graphical model

I have been reading about graphical models for a few days here and there. So on the plane (to NIPS 2011) I tried to draw a graphical model for a large-scale structure and weak-lensing jointly supported model of the density field and cosmological parameters. I think I am close. Marshall and I and anyone who can stand it are going to unconference on Friday and try to work it out.

finding absorption features by machine

Guangtun Zhu (JHU) stopped by this week (welcome back!) and came for a chat about modeling spectra, in particular quasar spectra that might have absorption-line systems redward of Lyman alpha. Most catalogs of absorption systems involve significant hand-vetting; we discussed methods to ameliorate or eliminate the by-hand aspects of the problem. Guess what I advocated? Justified probabilistic modeling of the spectra, plus some sensible priors.

exam time

Nothing to report.

space-filling curve

It is not clear it counts as research, since I did it purely for fun, but I wrote a recursive code to build this kind of space-filling curve. It is slow, but it works for any image (array) that is 2ⁿ×2ⁿ. If it has any research application, it is in image compression, but I officially don't care about that subject (this paper notwithstanding; it isn't about image compression; it is about information!).

I am so proud of this; check out how scale-free it is: The top-left 4×4 block has the same two-dimensional pattern for the ones digit as the whole chart has for the sixteens digit, but with the pixels being sixteen-cell blocks.

what would you do with the PTF data?

Price-Whelan returned to NYU for a day (from distant Columbia, where he is now a grad student) to discuss methodologies for some of the projects he is thinking about with Palomar Transient Factory imaging data. There are lots of things possible; like most big projects, PTF creates more possible projects than there are collaborators to do them; this is why astrophysics is a great field! I argued that he should use the things we have been thinking about with respect to image combination and source detection to detect—and then analyze—the sources below the single-exposure detection limit. Any project PTF has done at high signal-to-noise can be done for far more sources additionally if you are willing to work at lower signal-to-noise (and at image level rather than catalog level).

non-integrable galaxies

As my loyal reader knows, I am strongly against modeling non-integrable systems with integrable potentials. And guess what: No galaxy in any kind of realistic dark-matter mass distribution could ever have an integrable potential! During a meeting today (and yesterday also, which perhaps doesn't count according to the implicit parts of The Rules), Fadely, Foreman-Mackey, and I talked about how to generalize things like Schwarzschild modeling to non-integrable potentials and proper probabilistic inference. Glenn van de Ven (MPIA) does these things for a living, so we wrote him email and he sent back some great suggestions for how to proceed. Now the question is: Should we implement something, even if it means reinventing a few wheels? It didn't take us long to decide: Yes!. Ah, Camp Hogg prefers engineering to science these days. One set of ideas I am excited about is how to write down a prior over phase-space distribution functions when you don't have an integrable-potential action–angle language in which to write it. I love that hairball of problems.

why do we have five fingers?

After I used my five fingers to put my foot squarely in my mouth on the APOGEE Collaboration mailing list (yes folks, apparently you should be careful with that Reply All button), I was treated late in the day to a couple of excellent talks about NYU efforts that border on astrobiology; not astrobiology per se but close: Paul Chaikin (NYU Physics) talked about his work with Pine and Seamans to make artificial structures out of DNA or small particles painted with DNA or the like that can self-assemble and then self-replicate. Bud Mishra (NYU Math and Biology) talked about simulations of the interactions that lead to multi-cellularity, with the thought that multi-cellularity might be inexpensive and generic in the path from first life forms to complex ones.

One of Mishra's most gripping (ha ha) points was about the number of fingers we have: Is five a random accident or is it something you would expect to see on other worlds? He pointed to a nice math result of his that shows that for frictionless fingers to hold any arbitrary solid object under load, you need between 7 and 12 fingers. Given that fingers come at a cost, and that there is friction, five might be very close to optimal for a wide range of utility functions! That's an idea I will be thinking about for a while. I also got him to say a bit about higher dimensions: He said my problem wasn't crazy because an articulated object is like a solid object in higher dimensions!

spectroscopy, calibration

Douglas Brenner (AMNH) dropped by for a discussion of spectrum extraction, related to the coronography stuff we have been working on. I pointed him to this instant classic, a paper much discussed a few years ago in this forum (for example here). At lunch, Muna talked about software development for stellar population synthesis, which could have a big impact in the large-spectroscopic-survey world we are currently in. In the afternoon, Foreman-Mackey produced color-color plots for stars in his re-calibration (self-calibration) of SDSS Stripe 82 imaging. They are looking very good.

what is the probability of habitability?

At the end of yesterday, there was a discussion of astrobiology at NYU, and then this morning I had email from Rix about a probability calculation for p(habitable|data) where "habitable" is the bit "yes this exoplanet is habitable" or "no it isn't" and "data" is whatever you have measured about an exoplanetary system. He was inspired by this paper. We discussed briefly by email; one of the issues (of course) is that many of the parameters that might determine habitability do not even indirectly affect the observable (at present day) properties of the exoplanet. That's a nice problem; it reminds me of my conversations about extraterrestrial life: Prior information informs the current debate much more than the data do; it is a playground for Bayesian reasoners.

talking, and occupy!

In an all-talk day, I talked with Lang about next steps on our various slowly moving projects; I talked with Sjoert van Velzen (Amsterdam) and Foreman-Mackey about large surveys; I talked with Fadely yet again about the scope for paper zero on classification. This was a highlight: This time I think we have done it! The scope will be showing that hierarchical beats likelihood tests no matter how bad the models. In the afternoon, Nissanke (Caltech) gave a seminar about gravitational radiation detection, particularly in the light of coincident electromagnetic signals. At the end, discussion devolved into the insane data-secrecy policies of LIGO, which I abhor. Occupy LIGO!

software engineering for science

I had lunch today with Demitri Muna (NYU) of scicoder fame. We discussed software engineering, code-writing environments, and the idea that Muna should write a book about what he knows. He has been teaching scicoder workshops at NYU for a couple of years now, and there is no good book for scientists who code.

IDL

Almost no research got done today, with the exception of being impressed by David Mykytyn (NYU), my new undergraduate researcher, picking up IDL and using it within an hour. Of course we hate IDL here at Camp Hogg, but sometimes we can't avoid it!

editing

I spent my research time editing Holmes's nearly-ready paper on self-calibration of imaging surveys (what I have called "uber-calibration" previously). The results are so simple and so easy-to-use; I hope we have an influence on the next generation of surveys.

zip

No research today; gone till Monday.

oscillation model bad?

Hou, Goodman, and I have a radial-velocity linear damped oscillator model to capture non-exoplanet near-sinusoidal variability in radial velocity surveys (especially for giant stars). Unfortunately, it doesn't seem to be a good fit to the radial-velocity jitter in our test-case real star. So now we have to see if among the data we have there is any nail on which we can use this hammer.