My research focus is inflationary cosmology. Cosmic inflation is the hypothesis that within about 10-32 second after the Big Bang there was a very brief but exponential period of expansion. After inflation, the universe evolved as in the standard Big Bang scenario. This hypothesis, first proposed by Alan Guth in 1979, answers a number of otherwise perplexing questions about the nature of the universe. In particular, it explains why the universe appears so flat, homogeneous, and isotropic; further, it provides an explanation of the origin of galactic structure and the absence of copious magnetic monopoles which are produced by many models that unify the strong, weak, and electromagnetic forces (GUTs).
My research has mostly focused on “hybrid” models in the context of supersymmetry (SUSY). Hybrid inflation models involve two fields, only one of which drives inflation, and supersymmetry posits the existence of a superpartner for each particle. I am particularly interested in whether one can build such models that are phenomenologically realistic. Below I briefly summarize my research.
Although SUSY hybrid inflation is a simple and well-motivated class of inflation models, it does have some flaws. In the standard scenario, inflation occurs before symmetry breaking, leading to a monopole—and, in general, a topological defect—problem. With colleagues, I have shown that one can inflate the universe properly via a “shifted” track in a natural way, such that symmetry breaking must occur before inflation. In this way, one can preserve the critical features of the model while avoiding the overproduction of topological defects.
One drawback of SUSY, on the other hand, is that it predicts rapid proton decay without the imposition of additional symmetries. This is a general SUSY problem and not specific to SUSY hybrid inflation. The standard solution to this problem is the inclusion of R-symmetry, which specifically prohibits terms leading to rapid proton decay. There is no theoretical reason why this symmetry must be exact, however, and my research has shown that Planck-suppressed R-symmetry breaking terms enhance the tensor-to-scalar ratio, producing a more testable model.
One of the motivations for SUSY is that it naturally unifies the three coupling constants at high energies. To build a more phenomenologically realistic model, one may constrain the SUSY breaking scale to be the gauge coupling unification scale in MSSM (the minimal supersymmetric standard model). My research has shown that one can inflate the universe within such a scenario, keeping to GUTs which can be broken to the standard model without producing fatal topological defects, providing an extremely well-motivated model.
Civiletti, M., Pallis, C., & Shaﬁ, Q. (2014). Upper bound on the tensor-to-scalar ratio in GUT scale supersymmetric hybrid inﬂation. Physics Letters B, 733, 276-282
Civiletti, M., Rehman, M.U., Sabo, E., Shaﬁ,Q., & Wickman, J.(2013). R-symmetry breaking in supersymmetric hybrid inﬂation. Physical Review D, 88(10), 103514
Civiletti, M., Rehman, M.U., Shaﬁ, Q., & Wickman, J.(2011). Red spectral tilt and observable gravity waves in shifted hybrid inﬂation. Physical Review D, 84(10), 103505