initial conditions

In the astrophysics seminar today Tristan Smith (Swarthmore) convinced us, perhaps incidentally to his main point, that the Hubble Constant controversy could be resolved if the baryon acoustic feature (or peaks in the CMB) is moved relative to the vanilla CDM prediction. And it doesn't have to be moved far! So maybe there is just a bit of initial-conditions manipulation to make that happen, and then everything is in agreement! Interesting take on things.

My only other significant research for the day was a discussion with Kate Storey-Fisher (NYU) about the outline of our paper about correlation-function estimation, and a chat with Boris Leistedt (NYU) about a possible pedagogical piece on hierarchical modeling and graphical models.


R-M effect; failure

The day started with a great discussion with Luger (Flatiron) and Bedell (Flatiron) about the Rossiter–McLaughlin effect, which is the apparent velocity shift as a planet transits a rotating star. We discussed how this effect really is different from a radial-velocity shift; it is a line-shape change, and how we might model that within an extension to the wobble framework. That's a great idea and possibly an important contribution. The R–M effect has been important in exoplanets.

Late in the day, I experienced complete failure to produce a grant proposal. It was effectively due late last week, so I really had to produce today, but under the gun I failed. That was a hard blow! I love my job, but sometimes I find it to be difficult.



I spent the day today at CITA, which is my childhood home: My first-ever scientific paper was written here (when I was an undergraduate researcher) with Scott Tremaine (now IAS) and Gerry Quinlan. At the CITA weekly grass-roots discussion of matters cosmological, Deyan Mihaylov (Cambridge) spoke about gravitational-wave detection with Gaia. He made an amazing point (which, like most amazing points, is obvious in retrospect): The GW signature in Gaia has an earth term but no “pulsar term”, in the language of pulsar timing. That is, it only depends on local metric perturbations! That is extremely good for scaling and precision.

In that same forum, I spoke for the first time ever about the correlation function estimators I have been developing with Kate Storey-Fisher (NYU). I spoke extemporaneously—it's a discussion forum—but I realized that we do have a great story to tell. It includes context from the Landy-Szalay estimator world and context from the linear-fitting world. Plus some information theory for spice! It is a great audience at CITA and they helped me sharpen my case well.

A highlight of a long day of conversations was a chat with Katie Breivik (CITA) about binary population synthesis. She is interested in predicting gravitational-wave sources. But the issues are general. We discussed what aspects of the theory are most weak, and where we might be able to patch in a data-driven replacement. That conversation is only just started, but it's something I want to bring home to NYC and think more about.


listening at Toronto Physics

I spent the day today at CITA and UofT Physics in Toronto. The CITA Seminar was given by Alexander van Engelen (CITA), who spoke about the things we can learn from the CMB in the near future. He emphasized that there are still interesting things to learn about the primary CMB, which violates some beliefs I held prior to the talk! But he also put a lot of emphasis on the lensing or convergence map, which can be combined with other tracers to do a lot of science.

I had so many great conversations and discussions, too many to describe! But some highlights included the following: I chatted with Patrick Breysse (CITA) about testing cross-correlations and self-calibration for line-intensity mapping experiments with toy models. He has some nice ideas there. I chatted with visitor Deyan Mihaylov (Cambridge) about the possibility that Gaia might detect gravitational radiation! Bart Netterfield (Toronto) talked about very precisely pointed balloon-born optical telescope experiments. And Chris Thompson (CITA) had all sorts of crazy ideas about what might cause the fast radio bursts. His principal ideas involve cosmic strings and black holes!

I gave the UofT Physics Colloquium. I spoke about how Gaia and other kinematic surveys can measure the dark matter. I talked about the results that Ana Bonaca (Harvard) and Adrian Price-Whelan (Princeton) and Charlie Conroy (Harvard) and I will have on the arXiv on Monday!



I spent the day today at University of Waterloo, where I gave the Astro seminar. It was a great day! I prepared my talk on the bus from Toronto, which wasn't good from a nausea perspective! But I really find I give a better talk if I remake it from scratch before I give it. That is, old talks get stale, at least for me. So I have a brand-new talk about machine learning and data-driven models and criticisms thereof.

Before my talk, in the astro-ph discussion, and after my talk, with James Taylor (Waterloo) and with Mike Hudson (Waterloo), there were good ideas flowing about how to use galaxy morphologies and in particular galaxy granularity to determine galaxy distances and maybe also gravitational-lensing shear. This relates to photometric redshifts and also my ideas about making adversarial galaxies that don't reveal their shear via their ellipticities (or not strongly). Many other great conversations; too many to mention!

My visit ended with quality time with Dustin Lang (Perimeter), who always makes my day.


finding moons indirectly

The only research I personally did today was stressing out about the talks at Waterloo and Toronto that I haven't even started to prepare! And that isn't research either. However, Apurva Oza (Bern) gave a nice talk about sodium and potassium in the Solar System and in extra-solar systems. He pointed out that the outgassing / volcanism of Io means that there is a gas ring around Jupiter that might be visible in transit spectroscopy, and might permit the detection of moons even when there aren't visible moon transits. Or might confuse transit spectroscopy. In some cases the ring is partial and follows the moon, so it would lead to a predictable time-domain spectroscopic signal, in principle. Worth a search!


Finding planets near resonances

At breakfast, I had a long discussion with Megan Bedell (Flatiron) about what things should go into the discussion part of our wobble paper, in terms of the limitations and extensions of the model. We came up with quite a list! But I love any project that opens new paths.

I also had a long discussion with Rodrigo Luger (Flatiron) about searching for planets in Kepler data that are in 1:1 resonances. He is focused on the point that they will (in general) have large transit-timing variations. I would call these librations around their exact resonances. If we model these librations as approximately sinusoidal, the search space is tractable: A fixed period plus a TTV with some amplitude and period. That's a good idea! And Luger points out that there are strong priors on the amplitudes and periods of the librations. Of course there will be systems that even this setup will miss; there was a dispute between us and Foreman-Mackey (Flatiron) about what fraction. He argued for using a completely stochastic model for the librations. He might be right; but baby steps!

All this motivated by the possible discovery of a 1:1 by Mitchell Karmen (NYU). Of course the actual system he found almost certainly isn't a 1:1, we now think: It has many signs of “just” being an incredibly eccentric eclipsing binary with dilution from a third star.



I spent a good part of the day working through Fourier transform issues with Kate Storey-Fisher (NYU). We started out confused both about what the transform should be giving us and how to run the code correctly. So we switched to Gaussian functions for which we know the correct answer and at least understood the interface. Now to understand the correlation function!

In group meeting, two threads came together today. Bedell (Flatiron) asked for feedback about how to present wobble results for maximum impact. And Luger (Flatiron) took some of those wobble radial velocities and fit them with a model for the Rossiter-McLaughlin effect made by punking his own STARRY code for modeling photometric transits.


not much!

In a day obliterated by letters of recommendation, Humzah Kiani and I discussed extinctions in the Gaia footprint, and Kate Storey-Fisher and I discussed the Gaussian random fields we have been trying to simulate.


Trojans, Oort Cloud, greedy algorithm paradox

Early in the day, undergraduate Mitchell Karmen (NYU) blew me away by showing a possible Trojan satellite hiding in the Kepler false-positive bin. It probably has some other explanation, but damn it's exciting! I discussed this with Rodrigo Luger (Flatiron) who dampened my excitement (for good reasons).

At stars meeting, Michele Bannister (Belfast) spoke about ways in which we might use the properties of the outer Solar System (and especially the things past the Kuiper Belt and including the Oort Cloud) to constrain the birth environment and subsequent dynamical environment of the Sun at formation. It appears that these structures could be created early and are strongly modified by nearby stars and close passages. One implication is that different stars should have very different Oort Clouds. That's a great prediction; now how to test it?

Mike Blanton (NYU) showed some very cool results from the work being done on SDSS-V robot fiber positioners. As you might guess, the positioning of fibers on a focal plane by robot arms that can collide is an intractable problem in general—it's like traveling salesman. But you might also know that most NP problems are pretty well-served by sensible greedy algorithms. That is, you can usually do something akin to the simplest thing and still succeed most of the time.

Blanton showed the interesting thing (worked out by Conor Sayres at UW) that if they do a greedy algorithm to take the robot arms from the "home" state to the configuration they want, it is very slow and hard, and it still fails in many cases. But if they do the exact same greedy algorithm the other way—that is, to take the arms from the configuration they want back to the home state—it works fine! So they do that and then run the result backwards!

Crazy talk. And cool. And worthy of a lot more thought. And something about entropy? After all, the home state is like a crystal.



Today Michele Bannister (Belfast) gave a great talk about the outer Solar System. She was very clear that her observations do not rule out in any way the existence of Planet 9. But they do discredit every single shred of evidence in its favor! And she gave many other mechanisms that could explain the same data. That is, there really doesn't seem to be any reason to believe that there is an unknown planet hanging out in the outer Solar System. Lots of what she said relies on the following theoretical observation: When a planetesimal is perturbed by a massive body on an orbit interior to its perihelion, it tends to preserve its perihelion but change its semi-major axis. And the same but opposite when the massive body is outside it's aphelion. All planetesimal migration scenarios must respect these constraints.

Before that, Kate Storey-Fisher (NYU) and I had a long conversation in which we re-discovered our confusions about the differences between the continuous Fourier transform (which never exists in any real-data context) and the discrete Fourier transform (which is what's appropriate when the data are treated as a patch of a periodic function. We got confused and then un-confused, but I am still somewhat confused!


asteroids and dark-matter halos

Today Michele Bannister (Belfast) showed up. We spent time talking about how asteroids are characterized in time-domain imaging surveys. The idea is to make a fictitious absolute magnitude, which is what the asteroid would look like if it was simultaneously 1 AU from the Sun and 1 AU from the Earth, and observed with the Sun and Earth both getting it from the same angle. That's not real! We discussed how we might improve that situation.

I also spoke with Lauren Anderson (Flatiron) about how we might reduce the dimensionality of cosmological simulations of galaxies to a small parameterization of what's possible. The idea is to get a not-too-complex parameterization of the triaxiality of galaxy dark-matter halos and their dependences on time. I have a vision here, but it isn't clear it is possible to execute. We discussed the issues of using existing simulations or running our own.


simplest possible model

Today was a fun day at Flatiron. I saw Didier Queloz (Cambridge) and Karin Öberg (Harvard), who are in town for a Simons program on Origins of Life. With Queloz we discussed target selection for the Terra Hunting Experiment and with Öberg we discussed data-driven methods for finding planets embedded in proto-planetary disks observed by ALMA.

Today was the last day of the visit by Heather Knutson (Caltech). We decided to implement the simplest possible version of the data-driven models for planet and brown-dwarf spectroscopy that we have been talking about all week. This would mean one spectral template per object, and one telluric template per night. This might not be good enough, but it is worth a shot, and might teach us a lot. The idea is to structure the model very much like Bedell's wobble model.


target selection for THE@INT

This morning, Megan Bedell (Flatiron) and I joined a telecon relating to target selection for the new Terra Hunting Experiment with HARPS3 on the Isaac Newton Telescope. It was a great conversation; we are new to this project; we learned that the project has some good and sensible ideas:

One is that the target list must be at least twice as large as what they can handle, so that changes can be made on the fly during the project's 10-year baseline. Another is that target changes must be made algorithmically, to preserve the statistical value of the sample. Another is that the strategy cannot be as dumb as we might like because discovery rate is a driver of policy. And another is that the observing decisions will be made just-in-time, on the fly, at the telescope. Again, algorithmically. My loyal reader knows that I Love These Rules. Now to play!


Patel, Knutson, Rey

Ekta Patel (Arizona), former NYU undergraduate researcher extraordinaire, showed up at Flatiron today. We spoke about all the new ideas around making inferences about the Milky Way and it's formation and dynamics, given that we can't treat the Galaxy as a time-independent, symmetric, steady-state object (and we really can't, especially in the halo). Right now all the methods are either based on very questionable assumptions (like when can a time-dependent system be treated as a small perturbation away from a time-independent system, etc) or on super-brute-force methods (like find, among billions of simulated galaxies, a few that look like what we see!). Patel has been a pioneer in the latter, but there is lots more to do.

At Stars Meeting, Patel told us about possible strong selection effects in the MW-satellite game, which might mean that we are missing many! Missing satellite non-problem? Martin Rey (UCL) told us about how you might answer semantically causal questions about galaxy evolution with quantifiable and sensible adjustments to initial conditions in simulations. That got me all philosophical about causality in a unitary universe! And Heather Knutson (Caltech) told us about metallicity effects in the spectroscopy of directly detected exoplanets; it turns out her study is limited by the quality of the stellar metallicities. Maybe Birky (UCSD) and I could help with that?

All this after an early-morning discussion with Knutson about building a data-driven model with good causal structure to explain her exoplanet spectra. I argued that once you have the causal structure in place, good inferences become optimization (or sampling) problems. I hope this is true!