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Register a new userDynamics of holographic vacuum energy in the DGP model
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We consider the evolution of the vacuum energy in the DGP model according to the holographic principle under the assumption that the relation linking the IR and UV cut-offs still holds in this scenario. The model is studied when the IR cut-off is chosen to be the Hubble scale $H^{-1}$, the particle horizon $R_{rm ph}$ and the future event horizon $R_{rm eh}$, respectively. And the two branches of the DGP model are also taken into account. Through numerical analysis, we find that in the cases of $H^{-1}$ in the (+) branch and $R_{rm eh}$ in both branches, the vacuum energy can play the role of dark energy. Moreover, when considering the combination of the vacuum energy and the 5D gravity effect in both branches, the equation of state of the effective dark energy may cross -1, which may lead to the Big Rip singularity. Besides, we constrain the model with the Type Ia supernovae and baryon oscillation data and find that our model is consistent with current data within $1sigma$, and that the observations prefer either a pure holographic dark energy or a pure DGP model
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Based on the cosmic holographic conjecture of Fischler and Susskind, we point out that the average energy density of the universe is bound from above by its entropy limit. Since Friedmanns equation saturates this relation, the measured value of the cosmological energy density is completely natural in the framework of holographic thermodynamics: vacuum energy density fills the available quantum degrees of freedom allowed by the holographic bound. This is in strong contrast with traditional quantum field theories where, since no similar bound applies, the natural value of the vacuum energy is expected to be 123 orders of magnitude higher than the holographic value. Based on our simple calculation, holographic thermodynamics, and consequently any future holographic quantum (gravity) theory, resolves the vacuum energy puzzle.
The braneworld model proposed by Dvali, Gabadadze and Porrati (DGP) leads to an accelerated universe without cosmological constant or other form of dark energy for the positive branch $(epsilon =+1)$. For the negative branch $(epsilon =-1)$ we have investigated the behavior of a model with an holographic Ricci-like dark energy and dark matter, where the IR cutoff takes the form $alpha H^2 + beta dot{H}$, being $H$ the Hubble parameter and $alpha$, $beta$ positive constants of the model. We perform an analytical study of the model in the late-time dark energy dominated epoch, where we obtain a solution for $r_cH(z)$, where $r_c$ is the leakage scale of gravity into the bulk, and conditions for the negative branch on the holographic parameters $alpha$ and $beta$, in order to hold the conditions of weak energy and accelerated universe. On the other hand, we compare the model versus the late-time cosmological data using the latest type Ia supernova sample of the Joint Light-curve Analysis (JLA), in order to constraint the holographic parameters in the negative branch, as well as $r_cH_0$ in the positive branch, where $H_0$ is the Hubble constant. We find that the model has a good fit to the data and that the most likely values for $(r_cH_0, alpha, beta)$ lie in the permitted region found from an analytical solution in a dark energy dominated universe. We give a justification to use holographic cut-off in 4D for the dark energy in the 5 dimensional DGP model. Finally, using the Bayesian Information Criterion we find that this model it is disfavored compared with the flat $Lambda$CDM model.
We investigate the effect of the bulk contents in the DGP braneworld on the evolution of the universe. We find that although the pure DGP model cannot accommodate the transition of the effective equation of state of dark energy, once the bulk matter T^5_5 is considered, the modified model can realize the w_{eff} crossing -1. However this transition of the equation of state cannot be realized by just considering bulk-brane energy exchange or the GB effect while the bulk matter contribution is not included. T^5_5 plays the major role in the modified DGP model to have the w crossing -1 behavior. We show that our model can describe the super-acceleration of our universe with the equation of state of the effective dark energy and the Hubble parameter in agreement with observations.
We propose two improved parameterized form for the growth index of the linear matter perturbations: (I) $gamma(z)=gamma_0+(gamma_{infty}-gamma_0){zover z+1}$ and (II) $gamma(z)=gamma_0+gamma_1 frac{z}{z+1}+(gamma_{infty}-gamma_1-gamma_0)(frac{z}{z+1})^{alpha}$. With these forms of $gamma(z)$, we analyze the accuracy of the approximation the growth factor $f$ by $Omega^{gamma(z)}_m$ for both the $omega$CDM model and the DGP model. For the first improved parameterized form, we find that the approximation accuracy is enhanced at the high redshifts for both kinds of models, but it is not at the low redshifts. For the second improved parameterized form, it is found that $Omega^{gamma(z)}_m$ approximates the growth factor $f$ very well for all redshifts. For chosen $alpha$, the relative error is below 0.003% for the $Lambda$CDM model and 0.028% for the DGP model when $Omega_{m}=0.27$. Thus, the second improved parameterized form of $gamma(z)$ should be useful for the high precision constraint on the growth index of different models with the observational data. Moreover, we also show that $alpha$ depends on the equation of state $omega$ and the fractional energy density of matter $Omega_{m0}$, which may help us learn more information about dark energy and DGP models.
We investigate the quark-gluon mixed condensate based on an AdS/QCD model. Introducing a holographic field dual to the operator for the quark-gluon mixed condensate, we obtain the corresponding classical equation of motion. Taking the mixed condensate as an additional free parameter, we show that the present scheme reproduces very well experimental data. A fixed value of the mixed condensate is in good agreement with that of the QCD sum rules.
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