New probes of the physics of inflation have emerged in recent years. This proposal focuses on two topics: one is non-Gaussianity of primordial fluctuations and the other includes reheating and preheating after inflation. If inflation was accompanied by Gaussian fluctuations, this means that only correlations between two (density) fields are important. Non-Gaussianity is measured by there being non zero correlations between more than two fields. The PI has developed and is continuing to develop new quantitative tests of inflation with non-Gaussianity. Specifically, the PI along with his students proposes to study the following: 1. Higher correlation functions such as the Trispectrum (four-point function) which directly measures scattering effects and how they effect primordial non-Gaussianity; 2. Including non-linear effects such as scattering in the Boltzmann equation which describes the evolution of the fluctuations. This calculation will allow one to compare theoretical predictions and future CMB data from the Planck satellite. 3. The PI will develop the tools and formalism necessary to calculate gravitino production due to the non-linear scattering of inhomogeneous,time-dependent scalar fields. The project integrates research and education naturally by providing graduate students with opportunities to do cutting-edge research The PI intends to involve undergraduate students in research as well as outreach projects. The PI intends to use simulation data as an attractive and efficient tool for giving undergraduate students research experiences and outreach projects. As a part of undergraduate student?s outreach activities, he intends to create movies of the ?Big Bang at Work? using their simulations and make the movies publicly available via the World Wide Web.
What powered the Big Bang? What happened before the Big Bang? Thanks to the advance in technology and theoretical understanding of physics, we are now able to seek answers to these profound questions in cosmology using observations. We now think that the Big Bang was not necessarily the beginning of the universe, but rather the epoch at which the universe became hot. Then, what happened before then? Observations indicate that, shortly after the unverse was born, there was a period called "cosmic inflation," during which the universe expanded by a large factor - so large that the size of a subatomic particle would become the size of a solar system in a tiny fraction of a second. While this is a rather unusual situation, observational support for the existence of such a period has been accumulating. However, the existence of such a period has not been completely proven. Using the award from NSF, we have set out to answer the following question: "can we falsify inflation?" While the key idea of inflation, i.e., an exponential expansion of the early universe, is a generic feature of inflation models, what physics drove such an expansion remains a mystery. In fact, over the last three decades, it has become clear that vastly different physical models can achieve the exponential expansion, and thus it has become increasingly difficult to identify the physics that drove inflation. This also means that inflation has become somewhat difficult to falsify, as there are so many models that they seem to be able to explain anything. In order to answer this question, we have focused our attention on specific statistical properties of fluctuations generated during inflation. One of the major successes of inflation is that it can explain the origin of structures in the universe: galaxies, planets, and us. According to inflation, the universe became very big in a tiny fraction of a second. This implies that the universe was extremely small at the onset of inflation - so small that one has to take into account the physics that governs a small world: quantum physics. As a result, during inflation quantum fluctuations are constantly generated, and they become the seed for structures that we see today. As different models of inflation make specific, testable predictions for the statistical properties of quantum fluctuations, we can use the statistical proeprties of the observed structures - such as fluctuations in the cosmic microwave background and in the large-scale distribution of matter - in order to study the physics of inflation. The specific properties that we have focused on are called "non-Gaussianity." Most of inflation models predict that fluctuations obey Gaussian statistics. Specifically, we have focused on the 3-point correlation function of fluctuations, which would vanish for Gaussian fluctuations. As connecting 3-points forms a triangle, one can measure 3-point correlations with a variety of triangular shapes. Among various shapes, the so-called "squeezed shape" is the most important one, as this form cannot be produced by any inflation models which are driven by one energy component. We have investigated to what extent this can be used to rule out inflation models, and found that this is indeed a powerful way to rule out single-component inflation. Moreover, we have found that a slightly different version of the squeezed shape is sensitive to the initial state of quantum fluctuations. This allows us to look into the details of physics of inflation: namely, whether the initial state was a vacuum or it was already occupied by particles. Finally, we have investigated whether it is possible to falsify inflation regardless of the number of energy components during inflation. We have found that most inflation models should yield an inequality between the 3-point and 4-point functions with specific shapes regardless of the number of energy components; thus, detection of a violation of this inequality would challenge the inflationary paradigm as a mechanism for generating observed structures in the universe. All of these can be tested using the observational data which become available soon, including the cosmic microwave background data from ESA's Planck satellite.
