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Bamba, Kazuharu

Publications and source records attributed to Bamba, Kazuharu.

8 records

All sources NASA NTRS DOE OSTI

DOE OSTI2015

Bounce universe from string-inspired Gauss-Bonnet gravity

We explore cosmology with a bounce in Gauss-Bonnet gravity where the Gauss-Bonnet invariant couples to a dynamical scalar field. In particular, the potential and and Gauss-Bonnet coupling function of the scalar field are reconstructed so that the cosmological bounce can be realized in the case that the scale factor has hyperbolic and exponential forms. Furthermore, we examine the relation between the bounce in the string (Jordan) and Einstein frames by using the conformal transformation between these conformal frames. It is shown that in general, the property of the bounce point in the string frame changes after the frame is moved to the Einstein frame. Moreover, it is found that at the point in the Einstein frame corresponding to the point of the cosmological bounce in the string frame, the second derivative of the scale factor has an extreme value. In addition, it is demonstrated that at the time of the cosmological bounce in the Einstein frame, there is the Gauss-Bonnet coupling function of the scalar field, although it does not exist in the string frame.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Bounce cosmology from F(R) gravity and F(R) bigravity

We reconstruct F(R) gravity models with exponential and power-law forms of the scale factor in which bounce cosmology can be realized. We explore the stability of the reconstructed models with analyzing the perturbations from the background solutions. Furthermore, we study an F(R) gravity model with a sum of exponentials form of the scale factor, where the bounce in the early universe as well as the late-time cosmic acceleration can be realized in a unified manner. As a result, we build a second order polynomial type model in terms of R and show that it could be stable. Moreover, when the scale factor is expressed by an exponential form, we derive F(R) gravity models of a polynomial type in case of the non-zero spatial curvature and that of a generic type in that of the zero spatial curvature. In addition, for an exponential form of the scale factor, an F(R) bigravity model realizing the bouncing behavior is reconstructed. It is found that in both the physical and reference metrics the bouncing phenomenon can occur, although in general the contraction and expansion rates are different each other.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Future singularities and teleparallelism in loop quantum cosmology

We demonstrate how holonomy corrections in loop quantum cosmology (LQC) prevent the Big Rip singularity by introducing a quadratic modification in terms of the energy density ρ in the Friedmann equation in the Friedmann-Lemaître-Robertson-Walker (FLRW) space-time in a consistent and useful way. In addition, we investigate whether other kind of singularities like Type II,III and IV singularities survive or are avoided in LQC when the universe is filled by a barotropic fluid with the state equation P = −ρ−f(ρ), where P is the pressure and f(ρ) a function of ρ. It is shown that the Little Rip cosmology does not happen in LQC. Nevertheless, the occurrence of the Pseudo-Rip cosmology, in which the phantom universe approaches the de Sitter one asymptotically, is established, and the corresponding example is presented. It is interesting that the disintegration of bound structures in the Pseudo-Rip cosmology in LQC always takes more time than that in Einstein cosmology. Our investigation on future singularities is generalized to that in modified teleparallel gravity, where LQC and Brane Cosmology in the Randall-Sundrum scenario are the best examples. It is remarkable that F(T) gravity may lead to all the kinds of future singularities including Little Rip.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Generation of large-scale magnetic fields from inflation in teleparallelism

We explore the generation of large-scale magnetic fields from inflation in teleparallelism, in which the gravitational theory is described by the torsion scalar instead of the scalar curvature in general relativity. In particular, we examine the case that the conformal invariance of the electromagnetic field during inflation is broken by a non-minimal gravitational coupling between the torsion scalar and the electromagnetic field. It is shown that for a power-law type coupling, the magnetic field on 1 Mpc scale with its strength of ∼ 10{sup −9} G at the present time can be generated.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

Thermodynamics of cosmological horizons in f(T) gravity

We explore thermodynamics of the apparent horizon in f(T) gravity with both equilibrium and non-equilibrium descriptions. We find the same dual equilibrium/non-equilibrium formulation for f(T) as for f(R) gravity. In particular, we show that the second law of thermodynamics can be satisfied for the universe with the same temperature outside and inside the apparent horizon.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

Equation of state for dark energy in f(T) gravity

We study the cosmological evolutions of the equation of state for dark energy w{sub DE} in the exponential and logarithmic as well as their combination f(T) theories. We show that the crossing of the phantom divide line of w{sub DE} = −1 can be realized in the combined f(T) theory even though it cannot be in the exponential or logarithmic f(T) theory. In particular, the crossing is from w{sub DE} > −1 to w{sub DE} < −1, in the opposite manner from f(R) gravity models. We also demonstrate that this feature is favored by the recent observational data.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

Generic feature of future crossing of phantom divide in viable f(R) gravity models

We study the equation of state for dark energy and explicitly demonstrate that the future crossings of the phantom divide line w{sub DE} = −1 are the generic feature in the existing viable f(R) gravity models. We also explore the future evolution of the cosmological horizon entropy and illustrate that the cosmological horizon entropy oscillates with time due to the oscillatory behavior of the Hubble parameter. The important cosmological consequence is that in the future, the sign of the time derivative of the Hubble parameter changes from negative to positive in these viable f(R) gravity models.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

Cosmological evolution in exponential gravity

We explore the cosmological evolution in the exponential gravity f(R) = R+c{sub 1}(1−e{sup −c{sub 2}R}) (c{sub 1,2} = constant). We summarize various viability conditions and explicitly demonstrate that the late-time cosmic acceleration following the matter-dominated stage can be realized. We also study the equation of state for dark energy and confirm that the crossing of the phantom divide from the phantom phase to the non-phantom (quintessence) one can occur. Furthermore, we illustrate that the cosmological horizon entropy globally increases with time.

79 ASTRONOMY AND ASTROPHYSICS↗