No Arabic abstract
In present article we consider an axion F(R) gravity model and described with the help of holographic principle the cosmological models of viscous dark fluid coupled with axion matter in a spatially flat Friedmann-Robertson-Walker (FRW) universe. This description based on generalized infrared-cutoff holographic dark energy, proposed by Nojiri and Odintsov. We explored the Little Rip, the Pseudo Rip, and the power-law bounce cosmological models in terms of the parameters of the inhomogeneous equation of the state of viscous dark fluid and calculated the infrared cutoffs analytically. We represented the energy conservation equation for the dark fluid from a holographic point of view and showed a correspondence between the cosmology of a viscous fluid and holographic cosmology. We analyzed the autonomous dynamic system. In the absence of interaction between fluids, solutions are obtained corresponding to two cases. In the first case, dark energy is missing and the extension describes the component of dark matter. The second case corresponds to cosmological models with an extension due to dark energy. The solutions obtained are investigated for stability. For a cosmological model with the interaction of a special type, the stability of solutions of the dynamic system is also investigated.
Four-dimensional cosmological models are studied on a boundary of a five-dimensional Anti-de Sitter (AdS_5) black hole with AdS Reissner-Nordstrom and scalar charged Reissner- Nordstrom black hole solutions, where we call the former a Hairless black hole and the latter a Hairy black hole. To obtain the Friedmann-Robertson-Walker (FRW) spacetime metric on the boundary of the AdS_5 black hole, we employ Eddington-Finkelstein (EF) coordinates to the bulk geometry. We then derive modified Friedmann equations on a boundary of the AdS_5 black hole via AdS/CFT correspondence and discuss its cosmological implications. The late-time acceleration of the universe is investigated in our models. The contributions coming from the bulk side is treated as a dark energy source, and we perform MCMC analyses using observational data. Compared to the LCDM model, our models contain additional free parameters; therefore, to make a fair comparison, we use the Akaike information criterion (AIC) and the Bayesian information criterion (BIC) to analyze our results. Our numerical analyses show that our models can explain the observational data as reliable as the LCDM model does for the current data.
I show that a generic quantum phenomenon can drive cosmic acceleration without the need for dark energy or modified gravity. When treating the universe as a quantum system, one typically focuses on the scale factor (of an FRW spacetime) and ignores many other degrees of freedom. However, the information capacity of the discarded variables will inevitably change as the universe expands, generating quantum bias (QB) in the Friedmann equations [Phys. Lett. A 382, 36, 2555 (2018)|arXiv:1707.05789]. If information could be stored in each Planck-volume independently, this effect would give rise to a constant acceleration $10^{120}$ times larger than that observed, reproducing the usual cosmological constant problem. However, once information capacity is quantified according to the holographic principle, cosmic acceleration is far smaller and depends on the past behaviour of the scale factor. I calculate this holographic quantum bias, derive the semiclassical Friedmann equations, and obtain their general solution for a spatially-flat universe containing matter and radiation. Comparing these QB-CDM solutions to those of $Lambda$CDM, the new theory is shown to be falsifiable, but nonetheless consistent with current observations. In general, realistic QB cosmologies undergo phantom acceleration ($w_mathrm{eff}<-1$) at late times, predicting a Big Rip in the distant future.
We investigate the structure formation in the effective field theory of the holographic dark energy. The equation of motion for the energy contrast $delta_m$ of the cold dark matter is the same as the one in the general relativity up to the leading order in the small scale limit $kgg aH$, provided the equation of state is Quintessence-like. Our effective field theory breaks down while the equation of state becomes phantom-like. We propose a solution to this problem by eliminating the scalar graviton.
I give a critical review of the holographic hypothesis, which posits that a universe with gravity can be described by a quantum field theory in fewer dimensions. I first recall how the idea originated from considerations on black hole thermodynamics and the so-called information paradox that arises when Hawking radiation is taken into account. String Quantum Gravity tried to solve the puzzle using the AdS/CFT correspondence, according to which a black hole in a 5-D anti-de Sitter space is like a flat 4-D field of particles and radiation. Although such an interesting holographic property, also called gauge/gravity duality, has never been proved rigorously, it has impulsed a number of research programs in fields as diverse as nuclear physics, condensed matter physics, general relativity and cosmology. I finally discuss the pros and cons of the holographic conjecture, and emphasizes the key role played by black holes for understanding quantum gravity and possible dualities between distant fields of theoretical physics.
We discuss the Damour--Esposito-Far`ese model of gravity, which predicts the spontaneous scalarization of neutron stars in a certain range of parameter space. In the cosmological setup, the scalar field responsible for scalarization is subject to a tachyonic instability during inflation and the matter domination stage, resulting in a large value of the field today. This value feeds into the PPN parameters, which turn out to be in gross conflict with the Solar system measurements. We modify the original Damour--Esposito-Far`ese model by coupling the scalar to the inflaton field. This coupling acts as an effective mass for the scalar during inflation. For generic couplings that are not extremely small, the scalar (including its perturbations) relaxes to zero with an exponential accuracy by the beginning of the hot stage. While the scalar exhibits growth during the subsequent cosmological stages, the resulting present value remains very small---in a comfortable agreement with the Solar system tests.