Co-authors: K. Abazajian (UCI), N. Canac (UCI), S. Horiuchi (Virginia Tech), M. Kaplinghat (UCI)
Approximately 20% of the mass in the universe is composed of familiar, known particles, i.e. protons and electrons– the remaining 80% of the Universe’s mass, the so-called ‘dark matter’, thought to be constituted of a new, exotic particle whose existence is inferred through is gravitational effects on visible matter. Dark matter is one of the biggest mysteries that modern physics has yet to solve. Physicists know almost nothing about dark matter because it does not absorb or emit light and is therefore impossible to observe with telescopes… or is it?
Certain classes of well-motivated particle physics models of dark matter predict that annihilations between pairs of dark matter particles can produce high-energy gamma-rays. We analyze data from the Fermi gamma-ray space telescope to search for signs of excess gamma-ray emission in regions where observations of gravitational effects indicate a very high density of dark matter. We choose to focus on observations of the Milky Way center because of its high concentration of dark matter and its proximity. A large part of this analysis involves precise modeling of the background in the region, as the predicted dark matter signal is weak compared to the gamma-ray flux produced by more mundane astrophysical sources (e.g. supernovae and cosmic rays interactions). Careful background modeling helps us to avoid spurious dark matter detections arising from an undersubtraction of the background.
We find that there is indeed an excess gamma-ray signal present at the galactic center whose observed properties are remarkably consistent with expectations for a dark matter annihilation signal. Intriguingly, we also detect additional gamma-ray signals that are consistent with what might be produced by secondary interactions following the initial dark matter particles’ annihilation. However, we still have not completely ruled out the possibility that these signals might be produced by less exotic astrophysical processes. Further analysis and modeling of high-energy astrophysical sources will help to determine the true origin of the excess emission. If the galactic center gamma-ray excess is indeed the result of dark matter annihilations, it would shed light on dark matter’s particle properties and have enormous implications for fundamental particle physics.
Co-authors: Trevor Gamble (UCI), Eleanor Gillette (University of Maryland), Zuzanna Siwy (UCI)
Clean, reusable energy is a subject of critical importance both to science and modern culture. With the growing number of portable electronic devices, battery technology has received considerable attention and has seen improvements in lifespan, power, and efficiency. Though these improvements are considerable, the demand for smaller, more powerful batteries continues. Recent discoveries have revealed remarkable possibilities for energy storage in battery material structures at the nanometer scale. Several materials have been studied in particular manganese dioxide. Manganese dioxide (MnO2) is attractive since it is currently used in modern batteries, is abundant, inexpensive, non-toxic, and has staggering theoretical energy storing capability. Many studies have been made of MnO2 nanostructures, testing energy storage for a variety of architectures. Despite this, the nature of ion transport in, through, and around MnO2 at the nanoscale is not well understood. MnO2 is naturally porous. Therefore, we have designed an experiment to help us understand how well ions move through MnO2, the hope being such an experiment will allow us to further optimize structural design for controlled charging and discharging of batteries.
A useful tool to perform this experiment that has been used to understand ion transport at the nanoscale is the synthetic nanopore, a channel with a sub-micron diameter that has been etched or drilled into a thin membrane. Properties of the ion current passing through nanopores, that is the flow of ions through the channel, can reveal characteristics about the pore’s structure and surface charge.
In this study, we utilized synthetic nanopores to perform experiments on MnO2 to determine how easily it allows ions to move through the material, i.e. its conductivity, in different charged states. The measured ion current carried information on the pores in MnO2 and its surface charge. Membranes containing many nanopores as well as membranes containing only one nanopore were coated with gold, and then deposited with MnO2 ‘nanowires.’ The gold layer remained in direct contact with the MnO2 wires, which permitted charging and discharging of the wires with lithium ions, similar to conventional rechargeable batteries. Measurements of ion current through the wires after deposition, after charging, and after discharging revealed that each charge state demonstrated a different conductivity. Several different concentrations of KCl electrolyte in single nanowire studies caused changes in the response of MnO2, indicating that the pores in the MnO2 nanowire had changed as a result of the lithium charging and discharging.
Chemical and Materials Physics / Physics
Co-authors: Yongki Choi (Physics and Astronomy), Tivoli J. Olsen (Chemistry), Patrick C. Sims (Physics and Astronomy), Gregory A. Weiss (Chemistry), Philip G. Collins (Physics and Astronomy)
Despite significant advances in diagnosing and curing diseases scientists have made over the years, we still know very little about daily biological processes in our body. Annual doctor check-ups provide just a snapshot of the body’s activity and might be insufficient to diagnose health problems in their early stages. What if checking your body’s biological functions was as easy as checking your email? In principle, portable and low-power sensors integrated into wearable devices could provide such insights. We have recently designed tiny electronic sensors that report real-time activity of biomolecules responsible for anti-inflammatory, regulatory, and DNA replication functions. Their unique electronic fingerprints could be used to detect malfunctioning process. However, finding molecular abnormalities in biological signals requires sophisticated algorithms. We are developing algorithms that can identify anomalous molecular events in long time varying signals. Using an array of such sensors and computer algorithms “crunching numbers” in real-time one can imagine versatile wearable devices for human health monitoring, disease diagnosis and treatment.
Physics & Astronomy
Co-authors: James Bullock, UCI. Jose Onorbe, MPIA. Mike Boylan-Kolchin, UMD.
In the currently favored cosmological paradigm, galaxies are embedded within massive collapsed pockets — or “halos” — of a mysterious substance known as dark matter. According to this theory, dark matter is required for regular matter overcome the early expansion of the Universe and to collapse into the galaxies and stars that are required for life as we know it.
These dark halos that permeate the known Universe are themselves predicted to be filled with smaller dark matter clumps in a hierarchical manner. Observationally verifying the existence of these small clumps is one of the most important goals in modern cosmology.
We expect that the smallest dark halos should be largely free of stars, due to the ambient ionizing background radiation — emitted when the first stars formed — preventing them from accreting gas. But below what mass do we expect all dark halos to be completely dark? What are the masses of the smallest galaxies and can we observe them now or in the near future?
I use ultra-high resolution hydrodynamic simulations — the highest resolution ever run with realistic models of star-formation and energy released from star formation — to make predictions aimed at testing the validity of the prevailing paradigm. I predict that orbiting around isolated low mass galaxies — themselves thousands of times less massive than the Milky Way — we will find ultra-faint satellite galaxies that are only a few thousand times the mass of the sun. These galaxies are lower in mass than the lowest-mass galaxies predicted by many authors, and were able to form only because they managed to form their stars before the ionizing background heated their gas away.
The most massive of these satellites should be visible with current telescopes, and more powerful instruments coming on line in the next few years will be able to observe even the faintest of these objects. Verifying the predictions I have made would provide critical support to the prevailing cosmological paradigm, while failing to find these tiny galaxies would call into question much of what we think we know about the origin of the Universe. Either way, my research will have a deep impact on the astrophysical community and on society as a whole, because who hasn’t looked up on a dark night, seen the stars, the Milky Way, and the endless emptiness of space and wanted to know from where it all came?
Chemical and Materials Physics / Physics
Co-authors: Peter Taborek (Professor, UCI)
As the technology supporting modern life becomes increasingly complex we have begun to encounter serious obstacles to improving this technology. These issues stem not only from a lack of knowledge but also from the physical limitations inherent in the underlying materials. Things can only be made so small, so thin, etc. before small scale physics effects, phenomena we never experience in our daily lives, become significant. In some cases these effects present a serious stumbling block. For instance microchip architecture is now so compact that quantum physics must be taken into account. While these physical phenomena can be problematic we can also harness them to both improve existing technologies and create entirely new ones. One very promising avenue is to simply engineer new materials with these effects in mind. Such revolutionary materials have been termed “metamaterials”.
Our research focuses on a phenomenon known as evanescent heat transfer. This is a unique way in which heat moves between objects which are separated by very small distances, such as those separating microchip transistors. Using theoretical studies as a framework we have manufactured a “metamaterial” consisting of alternating microscopic layers of glass and a common ceramic. This material is designed such that it should experience a drastic enhancement of evanescent heat transfer. This means that over small distances it should emit heat at a greater rate than any existing material. Using our unique experimental apparatus we are probing the flow of heat from this novel material. Better understanding its unique characteristics promises numerous potential applications from improving nanotechnology to devising new ways of transforming heat into electrical energy.