Annika Peter

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It is an astonishing fact that we only know what ~5% of the observable Universe is made of, and that this percentage has only been established in the last couple of decades. The 5% that we know and think we understand encompasses atomic nuclei, electrons, neutrinos, and light. Everything from galaxies, stars, gas, planets, to IKEA coffee tables, cats, trees and airplanes are made up of these things. These substances, classified by astronomers as "baryons" (a much broader definition than particle physicists use), are fairly evenly distributed in the Universe, in the sense that if you weighed all the baryons in spheres with diameters many tens of millions of light years across, the spheres would all have about the same mass. There is more variation on smaller scales.

So, what is the other 95% of the Universe made of? The biggest percentage, ~70-75%, is a gravitationally repulsive substance that has been dubbed "dark energy". It is currently unknown if this gravitational repulsion is due to some sort of new form of matter, or if gravity behaves strangely on scales larger than we can currently measure. There is a strong observational push to see how dark energy evolves with time, which may yield insights as to what the dark energy actually is.

The other 20-25% of matter is more conventional. Gravitationally, it behaves just like baryons, but it cannot consist of baryons. Since this matter cannot emit (much, if any) light, it is called "dark matter". We have some ideas of what dark matter might be. Most likely, it consists of at least one type of particle that arises from a theory that encompasses the particle physics we currently understand and can observe in massive collider experiments (e.g., the Large Hadron Collider, LHC, at CERN, a massive complex on the Swiss/French border near Geneva).

There are currently a number of different types of experiment underway to try to detect dark matter and to figure out what it is. The experiments fall into three broad classes. First, it may be possible to make dark matter in a huge particle collider, such as the LHC at CERN. Collider experiments work by smashing electrons or atomic nuclei (or their anti-matter counterparts) at each other at high energies. The particles don't bounce off each other; instead, they destroy each other and create a ton of other particles as a result. This process is called "annihilation" or "inelastic scattering" (depending on the type of interaction) in the technical language of physics. Some of those particles could be dark matter. Second, dark matter particles may interact with either atoms or light (depending on the type of dark matter particle candidate), which may be observed in a lab since our Galaxy sits in a big pot of dark matter. Experiments of this type include CAST, CDMS, LUX, WARP, and XENON100, just to name a few. Third, dark matter particles can destroy each other when they come across each other floating around in space. A number of experiments are set up to observe any particles that arise from such interactions, such as Fermi, IceCube, and PAMELA.

I am interested in helping to figure out what the dark matter is and what its properties are. The way in which I have and continue to help in this effort is to figure out where the dark matter is and how fast it is moving. Why is this important? Because the signal in the latter two classes of experiment I just described depend on these things. It's easier to detect dark matter if there's a lot of it, and if it is moving slow with respect to our Solar System. In some dark matter models, it is easier to detect dark matter if two particles are slow with respect to each other. However, the signals tend to also depend on the mass of the dark matter particles (which can be ultra-light like axions, or the mass of hundreds to thousands of protons, like weakly interacting massive particles, also known as WIMPs) and on the strength of the particle-atom or particle-light interaction. In order to learn about the particle mass and interactions, which will help determine what the dark matter is, one must have some idea of how the dark matter is distributed both in space and in velocity, or at least a good idea of how to separate the details of where/how fast the particles are from their particle physics properties.

I have considered dark matter that is bound to the Solar System either by elastically scattering off nuclei in the Sun or by gravitational three-body interactions between the Sun, a planet, and a particle. I wrote my own code to simulate the particle orbits in a simpler version of our Solar System, and simulated a total of nearly 10 billion particle orbits. I used those orbits to figure out how many dark matter particles that are gravitationally bound to the Solar System pass through the Earth, and with what speeds. This is important because direct detection experiments are located on the surface on the Earth, while dark matter particles may become trapped in the Earth by hitting iron or silicon nuclei in the core or mantle and then annihilate into neutrinos, which may be seen in neutrino telescopes. I found that the population of dark matter bound to the Solar System is quite small. However, since the neutrino signal from the Earth depends so sensitively on the speed distribution of dark matter particles at the Earth, better simulations will be necessary to fully map out the effects of these particles on that particular signal. For more technical details, see these papers on the subject.

I also found that the planets may drastically decrease the neutrino signal from dark matter particles that get trapped in the Solar System by scattering off nuclei in the Sun if the particles are sufficiently massive. If the dark matter particles are very massive, more than a thousand proton masses each, then when these particles become trapped to the Solar System, they end up on pretty big orbits. By pretty big, I mean that the orbits can come close to the planets. In this case, the planets have a high probability of kicking those dark matter particles out of the Solar System before they have a chance to annihilate. The positive spin on this situation is that this is really only a problem if dark matter particles are heavy, so the promise of looking for a signature of dark matter in the annihilation of Solar System-bound particles should be realized if the particle mass is about a hundred proton masses. The negative spin on this is that it will be much more difficult to probe heavy dark matter particles in this way. More technical details may be found here.

Lately, I have been thinking about how to learn about the spatial and velocity distribution of dark matter from experiments. This is work in progress; once I have something more interesting to say on the subject, I will write about it here.

I have also been thinking about constraining decaying dark matter theories using existing observations. This work is in collaboration with Chris Moody (UCSC). In many WIMP models, dark matter is formed in the early Universe and then slowly putters around until it may end up in a galaxy. In these models, there are predictions, both from pure theory and simulations, on how dark matter should be distributed in the galaxies (although there are some fundamental uncertainties in this, but that is a rant for another day). However, this simple picture may not be accurate. Dark matter might have a higher probability of bouncing off itself without annihilating than is usually expected, or it might decay into another type of dark matter particle. We are investigating the latter case, trying to determine how the decays affect the structure of dark matter in the galaxies, and if we can put constraints on the properties of the decay based on existing observations of galaxies. Andrew Benson and I also considered constraints using Milky Way dwarf galaxies. We found some pretty interesting constraints and came up with some cool ideas of how to constrain different dark-matter models using dwarfs in the future (when more dwarfs are found).

In addition, I have been rethinking the very 1980's idea of dark matter constraints from stellar evolution. Again looking back at early work on the subject in the 1980's and 1990's, I have been thinking about how people estimate signals for direct detection and neutrino telescope experiments and whether these estimates will be "good enough" for the next generation of experiments, or if more careful calculations are necessary.

In the past year, I have become quite interested in how to learn about the distribution of dark matter in the Milky Way from upcoming large-scale astrometric observations. There is quite a bit of literature on the subject, and it is quite fascinating. I have not done any work in this field yet, but since this seems like an interesting field that will necessarily explode as these projects come online, I am reading lots of papers and doing some brainstorming.

Other things I am interested in include:

Resources: