I wrote the following post yesterday, but I fell asleep before I could do anything with it. It’s about the first set of results from the Baryon Oscillation Spectroscopic Survey (BOSS), part of Sloan Digital Sky Survey-III project, which we announced to the science community and to the press yesterday. How this whole project was picked up by the press in a way I hadn’t anticipated is the matter for another post. What really matters is the science, and the science – if you don’t mind my exceedingly biased opinion – is just excellent.
I’m now making my way back home from this year’s National Astronomy Meeting (NAM) 2012 in Manchester. I love NAM. It’s always a chance to see old friends and listen to good science, to catch up on gossip and long-promised pints. This year, I did almost none of these things. The reason is that one of the teams I’m working with announced a pretty exciting set of science results today, in the form of six papers. This involved submitting the papers to the journals and to the arXiV yesterday. And that means that yesterday we panicked about coordinating submission of 6 papers across the globe, and today we panicked about press conferences, press interviews and science talks.
So what was all the fuss about? In essence, it all comes down to a map. You see, 14 years ago a number of astronomers announced a pretty remarkable observation: that the Universe’s expansion is accelerating. If the Universe is made only of matter (dark or bright), and the only force acting on large scales is gravity, then the Universe’s expansion should be doing exactly the opposite. This discovery, based on the observation of distant supernovae type Ia explosions (awarded with the Nobel Prize for Physics last year, and that I’ve covered here before), shook our cosmological model to the ground; the job of the science community was to build it back up.
There are a number of proposed explanations for this phenomenon, which astronomers named Dark Energy. The simplest from a theoretical point of view (but not from a conceptual point of view – more on that later) is the idea of a Cosmological Constant – a simple constant term in Einstein’s equations of General Relativity that would naturally explain the acceleration. In physical terms, this would equate to a rather bizarre property of empty space, a sort of vacuum energy that would cause space itself to expand. Einstein originally included this constant in his equations to ensure that the Universe was static. When Hubble announced his observation that the Universe was expanding, Einstein removed this constant from his theory and allegedly referred to it later as is ‘biggest blunder’. It’s ironic that we now put it back in – albeit with the opposite sign – to explain an observation that probably never crossed Einstein’s mind.
At the other extreme of possible explanations for Dark Energy we have modified gravity models. Einstein’s theory of General Relativity has passed some stringent tests at solar-system scales. But if this theory breaks down on very large scales, or at very low densities, then perhaps this acceleration is just really an illusion that stems from a ill-conceived theory.
So which is it? How can we tell? That’s where this map comes along. It turns out that one of the most effective ways to learn more about Dark Energy is to map out how matter is distributed in the Universe. As most of the matter out there is in the form of Dark Matter, and because Dark Matter doesn’t lend itself to an easy detection, we map galaxies instead. Galaxies are faithful (if sometimes cheeky) tracers of Dark Matter, so by mapping their positions we are essentially mapping the matter in the Universe. We map their position in the sky, and we use spectrographs to measure a redshift to each individual galaxy – resulting in a three-dimensional map of our Universe.
This map gives us two main ways with which to study Dark Energy.
Firstly it gives us the means with which to measure the expansion rate of the Universe. If we want to learn about this new phenomenon, then we best start by carefully measuring its effects. We do this by finding a measuring rod in the Universe, something whose size is known. Thankfully, Nature was kind enough to provide us with such a measuring rod, and this is how it happened: shortly after the Big Bang, the Universe was so hot and dense that all matter was in the form of a plasma and the Universe behaved very much like a fluid. This fluid wasn’t all of uniform density – some parts were denser than others, and these small gradients set up pressure waves in the fluid that propagated at a speed related to the density of the fluid (that in turn depends on its composition, temperature, etc). They set up ripples if you like, of a characteristic wavelength, that can still be detected in the distribution of galaxies today.
These are called Baryon Acoustic Oscillations. ‘Baryon’ because they exist only due to the interaction of baryonic (normal, i.e., not dark) matter with radiation at extreme temperature and densities; Acoustic because they are pressure driven, just like sound is; and Oscillations because they are waves. BAO, for short.
We know this wavelength – or size – pretty well by looking for example at the cosmic microwave background. So if we can measure it at different times of the Universe’s history, we can map out its expansion in a very clean and robust way. And we can measure it, by looking at the distribution of galaxies at times in the past (or equivalently, at galaxies at different distances from us – thanks to the finite speed of light). This BAO size shows itself in the distribution of galaxies as a distance at which galaxies are preferentially distanced from one another. The magic number is around 500 million light-years. Let me quote Daniel Eisenstein on this, the director of SDSS-III,
“Because of the regularity of those ancient waves, there’s a slightly increased probability that any two galaxies today will be separated by about 500 million light-years, rather than 400 million or 600 million.”
And that is the whole point – to measure the distance at which we find a slight excess of finding pairs of galaxies. As it has been expanding with the size of the Universe, tracking it means tracking the cosmic expansion.
BOSS mapped the positions of 250,000 galaxies that are roughly 6 billion light years away for Earth. We measured the BAO scale with a phenomenal 1.7% precision. When we combined it with other BAO scale measurements, we found an expansion rate consistent with a Universe dominated by Dark Energy in the form of a cosmological constant, and otherwise consistent with the General Relativity.
But this isn’t the whole story. The trouble is that you can always modify the laws of gravity to match any expansion history measurement in such a way that it becomes almost impossible to tell the difference between a GR + Dark Energy combo, from a Modified Gravity (MG) and no Dark Energy model if we just measure the Universe’s expansion. There’s one more test we can do, however, that tests gravity directly.
The galaxy map we measured was shaped by the continual tug of war between the two dominant forces in the Universe – on one hand we have Dark Energy accelerating things apart, and on the other we have gravity, pulling everything together.
Gravity causes structure to form – small fluctuations in matter density in the early Universe gave rise to the largest structures in the Universe today (I’m talking about galaxies, and clusters of galaxies) simply due to gravity. Importantly for the question at hand, this growth allows us to test our understanding of the laws of gravity. If gravity behaves as Einstein suggested it did, even at these colossal scales, we know how fast structure should be growing in the Universe.
Gravity and dark energy affect the motions of galaxies in different ways. By large, the motion of a galaxy is set by the expansion of the Universe (hence the relevance of the BAO measurements). But if you look closely enough, if you have enough galaxies, you can also detect the velocity component due to growth; due to gravity.
Now, remember that our map is really a redshift map. On one direction and one direction alone – that is along our line of sight – it gives us a recession velocity, or redshift. As galaxies are infallling into clusters, as structure grows, galaxies appear to be closer than they really are along this direction. This distorts our map in a very peculiar way, squashing it along our line of sight. A faster growth of structure results in a more prominent ‘squashing’ signal. This gives the name to this technique – redshift space distortions.
My measuring these distortions we can measure the rate at which structure grows and we can test gravity. BOSS, with its large volume and number of galaxies, set a rigorous test of Einstein’s theory of gravity at these very large scales. And what is outstanding is that it still seems to work.
This is a profound statement about the physical nature of our Universe, and one that we don’t make lightly. Of course we need to continue to look ahead. As we gather more data (and BOSS is only one third of the way through collecting data), we can make more precise measurements and more interesting tests.
To quote the principal investigator for BOSS, David Schlegel: “If there are surprises lurking out there, we expect to find them.”
Lauren Anderson, Eric Aubourg, Stephen Bailey, Dmitry Bizyaev, Michael Blanton, Adam S. Bolton, J. Brinkmann, Joel R. Brownstein, Angela Burden, Antonio J. Cuesta, Luiz N. A. da Costa, Kyle S. Dawson, Roland de Putter, Daniel J. Eisenstein, James E. Gunn, Hong Guo, Jean-Christophe Hamilton, Paul Harding, Shirley Ho, Klaus Honscheid, Eyal Kazin, D. Kirkby, Jean-Paul Kneib, Antione Labatie, Craig Loomis, Robert H. Lupton, Elena Malanushenko, Viktor Malanushenko, Rachel Mandelbaum, Marc Manera, Claudia Maraston, Cameron K. McBride, Kushal T. Mehta, Olga Mena, Francesco Montesano, Demetri Muna, Robert C. Nichol, Sebastian E. Nuza, Matthew D. Olmstead, Daniel Oravetz, Nikhil Padmanabhan, Nathalie Palanque-Delabrouille, Kaike Pan, John Parejko, Isabelle Paris, Will J. Percival, Patrick Petitjean, Francisco Prada, Beth Reid, Natalie A. Roe, Ashley J. Ross, Nicholas P. Ross, Lado Samushia, Ariel G. Sanchez, David J. Schlegel Donald P. Schneider, Claudia G. Scoccola, Hee-Jong Seo, Erin S. Sheldon, Audrey Simmons, Ramin A. Skibba, Michael A. Strauss, Molly E. C. Swanson, Daniel Thomas, Jeremy L. Tinker, Rita Tojeiro, Mariana Vargas Magana, Licia Verde, Christian Wagner, David A. Wake, Benjamin A. Weaver, David H. Weinberg, Martin White, Xiaoying Xu, Christophe Yeche, Idit Zehavi, & Gong-Bo Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: Baryon Acoustic Oscillations in the Data Release 9
Spectroscopic Galaxy Sample arXiv arXiv: 1203.6594v1
Ashley J. Ross, Will J. Percival, Ariel G. Sanchez, Lado Samushia, Shirley Ho, Eyal Kazin, Marc Manera, Beth Reid, Martin White, Rita Tojeiro, Cameron K. McBride, Xiaoying Xu, David A. Wake, Michael A. Strauss, Francesco Montesano, Molly E. C. Swanson, Stephen Bailey, Adam S. Bolton, Antonio Montero Dorta, Daniel J. Eisenstein, Hong Guo, Jean-Christophe Hamilton, Robert C. Nichol, Nikhil Padmanabhan, Francisco Prada, David J. Schlegel, Mariana Vargas Magana, Idit Zehavi, Michael Blanton, Dmitry Bizyaev, Howard Brewington, Antonio J. Cuesta, Elena Malanushenko, Viktor Malanushenko, Daniel Oravetz, John Parejko, Kaike Pan, Donald P. Schneider Alaina Shelden, Audrey Simmons, Stephanie Snedden, & Gong-bo Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: Analysis of potential systematics arXiv arXiv: 1203.6499v1
Rita Tojeiro, W. J. Percival, J. Brinkmann, J. R. Brownstein, D. Eisenstein, M. Manera, C. Maraston, C. K. McBride, D. Duna, B. Reid, A. J. Ross, N. P. Ross, L. Samushia, N. Padmanabhan, D. P. Schneider, R. Skibba, A. G. Sanchez, M. E. C. Swanson, D. Thomas, J. L. Tinker, L. Verde, D. A. Wake, B. A. Weaver, & G. Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: measuring structure growth using passive galaxies arXiv arXiv: 1203.6565v1
Marc Manera, Roman Scoccimarro, Will J. Percival, Lado Samushia, Cameron K. McBride, Ashley Ross, Ravi Sheth, Martin White, Beth Reid, Ariel Sánchez, Roland de Putter, Xiaoying Xu, Lauren Anderson, Andreas A. Berlind, Jonathan Brinkmann, Bob Nichol, Francesco Montesano, Nikhil Padmanabhan, Ramin A. Skibba1, Rita Tojeiro, & Benjamin A. Weaver (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: a large sample of mock galaxy catalogues arXiv arXiv: 1203.6609v1
Ariel G. Sanchez, C. G. Scoccola, A. J. Ross, W. Percival, M. Manera, F. Montesano, X. Mazzalay, A. J. Cuesta, D. J. Eisenstein, E. Kazin, C. K. McBride, K. Mehta, A. D. Montero-Dorta, N. Padmanabhan, F. Prada, J. A. Rubino-Martin, R. Tojeiro, X. Xu, M. Vargas Magana, E. Aubourg, N. A. Bahcall, S. Bailey, D. Bizyaev, A. S. Bolton, H. Brewington, J. Brinkmann, J. R. Brownstein, J. Richard Gott III, J. C. Hamilton, S. Ho, K. Honscheid, A. Labatie, E. Malanushenko, V. Malanushenko, C. Maraston, D. Muna, R. C. Nichol, D. Oravetz, K. Pan, N. P. Ross, N. A. Roe, B. A. Reid, D. J. Schlegel, A. Shelden, D. P. Schneider, A. Simmons, R. Skibba, S. Snedden, D. Thomas, J. Tinker, D. A. Wake, B. A. Weaver, David H. Weinberg, Martin White, I. Zehavi, & G. Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: cosmological implications of the large-scale two-point
correlation function arXiv arXiv: 1203.6616v1
Beth A. Reid, Lado Samushia, Martin White, Will J. Percival, Marc Manera, Nikhil Padmanabhan, Ashley J. Ross, Ariel G. Sánchez, Stephen Bailey, Dmitry Bizyaev, Adam S. Bolton, Howard Brewington, J. Brinkmann, Joel R. Brownstein, Antonio J. Cuesta, Daniel J. Eisenstein, James E. Gunn, Klaus Honscheid, Elena Malanushenko, Viktor Malanushenko, Claudia Maraston, Cameron K. McBride, Demitri Muna, Robert C. Nichol, Daniel Oravetz, Kaike Pan, Roland de Putter, N. A. Roe, Nicholas P. Ross, David J. Schlegel, Donald P. Schneider, Hee-Jong Seo, Alaina Shelden, Erin S. Sheldon, Audrey Simmons, Ramin A. Skibba, Stephanie Snedden, Molly E. C. Swanson, Daniel Thomas, Jeremy Tinker, Rita Tojeiro, Licia Verde, David A. Wake, Benjamin A. Weaver, David H. Weinberg, Idit Zehavi, & Gong-Bo Zhao (2012). The clustering of galaxies in the SDSS-III Baryon Oscillation
Spectroscopic Survey: measurements of the growth of structure and expansion
rate at z=0.57 from anisotropic clustering arXiv arXiv: 1203.6641v1
So I ave a confession to make, this time last week I’d never seen the Northern Lights. Growing up near Edinburgh it’s a bit too far south and often cloudy when there are solar storms. Nijmegen and Heidelberg are a bit too far south and as for Hawai`i forget it. So a week ago, along with a few other astronomers (and an accountant) from Heidelberg, I set off for the Arctic Circle. This sounds a lot more intrepid than it actually was, booking a flight to Tromsø and hiring a car.
Anyway, enough about my jaunt, here’s some nice aurora pictures. I don’t have a fancy DSLR so I was using my Sony DSC-H2 which is apparently a “bridge camera”. Additionally the tripod is only a small pocket one so there is a small amount of camera-shake on some of the star images.
And here’s a time-lapse of the aurora changing over about 5 minutes. Note the Pleiades star cluster setting in the background.
Have you been watching MasterChef, the BBC1 programme where amateur chefs compete to be crowned the, well, Master Chef? I have, and I think that there’s a serious flaw the competition. Don’t worry though. I think I know how to fix it.
Each week the contestants cook various meals either using their own recipes or in professional kitchens. They also have to invent a dish with surprise ingredients they don’t know about in advance. However, I don’t see how the judges can tell whether one contestant’s dish is better than another when everyone cooks different things? No, what’s needed is one menu per episode which can be prepared by all competitors. There could even be a master version – the food-standard to be met. Something like this:
Unfortunately this solution is also flawed. The trouble is that in the early stages of the contest a different set of aspiring amateur chefs compete each week. The winners of each heat then go on to semi-finals, quarter-finals and finals, until one person emerges triumphant. Ok, that may seem like a good way to do things, but the same problem remains: how can you compare contestants from different heats satisfactorily?
My solution: Monte Carlo MasterChef.
The Monte Carlo method uses random sampling to approximate the solution to a problem. It’s used a lot in astronomy to figure out the effect uncertain measurements can have on results. For example, say you want to count how many galaxies there are 150 galactic-miles (GM) away (‘galactic-miles’ are your own, personal, galaxy-distance measure). However you know that there’s an error on the distance you’ve measured, such that a galaxy you put at 145 GM may really lie much nearer or further away. You also know that 68% of the time your measurement will be within 10 GM of the true value, but 0.2% of the time you’ll be wrong by 30 GM.
To investigate the effect this uncertainty has you randomly vary the measurements thousands of times, bearing how likely each possible distance is in mind, and taking care that it obeys what you know about the error distribution. You then count the number of galaxies that happen to lie at 150 GM each time. Congratulations, you now have an estimate of how accurate your original count was. How good an estimate it is depends on how many times you repeated this step – the more the better.
Right, back to MasterChef. My new version’s pretty simple: the same set of contestants would cook the same menu each and every week. Each time random factors would change how well each person’s dish turned out. Some differences would be small (oven fractionally too hot), and some large. At the end, the judges would be able to asses not only who produced the best tasting food by direct comparison, but also how consistently they did it. The winner would be the person who produced the best meals on average, with the smallest variation between them:
The only drawback that I can see with my suggestion is that it would probably turn MasterChef into one of the most boring programmes on TV!
Photo credit: The Guardian
I have a confession to make: when it comes to books I’m unable to resist a certain type of boys-own science fiction. Normally published in the first half of the last century, they imagine a futuristic world where space travel is normal and fantastic technology abounds. They were written at the beginning of the space race, when things were moving so fast that it seemed like people would be living in moon bases in a few years.
Last year, in a second-hand bookshop in Maine, I found an excellent example of the genre. ‘Stand by for Mars’ by Carey Rockwell is the first installment in the adventures of Tom Corbett, Space Cadet. It deals with his basic training at Space Academy (he’s a natural rocket pilot of course), and how he copes with the difficult job of leading his unit-mates: eager engineer Astro and supercilious navigator (and man with a chip on his shoulder) Roger Manning. He has space fever, eats spaceburgers, travels in space at space speed and plays a sport which I’m amazed isn’t named spaceball. He ends up, as you probably guessed from the title, on Mars, but can he save the day?
The book is definitely aimed at boys, and seems designed to encourage them into science careers. Girls, unfortunately, don’t fare so well:
The boys advanced toward the huge circular reception desk where a pretty girl with red hair waited to greet them.
“May I help you?” she asked. She flashed a dazzling smile.
“You’re a lucky girl,” said Roger. “It just so happens you can help me. We’ll have dinner together—just the two of us—and then we’ll go to the stereos. After which we’ll—”
“Just give us a nice room, Miss,” said Tom, cutting in. “And please excuse Manning. He’s so smart, he gets a little dizzy now and then. Have to take him over to a corner and revive him.” He glanced at Astro, who picked Roger up in his arms and walked away with him as though he were a baby.
“We came here to have fun, didn’t we?” demanded Roger.
“That doesn’t mean getting thrown out of the hotel because you’ve got to make passes at every beautiful girl.”
“What’s the matter with beautiful girls?” growled Roger. “They’re official equipment, like a radar scanner. You can’t get along without them!”
There is a role model for potential young female readers though. Meet Dr. Joan Dale:
Exactly one hour and ten minutes later, promptly at seven o’clock, the three members of Unit 42-D stood at attention in front of Dr. Joan Dale, along with the rest of the green-clad cadets.
When the catcalls and wolf whistles had died away, Dr. Dale, pretty, trim, and dressed in the gold and black uniform of the Solar Guard, held up her hand and motioned for the cadets to sit down.
Don’t worry about the quality of her scientific research though – it’s been checked over by a whole conference of men!
Joan Dale held the distinction of being the first woman ever admitted into the Solar Guard, in a capacity other than administrative work. Her experiments in atomic fissionables was the subject of a recent scientific symposium held on Mars. Over fifty of the leading scientists of the Solar Alliance had gathered to study her latest theory on hyperdrive, and had unanimously declared her ideas valid. She had been offered the chair as Master of Physics at the Academy as a result, giving her access to the finest laboratory in the tri-planet society.
The best thing about the story is the effort it makes to include accurate science, amongst the far-fetched ideas. For example, a stranded rocket ship needs to ditch its engine but
“It seems to me,” drawled Roger lazily, “that the two great heroes in their mad rush for the Solar Medal have forgotten an unwritten law of space. There’s no gravity out here—no natural force to pull or push the tube. The only way it could be moved is by the power of thrust, either forward or backward!”
“O.K. Then let’s push it out, just that way,” said Astro.
“How?” asked Roger cynically.
“Simple, Roger,” said Tom, “Newton’s Laws of motion. Everything in motion tends to keep going at the same speed unless influenced by an outside force. So if we blasted our nose rockets and started going backward, everything on the ship would go backward too, then if we reversed—”
On researching Tom for this post I discovered that he was also a radio, comic book and T.V. star. Here he is pursuing the deadly Grapes of Ganymede:
I can’t help loving this sort of book. The enthusiasm for this wonderful future we’re going to have is infectious, whilst the characters are unintentionally hilarious. After all, it’s instructive to find out what it would have been like to live in space in the 1950s?
All the Tom Corbett books are available for free via Project Gutenberg. If you like him, you might also be interested in wonder-teen inventor Tom Swift. There’s also a Martian adventure series by Patrick Moore.
Don’t forget, Tom’s waiting for you to “Join him for another exciting adventure in the world beyond tomorrow!”
You know how astronomy works, you look up at something with a telescope and “oh look”, Jupiter has moons or there’s a 7th planet. But you can also find nothing. One of the great things about science is that a null result is still a result. Hence by looking at your measurements carefully enough, you can actually say something interesting about what you haven’t seen.
On the 3rd of November 2005 a gamma-ray burst (GRB) was detected in the constellation of Ursa Major. Further examination found that there was a well-known galaxy in the vicinity, M81– Bode’s Galaxy. Could this violent event have come from one of amateur astronomy’s favourite objects?
During the latter half of the 20th century, astronomy moved away from being purely based on optical light to a wider range of wavelengths across the electromagnetic spectrum. From radio to submillimetre, infrared, UV, X-ray and gamma-ray, astronomers now have a vast array of tools for studying the visible universe. There are however other sources of information that come from astronomical sources.
Gravitational waves were first predicted by Einstein. While they haven’t been directly observed, their emission has been inferred from the orbit of a pair of neutron stars. Gravitational waves subtly stretch and compress spacetime. Hence to detect them you have to very accurately measure stretches and compressions. This is done at labs like LIGO where they measure this stretching over long distances (several miles). Such long distances are needed as the effect of gravitational waves is fractional. Hence the bigger the distance over which you measure the stretching, the bigger the stretch.
The gamma-ray burst in the vicinity of M81 was what is known as a short duration burst. While long duration bursts are the product of exploding massive stars, most short bursts are though to be formed when two compact objects (neutron stars or black holes) slam together after spiralling in due to energy lost by gravitational wave emission. However there is also another possible cause, a massive flare from a magnetar, a neutron star with an extremely high magnetic field.
To investigate this, a team from LIGO searched through their data for a signal that could come from either a magentar or colliding compact objects. They found nothing.
But nothing can be interesting. After going back and looking at their measurement errors they were able to set upper limits on the flux of gravitational waves received from this gamma-ray burst. Consequently, by examining the flux they would expect to receive from merging compact objects they were able to set lower limits on the distance this burst was from Earth. Based on these limits they excluded a black hole – neutron star merger in M81 as the source of this GRB to at least 93% confidence. The constraint on a neutron star – neutron star merger was slightly weaker, but would require the event to have a very weakly beamed jet (and GRBs are known to almost always have tight, collimated jets). Based on a fairly generously unbeamed jet the LIGO results (seeing nothing) exclude an black hole – neutron star merger in M81 to greater than 99% confidence and a neutron star – neutron star merger to over 98% confidence. However the expected gravitational wave flux from an erupting magnetar is too low to be detected at the Earth – M81 distance so the results don’t rule that out.
So what was the cause of the bright flash of gamma-rays seen in Ursa Major seven years ago? Dunno, but seeing no gravitational wave signals tells us that it’s highly unlikely to be two massive compact objects slamming together in one of the sky’s prettiest galaxies.
The LIGO Scientific Collaboration, J. Abadie, B. P. Abbott, T. D. Abbott, R. et al. (2012). Implications For The Origin Of GRB 051103 From LIGO Observations Preprint arXiv: 1201.4413v1
Ever wondered how astronomers study galaxy formation when we can’t actually see it happen (as it takes billions and billions of years)? It’s all explained in this excellent video from Andrew Pontzen:[youtube:http://www.youtube.com/watch?v=77ZoF7Y1pNk%5D