Hiroki Yajima
Current Organizations / Institutes
The organizations/institutes from the scholar's latest article
Research Subjects
Research Interests
Hiroki Yajima has a total of 4 co-authors. Here are the top co-authors by number of shared publications:
- Shinji Tsujikawa
Department of Physics Waseda University Shinjuku-ku 169-8555 Tokyo Japan
3 shared publications - Hidetoshi Ohkubo
Engineering Department, Yahama Corporation, Shizuoka 438-0192, Japan
2 shared publications - Kei-Ichi Maeda
Department of Physics Waseda University Okubo 3-4-1 169-8555 Shinjuku Tokyo Japan
2 shared publications - Takashi Tamaki
Department of Physics Waseda University Okubo 3-4-1 169-8555 Tokyo Japan
2 shared publications
Latest Publications
- Open Access
Massive fermion production in nonsingular superstring cosmology
We study massive spin-1/2 fermion production in nonsingular superstring cosmology, taking into account one-loop quantum corrections to a superstring effective action with dilaton and modulus fields. While no creation occurs in the massless limit, massive fermions can be produced by the existence of a time-dependent frequency. Due to the increase of the Hubble expansion rate during the modulus-driven phase, the occupation of number of fermions continues to grow until the point of the graceful exit, after which fermion creation ceases with the decrease of the Hubble rate.
Open AccessMassive fermion production in nonsingular superstring cosmology
We study massive spin-1/2 fermion production in nonsingular superstring cosmology, taking into account one-loop quantum corrections to a superstring effective action with dilaton and modulus fields. While no creation occurs in the massless limit, massive fermions can be produced by the existence of a time-dependent frequency. Due to the increase of the Hubble expansion rate during the modulus-driven phase, the occupation of number of fermions continues to grow until the point of the graceful exit, after which fermion creation ceases with the decrease of the Hubble rate.
- 12 February 2001
- Published by American Physical Society (APS) in Physical Review D
- Vol. 63 (6) , 064007
Black hole solutions in Euler-Heisenberg theory
We construct static and spherically symmetric black hole solutions in the Einstein-Euler-Heisenberg (EEH) system which is considered as an effective action of a superstring theory. We consider electrically charged, magnetically charged, and dyon solutions. We can solve analytically for the magnetically charged case. We find that they have some remarkable properties about causality and black hole thermodynamics depending on the coupling constant of the EH theory a and b, though they have a central singularity as in the Schwarzschild black hole. We restrict because it is natural if we think of EH theory as a low-energy limit of the Born-Infeld (BI) theory. (i) For the magnetically charged case, whether or not the extreme solution exists depends on the critical parameter For there is an extreme solution as in the Reissner-Nortström (RN) solution. The main difference from the RN solution is that there appear solutions below the horizon radius of the extreme solution and they exist till Moreover, for there is no extreme solution. For arbitrary a, the temperature diverges in the limit. (ii) For the electrically charged case, the inner horizon appears under some critical mass and the extreme solution always exists. The lower limit of the horizon radius decreases when the coupling constant a increases. (iii) For the dyon case, we expect a variety of properties because of the term which is peculiar to the EH theory. But their properties are mainly decided by the combination of the parameters We show that solutions have similar properties to the magnetically charged case in the limit for For it depends on the parameters- 12 February 2001
- Published by American Physical Society (APS) in Physical Review D
- Vol. 63 (6) , 064007
Black hole solutions in Euler-Heisenberg theory
We construct static and spherically symmetric black hole solutions in the Einstein-Euler-Heisenberg (EEH) system which is considered as an effective action of a superstring theory. We consider electrically charged, magnetically charged, and dyon solutions. We can solve analytically for the magnetically charged case. We find that they have some remarkable properties about causality and black hole thermodynamics depending on the coupling constant of the EH theory a and b, though they have a central singularity as in the Schwarzschild black hole. We restrict because it is natural if we think of EH theory as a low-energy limit of the Born-Infeld (BI) theory. (i) For the magnetically charged case, whether or not the extreme solution exists depends on the critical parameter For there is an extreme solution as in the Reissner-Nortström (RN) solution. The main difference from the RN solution is that there appear solutions below the horizon radius of the extreme solution and they exist till Moreover, for there is no extreme solution. For arbitrary a, the temperature diverges in the limit. (ii) For the electrically charged case, the inner horizon appears under some critical mass and the extreme solution always exists. The lower limit of the horizon radius decreases when the coupling constant a increases. (iii) For the dyon case, we expect a variety of properties because of the term which is peculiar to the EH theory. But their properties are mainly decided by the combination of the parameters We show that solutions have similar properties to the magnetically charged case in the limit for For it depends on the parameters- 27 November 2000
- Published by American Physical Society (APS) in Physical Review D
- Vol. 62 (12) , 123512
New constraints on multifield inflation with nonminimal coupling
We study the dynamics and perturbations during inflation and reheating in a multifield model where a second scalar field is nonminimally coupled to the scalar curvature When is positive, the usual inflationary prediction for large-scale anisotropies is hardly altered while the fluctuation in sub-Hubble modes can be amplified during preheating for large For negative values of however, long-wave modes of the fluctuation exhibit exponential increase during inflation, leading to the strong enhancement of super-Hubble metric perturbations even when is less than unity. This is because the effective mass becomes negative during inflation. We constrain the strength of and the initial by the amplitude of produced density perturbations. One way to avoid nonadiabatic growth of super-Hubble curvature perturbations is to stabilize the mass through a coupling to the inflaton. Preheating may thus be necessary in these models to protect the stability of the inflationary phase.- 27 November 2000
- Published by American Physical Society (APS) in Physical Review D
- Vol. 62 (12) , 123512
New constraints on multifield inflation with nonminimal coupling
We study the dynamics and perturbations during inflation and reheating in a multifield model where a second scalar field is nonminimally coupled to the scalar curvature When is positive, the usual inflationary prediction for large-scale anisotropies is hardly altered while the fluctuation in sub-Hubble modes can be amplified during preheating for large For negative values of however, long-wave modes of the fluctuation exhibit exponential increase during inflation, leading to the strong enhancement of super-Hubble metric perturbations even when is less than unity. This is because the effective mass becomes negative during inflation. We constrain the strength of and the initial by the amplitude of produced density perturbations. One way to avoid nonadiabatic growth of super-Hubble curvature perturbations is to stabilize the mass through a coupling to the inflaton. Preheating may thus be necessary in these models to protect the stability of the inflationary phase.- Open Access
New constraints on multi-field inflation with nonminimal coupling
We study the dynamics and perturbations during inflation and reheating in a multi-field model where a second scalar field $\chi$ is nonminimally coupled to the scalar curvature $(\frac12 \xi R\chi^2$). When $\xi$ is positive, the usual inflationary prediction for large-scale anisotropies is hardly altered while the $\chi$ fluctuation in sub-Hubble modes can be amplified during preheating for large $\xi$. For negative values of $\xi$, however, long-wave modes of the $\chi$ fluctuation exhibit exponential increase during inflation, leading to the strong enhancement of super-Hubble metric perturbations even when $|\xi|$ is less than unity. This is because the effective $\chi$ mass becomes negative during inflation. We constrain the strength of $\xi$ and the initial $\chi$ by the amplitude of produced density perturbations. One way to avoid nonadiabatic growth of super-Hubble curvature perturbations is to stabilize the $\chi$ mass through a coupling to the inflaton. Preheating may thus be necessary in these models to protect the stability of the inflationary phase.Open AccessNew constraints on multi-field inflation with nonminimal coupling
We study the dynamics and perturbations during inflation and reheating in a multi-field model where a second scalar field $\chi$ is nonminimally coupled to the scalar curvature $(\frac12 \xi R\chi^2$). When $\xi$ is positive, the usual inflationary prediction for large-scale anisotropies is hardly altered while the $\chi$ fluctuation in sub-Hubble modes can be amplified during preheating for large $\xi$. For negative values of $\xi$, however, long-wave modes of the $\chi$ fluctuation exhibit exponential increase during inflation, leading to the strong enhancement of super-Hubble metric perturbations even when $|\xi|$ is less than unity. This is because the effective $\chi$ mass becomes negative during inflation. We constrain the strength of $\xi$ and the initial $\chi$ by the amplitude of produced density perturbations. One way to avoid nonadiabatic growth of super-Hubble curvature perturbations is to stabilize the $\chi$ mass through a coupling to the inflaton. Preheating may thus be necessary in these models to protect the stability of the inflationary phase. - 26 June 2000
- Published by American Physical Society (APS) in Physical Review D
- Vol. 62 (2) , 024020
Generality of singularity avoidance in superstring theory: Anisotropic case
In the one-loop string effective action, we study a generality of nonsingular cosmological solutions found in the isotropic and homogeneous case. We discuss Bianchi type-I and -IX spacetimes. We find that nonsingular solutions still exist in the Bianchi type-I model around nonsingular flat Friedmann solutions. On the other hand, we cannot find any nonsingular solutions in the Bianchi type-IX model. The nonexistence of a nonsingular Bianchi type-IX universe may be consistent with the analysis of Kawai, Sakagami, and Soda; i.e., the tensor-mode perturbations against a nonsingular flat Friedmann universe are unstable, because the Bianchi type-IX model is regarded as a closed Friedmann universe with a single gravitational wave. With the stability analysis of Kawai, Sakagami, and Soda, the nonsingular universe found in the isotropic case is unstable, and a singularity avoidance may not work in generic spacetimes.
- 26 June 2000
- Published by American Physical Society (APS) in Physical Review D
- Vol. 62 (2) , 024020
Generality of singularity avoidance in superstring theory: Anisotropic case
In the one-loop string effective action, we study a generality of nonsingular cosmological solutions found in the isotropic and homogeneous case. We discuss Bianchi type-I and -IX spacetimes. We find that nonsingular solutions still exist in the Bianchi type-I model around nonsingular flat Friedmann solutions. On the other hand, we cannot find any nonsingular solutions in the Bianchi type-IX model. The nonexistence of a nonsingular Bianchi type-IX universe may be consistent with the analysis of Kawai, Sakagami, and Soda; i.e., the tensor-mode perturbations against a nonsingular flat Friedmann universe are unstable, because the Bianchi type-IX model is regarded as a closed Friedmann universe with a single gravitational wave. With the stability analysis of Kawai, Sakagami, and Soda, the nonsingular universe found in the isotropic case is unstable, and a singularity avoidance may not work in generic spacetimes.
- Open Access
Black hole solutions in Euler-Heisenberg theory
We construct static and spherically symmetric black hole solutions in the Einstein-Euler-Heisenberg (EEH) system which is considered as an effective action of a superstring theory. We considered electrically charged, magnetically charged and dyon solutions. We can solve analytically for the magnetically charged case. We find that they have some remarkable properties about causality and black hole thermodynamics depending on the coupling constant of the EH theory $a$ and $b$, though they have central singularity as in the Schwarzschild black hole.Open AccessBlack hole solutions in Euler-Heisenberg theory
We construct static and spherically symmetric black hole solutions in the Einstein-Euler-Heisenberg (EEH) system which is considered as an effective action of a superstring theory. We considered electrically charged, magnetically charged and dyon solutions. We can solve analytically for the magnetically charged case. We find that they have some remarkable properties about causality and black hole thermodynamics depending on the coupling constant of the EH theory $a$ and $b$, though they have central singularity as in the Schwarzschild black hole. - Open Access
Generality of Singularity Avoidance in Superstring Theory: Anisotropic Case
In one-loop string effective action, we study a generality of non-singular cosmological solutions found in the isotropic and homogeneous case. We discuss Bianchi I and IX type spacetimes. We find that nonsingular solutions still exist in Bianchi I model around nonsingular flat Friedmann solutions. On the other hand, we cannot find any nonsingular solutions in Bianchi IX model. Non-existence of nonsingular Bianchi IX universe may be consistent with the analysis by Kawai, Sakagami and Soda, i.e. the tensor mode perturbations against nonsingular flat Friedmann universe are unstable, because Bianchi IX model can be regarded as a closed Friedmann universe with a single gravitational wave. So based on these facts, we may conclude the nonsingular universe is found in isotropic case is generally unstable, a singularity avoidance may not work in the present model.
Open AccessGenerality of Singularity Avoidance in Superstring Theory: Anisotropic Case
In one-loop string effective action, we study a generality of non-singular cosmological solutions found in the isotropic and homogeneous case. We discuss Bianchi I and IX type spacetimes. We find that nonsingular solutions still exist in Bianchi I model around nonsingular flat Friedmann solutions. On the other hand, we cannot find any nonsingular solutions in Bianchi IX model. Non-existence of nonsingular Bianchi IX universe may be consistent with the analysis by Kawai, Sakagami and Soda, i.e. the tensor mode perturbations against nonsingular flat Friedmann universe are unstable, because Bianchi IX model can be regarded as a closed Friedmann universe with a single gravitational wave. So based on these facts, we may conclude the nonsingular universe is found in isotropic case is generally unstable, a singularity avoidance may not work in the present model.