Engineering topics

Nitta, Daisuke

Publications and source records attributed to Nitta, Daisuke.

4 records

DOE OSTI2016

Improved calculation of the gravitational wave spectrum from kinks on infinite cosmic strings

Gravitational wave observations provide unique opportunities to search for cosmic strings. One of the strongest sources of gravitational waves is discontinuities of cosmic strings, called kinks, which are generated at points of intersection. Kinks on infinite strings are known to generate a gravitational wave background over a wide range of frequencies. In this paper, we calculate the spectrum of the gravitational wave background by numerically solving the evolution equation for the distribution function of the kink sharpness. We find that the number of kinks for small sharpness is larger than the analytical estimate used in a previous work, which makes a difference in the spectral shape. Our numerical approach enables us to make a more precise prediction on the spectral amplitude for future gravitational wave experiments.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Modular invariant inflation

Modular invariance is a striking symmetry in string theory, which may keep stringy corrections under control. In this paper, we investigate a phenomenological consequence of the modular invariance, assuming that this symmetry is preserved as well as in a four dimensional (4D) low energy effective field theory. As a concrete setup, we consider a modulus field T whose contribution in the 4D effective field theory remains invariant under the modular transformation and study inflation drived by T. The modular invariance restricts a possible form of the scalar potenntial. As a result, large field models of inflation are hardly realized. Meanwhile, a small field model of inflation can be still accomodated in this restricted setup. The scalar potential traced during the slow-roll inflation mimics the hilltop potential V{sub ht}, but it also has a non-negligible deviation from V{sub ht}. Detecting the primordial gravitational waves predicted in this model is rather challenging. Yet, we argue that it may be still possible to falsify this model by combining the information in the reheating process which can be determined self-completely in this setup.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2012

Optimal limits on primordial magnetic fields from CMB temperature bispectrum of passive modes

We investigate bounds on the strength of the primordial magnetic field (PMF) from the cosmic microwave background (CMB) bispectra of the intensity (temperature) modes induced from the auto- and cross-correlated bispectra of the scalar and tensor components of the PMF anisotropic stress. At first, we construct a general formula for the CMB intensity and polarization bispectra from PMFs composed of any type of perturbation. Then we derive an approximate expression which traces the exact shape of the CMB bispectrum in order to reduce the computation time with respect to a large number of the multipole configurations, and also show that the non-Gaussian structure coming from PMFs is classified as the local-type configuration. Computing the signal-to-noise ratio on the basis of the approximate formula with the information of the instrumental noises and resolutions, we find expected upper bounds on the magnetic field strength, when the magnetic spectrum is nearly scale invariant (n{sub B} = −2.9), smoothed on 1Mpc scale at 95% confidence level from the WMAP and PLANCK experiments as B{sub 1Mpc} < 4.0−6.7nG and 3.8−6.5nG, respectively, depending on the energy scale of the magnetic field production from 10{sup 14}GeV to 10{sup 3}GeV. Our new consequences imply slight overestimations by the previous rough discussions.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

Formulation and constraints on decaying dark matter with finite mass daughter particles

Decaying dark matter cosmological models have been proposed to remedy the overproduction problem at small scales in the standard cold dark matter paradigm. We consider a decaying dark matter model in which one CDM mother particle decays into two daughter particles, with arbitrary masses. A complete set of Boltzmann equations of dark matter particles is derived which is necessary to calculate the evolutions of their energy densities and their density perturbations. By comparing the expansion history of the universe in this model and the free-streaming scale of daughter particles with astronomical observational data, we give constraints on the lifetime of the mother particle, Γ{sup −1}, and the mass ratio between the daughter and the mother particles m{sub D}/m{sub M}. From the distance to the last scattering surface of the cosmic microwave background, we obtain Γ{sup −1} > 30 Gyr in the massless limit of daughter particles and, on the other hand, we obtain m{sub D} > 0.97m{sub M} in the limit Γ{sup −1} → 0. The free-streaming constraint tightens the bound on the mass ratio as (Γ{sup −1}/10{sup −2}Gyr)∼<((1−m{sub D1}/m{sub M})/10{sup −2}){sup −3/2} for Γ{sup −1} < H{sup −1}(z = 3)

79 ASTRONOMY AND ASTROPHYSICS↗