Numerous observations indicate that an unidentified form of dark matter drives the growth of structure in the Universe. Significant experimental progress on this issue is anticipated from a number of independent projects, including direct searches for dark matter particles impacting nuclei in sophisticated experiments on Earth and indirect searches for signatures of dark matter in cosmic rays that bombard the Earth as well as the properties of stars and galaxies. A possible weak link in the dark matter identification program is that theoretical predictions for phenomena relevant to direct and indirect dark matter searches remain inadequate to address expected improvements in experimental techniques. The proposed project is a broad theoretical program to improve the predictions for signals in dark matter direct and indirect search experiments.
This project is also envisioned to have significant broader impacts. The graduate and undergraduate students participating in this research program will cultivate unique skill sets. The PI will continue numerous education and outreach projects under this proposal. These projects include the development of general education courses on the physics of energy and the physics of sports, public lectures at the Allegheny Observatory, educational events at the Carnegie Science Center, and participation in the Carnegie Science Center's programs to foster interest in science in school-aged children.
For more than eighty years, astrophysical observations have indicated that the amount of matter in the Universe must be greater than can be accounted for through observations of luminous material. The existence of this "dark matter" has been inferred only through its gravitational influence. The favored hypotheses are that dark matter is a wholly new form of matter. Determining the nature of the dark matter is among the highest priorities of physicists and may reveal unknown laws of physics. The identification of dark matter is among the scientific goals of a number of experiments and astronomical observatories including the Large Hadron Collider, the Hubble Space Telescope, and the Fermi Gamma-ray Space Telescope (FGST). This project was an effort to develop new methods to shed light on the properties of dark matter. The project aims were to build the theoretical understanding necessary to probe dark matter with observations of the cosmos, individual galaxies, and even individual stars. The project entailed three primary lines of inquiry. One line was to explore theories in which the dark matter may decay into lighter particles. We developed the theoretical basis for exploiting large-scale measurements of gravitational lensing to place constraints on the lifetime of the dark matter. We showed how to use forthcoming lensing measurements to probe models of unstable dark matter. We also used current measurements of the evolution of cosmological structure to test unstable dark matter theories. The result of this investigation was the most restrictive model-independent, experimental limits on the stability of the dark matter to date. A second line of inquiry involved developing better understanding of the detailed structures of the dark matter "halos" that observations show to surround all galaxies, including our own Milky Way. We applied these models in several ways. We predicted the statistics of gravitational lensing anomalies that would be characteristic of a class of theories called "cold dark matter." We predicted the properties of gamma ray signals from dark matter annihilation in nearby dark matter clumps that may be observable as unique signatures of dark matter by the FGST. Third, we built a model of the fine structure of the dark matter halo, including the dark matter associated with the Sagittarius Stream, a stream of stars ripped from the nearby Sagittarius galaxy by tidal forces from the Milky Way. Our work indicates that a stream of high-velocity dark matter particles originating from Sagittarius likely passes through the Earth. We demonstrated that this stream causes a number of unique experimental signatures that may aid in identifying dark matter experimentally. This work can help experimenters better determine the basic properties of dark matter. The last line of inquiry involved probing dark matter with individual stars. Many dark matter theories predict that the dark matter interacts with normal matter (for example, protons, neutrons, atoms, molecules), though weakly. Consequently, these theories predict that trace amounts of dark matter accumulate in stars (including the Sun). We showed how two classes of dark matter theories could be probed using stars. The first class of theories goes by the name "self-interacting dark matter" (SIDM) and is a class of hypothetical particles that interact strongly with themselves. This interaction leads to a large accumulation of dark matter in the Sun, which then causes the Sun to radiate high energy neutrinos at anomolously high rates, which may be seen in neutrino observatories, such as Ice Cube. In the second class of theories, "Asymmetric Dark Matter" (ADM), there is no anti-dark matter to accompany the dark matter. We showed that when ADM accumulates in stars, it cools stellar cores and alters the evolution of low-mass stars. It is possible that ADM could cause objects that would otherwise have been stars to become "giant" brown dwarfs. We showed that observations of low-mass stars could severely constrain ADM theories. The high-priorty goals of this project were achieved. This work resulted in a number of peer-reviewed publications describing our results. Final manuscripts being prepared for peer review will describe constraints on dark matter using stellar evolution and the stability of the dark matter from observations of cosmic structure. Our work required the development of software necessary to perform these analyses. This software is now available to other scientists through a web interface. Over the course of the work, the PI was engaged in an active program of graduate student education and mentoring as well as public education and outreach. Educational activities related to this work included public lectures, demonstrations at local schools, and participation in programs aimed at educating public school teachers about contemporary scientific questions and improving science education. Much of this work was performed as part of the doctoral dissertation of a graduate student at the University of Pittsburgh. The student received her Ph.D. degree in 2013 and is continuing as a postdoctoral researcher at Indiana University.
