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Friday, April 20th 2018

12:30 pm:

We propose a grand unified SU(5) x U(1)_X model, where the standard SU(5) grand unified theory is supplemented by minimal seesaw and a right-handed neutrino dark matter with an introduction of a global Z_2-symmetry. In the presence of three right-handed neutrinos (RHNs), the model is free from all gauge and mixed-gravitational anomalies. The SU(5) symmetry is broken into the Standard Model (SM) gauge group at M_GUT = 4 x 10^16 GeV in the standard manner, while the U(1)_X symmetry breaking occurs at the TeV scale, which generates the TeV-scale mass of the U(1)_X gauge boson (Z′ boson) and the three Majorana RHNs. A unique Z_2-odd RHN is stable and serves as the dark matter (DM) in the present Universe, while the remaining two RHNs work to generate the SM neutrino masses through the minimal seesaw. We investigate the Z′-portal RHN DM scenario in this model context, and find that the constraints from the DM relic abundance and the search results for a Z′ boson resonance at the Large Hadron Collider (LHC) are complementary to narrow down the allowed parameter region, which will be fully covered by the future LHC experiments (for the Z′ boson mass < 5 TeV). We also briefly discuss the successful implementation of baryogenesis and cosmological inflation scenarios in the present model.

Friday, April 27th 2018

12:30 pm:

How did inflation end? For a broad class of "simple", observationally consistent inflationary models, I will present results regarding: (1) Nonlinear fragmentation and soliton formation in the inflaton field, (2) the equation of state of the universe after inflation. For sufficiently complex models, I will highlight some universal results from stochastic particle production during inflation and reheating, and discuss their implications for initial curvature perturbations from the early universe.

Friday, May 4th 2018

12:30 pm:

Cosmic strings are generic cosmological predictions of many extensions

of the Standard Model of particle physics, such as a *U*(1)^{′}
symmetry breaking phase transition in the early universe or remnants of superstring theory. Unlike other topological defects, cosmic strings can reach a scaling regime that maintains a small fixed fraction of the total energy density of the universe from a very early epoch until today. If present, they will oscillate and generate gravitational waves with a frequency spectrum that imprints the dominant sources of total cosmic energy density throughout the history of the universe. In this talk I will demonstrate that current and future gravitational wave detectors, such as LIGO and LISA, could be capable of measuring the frequency spectrum of gravitational waves from cosmic strings and discerning the energy composition of the universe at times well before primordial nucleosynthesis and the cosmic microwave background where standard cosmology has yet to be tested. This work establishes a benchmark case that gravitational waves may provide an unprecedented, powerful tool for probing the evolutionary history of the very early universe.

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