Author

# Masa-aki Hashimoto

Bio: Masa-aki Hashimoto is an academic researcher from Kyushu University. The author has contributed to research in topics: Universe & Big Bang nucleosynthesis. The author has an hindex of 1, co-authored 3 publications receiving 2 citations.

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01 Jan 2018TL;DR: In this paper, the Friedmann equation is expressed in a convenient form in terms of density parameters and the Robertson-Walker metric is derived based on the fact that our universe is homogeneous and isotropic at large scales.

Abstract: We mention the fundamentals in the theory of general relativity, i.e., the principles of equivalence and general covariance. Based on the facts that our universe is homogeneous and isotropic at large scales, we derive the Robertson-Walker metric and subsequently the Friedmann equation which governs the expansion of the universe. The equation is expressed in a convenient form in terms of density parameters. Next, we describe thermonuclear reaction rates utilized in astrophysics, which involve resonant and nonresonant reactions, photodisintegration, electron capture, and β-decay. We review the significance of standard Big Bang nucleosynthesis (SBBN) and summarize the current situation in the observed primordial abundance of light elements,4He, D, and 7Li. Comparing the calculated abundance of the elements with observed values, we determine a reasonable range for the baryon-to-photon ratio. Moreover, we examine the dependence of the produced amount of 4He on the measured lifetimes of neutrons. Finally, we consider the magnitude-redshift relation of type Ia supernovae (SNe Ia) as an independent probe to cosmological models.

2 citations

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01 Jan 2018TL;DR: In this paper, it was shown that neutrinos or antineutrinos are degenerate and neutrons shift less neutrons through β-equilibrium, and the plausible ranges of the degeneracy parameter and the baryon-to-photon ratio were derived.

Abstract: Many alternative models are proposed, though remarkable agreement is obtained on the primordial abundance of standard BBN. As an approach to nonstandard models, first we investigate a possibility that neutrinos or antineutrinos are degenerate. An excess density of neutrinos speeds up the expansion of the universe, leaving more neutrons at the onset of nucleosynthesis. In addition, degenerate electron-neutrinos shift less neutrons through β-equilibrium. Performing χ2 analysis for the calculated and observed abundances of 4He and D, we determine the plausible ranges of the degeneracy parameter and the baryon-to-photon ratio. Next, we explore BBN under the brane-world cosmology. The Friedmann-like equation is derived in the five-dimensional universe. It is found that more 4He is produced than in standard BBN because of rapid expansion due to the interaction energy on the brane. Finally, we examine thermal evolution in the early universe including a decaying cosmological term which is treated as a source of the gravitational field.

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01 Jan 2018TL;DR: In this paper, the Brans-Dicke theory was modified with a variable Λ term which is a function of the scalar field and the fundamental equations were derived for the gravitational and scalar fields from the variational principle of the action.

Abstract: Concerning the decrease in the cosmological term Λ from a large magnitude in a very early universe to a small value at the present epoch, we explore the Brans-Dicke theory modified with a variable Λ term which is a function of the scalar field. The fundamental equations are derived for the gravitational and scalar fields from the variational principle of the action. In the framework of the Robertson-Walker metric, we obtain the expression for the expansion rate of the universe like the Friedmann equation. Then we turn to summarize the observational constraints on the intrinsic parameters contained in this theory. We confirm that the evolution of the universe deviates significantly from the standard model in the early stage. Calculations of Big Bang nucleosynthesis (BBN) are performed with the use of the nuclear reaction network. Comparing the resultant amounts of the light elements,4He, D, and 7Li, with the observed primordial abundances and using also the magnitude-redshift relation of Type Ia supernovae (SNe Ia), we derive reasonable ranges of the parameters.

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01 Jan 1988

TL;DR: The first detection of non-luminous matter from its gravitational effects occurred in 1844, when Friedrich Wilhelm Bessel announced that several decades of positional measurements of Sirius and Procyon implied that each was in orbit with an invisible companion of mass comparable to its own.

Abstract: Ann. Rev. Astron. Astrophys. 1987. 25: 425-72 Copyright © 1987 by Annual Reviews Inc. All rights reserved EXISTENCE AND NATURE OF DARK MATTER IN THE UNIVERSE Virginia T rimble Astronomy Program, University of Maryland, College Park, Maryland 20742, and Department of Physics, University of California, Irvine, California 92717 1. HISTORICAL INTRODUCTION AND THE SCOPE OF THE PROBLEM The ﬁrst detection of nonluminous matter from its gravitational effects occurred in 1844, when Friedrich Wilhelm Bessel announced that several decades of positional measurements of Sirius and Procyon implied that each was in orbit with an invisible companion of mass comparable to its own. The companions ceased to be invisible in 1862, when Alvan G. Clark turned his newly-ground 18%” objective toward Sirius and resolved the 10q‘ of the photons from the system emitted by the white dwarf Sirius B. Studies of astrometric and single-line spectroscopic binaries are the modern descendants of Bessel’s work. A couple of generations later, data implying nonluminous matter on two very different scales surfaced almost simultaneously. First, Oort (498, 499) analyzed numbers and velocities of stars near the Sun and concluded that visible stars fell shy by 30-50% of adding up to the amount of gravitating matter implied by the velocities. Then, in 1933, Zwicky (777) concluded that the velocity dispersions in rich clusters of galaxies required 10 to 100 times more mass to keep them bound than could be accounted for by the luminous galaxies themselves. The former result was taken much more seriously than the latter by contemporary and succeeding astronomers (being digniﬁed by the name “the Oort limit”), which is perhaps more a statement about the personalities of Oort and Zwicky than about anything else. 425 0066-4146/87/0915—0425$02.00 © Annual Reviews Inc. - Provided by the NASA Astrophysics Data System

40 citations

04 Mar 2006

TL;DR: In this paper, the total cross section for radiative neutron capture on a proton, np{yields}d{gamma}, is evaluated at big-bang nucleosynthesis (BBN) energies.

Abstract: The total cross section for radiative neutron capture on a proton, np{yields}d{gamma}, is evaluated at big-bang nucleosynthesis (BBN) energies. The electromagnetic transition amplitudes are calculated up to next-to-leading-order within the framework of pionless effective field theory with dibaryon fields. We also calculate the d{gamma}{yields}np cross section and the photon analyzing power for the d{gamma}(vector sign){yields}np process from the amplitudes. The values of low-energy constants that appear in the amplitudes are estimated by a Markov Chain Monte Carlo analysis using the relevant low-energy experimental data. Our result agrees well with those of other theoretical calculations except for the np{yields}d{gamma} cross section at some energies estimated by an R-matrix analysis. We also study the uncertainties in our estimation of the np{yields}d{gamma} cross section at relevant BBN energies and find that the estimated cross section is reliable to within {approx}1% error.