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My research is in theoreticl cosmology using analytical and numerical calculations, which involve the use of available data from NASA and other international databases. I use computational resources such as high performance computing clusters, sophisticated numerical algorithms and interactive simulations all integrated with recent observational data sets to pursue answers to some of the most compelling and still-lingering questions about the universe. How old is the universe? When and how did it begin? Was there something before the Big Bang? What is the cause of the current accelerated expansion of the universe (also called the cosmic acceleration)? What size and shape is the universe? What is the ultimate fate of the universe? Most of my current work involves determining the cause of the cosmic acceleration. As the universe expands, other galaxies accelerate away from us. Physicists do not yet know the cause of this accelerated expansion. It could be due to some form of as of yet unknown dark energy, i.e. Cosmological Constant, requiring a negative pressure and negative equation of state. Another popular explanation for the cosmic acceleration comes from extensions to the theory of general relativity that take effect at cosmological scales of distances, e.g. modified gravity. An important part of this project involves being able to observationally distinguish between these different theories and perform tests to falsify them. I explored and falsified many modified gravity models in my dissertation, Dynamics and Phenomenology of Higher Order Gravity Cosmological Models, and we developed parameterizations to falsify general relativity at cosmological scales with the Integrated Software in Testing General Relativity (ISiTGR), which will prove very useful as more precise data becomes available over the next few years.
Specifically, I test cosmological models against experimental data sets. Constraints on parameter values in the models will either support or rule out the theory behind the model. It is my desire, not only to answer some of these questions, but also be able to allow non-cosmologists to understand the explanations and cosmological methodologies. So, I also focus some of my time on cosmology education research. I began constructing cosmological modeling simulations with the help of Easy Java Simulations (EJS) in collaboration with my colleagues at Francis Marion University (FMU) and most recently at the University of Dallas (UD). These simulations are packaged as CosmoEJS, an interactive cosmology simulation that allows users to fit cosmological models to actual data sets using EJS. These simulations combine real-time plotting and numerical fitting to illustrate how theoretical models can be compared with experimental data sets to test a cosmological model. The simulations contain genuine data sets from recent cosmological surveys with options for testing more than one model at a time for easy visual and numerical comparisons. The visual comparisons allow users to directly see why some models match the data and others do not, while the numerical comparisons are for refining the parameter values for a best-fit model. Several colleagues at the Fall 2014 Meeting of the Texas Section of the American Physical Society (TSAPS) remarked that these programs were exactly what they were looking for in the field of cosmology. More information on these simulations can be found in our articles, Modern Cosmology: Interactive Computer Simulations using Recent Observational Data Sets, (Am. Jour. Phys., Vol. 81, Issue 6, 2013) and Exploring the constraints on cosmological models using CosmoEJS, (JCAP11(2018)011). (Both projects were completed with undergraduate students.)
At the 2019 Texas Symposium on Relativistic Astrophysics in Portsmouth, England, I presented a 30 minute talk on this work on CosmoEJS. This work was recently highlighted with the 2019 Nobel Prize in Physics. In the announcement of the Nobel Prize in Physics by the American Journal of Physics (AJP), our original CosmoEJS article in AJP is given as one of three Top Articles in Cosmology, along with those of the Nobel Laureate for content related to the 2019 Nobel Prize in Physics.
This is an exceptional arena from which undergraduates new to cosmology can begin their study because in addition to learning about the subject matter, they also help to develop tools for their peers. I had undergraduate students during my two years at FMU and continue to work with undergraduate students at UD in the past years contribute to these projects by adding a new simulations for new cosmological observations, as well as new functionalities for these programs. Once they become familiar with the material, they are better prepared to understand the theoretical research, which involves not only knowing the observations, but also how the theoretical models are constructed. We also make use of the simulations in the classroom and education outreach, like astronomy club. I have my students present their program to a non-majors class for the opportunity to try out their tools on audiences less familiar with the work. This gives them a chance not only to demonstrate their understanding, but also improves their ability to communicate about the field with their peers.
Undergraduate students with the desire to explore cosmological questions learn High Performance Computing (HPC) skills to test cosmological models with observational data sets. These tests are performed using a modified version of the publicly available Monte Carlo Markov Chain code for cosmology, cosmoMC, (A. Lewis and S. Bridle, Phys. Rev. D66, 2002). This code is used along with certain packages, or modules like ISiTGR, (J. Dossett, M. Ishak and JM, Phys. Rev. D84, 2011) designed to test for consistency with general relativity using elegant parameterizations for modeling the dark energy at different times in the universes evolution. My students and I collaborate with a group at The University of Texas at Dallas, DARKLIGHT Group Osservatorio Astronomico di Brera and also use the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for running these tests, and we have our own cluster of computing nodes on the UD campus. In all cases, not only do the students learn how to use HPC to efficiently test dark energy models, but they also learned about HPC in general through usage and design of an instructive computing cluster built by the students from repurposed materials in the department. In order to bridge the gap between undergraduate education and research, undergraduates need experience in HPC. Beyond simply logging into a remote terminal, truly understanding HPC is particularly difficult for undergraduate students because the HPC cluster consisting of large machines locked-away in secure datacenters, offers few opportunities for students to grasp the totality of HPC systems. A hands-on experience is critical to filling in the gaps in their understanding.
In addition to my cosmology research, I have interdisciplinary projects with the UD Biology Department. We have successfully developed a novel physical model of the human head and brain for studying concussive and sub-concussive level collisions. There is also new work with Dr. Stenesen on developing methods to detect the electrical signal in the eye of a fruit fly and image structures using the Atomic Force Microscope and X-ray Computed Tomography (CT) scans, see the work on the student research webpage.
My research, which was initially funded by the Nancy Cain Marcus and Jeffrey A. Marcus Chair in Science, involves photometric studies, data analysis, and modeling of cataclysmic variable stars (CVs). Since then we have discovered and classified a new dwarf nova, observed exoplanetary transits, and discovered new binaries.
Cataclysmic variables are binary systems in which a white orbits with a normal star. The orbital periods of CVs typically range from approximately 0.05 day to 0.6 day. These binaries are quite small by astronomical sizes. The evolution of these binaries occurs through mass transfer from the secondary to the white dwarf. The luminosity of CVs is dominated by the accretion disk. Strong tidal forces from the white dwarf pull hydrogen gas from the surrounding Roche lobe of the secondary and larger star through the L1 Lagrange point. The gas spirals down around the white dwarf and usually forms an accretion disk. If the white dwarf has a large magnetic field, the materials may be directed right onto the surface of the star. As gas accumulates, forming a shell around the white dwarf, thermonuclear fusion reactions can be re-ignited on the white dwarf.
My current research into CVs focuses on improved ways of determining the mass ratio through observations and accretion disk modeling and is carried out with collaborators Dr. Michele Montgomery at the University of Central Florida and Dr. Irina Voloshina at the Sternberg Institute of Moscow State University. His research students have accompanied him to Moscow and to Crimea for observations with Dr. Voloshina of new cataclysmic variable stars as well as to Colorado to gather data to search for exoplanet transits.
I also search for exoplanets with students and my colleague, Arthur Sweeney in our STExTS project--Small Telescope Extrasolar Transit Search. Starting in 2011 we set up telescopes during the summers in Pitkin, Colorado (elevation 9210 ft) and later in the Upper Peninsular. For the past two years we have used our telescopes at the Monroe Robotic Observatory of the University of North Texas to gather data. Each summer we gather data on approximately 3500 - 5000 stars for 30 - 38 nights and then sift through the light curves looking for transits. We currently have seven candidates for new exoplanet transits.
During our transit searches we have discovered numerous new stars, many of which are now listed in the Variable Star Index of the American Association of Variable Star Observers (AAVSO) and which we are currently writing up for publications. These discoveries include RR Lyrae stars exhibiting the Blazhko effect and delta Scuti stars.
The focus of my research for the last several years has been the investigation of neutron scattering from materials that are important for fission reactor applications. This work is done in collaboration with scientists from the University of Dallas, the United States Naval Academy, the University of Kentucky, and Idaho National Laboratory. The work is supported primarily by grants from the Department of Energy NEUP Program. This research concentrates on measurements of neutron elastic and inelastic scattering differential cross sections from the structural materials 54,56Fe and on 23Na which is a coolant in future generation fast reactors. University of Dallas physics majors have been heavily involved in the measurements.
All measurements for this project have been completed at the University of Kentucky Low-Energy Accelerator Facilities. There pulsed bunches of protons are accelerated using the 7 MV Model CN Van de Graaff shown at the left. These proton pulses produce neutrons for scattering via the T(p,n) reaction.