracings of three photons of differing wavelengths that had been recorded by NASA's Fermi Gamma-ray Space Telescope in May 2009. The photons originated about 7 billion light years away from Earth in one of three pulses from a gamma-ray burst and arrived at the orbiting telescope just one millisecond apart, in a virtual tie. Gamma-ray bursts are short-lived bursts of gamma-ray photons, the most energetic form of light. They can originate far across the universe, and astronomers believe many are caused by giant stars collapsing, often billions of years before the Earth was formed. "Gamma-ray bursts can tell us some very interesting things about the universe," Nemiroff said. In this case, those three photons recorded by the Fermi telescope suggest that spacetime may not be not as bubbly as some scientists think.

Read more at: http://phys.org/news/2012-08-spacetime-smoother-brew-knew.html#jCp
Or so an intergalactic photo finish would suggest. Physicist Robert Nemiroff of Michigan Technological University reached this heady conclusion after studying the tracings of three photons of differing wavelengths that had been recorded by NASA's Fermi Gamma-ray Space Telescope in May 2009. The photons originated about 7 billion light years away from Earth in one of three pulses from a gamma-ray burst and arrived at the orbiting telescope just one millisecond apart, in a virtual tie. Gamma-ray bursts are short-lived bursts of gamma-ray photons, the most energetic form of light. They can originate far across the universe, and astronomers believe many are caused by giant stars collapsing, often billions of years before the Earth was formed. "Gamma-ray bursts can tell us some very interesting things about the universe," Nemiroff said. In this case, those three photons recorded by the Fermi telescope suggest that spacetime may not be not as bubbly as some scientists think.

Read more at: http://phys.org/news/2012-08-spacetime-smoother-brew-knew.html#jCp
Or so an intergalactic photo finish would suggest. Physicist Robert Nemiroff of Michigan Technological University reached this heady conclusion after studying the tracings of three photons of differing wavelengths that had been recorded by NASA's Fermi Gamma-ray Space Telescope in May 2009. The photons originated about 7 billion light years away from Earth in one of three pulses from a gamma-ray burst and arrived at the orbiting telescope just one millisecond apart, in a virtual tie. Gamma-ray bursts are short-lived bursts of gamma-ray photons, the most energetic form of light. They can originate far across the universe, and astronomers believe many are caused by giant stars collapsing, often billions of years before the Earth was formed. "Gamma-ray bursts can tell us some very interesting things about the universe," Nemiroff said. In this case, those three photons recorded by the Fermi telescope suggest that spacetime may not be not as bubbly as some scientists think.

Read more at: http://phys.org/news/2012-08-spacetime-smoother-brew-knew.html#jCp
Or so an intergalactic photo finish would suggest. Physicist Robert Nemiroff of Michigan Technological University reached this heady conclusion after studying the tracings of three photons of differing wavelengths that had been recorded by NASA's Fermi Gamma-ray Space Telescope in May 2009. The photons originated about 7 billion light years away from Earth in one of three pulses from a gamma-ray burst and arrived at the orbiting telescope just one millisecond apart, in a virtual tie. Gamma-ray bursts are short-lived bursts of gamma-ray photons, the most energetic form of light. They can originate far across the universe, and astronomers believe many are caused by giant stars collapsing, often billions of years before the Earth was formed. "Gamma-ray bursts can tell us some very interesting things about the universe," Nemiroff said. In this case, those three photons recorded by the Fermi telescope suggest that spacetime may not be not as bubbly as some scientists think.

Read more at: http://phys.org/news/2012-08-spacetime-smoother-brew-knew.html#jCp
Or so an intergalactic photo finish would suggest. Physicist Robert Nemiroff of Michigan Technological University reached this heady conclusion after studying the tracings of three photons of differing wavelengths that had been recorded by NASA's Fermi Gamma-ray Space Telescope in May 2009. The photons originated about 7 billion light years away from Earth in one of three pulses from a gamma-ray burst and arrived at the orbiting telescope just one millisecond apart, in a virtual tie. Gamma-ray bursts are short-lived bursts of gamma-ray photons, the most energetic form of light. They can originate far across the universe, and astronomers believe many are caused by giant stars collapsing, often billions of years before the Earth was formed. "Gamma-ray bursts can tell us some very interesting things about the universe," Nemiroff said. In this case, those three photons recorded by the Fermi telescope suggest that spacetime may not be not as bubbly as some scientists think.

Read more at: http://phys.org/news/2012-08-spacetime-smoother-brew-knew.html#jC

Sunday, 19 August 2012

The origin of the split red clump in the Galactic bulge of the Milky Way

I have been swamped recently, not only at work (I am teaching electromagnetism to first year students) but also at home where we are renovating (and I am not too geeky enough to wield a sledge hammer). So, apologies for the sparse posting. The renovating is slowly approaching its conclusions (although we have lived almost a month with no doors inside the house), and research is trundling, so I will try and get back on top of posting.

I should have posted this a little while ago, but the first paper in a new survey of the Galactic Centre was accepted for publication. The ARGOS project, as it is known, was originally intended to be a large scale international survey of a large chunk of sky, but we were not awarded time for that, and now it is a program focused upon the Galactic Bulge.

It might seem strange, but while we can plainly see the centre of the Galaxy from here in Australia, there is a lot we don't know about it. It was only in the last 10-20 years we have come to understand that the Milky Way's bulge possesses a prominent bar that had not been seen before. In fact, it is more complicated than that, as there appears to be more than one bar structure. Here's an artists interpretation of the Milky Way (pinched from Wikipedia, formally of NASA).
So, how do we understand what's going on? We need to basically measure the speed and chemistry of lots and lots of stars, and to do this we need to do spectroscopy. Generally, this is very time consuming as, with even a large-ish telescope (like the 3.9m Anglo-Australian Telescope), as you need to spend about an hour per star to get enough signal to make meaningful measurements.

I've written before about the solution to this, essentially you use lots of optical fibres at once to collect the light from a lot of stars. For ARGOS, we used the 2dF/AAOmega spectrograph to look at around 400 stars at once. In fact, we got spectral for more than 27000 stars. Cool eh?

The first paper from this data has a title which may seem mysterious to the lay person. It is "The origin of the split red clump in the Galactic bulge of the Milky Way". OK, some of the words make sense, but what is a red clump, and why it is split?

The Red Clump is a "clump" (i.e high density) of stars on a colour-magnitude diagram (more traditionally called a Hertzsprung-Russell diagram. You do what it says on the tin, namely plot a point for each star on a plane, where one axis is colour, and the other is brightness (or magnitude).

The clump is the blob of stars at about (1,0) in this plot

(taken from the webpage of the late, great polish astronomer, Bohdan Paczynski). I won't go too much into stellar evolution here, but where you see clumps on the colour-magnitude diagram, this indicates relatively stable phases of the life of a star (so stars hang about there for a bit longer).

The cool thing about the clump is that is seems (modulo a few physical things) that the stars in there have roughly the same intrinsic brightness and so you can use them to do the most difficult thing in astronomy, namely measuring distances.

So, what does a "split red clump" mean? Well, it means that what we are seeing in the bulge is not a nice, simple "blob" of stars at some distance, but there is more complex 3-D structure, and chunks along the line of sight give us different features in the structure of the red clump at different locations.

What ARGOS shows us it that this structure means that the bulge has "boxy/peanut" orbits (I will write about these in the future) and that these orbits grew due to a complex interplay between the disk of stars and a bar instability. Make sense? Essentially, what it boils down to is the growth of galaxies, even on small scale, like the formation of an individual bulge, is complicated. None of the nice, simple pictures presented in textbooks are actually realised. It's all a mess. Fantastic! (don't take mess to mean that we don't understand what is going on!)

This is a great piece of work, lead by ANU PhD student, Melissa Ness. Well done Melissa!

The origin of the split red clump in the Galactic bulge of the Milky Way 


Near the minor axis of the Galactic bulge, at latitudes b < -5 degrees, the red giant clump stars are split into two components along the line of sight. We investigate this split using the three fields from the ARGOS survey that lie on the minor axis at (l,b) = (0,-5), (0,-7.5), (0,-10) degrees. The separation is evident for stars with [Fe/H] > -0.5 in the two higher-latitude fields, but not in the field at b = -5 degrees. Stars with [Fe/H] < -0.5 do not show the split. We compare the spatial distribution and kinematics of the clump stars with predictions from an evolutionary N-body model of a bulge that grew from a disk via bar-related instabilities. The density distribution of the peanut-shaped model is depressed near its minor axis. This produces a bimodal distribution of stars along the line of sight through the bulge near its minor axis, very much as seen in our observations. The observed and modelled kinematics of the two groups of stars are also similar. We conclude that the split red clump of the bulge is probably a generic feature of boxy/peanut bulges that grew from disks, and that the disk from which the bulge grew had relatively few stars with [Fe/H] < -0.5


And here's a bonus question. Some of the words in this post were originally in the 1980s movie, and song of the same name, Flash. Can someone find them?