arXiv subjects

Shigeo Kimura

Publications and source records attributed to Shigeo Kimura.

6 recordsLinked to original sources

arXiv2016

General form of entropy on the horizon of the universe in entropic cosmology

Entropic cosmology assumes several forms of entropy on the horizon of the universe, where the entropy can be considered to behave as if it were related to the exchange (the transfer) of energy. To discuss this exchangeability, the consistency of the two continuity equations obtained from two different methods is examined, focusing on a homogeneous, isotropic, spatially flat, and matter-dominated universe. The first continuity equation is derived from the first law of thermodynamics, whereas the second equation is from the Friedmann and acceleration equations. To study the influence of forms of entropy on the consistency, a phenomenological entropic-force model is examined, using a general form of entropy proportional to the $n$-th power of the Hubble horizon. In this formulation, the Bekenstein entropy (an area entropy), the Tsallis--Cirto black-hole entropy (a volume entropy), and a quartic entropy are represented by $n=2$, $3$, and $4$, respectively. The two continuity equations for the present model are found to be consistent with each other, especially when $n=2$, i.e., the Bekenstein entropy. The exchange of energy between the bulk (the universe) and the boundary (the horizon of the universe) should be a viable scenario consistent with the holographic principle.

gr-qc↗

arXiv2015

Cosmic microwave background radiation temperature in a dissipative universe

The relationship between the cosmic microwave background radiation temperature and the redshift, i.e., the $T$--$z$ relation, is examined in a phenomenological dissipative model. The model contains two constant terms, as if a nonzero cosmological constant $Λ$ and a dissipative process are operative in a homogeneous, isotropic, and spatially flat universe. The $T$--$z$ relation is derived from a general radiative temperature law, as appropriate for describing nonequilibrium states in a creation of cold dark matter (CCDM) model. Using this relation, the radiation temperature in the late universe is calculated as a function of a dissipation rate ranging from $\tildeμ =0$, corresponding to a nondissipative $Λ$CDM model, to $\tildeμ =1$, corresponding to a fully dissipative CCDM model. The $T$--$z$ relation for $\tildeμ =0$ is linear for standard cosmology and is consistent with observations. However, with increasing dissipation rate $\tildeμ$, the radiation temperature gradually deviates from a linear law because the effective equation-of-state parameter varies with time. When the background evolution of the universe agrees with a fine-tuned pure $Λ$CDM model, the $T$--$z$ relation for low $\tildeμ$ matches observations, whereas the $T$--$z$ relation for high $\tildeμ$ does not. Previous work also found that a weakly dissipative model accords with measurements of a growth rate for clustering related to structure formations. These results imply that low dissipation is likely for the universe. The weakly dissipative model should be further constrained by recent observations.

astro-ph.CO↗

arXiv2014

Entropic cosmology in a dissipative universe

The bulk viscosity of cosmological fluid and the creation of cold dark matter both result in the generation of irreversible entropy (related to dissipative processes) in a homogeneous and isotropic universe. To consider such effects, the general cosmological equations are reformulated, focusing on a spatially flat matter-dominated universe. A phenomenological entropic-force model is examined that includes constant terms as a function of the dissipation rate ranging from $\tildeμ =0$, corresponding to a nondissipative $Λ$CDM (lambda cold dark matter) model, to $\tildeμ =1$, corresponding to a fully-dissipative CCDM (creation of cold dark matter) model. A time evolution equation is derived for the matter density contrast, in order to characterize density perturbations in the present entropic-force model. It is found that the dissipation rate affects the density perturbations even if the background evolution of the late universe is equivalent to that of a fine-tuned pure $Λ$CDM model. With increasing dissipation rate $\tildeμ$, the calculated growth rate for the clustering gradually deviates from observations, especially at low redshifts. However, the growth rate for low $\tildeμ$ (less than 0.1) is found to agree well with measurements. A low-dissipation model predicts a smaller growth rate than does the pure $Λ$CDM model (for which $\tildeμ =0$). More detailed data are needed to distinguish the low-dissipation model from the pure $Λ$CDM one.

astro-ph.CO↗

arXiv2014

Evolution of the universe in entropic cosmologies via different formulations

We study two types of entropic-force models in a homogeneous, isotropic, spatially flat, matter-dominated universe. The first type is a `$Λ(t)$ type' similar to $Λ(t)$CDM (varying-lambda cold dark matter) models in which both the Friedmann equation and the acceleration equation include an extra driving term. The second type is a `BV type' similar to bulk viscous models in which the acceleration equation includes an extra driving term whereas the Friedmann equation does not. In order to examine the two types systematically, we consider an extended entropic-force model that includes a Hubble parameter ($H$) term and a constant term in entropic-force terms. The $H$ term is derived from a volume entropy whereas the constant term is derived from an entropy proportional to the square of an area entropy. Based on the extended entropic-force model, we examine four models obtained from combining the $H$ and constant terms with the $Λ(t)$ and BV types. The four models agree well with the observed supernova data and describe the background evolution of the late universe properly. However, the evolution of first-order density perturbations is different in each of the four models, especially for low redshift, assuming that an effective sound speed is negligible. The $Λ(t)$ type is found to be consistent with the observed growth rate of clustering, in contrast with the BV type examined in this study. A unified formulation is proposed as well, in order to examine density perturbations of the two types systematically.

astro-ph.CO↗

arXiv2013

Entropic cosmology for a generalized black-hole entropy

An entropic-force scenario, i.e., entropic cosmology, assumes that the horizon of the universe has an entropy and a temperature. In the present study, in order to examine entropic cosmology, we derive entropic-force terms not only from the Bekenstein entropy but also from a generalized black-hole entropy proposed by C. Tsallis and L.J.L. Cirto [Eur. Phys. J. C \textbf{73}, 2487 (2013)]. Unlike the Bekenstein entropy, which is proportional to area, the generalized entropy is proportional to volume because of appropriate nonadditive generalizations. The entropic-force term derived from the generalized entropy is found to behave as if it were an extra driving term for bulk viscous cosmology, in which a bulk viscosity of cosmological fluids is assumed. Using an effective description similar to bulk viscous cosmology, we formulate the modified Friedmann, acceleration, and continuity equations for entropic cosmology. Based on this formulation, we propose two entropic-force models derived from the Bekenstein and generalized entropies. In order to examine the properties of the two models, we consider a homogeneous, isotropic, and spatially flat universe, focusing on a single-fluid-dominated universe. The two entropic-force models agree well with the observed supernova data. Interestingly, the entropic-force model derived from the generalized entropy predicts a decelerating and accelerating universe, as for a fine-tuned standard $Λ$CDM (lambda cold dark matter) model, whereas the entropic-force model derived from the Bekenstein entropy predicts a uniformly accelerating universe.

astro-ph.CO↗

arXiv2013

Non-adiabatic-like accelerated expansion of the late universe in entropic cosmology

In `entropic cosmology', instead of a cosmological constant $Λ$, an extra driving term is added to the Friedmann equation and the acceleration equation, taking into account the entropy and the temperature on the horizon of the universe. By means of the modified Friedmann and acceleration equations, we examine a non-adiabatic-like accelerated expansion of the universe in entropic cosmology. In this study, we consider a homogeneous, isotropic, and spatially flat universe, focusing on the single-fluid (single-component) dominated universe at late-times. To examine the properties of the late universe, we solve the modified Friedmann and acceleration equations, neglecting high-order corrections for the early universe. We derive the continuity (conservation) equation from the first law of thermodynamics, assuming non-adiabatic expansion caused by the entropy and temperature on the horizon. Using the continuity equation, we formulate the generalized Friedmann and acceleration equations, and propose a simple model. Through the luminosity distance, it is demonstrated that the simple model agrees well with both the observed accelerated expansion of the universe and a fine-tuned standard $Λ$CDM (lambda cold dark matter) model. However, we find that the increase of the entropy for the simple model is likely uniform, while the increase of the entropy for the standard $Λ$CDM model tends to be gradually slow especially after the present time. In other words, the simple model predicts that the present time is not a special time, unlike for the prediction of the standard $Λ$CDM model.

astro-ph.CO↗