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Riou Nakamura

Other affiliations: Kyushu University
Bio: Riou Nakamura is an academic researcher from Kurume Institute of Technology. The author has contributed to research in topics: Universe & Big Bang nucleosynthesis. The author has an hindex of 3, co-authored 8 publications receiving 33 citations. Previous affiliations of Riou Nakamura include Kyushu University.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors review the recent progress in the Big-Bang nucleosynthesis which includes the standard and non-standard theory of cosmology, effects of neutrino degeneracy, and inhomogeneous nucleosynthetic within the framework of a Friedmann model.
Abstract: We review the recent progress in the Big-Bang nucleosynthesis which includes the standard and non-standard theory of cosmology, effects of neutrino degeneracy, and inhomogeneous nucleosynthesis within the framework of a Friedmann model. As for a non-standard theory of gravitation, we adopt a Brans-Dicke theory which incorporate a cosmological constant. We constrain various parameters associated with each subject.

16 citations

Posted Content
TL;DR: In this article, the authors investigated the observational constraints on the inhomogeneous big-bang nucleosynthesis that Matsuura et al. suggested the possibility of heavy element production beyond Ni in the early universe.
Abstract: We investigate the observational constraints on the inhomogeneous big-bang nucleosynthesis that Matsuura et al. suggested the possibility of the heavy element production beyond ${}^7$Li in the early universe. From the observational constraints on light elements of ${}^4$He and D, possible regions are found on the plane of the volume fraction of the high density region against the ratio between high-and low-density regions. In these allowed regions, we have confirmed that the heavy elements beyond Ni can be produced appreciably, where $p$- and/or $r$-process elements are produced well simultaneously.

7 citations

Posted Content
TL;DR: Based on a scenario of the inhomogeneous big-bang nucleosynthesis (IBBN), the authors investigated the detailed nucleosynthetic that includes the production of heavy elements beyond 7 Li, where p-and r-process elements are produced well simultaneously compared to the solar system abundances.
Abstract: Based on a scenario of the inhomogeneous big-bang nucleosynthesis (IBBN), we investigate the detailed nucleosynthesis that includes the production of heavy elements beyond 7 Li. From the observational constraints on light elements of 4 He and D for the baryon-to-photon ratio given by WMAP, possible regions found on the plane of the volume fraction of the high density region against the ratio between high- and low-density regions. In these allowed regions, we have confirmed that the heavy elements beyond Fe can be produced appreciably, where p- and/or r-process elements are produced well simultaneously compared to the solar system abundances. We suggest that recent observational signals such as 4 He overabundance in globular clusters and high metallicity abundances in quasars could be partly due to the results of IBBN. Possible implications are given for the formation of the first generation stars.

7 citations

Book ChapterDOI
01 Jan 2018
TL;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

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the cosmic thermal evolution with a vacuum energy which decays into photon at the low redshift and found that the effects of a decaying vacuum energy on the cosmic expansion rate should be very small but could be possible for z < 1.5.
Abstract: We investigate the cosmic thermal evolution with a vacuum energy which decays into photon at the low redshift. We assume that the vacuum energy is a function of the scale factor that increases toward the early universe. We put on the constraints using recent observations of both type Ia supernovae (SNIa) by Union-2 compilation and the cosmic microwave background (CMB) temperature at the range of the redshift 0.01

1 citations


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01 Jan 1988
TL;DR: In this article, a new mechanism for baryogenesis was proposed, where it was argued that the scalar quarks and leptons of a supersymmetric GUT may have large expectation values.
Abstract: A new mechanism for baryogenesis is proposed. It is argued that, after inflation, the scalar quarks and leptons of a supersymmetric GUT may have large expectation values. The subsequent evolution of such a system is shown to generate a significant baryon density. For typical values of the parameters, n B / n γ may be as large as 10 3 .

757 citations

01 Jan 1988
TL;DR: In this paper, the observational consequences of a vacuum energy which decays in time were examined and it was shown that in both radiation and matter dominated eras, the ratio of the vacuum energy to the total energy density of the universe must be small.
Abstract: Motivated by recent attempts to solve the cosmological constant problem, we examine the observational consequences of a vacuum energy which decays in time. In both radiation and matter dominated eras, the ratio of the vacuum to the total energy density of the universe must be small. Although the vacuum cannot provide the “missing mass” required to close the universe today, its presence earlier in the history of the universe could have important consequences. Element abundances from primordial nucleosynthesis require the ratio x = ϱ vac /( ϱ vac + ϱ rad ) ⩽ 0.1 of neutrino (or equivalent light) species to exceed N ν > 4, a case ruled out in the standard cosmological model. If the vacuum decays into low energy photons, the lack of observed spectral distortions in the microwave background gives tighter bounds, x −4 . In the matter-dominated era, the presence of a vacuum term may allow more time for growth of protogalactic perturbations.

270 citations

Journal ArticleDOI
TL;DR: In this paper, a review on massive primordial black holes in the early universe is presented in Russian to Uspekhi Fizicheskikh Nauk (Physics-Uspekhi).
Abstract: This is the translation into English of the introduction, conclusion, and the list of references of the review on massive primordial black holes, which is submitted in Russian to Uspekhi Fizicheskikh Nauk (Physics-Uspekhi). If accepted, this review is translated into English by the Journal and published in Russian and a little later in English. The review concerns the recent astronomical data which show that massive primordial black holes play much more significant role in the universe than it was previously believed. This is true both for the the contemporary and the early universe at the red-shifts about 10. The mechanism, proposed in 1993, of primordial creation of heavy and superheavy black holes in the very early universe is discussed. This mechanism predicts the log-normal mass spectrum of the primordial black holes, which became very popular during the last couple of years. The proposed mechanism presents a natural explanation of a large amount of the recent observational data, which do not fit the standard cosmology and astrophysics.

47 citations

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