arXiv subjects

Shin'ichi Nojiri

Publications and source records attributed to Shin'ichi Nojiri.

At least 235 records · Page 13Linked to original sources

arXiv2005

One-loop f(R) gravity in de Sitter universe

Motivated by the dark energy issue, the one-loop quantization approach for a family of relativistic cosmological theories is discussed in some detail. Specifically, general $f(R)$ gravity at the one-loop level in a de Sitter universe is investigated, extending a similar program developed for the case of pure Einstein gravity. Using generalized zeta regularization, the one-loop effective action is explicitly obtained off-shell, what allows to study in detail the possibility of (de)stabilization of the de Sitter background by quantum effects. The one-loop effective action maybe useful also for the study of constant curvature black hole nucleation rate and it provides the plausible way of resolving the cosmological constant problem.

hep-th↗

arXiv2005

Consistent modified gravity: dark energy, acceleration and the absence of cosmic doomsday

We discuss the modified gravity which includes negative and positive powers of the curvature and which provides the gravitational dark energy. It is shown that in GR plus the term containing negative power of the curvature the cosmic speed-up may be achieved, while the effective phantom phase (with $w$ less than -1) follows when such term contains the fractional positive power of the curvature. The minimal coupling with matter makes the situation more interesting: even 1/R theory coupled with the usual ideal fliud may describe the (effective phantom) dark energy. The account of $R^2$ term (consistent modified gravity) may help to escape of cosmic doomsday.

hep-th↗

arXiv2004

Dark Energy and Cosmic Speed-Up from Consistent Modified Gravity

We review some modified gravity models which describe the gravitational dark energy and the possibility of cosmic speed-up. The new consistent version of such theory which contains inverse and HD curvature terms as well as new type of coupling with matter is proposed. The accelerating cosmologies are discussed there. The structure of finite-time (sudden) singularities is investigated.

hep-th↗

arXiv2004

Late-time phantom universe in ${\bf SO(1,1)}$ dark energy model with exponential potential

We discuss the late-time property of universe and phantom field in the SO(1,1) dark energy model for the potential $V=V_0e^{-βΦ^α}$ with $α$ and $β$ two positive constants. We assume in advance some conditions satisfied by the late-time field to simplify equations, which are confirmed to be correct from the eventual results. For $α<2$, the filed falls exponentially off and the phantom equation of state rapidly approaches -1. When $α=2$, the kinetic energy $ρ_k$ and the coupling energy $ρ_c$ become comparable but there is always $ρ_k<-ρ_c$ so that the phantom property of field proceeds to hold. The analysis on the perturbation to the late-time field $Φ$ illustrates the square effective mass of the perturbation field is always positive and thus the phantom is stable. The universe considered currently may evade the future sudden singularity and will evolve to de Sitter expansion phase.

gr-qc↗

arXiv2004

The final state and thermodynamics of dark energy universe

As it follows from the classical analysis, the typical final state of the dark energy universe where dominant energy condition is violated is finite time, sudden future singularity (Big Rip). For a number of dark energy universes (including scalar phantom and effective phantom theories as well as specific quintessence model) we demonstrate that quantum effects play the dominant role near Big Rip, driving the universe out of future singularity (or, at least, making it milder). As a consequence, the entropy bounds with quantum corrections become well-defined near Big Rip. Similarly, black holes mass loss due to phantom accretion is not so dramatic as it was expected: masses do not vanish to zero due to transient character of phantom evolution stage. Some examples of cosmological evolution for negative, time-dependent equation of state are also considered with the same conclusions. The application of negative entropy (or negative temparature) occurence in the phantom thermodynamics is briefly discussed.

hep-th↗

arXiv2004

Is brane cosmology predictable?

The creation of the inflationary brane universe in 5d bulk Einstein and Einstein-Gauss-Bonnet gravity is considered. We demonstrate that emerging universe is ambigious due to arbitrary function dependence of the junction conditions (or freedom in the choice of boundary terms). We argue that some fundamental physical principle (which may be related with AdS/CFT correspondence) is necessary in order to fix the 4d geometry in unique way.

hep-th↗

arXiv2004

Gravity assisted dark energy dominance and cosmic acceleration

It is proposed that dark energy may become dominant over standard matter due to universe expansion (curvature decrease). Two models: non-linear gravity-matter system and modified gravity may provide the effective phantom or effective quintessence dark energy which complies with the conjecture. It is interesting that future of such universe is not necessary finite time singularity (Big Rip). The effective quintessence naturally describes current cosmic speed-up.

astro-ph↗

arXiv2004

Dark Energy and Modified Gravities

We review the several models of the dark energy, which may generate the accelerated expansion of the present universe. We also discuss the the Big Rip singularity, which may occur when the equation of the state parameter w is less than -1. We show that the quantum correction would be very important near the singularity.

hep-th↗

arXiv2004

Far-Ultraviolet Surveys of Globular Clusters: Hunting for the Products of Stellar Collisions and Near Misses

Globular clusters are gravitationally bound stellar systems containing on the order of 100,000 stars. Due to the high stellar densities in the cores of these clusters, close encounters and even physical collisions between stars are inevitable. These dynamical interactions can produce exotic types of single and binary stars that are extremely rare in the galactic field, but which may be important to the dynamical evolution of their host clusters. A common feature of these dynamically-formed stellar populations is that many of their members are relatively hot, and thus bright in the far-ultraviolet (FUV) waveband. In this short review, I describe how space-based FUV observations are being used to find and study these populations.

astro-ph↗

arXiv2004

Surprises in nonperturbative dynamics in sigma-model at finite density

The linear $SU(2)_L \times SU(2)_R$ sigma-model occupies a unique place in elementary particle physics and quantum field theory. It has been recently realized that when a chemical potential for hypercharge is added, it becomes a toy model for the description of the dynamics of the kaon condensate in high density QCD. We review recent results in nonperturbative dynamics obtained in the ungauged and gauged versions of this model.

hep-ph↗

arXiv2004

Quantum escape of sudden future singularity

We consider finite-time, future (sudden or Big Rip type) singularities which may occur even when strong energy condition is not violated but equation of state parameter is time-dependent. Recently, example of such singularity has been presented by Barrow, we found another example of it. Taking into account back reaction of conformal quantum fields near singularity, it is shown explicitly that quantum effects may delay (or make milder) the singularity. It is argued that if the evolution to singularity is realistic, due to quantum effects the universe may end up in deSitter phase before scale factor blows up. This picture is generalized for braneworld where sudden singularity may occur on the brane with qualitatively similar conclusions.

hep-th↗

arXiv2004

Late-time cosmology in (phantom) scalar-tensor theory: dark energy and the cosmic speed-up

We consider late-time cosmology in a (phantom) scalar-tensor theory with an exponential potential, as a dark energy model with equation of state parameter close to -1 (a bit above or below this value). Scalar (and also other kinds of) matter can be easily taken into account. An exact spatially-flat FRW cosmology is constructed for such theory, which admits (eternal or transient) acceleration phases for the current universe, in correspondence with observational results. Some remarks on the possible origin of the phantom, starting from a more fundamental theory, are also made. It is shown that quantum gravity effects may prevent (or, at least, delay or soften) the cosmic doomsday catastrophe associated with the phantom, i.e. the otherwise unavoidable finite-time future singularity (Big Rip). A novel dark energy model (higher-derivative scalar-tensor theory) is introduced and it is shown to admit an effective phantom/quintessence description with a transient acceleration phase. In this case, gravity favors that an initially insignificant portion of dark energy becomes dominant over the standard matter/radiation components in the evolution process.

hep-th↗

arXiv2004

Entropy and universality of Cardy-Verlinde formula in dark energy universe

We study the entropy of a FRW universe filled with dark energy (cosmological constant, quintessence or phantom). For general or time-dependent equation of state $p=wρ$ the entropy is expressed in terms of energy, Casimir energy, and $w$. The correspondent expression reminds one about 2d CFT entropy only for conformal matter. At the same time, the cosmological Cardy-Verlinde formula relating three typical FRW universe entropies remains to be universal for any type of matter. The same conclusions hold in modified gravity which represents gravitational alternative for dark energy and which contains terms growing at low curvature. It is interesting that BHs in modified gravity are more entropic than in Einstein gravity. Finally, some hydrodynamical examples testing new shear viscosity bound, which is expected to be the consequence of the holographic entropy bound, are presented for the early universe in the plasma era and for the Kasner metric. It seems that the Kasner metric provides a counterexample to the new shear viscosity bound.

hep-th↗

arXiv2004

Multisupergravity from latticized extra dimension

The construction of the linearized four-dimensional multisupergravity from five-dimensional linearized supergravity with discretized fifth dimension is presented. The one-loop vacuum energy is evaluated when (anti)periodic boundary conditions are chosen for (bosons) fermions, respectively or vice-versa. It is proposed that the relation between discretized M-theory and strings may be found in the same fashion.

hep-th↗

arXiv2004

Freedom in electroweak symmetry breaking and mass matrix of fermions in dimensional deconstruction model

There exists a freedom in a class of four-dimensional electroweak theories proposed by Arkani-Hamed et al. relying on deconstruction and Coleman-Weinberg mechanism. The freedom comes from the winding modes of the link variable (Wilson operator) connecting non-nearest neighbours in the discrete fifth dimension. Using this freedom, dynamical breaking of SU(2) gauge symmetry, mass hierarchy patterns of fermions and Cabbibo-Kobayashi-Maskawa matrix may be obtained.

hep-th↗

arXiv2003

Modified gravity with $\ln R$ terms and cosmic acceleration

The modified gravity with $\ln R$ or $R^{-n} (\ln R)^m$ terms which grow at small curvature is discussed. It is shown that such a model which has well-defined newtonian limit may eliminate the need for dark energy and may provide the current cosmic acceleration. It is demonstrated that $R^2$ terms are important not only for early time inflation but also to avoid the instabilities and the linear growth of the gravitational force. It is very interesting that the condition of no linear growth for gravitational force coincides with the one for scalar mass in the equivalent scalar-tensor theory to be very large. Thus, modified gravity with $R^2$ term seems to be viable classical theory.

hep-th↗

arXiv2003

The one-loop vacuum energy and RG flow induced by double-trace operators in AdS/CFT and dS/CFT correspondence

We calculate the difference of one-loop vacuum energies for massive scalar field in five-dimensional AdS black hole. (The same is done in five-dimensional deSitter space). In each case this difference is specified by the boundary conditions corresponding to the double-trace operator (massive term) and it describes RG flow in the manner discussed by Gubser-Mitra for pure AdS space. For AdS black hole there occurs instability which is the manifestation of the Hawking-Page phase transition. For stable phase of AdS black hole as well as for deSitter bulk, c-function found beyond the leading order approximation shows the monotonic behaviour consistent with c-theorem.

hep-th↗

arXiv2003

Cosmological anti-deSitter space-times and time-dependent AdS/CFT correspondence

We study classes of five-dimensional cosmological solutions with negative curvature, which are obtained from static solutions by an exchange of a spatial and temporal coordinate, and in some cases by an analytic continuation. Such solutions provide a suitable laboratory to address the time-dependent AdS/CFT correspondence. For a specific example we address in detail the calculation of the boundary stress-energy and the Wilson line and find disagreement with the standard AdS/CFT correspondence. We trace these discrepancies to the time-dependent effects, such as particle creation, which we further study for specific backgrounds. We also identify specific time-dependent backgrounds that reproduce the correct conformal anomaly. For such backgrounds the calculation of the Wilson line in the adiabatic approximation indicates only a Coulomb repulsion.

hep-th↗