Blogs / Cosmic Variance

Travel is broadening, and in particle physics we get to do a lot of it. In July, having temporarily settled my father into a nursing home after being hospitalized (the subject of my last post, Part 1), I was able to meet my commitment to travel to Krakow, Poland, to give a plenary talk on the search for the Higgs boson at the annual Europhysics conferenceheld at the Jagiellonian University there (where Copernicus studied for four years, 1491-1495).

Central Krakow emerged from World War II, which began nearly exactly 70 years ago, nearly unscathed. The central square is one of the more beautiful in Europe, similar in a way to that of Prague. But it was hard to avoid waling there without imagining what it must have looked like during the war, occupied by German soldiers who had made Krakow the center of their regional government during the war.

From the square one can take tours in little golf-cart-like jitneys, and see some of the interesting historical sites, including the Jewish Quarter (Kazimierz) and Schindler’s famous enamelware factory. Some of the apartment buildings in Kazimierz are still in the state they were at the end of the war, a rather grim reminder of the central role Krakow played in the Holocaust.

From Krakow one can take day trips to a number of interesting places, and we visited the spectacular salt mines of Wielicka, a UNESCO World Heritage site, which have amazing, huge rooms carved out of the rock.

But there was another interesting place to tour that we were hesitant about – Auschwitz. Others who took the tour came back saying that it was well worth the journey, over an hour by bus each way, but tended not to say much more about it…hmmm.

So on our last free day we took the plunge, signed up for the tour, and went. The bus traveled through quite rural countryside on two-lane roads, past farms and villages, roughly following the Vistula river, until reaching the town of Oswiecim, which the Germans called Auschwitz.

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I’m spending most of this week at the 2009 meeting of the Division of Particle and Fields of the American Physical Society (DPF2009), which is being held at Wayne State University in Detroit. I’m here giving a talk, and convening, with Corbin Covault from Case Western, a couple of sessions on particle astrophysics and cosmology.

I’m enjoying the conference, and have managed to fit in a number of very illuminating discussions with colleagues from other institutions, which is really the most useful part of conferences anyway, as everyone knows. Plus, I finally got to meet the only one of my co-bloggers that I’d never met before. Furthermore, the talks have generally been good – well thought through, and eloquently presented – and we had a lovely reception last evening at the beautiful Detroit public library, where I snapped the mural, which seemed appropriate (except for the pecs). Nevertheless, the overwhelming vibe that I’m getting here is one of extreme impatience and anticipation. This, of course, is all about the Large Hadron Collider (LHC).

There have been talks presenting rather recent and significant results, of course – for example Angela Olinto’s talk about high energy cosmic rays and gamma rays was a lovely survey of the combined data from Fermi, Auger, PAMELA, ATIC, and other experiments; Josh Frieman’s talk on cosmology, and particularly cosmic acceleration, provided a clear picture of the vibrancy of the field and the great progress that has been made over the last decade; and there are numerous other interesting talks coming up on QCD, heavy ion physics, neutrinos, etc.. But in high-energy particle physics I think we’re mostly seeing talks, albeit good talks, summarizing things we’ve seen again and again for a long time. The details of the LHC detectors (ATLAS, CMS, LHCb and ALICE); how one hopes to tease out evidence for the Higgs from the data; ditto for supersymmetric particles, and those arising from large extra dimensions; and a talk by Lyn Evans summarizing the progress towards getting the LHC back online after last year’s calamity.

Particle physics is screaming out for a new result pointing the way to the physics that we know must lie beyond the unreasonably successful standard model. We know this physics should be there because of purely particle physics problems, such as the hierarchy problem – why is the weak scale so much lower than the Planck scale, and stable against quantum corrections – but also because cosmological observations such as the matter antimatter asymmetry of the universe and dark matter tell us that new particles and interactions must be out there, perhaps at the energy scale of the LHC.

People aren’t sitting around twiddling their thumbs and just waiting for the machine to turn back on, of course. And none of what I’ve written above is in any way intended as a criticism. Ongoing work at existing experiments (such as those at the Tevatron, for example) is placing new limits (for example the recent claimed exclusion of a Higgs mass between 160 and 170 GeV), and experimentalists are busy refining their techniques for extracting the maximum amount of information from the upcoming LHC data. This is all extremely important work, and certainly very interesting. But it doesn’t change the fact that people really want the LHC.

And it isn’t just pure particle physicists who feel this way. Those of us in particle cosmology have been getting a wealth of data from cosmology for a while now. But this has left us in a position where dark matter and cosmic acceleration are on such a firm footing that more than ever we desperately need to understand how these phenomena fit into our understanding of fundamental physics. The LHC is the next essential tool in this quest. New physics discovered there may have direct implications for cosmology. And if it doesn’t, then proposed theoretical explanations will be constrained by, and may well open up new vistas for, cosmology.

So we’re all, particle physicists and cosmologists, keeping our fingers tightly crossed for the planned turn on later this year.

For a long time, the government has been responsible for space travel in the United States. That’s about to change.

Government is the appropriate agent for certain forms of collective action: roads, public schools, national defense. It’s also good for big-picture things without immediate financial payoff, like support for the arts or basic scientific research. It makes perfect sense for the government to shoulder the burden for developing the technologies to get us into space, and it will continue to make sense for them to play an active role in astronomical research in space. But for commercial purposes, like launching satellites, it ultimately makes a lot more sense for space travel to be a private-sector enterprise. We’re on the brink of seeing it happen.

SpaceX is a private company founded by Elon Musk, who previously co-founded PayPal and the electric car company Tesla Motors. For a while now, SpaceX has been developing reusable launch vehicles and space capsules. They’ve been awarded a contract from NASA to take over re-supplying the International Space Station after the Shuttle fleet is mothballed next year. And they’ve had one launch that reached orbit, but also a few failures; until yesterday, they hadn’t succeeded in putting a satellite into orbit.

But now they’ve done it. I was watching on live webcam last night as the Falcon 1 rocket launched a Malaysian satellite into orbit.

It’s incredibly exciting, but just the beginning. The idea behind the Shuttle was to make trips to orbit cheap, reliable, and routine; it failed spectacularly on all counts, and NASA’s capabilities and plans for space flight have become somewhat disjointed (while its science missions continue to have amazing success). Hopefully we’re moving past the point where we have to rely on the government to get us to space.

The internets have spoken, and it’s a good thing I listened. A few months ago I had the idea to organize a session at the upcoming meeting of the American Association for the Advancement of Science, in San Diego next February. It’s a giant cross-scientific-disciplinary meeting, offering a great chance for journalists and scientists in diverse fields to catch up on what’s happening in other areas.

But I couldn’t decide between two possible topics, both of which are close to my heart: “The Origin of the Universe” or “The Arrow of Time.” (My original book subtitle was “The Origin of the Universe and the Arrow of Time,” before that was squelched by the marketing department and replaced with “The Quest for the Ultimate Theory of Time.” Quests are big these days, apparently.) So I did the natural thing: I Tweeted the question. And the internet spoke with a fairly unambiguous voice: “Arrow of Time” sounded more interesting. So that’s what I proposed.

And now we’ve just been accepted, so it’s on for San Diego 2010. We have a fantastic line-up of speakers (and also me), spanning quite a range of topics:

• Me, setting the stage about the arrow of time and the Second Law, and drawing some connections to cosmology.
• Huw Price, director of the Centre for Time, Department of Philosophy, University of Sydney, and author of Time’s Arrow and Archimedes’ Point. Huw will be talking about the arrow of time in philosophy.
• Anthony Leggett, winner of the Nobel Prize in 2003 for his work on superfluidity. Tony will be talking about the arrow of time in quantum mechanics and condensed matter physics.
• Michael Lässig, University of Cologne. Michael specializes in statistical physics and quantitative biology, and will be speaking on the arrow of time in biology, in particular in evolution. He recently gave a talk on this topic at the KITP.
• Daniel Schacter, Harvard, author of The Seven Sins of Memory. Dan was the leader of the studies showing that the brain predicts the future in the same way that it remembers the past. He’ll be talking about the arrow of time in psychology.

That’s the fun part about this topic; it ranges naturally from the birth of the universe to the operation of your brain. Should be a good symposium.

Update: Unfortunately, Daniel Schacter won’t be able to make the symposium. Instead, we are very fortunate to have Kathleen McDermott of Washington University in St. Louis. Her research involves how we remember the past and forecast the future.

I’m back from the World Science Festival, which was a rousing success, leaving thousands of smiling attendees chattering excitedly about the mysteries of the universe as they dispersed through the streets of Manhattan. So naturally I want to talk about how it could be improved. Writing about one’s travels can be one of the least compelling arrows in the blogger’s quiver, but it would be great if the science-festival idea caught on more widely, so perhaps there is something to be learned from the experience.

A science festival, one presumes, aims to bring science to a wide audience through a series of events concentrated in space and time. But there are a lot of different approaches we could imagine taking to achieve that goal. Kirsten Sanford insightfully compares the WSF to the San Diego Science Festival — two similar-sounding events that end up having a very different look and feel. The WSF appeals to the cultural and cool, while the SDSF aspires to be a noisy bring-the-family affair. Neither is right or wrong, and in these cases each is appropriate to the venue; but the choices of how to proceed should be made consciously.

Public events for science can be placed in a two-dimensional parameter space, where one axis ranges from “observational” to “participatory,” and the other ranges from “inspiring” to “informative.” Again, none of these reflects a normative judgment; inspiring and informing are both laudable goals, and sometimes the best way to achieve those goals is to have the audience observe a performance, while other times it’s better to have them participate more directly. The point is not to say what’s better or worse, it’s to figure out what is appropriate for the circumstances.

The parts of the WSF I experienced directly — the opening gala, the two events in which I participated, and two events where I sat in the audience — were roughly speaking more observational than participatory, and more inspiring than informative. For the three events I watched, I think that was exactly right, but for the two events I participated in, I think they could have been even better had the balance been shifted. (Which obviously raises the possibility of some sort of bias on my part, left for you to decide.) In other words, I think a slightly more diversified portfolio of approaches could be beneficial to future science festivals.

The opening gala, a science-and-art extravaganza that both set the stage for the festival and celebrated E.O. Wilson’s 80th birthday, was a great example of an event for which the inspiring/observational paradigm worked perfectly. It was a big production, at Lincoln Center, with a rapid-fire series of performances bridging the gap between art and science; it would have been crazy to try to invite audience participation. And inspiration is just what you need to kick off a big festival. Brian Greene, who along with Tracy Day (”the first couple of New York science“) founded the WSF, did a tag-team presentation with violinist Joshua Bell. Brian would talk a bit about string theory or various wonders of the cosmos, while videos from The Elegant Universe played in the background, and then Bell would play some music appropriate to the mood. Very little educational was going on — nobody came out of the performance considerably more knowledgeable about the secrets of string theory than they went in. But it was an artistic success, putting people in the frame of mind to excitedly tackle meatier fare over the next few days.

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I spent most of last week back in Cleveland, at Case Western Reserve University, where I spent three delightful years working with Tanmay Vachaspati, Glenn Starkmann, and Lawrence Krauss (who recently left). The occasion was a workshop on Tests of Gravity and Gravitational Physics, organized by the Center for Education and Research in Cosmology and Astrophysics (CERCA).

An interesting mix of theoretical cosmologists, relativists, particle physicists, observers and experimentalists participated, and the aim was to pull and tug at the loose threads in various ideas of modifying gravity, while talking about how we might perform sensible tests of the theory in new regimes in the near future. Most of the program consisted of talks, both theoretical and experimental, which you can download from the main site. The theory talks ranged from Nima Arkani-Hamed’s talk on why general relativity (GR) is so remarkably robust, with the take-home message “Don’t modify gravity – understand it”, to Bill Unruh’s description of his “dumb Hole” analog models for black holes, and the discussion of the associated analogous phenomenon to Hawking radiation. On the experimental side, there were great talks covering tabletop tests of sub-millimeter gravity – like the one by Blayne Heckel – all the way up to tests of gravity on the scale of galaxies and above – like Stacy McGaugh’s talk on tests of MOND.

As well as taking part in the general discussions and chairing a session, my role in all this was to sit on a panel for a discussion of “Modified Gravity: What does it buy? and at What Price?”

Nima moderated the discussion, and the other panel members were Eanna Flanagan, Nemanja Kaloper, Glenn Starkman and Bob Wald. While everyone had their own particular take on this, a common theme was that the theoretical and observational constraints are such that we currently do not have any modifications to GR that are compelling, despite our attempts to find a way to address the cosmological constant problem, explain cosmic acceleration, and search for alternatives to the dark matter paradigm. This doesn’t mean it isn’t worth trying, of course, although Nima made a spirited argument that it is hard to maintain the important holographic (and hence nonlocal) features of general relativity in any modified theory that isn’t of the simple scalar-tensor type, and that this is an argument that we shouldn’t expect nontrivial modifications. Rather, he argued, we should search for a dual S-matrix description that applies in flat space (unlike AdS/CFT) and that may allow us a better understanding of, for example, the cosmological constant problem.

This was all heavy stuff, but it didn’t consume our entire time in Cleveland. Tuesday evening saw the workshop dinner and it turned out that our hosts had arranged some after dinner entertainment, in the form of an improvisational comedy team. These guys were big on audience participation, and I was one of the first people chosen to mildly (I hope) embarrass themselves. At first this seemed like a heavy price to pay for dinner. However, trust me, subsequently seeing Tanmay Vachaspati, Bill Unruh and George Pickett up there made me realize it was well worth every blush and squirm.

Update: Dmitry Podolsky has a couple of very nice posts over at NEQNET with quite a few details about the talks, including Nima’s.

Springtime in the Imperial Palace gardens, Tokyo:

Nikon D200.

It’s been a bit of a hectic week. Last Monday I took the train (can’t tell you how happy I am whenever I avoid flying these days) to Washington DC, for a couple of days of government work, finishing up quite late on Wednesday. The next morning I flew from DC to Vancouver (you can’t always avoid it), where on Friday I delivered the departmental colloquium at Simon Fraser University. My friend, and former postdoc, Levon Pogosian is a Professor there, and I spent a very stimulating couple of days discussing physics with him and his colleague Andrei Frolov. Everyone is always telling me how beautiful Vancouver is, but on my previous visits there has been nothing but rain and cloud, and I haven’t seen anything of the area’s natural assets. This time however, it was fifty degrees and sunny, without a cloud in the sky for the entire visit, and I was just blown away by the breathtaking scenery entirely surrounding the city. Completely gorgeous! I also managed to squeeze in a wonderful dinner with friend, UBC Professor and blogger, Moshe Rozali.

On Saturday morning I was up bright and early to fly back to Philadelphia, arriving late on Saturday, in time for a quick dinner and sleep before the firefighter and I had to drive up to Syracuse for some personal business that was going to take 25 minutes, but needed us to be there in person. We completed that this morning, and then I rented a car and drove immediately back to Philadelphia.

After a week like that, I’d normally think about taking a day off before driving back, but in this case that wasn’t an option because tomorrow is a big day for us here at Penn. You may recall me mentioning that the university was setting up a new Center for Particle Cosmology. Well, tomorrow is our launch event! The first part is a panel discussion, with program

The Center for Particle Cosmology presents Of Telescopes and Microscopes
A Panel Discussion and Q&A

Tuesday, February 24, 2009
4:30 – 5:30 p.m.
Bodek Lounge, Houston Hall
3417 Spruce Street

The study of the microscopically small and the unimaginably large are no longer distinct realms of inquiry. In the first of a series of events hosted by the Center for Particle Cosmology, leading experts will consider what insights modern observations might provide into phenomena such as dark matter, dark energy and the physics of the early universe.

Panelists Include:
Vijay Balasubramanian, Associate Professor of Physics and Astronomy
Bhuvnesh Jain, Associate Professor of Physics and Astronomy
Janna Levin, Assistant Professor of Physics and Astronomy (Barnard College of Columbia University) and author of A Madman Dreams of Turing Machines, winner of the 2007 PEN/Robert Bingham Fellowship for Writers
Mark Trodden, Professor of Physics and Astronomy

Moderated By:
Gino Segre, Emeritus Professor of Physics and Astronomy and author of Faust in Copenhagen: A Struggle for the Soul of Physics

Introduction By:
Tom Lubensky, Chair of the Department of Physics and Astronomy

Sponsored by the Center for Particle Cosmology and the School of Arts and Sciences.

This is then followed by:

Guests are invited to mingle with Center faculty who will be happy to answer questions and talk about their research. As part of the reception, attendees will also enjoy multimedia presentations by:

Mark Devlin, Professor of Physics and Astronomy and a principal investigator of BLAST (the “Balloon-borne Large-Aperture Submillimeter Telescope”)

Evelyn Thomson, Assistant Professor of Physics and Astronomy and a member of the ATLAS collaboration, a particle physics experiment at the Large Hadron Collider at CERN

Refreshments will be served.

which we expect to go on until around 7pm. Well, we’ll see about that “leading experts” stuff, but I’m hoping it’ll be a lot of fun. And with Gino and Janna involved, how could it not be? After all, if Janna can handle Stephen Colbert, and Gino can hold his own in the New York Times, they should be OK with us.

If you’re in the Philadelphia area you’re welcome to attend – you just need to register. This has taken up quite some time over the last two weeks, and so on Wednesday it’s back to work finishing a couple of papers that are begging for final edits.

I just got back from Pittsburgh, a city famous for honoring football players along with Fathers of our country. Apparently they recently won some sort of sporting contest, so the citizens were generally in good spirits.

I was visiting to Center for the Philosophy of Science at the University of Pittsburgh, to speak in their annual lecture series. The Center, along with the Department of History and Philosophy of Science, help make Pittsburgh one of the world’s leading institutions for studying philosophy of science.

The Center is also a remarkably friendly place, and I had a great time during my visit. The highlight, predictably, was lunch with some of the graduate students, where we got to let our hair down and talk about big ideas concerning time and causality and determinism. (Almost all professional academics start out fascinated by big ideas, but the interest is gradually beaten out of them along the way by the demands of professionalism and career advancement. Grad school is probably the peak combination of background knowledge and willingness to confront the hard problems.) I also got to chat with Adolf Grünbaum, whose declamations concerning the Primordial Existential Question had impressed me a year and a half ago. And I got to meet some fellow bloggers in the flesh — the formidable Cosma Shalizi, who helped me understand how to augment the principle of indifference with conditionalizing over the past hypothesis, and Bryan Roberts of Soul Physics, who was one of the aforementioned grad students.

But if I’m really honest, my favorite part of the trip was probably the building. The Center for the Philosophy of Science is housed in the Cathedral of Learning, a looming structure on the University’s campus — the second-tallest academic building in the world, after one at Moscow State University. Despite my lack of religious sympathies, I love cathedrals — the looming structures, swooping curves, open spaces, all designed to elicit a certain emotional response going far beyond their direct practical purpose. (Not that different from the best casinos in Vegas, really.) And I love learning! So the Cathedral of Learning is pretty much the perfect building.

And it really does work as a building. What everyone points to are the many Nationality Rooms scattered throughout the building — a series of 27 spaces decorated in the style of various different countries, often with the input (and financial assistance) of the respective governments, which work as display pieces but are also functioning classrooms. (I was told that prospective students are sometimes convinced to come to Pittsburgh by a visit to the room corresponding to their personal heritage.) But what I liked was the immense Commons Room (pictured), with impossibly high ceilings, which is just a place where people can sit down and read and talk and think. Such places are very precious, and the world should have a lot more of them.

If Wikipedia is to be believed, the Cathedral grew out of a vision of Chancellor John Gabbert Bowman in the 1920’s. He insisted that the Commons Room be built on the principles of true Gothic architecture, with self-supporting arches. When told that these things cost money, he replied:

“You cannot build a great University with fraud in it.”

I’m not sure if that’s strictly true, but it’s an honorable principle to strive towards.

As Daniel has already mentioned, I recently spent a week in Aspen, at a workshop on Understanding the Dark Sector: Dark Matter and Dark Energy, which I organized in collaboration with Rachel Bean, Carlo Contaldi, Marc Kamionkowski and Jochen Weller.

The Aspen Center for Physics is wonderful place. Unlike many physicists, I’m not a big skier, or a hiker, preferring to get my exercise through ball sports and in the gym, and so I don’t love Aspen in the way that many people do. But I do really like the Center, and I always have great physics conversations there, learn a lot, and am able to actually get work done. So I had been looking forward to our workshop for a while, and I wasn’t disappointed at all.

The conference spanned five days. We flew in on Sunday in time to register and have dinner, and then got started properly on the Monday morning. At many conferences, academic and otherwise, the day is full of sessions, and then the evenings are free. At Aspen, in the winter, this is modified somewhat, with the mornings, late afternoons and evenings full of sessions, and the middle part of the day free so that people can ski if they want to. We spend the same amount of time working as at a regular conference, but the time is just distributed differently. This last week, it was good that I don’t ski, since I had let a number of important deadlines slip to the point that I had to work each afternoon. I did have a great view while I was doing it though!

Our speakers covered a wide range of topics, experimental, observational and theoretical, covering dark matter and cosmic acceleration separately, as well as the possible interactions between them. I can’t possibly discuss all of the topic or talks here (but you can take a look at a slightly outdated program), but let me pick out one talk that particularly caught my attention, and one event.

The first morning we kicked off with a very nice talk by Doug Finkbeiner (Harvard), discussing his (and others’) recent work constructing a particle physics model of dark matter that is designed to account for the PAMELA and ATIC results. This was followed by later theoretical talks on related topics by Dan Hooper and Graham Kribs.The PAMELA results were a recurring theme at the workshop, and there were plenty of discussions in the free time and over dinner about the results themselves, their interpretation, and the models that a number of groups have constructed.

One talk that I particularly enjoyed was by Greg Tarle (Michigan), who gave an (originally unplanned) talk on the immense challenge of making the precise measurements that these quite beautiful experiments claim. From my perspective what was wonderful about Greg’s talk was that although he presented quite a few experimental details in the latter half, he constructed the first half in such a way as to get across some of the key things for theorists to keep in mind while they are rightfully getting excited by the data.

What I took away from it (although any mistakes here are naturally due to me, and not to Greg) is that, as we’ve discussed before, the big deal for PAMELA (and the earlier HEAT experiment) is an accurate measurement of the flux of cosmic ray positrons. The single largest challenge in measuring these is discriminating against the vast proton background, since protons have the same charge as positrons and are copious in cosmic rays. Ideally one needs to be able to reject the proton signal at the 10^-6 level. If this is done completely reliably and the dramatic rise of the spectrum at high energies persists, then we may indeed be seeing a signature of dark matter annihilation, or something more mundane, like nearby pulsars.

On the other hand, it is possible that the signal is due to protons leaking in. Obviously, the key to figuring this out is particle identification. One point of caution Greg pointed out is that on PAMELA, the particle ID is solely dependent on calorimetry. Greg certainly wasn’t saying that he believes that the PAMELA signal is contaminated, but he did want to point out how a possible contamination might lead to an erroneous result. In his talk, to demonstrate this, he showed how to reproduce the observed effect (at high energies) with a model of proton contamination at the 3×10^-4 level. Note though, that PAMELA claims proton rejection at the 10^-5 level, but one can at least get a simple idea of what a small error might yield. Again, I as a theorist can’t comment on this at all, and my understanding is that the PAMELA experiment is an excellent one. Still, it is nice to have a feeling for the possible issues involved. All should be settled by FERMI (GLAST) hopefully.

On the Tuesday evening we had a couple of public events as part of the workshop. In the first, Dan Hooper, particle phenomenologist, dark matter expert and author extraordinaire, teamed up with me to conduct a Physics Cafe in the mezzanine of the Wheeler Opera House. I’m awful at estimating numbers, but I’d say somewhere between fifty and a hundred people, members of the public who either live in or were visiting Aspen, turned up to ask us any kinds of questions they liked about particle physics and cosmology. This was terrific fun, and we got some wonderful questions, my personal favorite of which was “What does entropy have to do with gravity?” Not something to which one can give a full answer (in public or professionally), but a great chance to talk about the general issues that gravity raises for thermodynamics.

I participate in a lot of events like these, and one thing that never ceases to surprise me is the general perception that scientists treat theories like general relativity as received wisdom, about which one should not ask questions. Dan and I had to go out of our way to explain that science is, in fact, about challenging ideas, and forcing them to stand up to evidence and scrutiny. This is not the fault of the audience, of course, but reflects the way in which I think science is often taught and portrayed.

Directly after this, also at the Wheeler Opera House, my new colleague Bhuvnesh Jain gave a wonderful public lecture on Einstein Rings and Giant Arcs: Mapping Dark Matter with Gravitational Lensing to a large and lively audience. Bhuv has some great animations and movies, plus some fun props – optical lenses that are machined to reproduce the effects of gravitational lenses – and he did an admirable job of the lecture.

My own talk at the workshop was as part of a trio of talks, the other two of which were delivered by my collaborator Rachel Bean and her student Istvan Laszlo. I spoke about work with Rachel and Eanna Flanagan on adiabatic instabilities in coupled dark matter-dark energy models, and Istvan and Rachel covered different parts of a longer paper the three of us co-authored with Flanagan a few months ago, mapping out general constraints on such couplings.

Speaking of which, I have a seminar at Penn State to give on this topic on Wednesday, and I should probably get to finishing that!