MY RESEARCHWelcome to my research page. I am a cosmologist, and spend most of my time trying to figure out why the expansion of the universe is accelerating. That sort of research can sometimes take you down surprising paths, as some of the summaries below will show. Here's some highlights of what I get up to. Follow the links for some more detailed descriptions of the more exciting and weird aspects of the universe. |
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BARYON ACOUSTIC OSCILLATIONS:Who says sound waves can't propagate in space? Well, they can't now, but way back in the very early universe matter and light were easily dense enough to sustain sound waves. The ripples left over from these enormous universe-wide vibrations have left their imprint on the pattern of galaxies we now see in the sky. These are known as Baryon Acoustic Oscillations, and now telescopes and spectrographs are getting strong enough that we can actually measure these remnant ripples. |
WiggleZI moved to UQ in 2008 to start working with the WiggleZ Dark Energy Survey. This is a baryon acoustic oscillation survey run on the Anglo Australian Telescope which has now observed over 200,000 galaxies out to a redshift of 1.0, i.e. out to half the age of the universe ago. This is the largest scale galaxy redshift survey ever undertaken and with it we will be able to do all sorts of cool science. I'm particularly interested in measuring the mass of the neutrino as well as measuring the properties of dark energy and dark matter. Our first data release was at the end of 2009, and our final one will be coming soon. For the moment, you can go pick up your favourite galaxy from our database: https://wigglez.swin.edu.au/dr1/ WiggleZ homepage: http://astronomy.swin.edu.au/wigglez/WiggleZ/site/ BREAKING NEWS!!! Observations were completed at the beginning of 2011, and our first major results papers are now online with many more to come soon. The WiggleZ Dark Energy Survey: survey design and first data release: Drinkwater et al. 2009, MNRAS, 401(3), 1429-1452 The WiggleZ Dark Energy Survey: small-scale clustering of Lyman-break galaxies at z < 1: Blake et al. 2009, MNRAS, 395(1), 240-254 The WiggleZ Dark Energy Survey: Blake et al. 2009, Astronomy & Geophysics, 49(5), 5.19-5.24 |
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SUPERNOVA COSMOLOGY:Supernovae are exploding stars. In the early 1990s it was discovered that one particular type of supernova makes a great standard candle. That means that we know how intrinsically bright they are. In astronomy that is incredibly useful, because just by measuring how bright it appears you can measure how far it is away. We can also measure the redshift of the supernovae, and thus determine how fast they are receding from us. This discovery allowed astronomers to measure the expansion rate of the universe in the distant past and compare it to the expansion rate now. Much to our surprise the expansion appears to be accelerating, contrary to everything we thought we knew about gravity. Here's some of the work I do on supernova cosmology. |
Sloan Digital Sky Survey (SDSS) — supernova survey
The SDSS Supernova Survey has discovered hundreds of supernovae in the region between redshifts 0.1 and 0.3, known as the 'redshift desert'. Ironically this intermediate redshift range was more difficult to get to than the more distant supernovae because you need a telescope with an enormous field of view in order to be able to cover a large enough region of sky. In late 2009 we published a series of three papers covering the results from the first year of data. An additional 103 supernovae revealed some interesting issues, perhaps most importantly that we need to better understand the supernovae and their light curves. There are tantalising hints that some of the more exotic cosmological models may improve on our standard cosmological constant explanation of dark energy, but as yet it is too early to make any conclusions. SDSS homepage: http://sdssdp47.fnal.gov/sdsssn/sdsssn.html The three first results papers:
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ESSENCE — Using supernovae to measure the properties of dark energy
ESSENCE is a collaboration that aims to measure the equation of state of dark energy to better than 10%. To do that we aim to measure 200 type Ia supernovae between redshifts 0.3 and 0.8. The project is in its sixth and final year (2007). This year we released our first cosmological results based on the first three years of data. These came out in a series of three papers in the September 2007 edition of the Astrophysical Journal. I led the third of these, which tested exotic cosmological models using model selection criteria. You can read more about my part of the project here. As part of our Davis et al. 2007 paper, Scrutinizing Exotic Cosmological Models we released the most complete combined supernova data set to date, including Supernova Legacy Survey, ESSENCE and Higher-Z data with the nearby sample. You can download this data set and use it to do your own cosmology fits from http://www.dark-cosmology.dk/archive/SN.
The three first results papers:
Spectroscopy papers:
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SuperNova Acceleration Probe (SNAP) — Building a space telescope
When the current generation of supernova surveys are done we will still need better data if we are going to determine what is causing the universe to accelerate. SNAP is one possibility for the Joint Dark Energy Mission (JDEM) - a joint mission between the U.S. Dept. of Energy and NASA to build a probe to measure dark energy. In 2007 JDEM was named the priority mission in NASA's Beyond Einstein program, so now competition is heating up to see which version of JDEM gets selected. SNAP is a fantastic multi-purpose instrument. It will be a wide-field telescope in space, able to take pictures with the resolution of the Hubble Space Telescope but covering a region the size of the full moon (rather than something the size of a crater). It will take images in 9 different filters from the optical through to the infrared and will also carry a spectrograph. It will measure 2000 type Ia supernovae between redshifts 0.3 and 1.7. Although named the "SuperNova Acceleration Probe", SNAP is concentrating equally on Weak Lensing - a method of measuring dark energy using the gravitational warping of light by distant galaxies. It will also be able to measure Baryon Acoustic Oscillations (see my description of BAO below). Because SNAP is so versatile there is a huge amount of auxillary science that can be done with the data that will be collected and then with the instrument when it's main dark energy survey is done. SNAP homepage: http://snap.lbl.gov I've been working on the supernova optimisation of SNAP, in particular the bandpasses we will use.
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Making supernovae better standard candles.Supernovae are some of the best standard candles we have, but they're not perfect. Their peak brightness actually varies by about an order of magnitude, but in a calibratable way. The ones that are brighter glow for longer. So by measuring how long a supernova takes to brighten and fade we can calculate how bright it actually was, reducing the uncertainty to about 10%. As our measurements get more accurate and the models we're testing get more demanding this 10% accuracy is no longer enough. Our aim, therefore, is to find a better way to calibrate the supernovae. Something that lets us know the intrinsic brightness down to 1%. We're using a form of principal component analysis (PCA) to try to find patterns in the spectra that can be used to improve the calibration. PCA is a technique that is used in things like image compression to extract a simple description from complicated information. We've published some proof-of-concept papers, and are now working on doing a much better job with bigger data sets. Papers:
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GAMMA RAY BURSTS
Gamma ray bursts (GRBs) are the most energetic explosions ever seen. In a few seconds they emit more energy than our entire galaxy does in 100 years. They were initially detected in the 1960s by American satelites, launched to watch for Soviet nuclear tests, and their existence remained classified for several years. After their existence was made public in the early 1970s it took another 20 years before scientific satellites were built that were able to detect them. Now a series of satellites have given us a much clearer picture of these enigmatic objects. The latest satellite is Swift, which detects on average two or three GRB's each week. We believe there are two types, long and short, and we believe the long ones are the result of a massive stellar explosion as a star ends its life in a supernovae. But even as we converged on this consensus a stranger-than-usual burst appeared in the sky. Our group at the Dark Cosmology Centre, as well as some other groups around the globe, followed up on a burst that should have had a supernova afterglow but didn't. This shook us from our comfort zone and showed that even if we now understand a lot more about these bursts than we did in the 1990s, there is still a lot to learn. Papers:
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EXPANDING CONFUSION:
Who has not looked up at the stars and marvelled at how the universe works? In the world of physics, so often strewn with abstract equations, much effort goes into relating those abstract equations to the world we see. This quest is impeded by two major stumbling blocks. One is simply the imprecision of language. Words are rarely as precise as the equations they're explaining, so sometimes debate arises over apparent contradictions in the literature which are actually no more than sloppy wording. The more significant stumbling block is the difficulty in explaining the unintuitive concepts of quantum physics and relativity in terms of everyday analogues. Some concepts in modern physics simply don't have a direct correspondence to our everyday experience. Thus the imprecision of language compounds the impossibility of describing things we don't naturally have words for. Yet to truly understand physics it is crucial to relate our mathematical solutions to the observable world. I've spent some time working with this problem, and trying to explain the expansion of the universe is a way that is clear and accurate. It is my hope that by explaining some of the bizarre features of the universe clearly some of the strange features of general relativity might also become more intuitive. |
Fundamental aspects of the expansion of the universe and cosmic horizons.
For the first part of my PhD thesis I explored the meaning of the expansion of the universe and attempted to debunk common misconceptions. Read this and learn how galaxies can recede faster than the speed of light without violating special relativity, how event horizons form in an accelerating universe so our observable universe resembles and inverted black hole, how light travelling in our direction can be dragged away from us by the expansion of space, and how many of the galaxies we observe in pictures such as the Hubble Deep Field are already beyond our event horizon so that we can never communicate with them. Papers:
You may also be interested in these, which are some of the best papers arising from the topics I broached in my thesis:
And in this, which picks up a mistake in Section 4.2 of my 2004 PASA paper:
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Misconceptions about the Big BangFollowing on from the "Expanding Confusion" papers Scientific American was kind enough to ask me to write a review article on the topic. In it we answer such questions as: What kind of explosion was the big bang? Can galaxies recede faster than light? What is the cosmological redshift? How large is the observable universe? What expands? Get the article here. Get a series of spacetime diagrams depicting our universe here. 
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BLACK HOLES, EVENT HORIZONS and ENTROPYFor the second half of my PhD I was lucky enough to work with Paul Davies on several topics to do with black holes and event horizons. |
Black hole vs cosmological horizon entropy.
Sir Arthur Eddington, in The Nature of the Physical World (1927), wrote the famous lines: "The law that entropy always increases, holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation." Inspired by this perception and by the newly discovered acceleration of the universe we embarked on a project to measure whether the entropy in the universe could be reduced when black holes disappeared over the cosmological event horizon. Accelerating universes have event horizons, i.e. distances beyond which we can never communicate, and never travel. Like a black hole's event horizon these cosmological event horizons are expected to quantify entropy. So we tested whether the trade-off in entropy as black holes disappeared over the cosmological horizon balanced with the second law of thermodynamics. It does. Time and the universe are safe. Papers:
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Do the fundamental constants of Nature Vary?Are the constants really constant? Things like the speed of light, the charge on the electron, the strength of gravity, do these change with time (or with position) in the universe? (Of course it doesn't mean much to talk about the change in a number that has dimensions, like meters for example, because that could just tell you your definition of a meter, for example, is changing. But we can make dimensionless combinations of the constants, which are just numbers, in which case any change has to be fundamental. But I digress.) While at UNSW my office-mate, Michael Murphy, was looking into possible changes in the fine structure constant, α (Murphy et al. 2003). Meanwhile I was working on the entropy of black holes. It was natural to notice that α appeared in the equation for black hole entropy and wonder if a changing α could violate the second law of thermodynamics. We thought it could, and Paul Davies, Charley Lineweaver and I wrote a Nature article about it. In the ensuing frenzy several authors suggested ways for entropy to be saved and others argued that it proved the constants really are just that. Interesting times, but the observational evidence for varying constants remains inconclusive. Papers:
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ARE WE ALONE? — The scientific basis of the search for extraterrestrial life
The question of whether there is life in the universe beyond our Earth is one that until recently has been the speculation of science fiction. That is no longer the case. Our telescopes and technology are becoming good enough to allow us to probe this question scientifically. We now know of over 200 planets that orbit other stars and are attempting to observe spectra of the atmospheres of those planets whose orbits fortuitously take them directly between us and their host star, with the hope of detecting the tracers of life. We have discovered amino acids, the building blocks of life, floating free in the molecular clouds from which stars form. We're now sending probes to Mars and the moons of our large planets to search for hints of life, particularly where there might be liquid water. In short it is an exciting time to be an astrobiologist. My contributions to this field remain humble. Charley Lineweaver (my PhD supervisor) and I investigated whether the statement "Life began quickly on Earth, therefore life must be common in the cosmos", has statistical validity. The answer is a guarded "yes", with the hesitation arising because of the possibility that life on Earth might have been seeded by life on Mars, and because we don't know how precisely the conditions on early Earth need to be mimicked in order for life to begin as easily elsewhere. Papers:
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MOPRA — MOLECULAR CLOUD PHYSICS
Where would a list of my science projects be without including Mopra. Mopra, the mm-wave telescope (see the picture on the left... isn't she beautiful), is run by the University of New South Wales, my PhD institution. So all us Ph.D. students got to go and work with Mopra for a couple of weeks each year. Donning the hardhats and spanners to get her up to scratch when necessary, listening to her sweet array of warning alarms, dealing with the kangaroos and cows in the enclosure. Yes, we all have a special place in our hearts for Mopra. Oh, and we did some science too. Although I really did no more for these papers than proof-read, my status as an observer still got me on the author list. We examine molecular cloud physics and look at the chemistry of star forming regions - what mm-wave telescopes are best at. Papers:
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