The brief period in the early universe from the end of inflation until the development of a thermalized plasma is one of the least understood eras in cosmology. During that era, which is known as "reheating," the particles, fields, and effective low-energy interactions we see today all developed, but the high-energy physics required to describe that time is not known and modeling reheating requires large-scale numerical computations. The PI has coauthored LATTICEEASY, the most widely used program for simulating the reheating era. Most simulations to date, however, have been limited to unrealistically simple models of interacting scalar fields. The PI intends to extend the capabilities of his code to run on clusters of parallel processors and to include gauge fields in its calculations. Using those extended capabilities he will simulate reheating in complex models of high-energy physics and extract testable predictions. An understanding of preheating is important because it introduces extra "handles" (gravitational waves, topological defects) that one might use to constrain the physics involved in inflation. Other observables related to inflationary physics such as the CMB provide constraints on inflation models, but by their nature don't tell us anything about how the inflaton couples to other fields. Information about the couplings will be crucial if we are to move beyond phenomenological models of inflation and really start talking about embedding the inflaton in a model capable of making contact with other areas of particle physics. The PI intends to develop a program for training undergraduate students in numerical computation. This will include involving undergraduate research assistants in writing and running numerical simulations, creating computational units for two of the required courses in the Smith physics major and developing a course on computation built around examples drawn from the PI's cosmological research.
Cosmologists today believe that the early universe underwent a period of rapid expansion known as "inflation." This period lasted only a fraction of a second, but it left the universe extremely large, uniform, and completely devoid of matter. All of the energy in the universe at that time was in the form of a type of radiation known as a "scalar field." Over time that energy had to decay in order to produce the matter we see around us today. That process, known as "reheating," is still poorly understood. In this project I used computer simulations to test different models of inflation and reheating. Because the density and temperature at the time was much higher than we can reproduce in a lab, we cannot directly test what types of fields, particles, and interactions may have been present at the time, so we test different models by simulating them and seeing whether they would have produced a late-time universe like the one we observe. For those models that are compatible with our current observations, we then try to make predictions of what new observations we might make to test the models. The most important part of the work in this grant concerned one type of effect from the early universe that could help us to distinguish between competing models of high energy physics. At extremely high densities and temperatures object produce a type of radiation known as gravity waves. If such waves had been produced in the early universe they would still be around today, almost completely unchanged. In the next couple of decades several observatories are planned that could detect gravity waves. In this project my student assistants and I simulated a number of common models of inflation and reheating to see what types of gravity waves would have been produced by each one. We made detailed predictions about which models would be expected to have produced observable gravity waves, and what they would look like for each one. Those predictions may be testable in the next 10-20 years and could lead to a much better understanding of both the early universe and more generally of high energy physics. This work was all carried out at Smith College with undergraduate women. In addition to learning cosmology, these students learned computer skills, math skills, and other critical research skills such as data analysis and clear presentation of results.
