No abstract

Gravitational waves (GWs) are one of the key signatures of cosmic strings. If GWs from cosmic strings are detected in future experiments, not only their existence can be confirmed but also their properties might be probed. In this paper, we study the determination of cosmic string parameters through direct detection of GW signatures in future ground-based GW experiments. We consider two types of GWs, bursts and the stochastic GW background, which provide us with different information about cosmic string properties. Performing the Fisher matrix calculation on the cosmic string parameters, such as parameters governing the string tension Gµ and initial loop size α and the reconnection probability p, we find that the two different types of GW can break degeneracies in some of these parameters and provide better constraints than those from each measurement. * skuro@icrr.u-tokyo.ac.jp Cosmic strings are one-dimensional topological defects which can be formed at a phase transition in the early universe [1] (as a review, see [2]). Strings in superstring theory which have cosmological length, so-called cosmic super strings, can also appear after the stringy model of inflation and behave like cosmic strings [3,4,5]. They form the highly complicated string network, including infinite strings, which stretch across Hubble horizon, and closed loops, whose size is much smaller than the Hubble scale, and leave cosmological effects in many ways through their nonlinear evolution. They therefore have attracted strong attention since the possibility of their existence was pointed out. Considerable research concerning their observational signals and their detectability have been done so far. If their signals can be observed precisely enough, not only the existence of cosmic strings will be confirmed but their properties also might be studied. Since we can probe physics beyond the standard model of particle physics such as grand unified theory or superstring theory through the study of cosmic strings, it is very interesting and meaningful to examine how the properties of cosmic strings can be constrained through future experiments and observations.Among important signals of cosmic strings are gravitational waves (GWs) [6,7,8,9,10,11,12,13,14,15,16,17]. The main source of GWs in the string network is cusps on loops. 1 A cusp is a highly Lorentz boosted region on a loop which appears O(1) times in an oscillation period of the loop. Beamed GW bursts are emitted from cusps and can be detected directly as strong but infrequent bursts as well as in the form of a stochastic GW background, which consists of many small bursts overlapping each other [10,11]. The rate of GW bursts and the spectrum of the GW background depend on the parameters which characterize the string network. Conversely, we can constrain the parameters from the fact that GWs from cosmic strings have never been detected, or even we can determine the values of them through future observations if GWs from cosmic strings are detected.So far, constraints on cosmic string paramet...

Gravitational waves emitted by kinks on infinite strings are investigated using detailed estimations of the kink distribution on infinite strings. We find that gravitational waves from kinks can be detected by future pulsar timing experiments such as SKA for an appropriate value of the the string tension, if the typical size of string loops is much smaller than the horizon at their formation.Moreover, the gravitational wave spectrum depends on the thermal history of the Universe and hence it can be used as a probe into the early evolution of the Universe.

No abstract

Recent cosmological observations, including measurements of the CMB anisotropy and the primordial helium abundance, indicate the existence of an extra radiation component in the Universe beyond the standard three neutrino species. In this paper we explore the possibility that the extra radiation has isocurvatrue fluctuations. A general formalism to evaluate isocurvature perturbations in the extra radiation is provided in the mixed inflaton-curvaton system, where the extra radiation is produced by the decay of both scalar fields. We also derive constraints on the abundance of the extra radiation and the amount of its isocurvature perturbation. Current observational data favors the existence of an extra radiation component, but does not indicate its having isocurvature perturbation. These constraints are applied to some particle physics motivated models. If future observations detect isocurvature perturbations in the extra radiation, it will give us a hint to the origin of the extra radiation.