inflation, smackdown

I spent the day at CITA today. The morning was spent with talks on inflation, and they nearly convinced me that it might make some testable predictions! In the afternoon, Rocky Kolb (Chicago) and Simon White (MPA) faced off in the Dark Energy Smackdown. In the end there was no smacking, because they agreed. But I think they were being too conciliatory: Although we all agree that both large dark energy projects and chaotic astronomy are good parts of our future, I do think there are deep disagreements about how to balance these and how to protect ourselves from the secretive culture of high-energy experiment.


Perimeter Institute

I spent the day at Perimeter, where there was a full schedule of talks about cosmological observations. There were many highlights; indeed all the talks were great. In particular, Olivier Doré (CITA) showed that simple inflation-inspired non-gaussianity can lead to very strong scale-dependent bias, even at large scales where we think that locality ensures linear bias. This may provide new ways to demonstrate gaussianity in the near term with galaxy surveys.

Gil Holder (McGill) told us about the brave new world of CMB, now that the primary anisotropy is nailed. He even gave a conceivable methodology for working out the z=1100 initial conditions for a large volume of the Universe using cross-correlations of CMB polarization and 21cm emission.

Dan Holz (LANL) showed us how black hole mergers are unprecedented standard sirens for cosmology.

Mike Kesden (CITA) used simple symmetries including parity and exchange symmetries to put constraints on the possible outcomes of (no-hair) black-hole—black-hole collisions; he has produced some wonderful results with symmetries alone. He can predict the outcomes of numerical simulations (which are ridiculously expensive). Bravo!



I spent the morning closing tickets, opening tickets, adjusting web pages, and generally tidying and cleaning up in Astrometry.net. There is lots more to do. We have had huge numbers of requests for code and web access after we appeared on space.com (disclaimer: I never said that I was the leader of this project).


magnetic fields

Today I attended an excellent candidacy exam by Ronnie Jansson (NYU), who is interested in reconstructing a three-dimensional model of the magnetic field of the Milky Way using Faraday Rotation, synchrotron radiation, stellar polarization, and Zeeman splitting.


objective galaxy classification

I hate to mention galaxy classification here, because that phrase is loaded, and most astronomers associate classification with what I would call classical morphological classification, which I believe has been a very unproductive method in astronomy (despite its incredibly successful adoption and widespread use). But that polemic is the subject of another post!

Today, Yann LeCun (NYU), Barron, and I discussed the possibility of performing an objective galaxy classification not in the space of 2-d galaxy images, but in a 3-d shape space, with the 2-d images constraining the shapes and setting the diversity of 3-d types. This is like an inverse of the computer graphics problem, that is, it is not here are the 3-d objects, what is the scene? but rather here is the scene, what are the 3-d objects? The key to solving the problem is the hypothesis that the great diversity of galaxies emerges from a small number of (parameterized) types, plus viewing peculiarities. Will it work? We shall see.


star positional errors

Barron and I spent part of the afternoon discussing various approximations by which we can estimate the uncertainty in a measure of a star's centroid. In principle this is easy, when the centroid is some kind of first moment. However, astronomers have learned that first moments do not deliver the most accurate centroids. The best centroids come from the fitting of paraboloidal tops to star peaks in the image, and the errors are determined by propagation through the fit. However, this propagation is non-trivial to perform correctly; it is even non-trivial to perform it approximately!


Spitzer peak-up, exact cosmology

Michael Joyce (Paris) gave a great group-meeting talk in which he showed that he can perform exact analyses of discrete N-body simulations for finite particle density, in the linear regime, and explain the output of numerical codes. He focused on differences between numerical simulations and the real world that arise from discreteness. All this matters for precision cosmology.

In the afternoon, Ed Bertschinger (MIT) used the nice property that the growth of fluctuations in the Universe can be calculated in a picture in which you treat each (sufficiently large) fluctuation as its own little FRW Universe to calculate growth of structure in some non-Einstein gravity theories. He showed some beautiful exact solutions in a suite of departures from GR.

In the early morning I worked on my tesselations for our Spitzer proposal. I had to reverse engineer the IRS peak-up star flux criteria, because I have to make my observation plans automatically with code, not manually with the (nice) observation planning tool.



I spent the day tesselating the galaxies onto a segmented sky, with a Spitzer/IRS peak-up star at the center of each segment, to make highly efficient observing modes for my proposal with Schiminovich.


Astrophysics 2020

I spent the day at Astrophysics 2020 at STScI. I was inserted into the extrasolar planet session, which was not totally appropriate, but I was cool with it, because I learned a huge amount. In particular, I learned that a lot of the figures of merit for extrasolar Earth-like planet detection go like the telescope diameter to the fourth power, and we are talking about telescopes in space, of course! But I also learned that there are dozens of transiting planets, some of which have both primary and secondary eclipses measured, and some of which are only tens of Earth masses in size. After the meeting I have no doubt that we are going to find Earth-like planets soon; I agree with the speakers that it is imperative that it be a top NASA priority moving forward.


Spitzer spectroscopy

Schiminovich and I spent much of the day preparing our Spitzer proposal, to take mid-infrared spectra of the most normal galaxies we can.


build-up of mass through mergers

I read a few papers today on the build-up of galaxy masses through mergers. The (very constructive) referee on our recent Masjedi et al paper pointed us at some new papers on the subject that find very small amounts of evolution since redshifts around unity. Given cosmic variance and sample sizes, plus uncertainties in K and evolution corrections, nothing is nearly as precise as our direct measure of the accretion rate, so they only confirm our result at the order-of-magnitude level.


plasma and habitats

Two seminars today, one on inferring animal habitats from recorded sightings plus environmental data, the other on magnetic reconnection in plasmas. Research postponed until the weekend!


how the ear works

My respite from NSF proposals was a nice seminar by Tom Duke (UCL) about how the ear hears. He emphasized the amazing range of the ear: a factor of 1000 in frequency, and a factor of 1012 in loudness. The faintest sounds are heard just at the thermal limit, ie, when the energy per cycle is about kT. He showed that the hair cells that do the heavy lifting are very dissipative objects but driven nonlinearly so as to act like incredibly sensitive oscillators, and that they transmit their information to the auditory nerve in the form of pulse distributions, or really the time delay distribution of delays between pulses. Incredible, all around.


NSF proposals, big data

I have been breaking my rules about posting! This is just to protect my reader from the terrifying fact that I have spent every spare second working on my NSF proposals. The only exception was Big Data Lunch, an event I co-organized at NYU, in which Rob Fergus (CS) gave a short seminar about his attempt to organize and classify a set of 80 million images harvested from the web. His results are impressive: He has so many images that he can use brute-force techniques like least-squares to find similar images! The discussion, among about 30 people from around the University, was very lively.


SDSS meeting, day 1

I am at the SDSS meeting at Fermilab. I spoke about how we are going about looking at the huge galaxies—galaxies comparable to or larger than an individual SDSS field. We have to build mosaics; building mosaics is non-trivial if you want to preserve all the information in the data. For me, this is part of our Gunn Atlas project.

There were incredibly impressive talks from the Supernovae team (working with the multiple epochs in the SDSS Southern Stripe that my reader knows well). They are taking systematics much more seiously than I have ever seen before. This is a good thing about getting particle experimentalists involved in precision cosmology. The scariest systematic (presented by Kessler of Fermilab) was a near-degeneracy between dust absorption, the intrinsic scatter in supernovae colors, and the world model.

Related to that, Holtzman (NMSU) gave a remarkable talk about how they do the time-variable supernova photometry; it involves making a complete fit to all of the pixels in all of the images at all epochs! This is not unlike the proper-motion stuff I was doing this summer, but much, much more sophisticated.

There were remarkable talks all day; two others that stood out were about clear detections of the MgII-absorber–galaxy cross-correlation function by Nestor (Cambridge) and the scale and success and potential of the galaxyzoo.com project by Thomas (Portsmouth). This latter project has some similarities in spirit to Astrometry.net, although they have much less technology under the hood.