Exploring cosmic strings: Observable effects and cosmological constraints

Observation of cosmic (super)strings can serve as a useful hint to understand the fundamental theories of physics, such as grand unified theories (GUTs) and/or superstring theory. In this regard, I present new mechanisms to produce particles from cosmic (super)strings, and discuss their cosmological and observational effects in this dissertation.;The first chapter is devoted to a review of the standard cosmology, cosmic (super)strings and cosmic rays.;The second chapter discusses the cosmological effects of moduli. Moduli are relatively light, weakly coupled scalar fields, predicted in supersymmetric particle theories including string theory. They can be emitted from cosmic (super)string loops in the early universe. Abundance of such moduli is constrained by diffuse gamma ray background, dark matter, and primordial element abundances. These constraints put an upper bound on the string tension as strong as Gmu ≲ 10-28 for a wide range of modulus mass m. If the modulus coupling constant is stronger than gravitational strength, modulus radiation can be the dominant energy loss mechanism for the loops. Furthermore, modulus lifetimes become shorter for stronger coupling. Hence, the constraints on string tension Gmu and modulus mass m are significantly relaxed for strongly coupled moduli predicted in superstring theory. Thermal production of these particles and their possible effects are also considered.;In the third chapter, moduli emitted from cosmic string cusps are studied. Highly boosted modulus bursts emanating from cusps subsequently decay into gluons and generate hadronic cascades which in turn produce large number of neutrinos. For reasonable values of the modulus mass and coupling constant, observable ultra high energy neutrino fluxes can be produced for a wide range of string tension Gmu.;The fourth chapter discusses cosmic rays produced by the charged particles ejected from cusps of superconducting cosmic strings. In many particle physics theories, cosmic strings respond to external magnetic fields, e.g., in clusters of galaxies, and develop currents. Observable UHE neutrino fluxes can be achieved for a range of symmetry breaking scale of strings. In this model, neutrinos with E ≳ 1011 GeV are expected in correlation with clusters of galaxies. Another unique signature of the model is simultaneous appearance of several neutrino-produced showers in the field of view of very large detectors, such as JEM-EUSO. The flux of UHE protons from cusps may account for a large fraction of the observed events at the highest energies.;The dissertation ends with overall conclusions.