black holes as the dark matter

Today Cameron Norton (NYU) gave a great brown-bag talk on the possibility that the dark matter might be asteroid-mass-scale black holes. This is allowed by all constraints at present: If the masses are much smaller, the black holes evaporate or emit observably. If the black holes are much smaller, they would create observable microlensing or dynamical signatures.

She and Kleban (NYU) are working on methods for creating such black holes primordially, by modifying hte potential at inflation, creating opportunities for bubble nucleations in inflation that would subsequently collapse into small black holes after the Universe exits inflation. It's speculative obviously, but not ruled out at present!

An argument broke out during and after the talk whether you would be injured if you were intersected by a 10²⁰ g black hole! My position is that you would be totally fine! Everyone else in the room disagreed with me, for many different reasons. Time to get calculating.

Another great idea: Could we find stars that have captured low-mass black holes by looking for the radial-velocity signal? I got really interested in this one at the end.

discovering and measuring horizon-scale gradients in large-scale structure

Kate Storey-Fisher (NYU) and I are advising an undergraduate research project for Abby Williams (NYU) in cosmology. Williams is looking at the question: How precisely can we say that the large-scale structure in the Universe is homogenous? Are there gradients in the amplitude of galaxy clustering (or other measures)? Her plan is to use Storey-Fisher's new clustering tools, which can look at variations in clustering without binning or patchifying the space. In the short term, however, we are starting in patches, just to establish a baseline. Today things came together and Williams can show that if we simulate a toy universe with a clustering gradient, she can discover and accurately measure that gradient, using analyses in patches. The first stage of this is to do some forecasts or information theory.

CMB

My only research time today was a nice astro seminar by Chris Sheehy (BNL), who convinced us that detecting primordial gravitational radiation in the b-mode polarization of the CMB at large scales will be hard but possible. It depends on some inflation physics, of course! He also showed some novel uses of what you might call “compressive sensing” to CMB foreground analyses, scooping some of my thoughts on the subject!

writing and listening

As per usual, Tuesdays are low-research days. But I did get in some time with Bonaca (Harvard) about her next projects on Milky Way and tidal streams. And some time with Leistedt (NYU) about our possible pedagogical paper on hierarchical inference and models. We got through half of a paper outline.

The Astro Seminar was Jo Dunkley (Princeton) talking about Simons Observatory. It is a beautiful project and many things will come of it. But it is not obvious to me that it will be possible to see the gravitational radiation from inflation. Obviously we should look, though!

how to write a discussion section

In a low-research day, a highlight was a long conversation with Bonaca (Harvard) about the writing of her paper on the GD-1 stream interaction. We discussed structure, and especially the discussion. In a discussion, I like a humble sandwich on proud bread: Start by saying what's most impressive about what we've done, then go into caveats, limitations, approximation wrongness, and the consequences of all that. And then end on a positive note about what kinds of great new things this work will enable going forward.

Late in the day, Alex Kusenko (UCLA, IPMU) spoke about a very wide range of subjects. He claims to have a full explanation for why we don't see the cutoff in the gamma-ray occurrence rate required by photon–photon interactions with the infrared background. He claims that the gamma rays we see from blazars are really reprocessed from cosmic rays. Plausible! But I would need to know a lot more. He also claims to have a way to naturally make primordial black holes in the end stages of inflation, and make all of the dark matter that way. That's interesting. Unfortunately it was such a long and tiring day I couldn't get it together to really check either of these ideas carefully.

FML, and the Big Bounce

The day started with a realization by Price-Whelan (Princeton) and me that, in our project The Joker, because of how we do our sampling, we have everything we need at the end of the sampling to compute precisely the fully marginalized likelihood of the input model. That's useful, because we are not just making posteriors, we are also making decisions (about, say, what to put in a table or what to follow up). Of course (and as my loyal reader knows), I don't think it is ever a good idea to compute the FML!

At lunch, Paul Steinhardt (Princeton) gave a great black-board talk about the idea that the Universe might have started in a bounce from a previously collapsing universe. His main point (from my perspective; he also has particle-physics objectives) is that the work that inflation does with a quantum mechanism might be possible to achieve with a classical mechanism, if you could design the bounce right. I like that, of course, because I am skeptical that the original fluctuations are fundamentally quantum in nature. I have many things to say here, but I'll just say a few random thoughts: One is that the strongest argument for inflation is the causality argument, and that can be achieved with other space-time histories, like a bounce. That is, the causality (and related problems) are fundamentally about the geometry of the space and the horizon as a function of time, and there are multiple possible universe-histories that would address the problem. So that's a good idea. Another random thought is that there is no way to make the bounce happen (people think) without violating the null-energy condition. That's bad, but so are various things about inflation! A third thought is that the pre-universe (the collapsing one) probably has to be filled with something very special, like a few scalar fields. That's odd, but so is the inflaton! And those fields could be classical. I walked into this talk full of skepticism, and ended up thinking it's a pretty good program to be pursuing.

radio-telescope calibration to string theory

At #NYCastroML, Josh Peek and I had a public conversation about how to self-calibrate single-dish radio observations of the Milky Way HI gas. We were able to cast it into a linear-ish form and develop the hope that he could use a sparse-coding method advocated (effectively) in the Ivezic et al book.

In the afternoon, Kilian Walsh presented the work he has done on cosmology to his PhD committee. He can use the void probability function to constrain the halo model beyond the abundance and clustering constraints usually in use. He is now a PhD candidate; congratulations!

At lunch time, Roberto Gobbetti (NYU) defended his PhD. He presented a mechanical model for slow-roll inflation based on flux discharge around compact dimensions in the string landscape. It is a beautiful model in which a great many things are computable. Let's hear it for the observational consequences of string-like theories! And let's congratulate Dr Gobbetti.

causality, web scraping for science, visualization

A great day! Avoiding proposal-writing, I guess: I met with Jennifer Hill (NYU Steinhardt) to discuss ways we might bring causal inference to astronomy. The idea is not to improve what is being done or concluded in astronomical studies, but rather to clarify the implicit assumptions. We tentatively decided to look at galaxy evolution, because it is a sub-field in which the models are currently very data-driven and aren't backed by strong theoretical ideas. It all might fail, of course, because it is hard to think of things like galaxy environments as "treatments" and things like star-formation rates as "outcomes". But the problems do map onto each other, I think.

At lunch, Raphael Flauger (NYU) gave a beautiful talk on foreground uncertainties related to the BICEP2 results. He built his foreground models as did the BICEP2 team by scraping data out of Keynote (tm) presentations posted on the web! I have to say that again: The Planck team showed some maps of foregrounds in some Keynote presentations and posted them on the web. Flauger (and also the BICEP2 team before him) grabbed those presentations, scraped them for the all-sky maps, calibrated them using the scale bars, and worked from there. The coolest thing is that Flauger also simulated this whole process to account in his analysis for the digitization (scraping?) noise. Awesome! He concludes that the significance of the BICEP2 results is much lower than stated in the paper, which makes him (and many others) sad: He has been working on inflation models that produce large signals.

Late in the day, Enrico Bertini (NYU Poly) rocked the Data Science Showcase series with a nice talk on visualization. Among many other things, he showed that there are known rankings of the value of different kinds of visual cues (bar lengths, dot positions, color, and so on) for displaying different kinds of quantities (real value, rank, binary). Afterwards, there were some conversations about what we should do about scientists who are blind. That's an important question we need to address when we speak about the importance of visualization!

Dr Schillo

Marjorie Schillo (NYU) defended her PhD today. She showed that eternal inflation implies a non-zero spatial curvature for the Universe. More specifically, it links the CMB low-ell amplitudes to the spatial curvature. Since all string-based inflation models are eternally inflating, she can construct hard tests of string theory by measurement of the spatial curvature. Right now there are just upper limits, but there is some chance her tests will be executed. She gave a great talk and handled some heavy interrogation from the floor. Congratulations, Dr Schillo.

Very late in this low-research day, I did some work on the 2014 Kentucky Derby. As of writing, I like Dance With Fate, Wicked Strong, and Medal Count. But that's not investment advice!

Spergel and BICEP2

David Spergel (Princeton) came into town and talked about CMB cosmology. He spent a good fraction of his talk throwing doubt on the new, hot BICEP2 / Keck result on B-mode polarization on large scales. He is concerned that the experiments don't seem to satisfy the standard null tests. He laid out a set of requests, which are easy to satisfy. He noted that six new experiments can confirm the result if it is real, so we will know extremely soon. One amusing thing about Spergel's talk was the repeated point (obvious, but often overlooked) that because all CMB experiments are observing the same, single sky, they ought to agree to better than one-sigma, especially on large scales where cosmic variance dominates.