DGP Model Research Articles

Out of the white hole: a holographic origin for the Big Bang

While most of the singularities of General Relativityare expected to be safely hidden behind event horizons by thecosmic censorship conjecture, we happen to live in the causalfuture of the classical Big Bang singularity, whoseresolution constitutes the active field of early universecosmology. Could the Big Bang be also hidden behind a causalhorizon, making us immune to the decadent impacts of a nakedsingularity? We describe a braneworld description of cosmologywith both 4d induced and 5D bulk gravity (otherwise known asDvali-Gabadadze-Porati, or DGP model), which exhibits thisfeature: the universe emerges as a spherical 3-brane out of theformation of a 5D Schwarzschild black hole. In particular, weshow that a pressure singularity of the holographic fluid,discovered earlier, happens inside the white hole horizon, andthus need not be real or imply any pathology. Furthermore, weoutline a novel mechanism through which any thermal atmosphere forthe brane, with comoving temperature of ∼20% of the 5DPlanck mass can induce scale-invariant primordial curvatureperturbations on the brane, circumventing the need for a separateprocess (such as cosmic inflation) to explain current cosmologicalobservations. Finally, we note that 5D space-time isasymptotically flat, and thus potentially allows an S-matrix or(after minor modifications) an AdS/CFT description of thecosmological Big Bang.

Statefinder hierarchy of bimetric and galileon models for concordance cosmology

In this paper, we use Statefinder hierarchy method to distinguish betweenbimetric theory of massive gravity, galileon modified gravity and DGP models applied to late time expansion of the universe. We also carry out comparison between bimetric and DGP models using Statefinder pairs {r,s} and {r,q}. We show that statefinder diagnostic can differentiate between ΛCDM and above mentioned cosmological models of dark energy, and finally show that Statefinder S2 is an excellent discriminant of ΛCDM and modified gravity models.

Cosmological dynamics of a quintom field on the warped DGP brane

As a generalization of the Brans-Dicke type scalar-tensor gravity in a braneworld context, we study cosmological phase space of a braneworld model with induced gravity in the presence of a scalar field on the brane. We consider a quintom field minimally or non-minimally coupled to induced gravity on the warped DGP brane and we present a detailed analysis of the critical points, their stability and late-time cosmological viability of the solutions within a phase space approach. In particular, de Sitter solutions, different from the famous self-accelerated branch of the DGP model are found and the phase-space analysis for checking their attractor properties is performed. We analyze also the possibility of crossing of the phantom divide by the effective equation of state parameter of the model. We also focus on the classical stability of the solutions in w–w′ phase plane.

An introduction to the Vainshtein mechanism

We introduce the Vainshtein mechanism which plays a crucial role in massive gravities, as well as in related theories such as Galileons and their extensions. This mechanism, also known as k-mouflage, allows to hide via nonlinear effects—typically for source distances smaller than a so-called Vainshtein radius which depends on the source and on the theory considered—some degrees of freedom whose effects are then only left important at large distances, e.g. for cosmology. It is introduced here in nonlinear Fierz–Pauli theories (massive gravities), including the dRGT theories, in their decoupling limits, as well as in other models such as the DGP model or generalized Galileons. This presentation is self-contained, and before discussing the Vainshtein mechanism we introduce some useful results and concepts concerning massive gravity, such as the vDVZ discontinuity, the decoupling limits or the Boulware–Deser ghost.

Testing General Relativity using the growth rate of structure

We place tight constraints on the growth index γ by using the recent growth history results of 2dFGRS, SDSS-LRG, VIMOS-VLT deep Survey (VVDS), and WiggleZ datasets. Utilizing a standard likelihood analysis, we find that the use of the combined growth data provided by the previous mentioned galaxy surveys, puts the most stringent constraints on the value of the growth index. Assuming a constant growth index, we obtain that γ = 0.602±0.055 for the concordance ΛCDM expansion model. Based on the Dvali-Gabadadze-Porrati gravity model, we find γ = 0.503±0.06 which is lower, and almost 3σ away, from the theoretically predicted value of γDGP ≊ 11/16 implying that the present growth rate data disfavor the DGP gravity.

Cosmological dynamics of a non-minimally coupled bulk scalar field in DGP setup

We consider cosmological dynamics of a canonical bulk scalar field, which is coupled non-minimally to 5-dimensional Ricci scalar in a DGP setup. We show that presence of this non-minimally coupled bulk scalar field affects the jump conditions of the original DGP model significantly. Within a superpotential approach, we perform some numerical analysis of the model parameter space and consider bulk-brane energy exchange in this setup. Also we show that the normal, ghost-free branch of the DGP solutions in this case has the potential to realize a self-consistent phantom-like behavior and therefore explains late time acceleration of the universe in a consistent way.

Intermediate inflation on the brane and warped DGP models

Brane and warped Dvali-Gabadadze-Porrati inflationary universe models in the context of intermediate inflation are studied. In both models we consider that the energy density is a standard scalar field and we discuss the evolution of the universe during the slow-roll inflation. Also, we describe the conditions for these models. The corresponding Wilkinson Microwave Anisotropy Probe seven year data are utilized to fix some parameters in our models. Our results are compared to those found in General Relativity.

Exploring Vainshtein mechanism on adaptively refined meshes

There has been a lot of research interest in modified gravity theories which utilise the Vainshtein mechanism to recover standard general relativity in regions with high matter density, such as the Dvali-Gabadadze-Porrati and Galileon models. The strong nonlinearity in the field equations of these theories implies that accurate theoretical predictions could only be made using high-resolution cosmological simulations. Previously, such simulations were usually done on regular meshes, which limits both their performance and the accuracy. In this paper, we report the development of a new algorithm and code, based on ECOSMOG, that uses adaptive mesh refinements to improve the efficiency and precision in simulating the models with Vainshtein mechanism. We have made various code tests against the numerical reliability, and found consistency with previous simulations. We also studied the velocity field in the self-accelerating branch of the DGP model. The code, parallelised using MPI, is suitable for large cosmological simulations of Galileon-type modified gravity theories.

Cosmological equivalence between the Finsler-Randers space-time and the DGP gravity model

We perform a detailed comparison between the Finsler-Randers cosmological model and the Dvali, Gabadadze and Porrati braneworld model. If we assume that the spatial curvature is strictly equal to zero then we prove the following interesting proposition: {\it despite the fact that the current cosmological models have a completely different geometrical origin they share exactly the same Hubble expansion}. This implies that the Finsler-Randers model is cosmologically equivalent with that of the DGP model as far as the cosmic expansion is concerned. At the perturbative level we find that the Finsler-Randers growth index of matter perturbations is $\gamma_{FR} \simeq 9/14$ which is somewhat lower than that of DGP gravity ($\gamma_{DGP} \simeq 11/16$) implying that the growth factor of the Finsler-Randers model is slightly different ($\sim 0.1-2%$) from the one provided by the DGP gravity model.

Gauss-Bonnet Braneworld Cosmology with Modified Induced Gravity on the Brane

We analyze the background cosmology for an extension of the DGP gravity with Gauss-Bonnet term in the bulk and $f(R)$ gravity on the brane. We investigate implications of this setup on the late-time cosmic history. Within a dynamical system approach, we study cosmological dynamics of this setup focusing on the role played by curvature effects. Finally we constraint the parameters of the model by confrontation with recent observational data.

A comprehensive comparison of cosmological models from the latest observational data

We investigate in detail some popular cosmological models in light of the latest observational data, including the Union2.1 supernovae compilation, the baryon acoustic oscillation measurements from the WiggleZ Dark Energy Survey, the cosmic microwave background information from the WMAP 7-year observations along with the observational Hubble parameter data. Based on the model selection statistics such as the Akaike and the Bayesian information criterias, we compare different models to assess the worth of them. We do not assume a flat universe in the fitting. Our results show that the concordance $\Lambda$CDM model remains the best one to explain the data, while the DGP model is clearly disfavored by the data. Among these models, those whose parameters can reduce themselves to the $\Lambda$CDM model provide good fits to the data. These results indicate that for the current data, there is no obvious evidence supporting any more complex models over the simplest $\Lambda$CDM model.

Holographic dark energy in the DGP model

The braneworld model proposed by Dvali, Gabadadze and Porrati leads to an accelerated universe without cosmological constant or other form of dark energy. Nevertheless, we have investigated the consequences of this model when an holo- graphic dark energy is included, taken the Hubble scale as IR cutoff. We have found that the holographic dark energy leads to an accelerated universe flat (de Sitter like expansion) for the two branch: {\ko} = \pm1 of the DGP model. Nevertheless, in universes with no null curvature the dark energy presents an EoS corresponding to a phantom fluid during the present era and evolving to a de Sitter like phase for future cosmic time. In the special case in which the holographic parameter c is equal to one we have found a sudden singularity in closed universes. In this case the expansion is decelerating. Manuscript

Virial mass in warped DGP-inspired $\mathcal{L}(R)$ gravity

A version of the virial theorem is derived in a brane-world scenario in the framework of a warped DGP model where the action on the brane is an arbitrary function of the Ricci scalar, \(\mathcal{L}(R)\). The extra terms in the modified Einstein equations generate an equivalent mass term (geometrical mass), which give an effective contribution to the gravitational energy and offer viable explanation to account for the virial mass discrepancy in clusters of galaxies. We also obtain the radial velocity dispersion of galaxy clusters and show that it is compatible with the radial velocity dispersion profile of such clusters. Finally, we compare the result of the model with \(\mathcal{L}(R)\) gravity theories.

New accelerated solutions in a DGP-inspired model

We study the late-time cosmological viability of the solutions in the DGP braneworld scenario. We consider a quintessence field trapped on the normal branch of the DGP model and we suppose this scalar field is both minimally and non-minimally coupled to induced gravity on the brane. Since a successful cosmological model should therefore admit for a sequence of epochs: a radiation era, a sufficiently long matter dominated era and a final stable positively accelerated scaling solution, we analyze the cosmological properties of system in its critical points.

Testing Einstein gravity with cosmic growth and expansion

We test Einstein gravity using cosmological observations of both expansion and structure growth, including the latest data from supernovae (Union2.1), CMB (WMAP7), weak lensing (CFHTLS) and peculiar velocity of galaxies (WiggleZ). We fit modified gravity parameters of the generalized Poisson equations simultaneously with the effective equation of state for the background evolution, exploring the covariances and model dependence. The results show that general relativity is a good fit to the combined data. Using a Pad{\'e} approximant form for the gravity deviations accurately captures the time and scale dependence for theories like $f(R)$ and DGP gravity, and weights high and low redshift probes fairly. For current observations, cosmic growth and expansion can be fit simultaneously with little degradation in accuracy, while removing the possibility of bias from holding one aspect fixed.

The growth index of matter perturbations and modified gravity

We place tight constraints on the growth index $\gamma$ by using the recent growth history results of 2dFGRS, SDSS-LRG, VIMOS-VLT deep Survey (VVDS) and {\em WiggleZ} datasets. In particular, we investigate several parametrizations of the growth index $\gamma(z)$, by comparing their cosmological evolution using observational growth rate data at different redshifts. Utilizing a standard likelihood analysis we find that the use of the combined growth data provided by the 2dFGRS, SDSS-LRG, VVDS and {\em WiggleZ} galaxy surveys, puts the most stringent constraints on the value of the growth index. As an example, assuming a constant growth index we obtain that $\gamma=0.602\pm 0.055$ for the concordance $\Lambda$CDM expansion model. Concerning the Dvali-Gabadadze-Porrati gravity model, we find $\gamma=0.503\pm 0.06$ which is lower, and almost $3\sigma$ away, from the theoretically predicted value of $\gamma_{DGP}\simeq 11/16$. Finally, based on a time varying growth index we also confirm that the combined growth data disfavor the DGP gravity.

Nonlinear cosmological consistency relations and effective matter stresses

We propose a fully nonlinear framework to construct consistency relations for testing generic cosmological scenarios using the evolution of large scale structure. It is based on the covariant approach in combination with a frame that is purely given by the metric, the normal frame. As an example, we apply this framework to the ΛCDM model, by extending the usual first order conditions on the metric potentials to second order, where the two potentials start to differ from each other. We argue that working in the normal frame is not only a practical choice but also helps with the physical interpretation of nonlinear dynamics. In this frame, effective pressures and anisotropic stresses appear at second order in perturbation theory, even for ``pressureless'' dust. We quantify their effect and compare them, for illustration, to the pressure of a generic clustering dark energy fluid and the anisotropic stress in the DGP model. Besides, we also discuss the effect of a mismatch of the potentials on the determination of galaxy bias.

On asymmetric brane creation

We exhaust the brane instanton solutions---an Einstein brane inhabiting at different positions in a 5-dimensional Einstein bulk with negative curvature. We construct a brane instanton model consisting of a brane with asymmetric bulk along two sides of the brane. And the junction condition of the resulting space-time is analyzed in the frame of induced gravity (DGP model). In spirits of quantum gravity of path integral formulism we calculate the Euclidean actions on three canonical paths and then compare the Euclidean actions of different instantons per unit 4-volume. We also compare the Euclidean actions per unit 4-volume of instantons, which consist of a brane gluing to a fixed half, with other Euclidean actions of halves possessing different cosmological constants.

EVOLUTION OF THE DENSITY PARAMETER IN THE ANISOTROPIC DGP COSMOLOGY

Evolution of the density parameter in the anisotropic DGP braneworld model is studied. The role of shear and cross-over scale in the evolution of Ωρ is examined for both the branches of solution in the DGP model. The evolution is modified significantly compared to the FRW model and further it does not depend on the value of γ alone. Behavior of the cosmological density parameter Ωρ is unaltered in the late universe. The study of decceleration parameter shows that the entry of the universe into self-accelerating phase is determined by the value of shear. We also obtain an estimate of the shear parameter [Formula: see text], which is in agreement with the constraints obtained in the literature using data.

Closed timelike curves in the Galileon model

It has long been known that generic solutions to the nonlinear DGP and Galileon models admit superluminal propagation. In this note we present a solution of these models which also admits closed timelike curves (CTCs). We observe that these CTCs only arise when, according to each observer, there exists some region in which the higher derivative terms are larger than the 2-derivative kinetic term.