On Friday evening I emerged from what have been a crazy couple of months, during which I’ve been in a paper crunch. When one works on a number of different projects at any one time, with a variety of collaborators, postdocs and students, the resulting papers generally seem to see the light of day at seemingly random times. Sometimes a few papers come out more closely together than at other times.
But sometimes it gets a bit ridiculous, and that’s what happened this summer, with a lot of projects suddenly getting close to completion about the same time – five papers to finish up in the space of a couple of months. I can’t complain – I love the work – but I found this particular random bunching of projects reaching completion to be a bit much, and I haven’t been getting much free time or even time to sit back and spend time focusing on any new ideas (or even time to blog!!).
But on Friday my collaborators (Rachel Bean and Eanna Flanagan, from Cornell) and I finished up two of the papers we’ve been working on. We’ve been studying a class of instabilities that can occur if dark matter and dark energy are coupled in a nontrivial way. There are a number of models in which this happens, and indeed one might think that if one tried to include both dark components of the cosmic energy budget into a simple particle physics theory, then there might quite naturally be couplings other than gravity between them.
We’ve done a great deal of work on this problem, and our long paper is sufficiently involved that we accompanied it by a letter that covers the main points and physical interpretations without all the analysis and examples that we have in the longer paper. For an idea of what we’re about, the abstract to the longer paper reads
We consider theories in which there exists a nontrivial coupling between the dark matter sector and the sector responsible for the acceleration of the universe. Such theories can possess an adiabatic regime in which the quintessence field always sits at the minimum of its effective potential, which is set by the local dark matter density. We show that if the coupling strength is much larger than gravitational, then the adiabatic regime is always subject to an instability. The instability, which can also be thought of as a type of Jeans instability, is characterized by a negative sound speed squared of an effective coupled dark matter/dark energy fluid, and results in the exponential growth of small scale modes. We discuss the role of the instability in specific coupled CDM and Mass Varying Neutrino (MaVaN) models of dark energy, and clarify for these theories the regimes in which the instability can be evaded due to non-adiabaticity or weak coupling.
It’s very satisfying to complete a project, although also a little sad, since one usually has such fun working on them, and learns a great deal. In this case, we’re still working on more projects and have lots of new ideas, so I’ll still be enjoying the collaboration, even though this particular project is over. Plus, I can now devote a little more time to a few other ideas I’ve been working on with my graduate students, who’ve been putting in the bulk of the calculational work on them over the last couple of months.