galaxies, stellar dynamics, exoplanets, and fundamental astronomy
I wrote code this weekend to look at the question of how we should visit a star in the upcoming Terra Hunting Experiment . The current (straw-person) plan is that we will observe each visible star once per night for ten years, with one exposure of a sensibly-chosen exposure time at each visit. Is this a good idea? I was interested in this problem for two reasons. The first is that binning is…
Today my rant on LLMs and the practices of our field hit the arXiv . I was scared to post it, because it is such a weird contribution, and it is so revealing about myself and my own political positions and hangups. But I have to say: I got great and supportive feedback all day. I got two comments on saying ACAB in the literature. The Astronomer Royal of Scotland quoted (on BlueSky) the last…
I spent the day at the NSBP / NSHP meeting in San José . My favorite session of the day was the morning astro session, which was entirely about brown dwarfs. I learned a lot in a very short time. Caprice Phillips (UCSC) introduced the session with an introduction to the scientific and technical questions in play. She put a lot of emphasis on using binaries and clusters to put detailed abundance…
Further insipired by yesterday's post about binary fitting , I worked today on the treatment of nuisance parameters that have known distributions . These can be treated as noise sometimes. Let me explain: If I had to cartoon inference (or measurement) in the face of nuisance parameters, I would say that frequentists profile (optimize) over the nuisances and Bayesians marginalize (integrate) over…
On Friday, Kareem El-Badry (Caltech) gave a seminar about looking for (and finding!) stars in binary orbits around dark or much darker companions, like black holes, neutron stars, and white dwarfs. He showed results that involve ESA Gaia astrometry, where he noted that the Gaia Mission has no sensitivity to periods right at (or within an inverse mission-length frequency difference of) one-year…
So imagine that you have a unique data set Y , and in that data set Y you measure a bunch of parameters θ by a bunch of different methods. Then you find, in your favorite analysis, your estimate of one particular parameter is way out of line: All of physics must be wrong! How do you figure out the significance of your result? If you only ever have data Y , you can't answer this question very…
I showed my robust spectral decomposition (dimensionality reduction) and residuals to the MPIA Binaries group today. There was much useful feedback (including that my H-gamma was actually H-delta; embarassing!). One comment was that the model isn't truly a causal separation between star and lines, so there will be some mean lines in the star model; lines aren't entirely outliers. That's true! The…
My goal this year in Heidelberg is to move forward all writing projects. I didn't really want to start new projects, but of course I can't help myself, hence the previous post. But today I crushed the writing: I wrote four pages in the book that Rix (MPIA) wants me to write, and I got more than halfway done with a Templeton Foundation pre-proposal that I'm thinking about, and I partially wrote up…
Over the last two weeks, I built a new robust dimensionality-reduction method called Robust-HMF . This method is a hammer, looking for a nail. This week, Hans-Walter Rix (MPIA) suggested that I use the method on young stellar objects observed in SDSS-V BOSS spectra. I did that, and I found hundreds of young stars with narrow H-alpha emission lines. It turns out that the Robust-HMF method does…
Dustin Lang (Perimeter) and I spoke today about many things, but the conversation got de-railed when Lang showed me his visualizations of the brand-new NASA SPHEREx data. Oh. My. Goodness. First of all, the data are being released daily, before the team has done its analysis, so that anyone in the world can do anything with it. Talk about open! Talk about international! Talk about everything I…
Is it research to have a set of conversations about whether to write a book on data analysis? Hans-Walter Rix (MPIA) thinks I should put together my arXiv-only submissions plus a lot more into a book about data analysis. His point of view is that the most important thing is how to convert an ill-posed question about the Universe into a well-posed operation on data.
There is a very nice algorithm and set of methods called “Robust PCA,” originating in a paper by Candès . This method makes use of ideas from convex optimization to simultaneously learn a low-rank representation of the data plus a sparse representation for the outliers. This kind of situation comes up in astronomy all the time. Way back, Tsalmantza and I made a replacement for PCA…
A nice talk at MPIA by Hanna Ũbler (MPE) about very high-redshift (redshifts 8 to 14 even) galaxies and their black-hole contents started some nice discussions in the audience and afterwards about the formation of black holes. Because of the Eddington limit (which is a limit on luminosity), black-hole growth is probably limited. The limit is on luminosity, not mass accretion rate, and the…
Coherent oscillators—astronomical sources that pulse or oscillate in a phase-stable way over long timescales—have been useful astrophysical tools. For two examples: Stably pulsing pulsars were used to discover gravitational radiation (and are being used to find the stochastic background). Delta-scuti star asteroseismic modes were used to find orbital companions. This led undergrad Nana…
There is an informal meeting at MPIA every Wednesday regarding binary stars, with a bit of a focus on massive binaries. Today there was a very nice presentation by Jakob Stegmann (MPA) about some anomalies among the (only six) black-hole–neutron-star binaries discovered by NSF LIGO . He showed the example of GW 200105, which shows a large eccentricity (0.15-ish). This eccentricity is very…
Dimensionality reduction (the basic being PCA) is very sensitive to outliers: A single bad pixel can dominate most objectives and thus create a spurious dimension. One of the best and most classic solutions to this is the robust PCA method, which is presented in a (very long) paper with impressive math and beautiful results. Yesterday Hans-Walter Rix (MPIA) and I coded it up and applied it to ESA…
In the Milky Way meeting at MPIA today, a bit of a discussion broke out about using stellar twins, inspired by work by Yuan-Sen Ting (OSU). The idea is: If you have two stars with very similar overall metallicity, and very similar temperature and surface gravity, then it should be possible to measure accurate element abundnace anomalies between the two stars, even in the absence of an extremely…
I saw a very nice talk today by Philippine Griveaud (MPIA) about how the Solar System formed. The idea is that the giant planets formed in an accretion disk. Their formation opened gaps and caused migration (first Type I and then Type II, if you must know :). That migration pulled them into a resonant chain. That is, if the giant planets formed the way we think they formed, they must have been in…
In work on Galaxy dynamics, from stellar kinematics, we measure relative velocities and relative positions, of nearby stars relative to the Sun (or really the Solar System barycenter). These relative positions and velocities are coordinate free, in the sense that they don't imply a rest frame for anything (and indeed, the SS barycenter is not anywhere near the rest-frame position or rest-frame…
Distant objects in the Universe look the same as nearby ones, in the sense that a redshift 8 quasar looks like a point source just the same as a nearby M dwarf star looks like a point source. This must, somehow, strongly constrain the scattering properties of the Universe. The Universe is very transparent, but there definitely is scattering and absorption (by dust, by gas, by rocks); why don't we…
In group meeting last week, Stefan Rankovic (NYU undergrad) presented results on a very low-amplitude possible transit in the lightcurve of a candidate long-period eclipsing binary system found in the NASA Kepler data. The weird thing is that (even though the period is very long) the transit of the possible planet looks just like the transit of the secondary star in the eclipsing binary. Like just…
OMG I actually just submitted an actual paper, with me as first author. I submitted to the AAS Journals, with a preference for The Astronomical Journal . I don't write all that many first-author papers, so I am stoked about this. If you want to read it: It should come out on arXiv within days, or if you want to type pdflatex a few times, it is available at this GitHub repo . It is about how to…
Today was day two of a meeting on generative AI in physics, hosted by MIT. My favorite talks today were by Song Han (MIT) and Thea Aarestad (ETH), both of whom are working on making ML systems run ultra-fast on extremely limited hardware. Themes were: Work at low precision. Even 4-bit number representations! Radical. And bandwidth is way more expensive than compute: Never move data, latents, or…
Today was the first day of a two-day symposium on the impact of Generative AI in physics. It is hosted by IAIFI and A3D3, two interdisciplinary and inter-institutional entities working on things related to machine learning. I really enjoyed the content today. One example was Anna Scaife (Manchester) telling us that all the different methods they have used for uncertainty quantification in…
At the end of the day I got a bit of quality time in with Danny Horta (Flatiron) and Adrian Price-Whelan (Flatiron), who have just (actually just before I met with them) created a new implementation of The Cannon (the data-driven model of stellar photospheres originally created by Melissa Ness and me back in 2014/2015). Why!? Not because the world needs another implementation. We are building a…