Phantom Braneworld and the Hubble Tension

Braneworld models with induced gravity exhibit phantom-like behavior of the effective equation of state of dark energy. They can, therefore, naturally accommodate higher values of H 0, preferred by recent local measurements while satisfying the cosmic microwave background constraints. We test the background evolution in such phantom braneworld scenarios with the current observational data sets. We find that the phantom braneworld prefers a higher value of H 0 even without the R19 prior, thereby providing a much better fit to the local measurements. Although this braneworld model cannot fully satisfy all combinations of cosmological observables, among existing dark energy candidates the phantom brane provides one of the most compelling explanations of cosmic evolution.

Modified Chaplygin gas in anisotropic universes on the brane

In this paper, we study anisotropic universes with Modified Chaplygin gas (MCG) in the context of Randall Sundrum-2 (RS2) braneworld model. The cosmological solutions for Kantowski–Sachs (KS) and Bianchi-I universes with MCG are obtained on the RS2 braneworld. The solutions are found to be dependent on MCG parameters but are modified from the GR solutions due to the braneworld correction term arising from high-energy effects. The anisotropy and deceleration parameters are obtained for each solution and the possibility of occurrence of future singularities is considered. Interestingly, we find that one drawback of the relativistic picture can be overcome in this model giving a universe close to the presently observed state.

Time-dependent quantum correlations in two-dimensional expanding spacetime

In expanding universes, the entanglement entropy must be time-dependent because the background geometry changes with time. For understanding time evolution of quantum correlations, we take into account two distinct holographic models, the dS boundary model and the braneworld model. In this work, we focus on two-dimensional expanding universes for analytic calculation and comparison. Although two holographic models realize expanding universes in totally different ways, we show that they result in the qualitatively same time-dependence for eternal inflation. We further investigate the time-dependent correlations in the radiation-dominated era of the braneworld model. Intriguingly, the holographic result reveals that a thermal system in the expanding universe is dethermalized after a critical time characterized by the subsystem size.

Smooth braneworld in 6-dimensional asymptotically AdS spacetime

In this paper, we investigate a six-dimensional smooth thick braneworld model which contains a compact extra dimension and an infinite large one. The braneworld is generated by a real scalar field with a ϕ6 potential and the bulk is an asymptotically AdS6 spacetime. The geometry achieves the localization of the free U(1) gauge field, which is a problem in five-dimensional Randall-Sundrum-like models. In addition, we analyze the stability of the braneworld system and the localization of the graviton.

Islands and complexity of eternal black hole and radiation subsystems for a doubly holographic model

We study the entanglement islands and subsystem volume complexity corresponding to the left/ right entanglement of a conformal defect in d-dimensions in Randall-Sundrum (RS) braneworld model with subcritical tension brane. The left and right modes of the defect mimic the eternal black hole and radiation system respectively. Hence the entanglement entropy between the two follows an eternal black hole Page curve which is unitarity compatible. We compute the volumes corresponding to the left and right branes with preferred Ryu-Takanayagi (RT) surfaces at different times, which provide a probe of the subregion complexity of the black hole and the radiation states respectively. An interesting jump in volume is found at Page time, where the entanglement curve is saturated due to the inclusion of the island surfaces. We explain various possibilities of this phase transition in complexity at Page time and argue how these results match with a covariant proposal qualitatively.

Galaxy formation in the brane world I: overview and first results

We carry out “full-physics” hydrodynamical simulations of galaxy formation in the normal-branch Dvali-Gabadadze-Porrati (nDGP) braneworld model using a new modified version of the Arepo code and the IllustrisTNG galaxy formation model. We simulate two nDGP models (N5 and N1) which represent, respectively, weak and moderate departures from GR, in boxes of sizes 62 h−1Mpc and 25 h−1Mpc using 2 × 5123 dark matter particles and initial gas cells. This allows us to explore, for the first time, the impact of baryonic physics on galactic scales in braneworld models of modified gravity and to make predictions on the stellar content of dark matter haloes and galaxy evolution through cosmic time in these models. We find significant differences between the GR and nDGP models in the power spectra and correlation functions of gas, stars and dark matter of up to ∼25 per cent on large scales. Similar to their impact in the standard cosmological model (ΛCDM), baryonic effects can have a significant influence over the clustering of the overall matter distribution, with a sign that depends on scale. Studying the degeneracy between modified gravity and galactic feedback in these models, we find that these two physical effects on matter clustering can be cleanly disentangled, allowing for a method to accurately predict the matter power spectrum with baryonic effects included, without having to run hydrodynamical simulations. Depending on the braneworld model, we find differences compared with GR of up to ∼15 per cent in galaxy properties such as the stellar-to-halo-mass ratio, galaxy stellar mass function, gas fraction and star formation rate density. The amplitude of the fifth force is reduced by the presence of baryons in the very inner part of haloes, but this reduction quickly becomes negligible above ∼0.1 times the halo radius.

Global monopole as a generator of a bulk-brane structure in Brans–Dicke bulk gravity

We investigate a braneworld model generated by a global monopole in the context of Brans–Dicke gravity. After solving the dynamical equations we found a model capable to alleviate the so-called hierarchy problem. The obtained framework is described by a hybrid compactification scheme endowed with a seven-dimensional spacetime, in which the brane has four non-compact dimensions and two curled extra dimensions. The relevant aspects of the resulting model are studied and the requirements to avoid the well known seesaw-like behavior are discussed. We show that under certain conditions it is possible to circumvent such a pathological behavior that characterizes most of the models that exhibit hybrid compactification. Lastly, we deepen our analysis by considering possible extensions of this model to a setup with multiple branes and orbifold-like extra dimension. For this, we compute the consistency conditions to be obeyed by this more general configuration as predicted by the braneworld sum rules formalism. This study indicates the possibility of exclusively positive brane tensions in the model.

Dynamical modelling of disc vertical structure in superthin galaxy ‘UGC 7321’ in braneworld gravity: an MCMC study

ABSTRACT Low surface brightness (LSBs) superthins constitute classic examples of very late-type galaxies, with their disc dynamics strongly regulated by their dark matter haloes. In this work, we consider a gravitational origin of dark matter in the braneworld scenario, where the higher dimensional Weyl stress term projected on to the three-brane acts as the source of dark matter. In the context of the braneworld model, this dark matter is referred to as the ‘dark mass’. This model has been successful in reproducing the rotation curves of several LSB and high surface brightness galaxies. Therefore, it is interesting to study the prospect of this model in explaining the vertical structure of galaxies which has not been explored in the literature so far. Using our two-component model of gravitationally coupled stars and gas in the external force field of this dark mass, we fit the observed scale heights of stellar and atomic hydrogen (H i) gas of superthin galaxy ‘UGC7321’ using the Markov Chain Monte Carlo approach. We find that the observed scale heights of ‘UGC7321’ can be successfully modelled in the context of the braneworld scenario. In addition, the model predicted rotation curve also matches the observed one. The implications on the model parameters are discussed.

N-body simulations for parametrized modified gravity

ABSTRACT We present MG-evolution, an N-body code simulating the cosmological structure formation for parametrized modifications of gravity. It is built from the combination of parametrized linear theory with a parametrization of the deeply non-linear cosmological regime extrapolated from modified spherical collapse computations that cover the range of known screening mechanisms. We test MG-evolution, which runs at the speed of conventional ΛCDM simulations, against a suit of existing exact model-specific codes, encompassing linearized and chameleon f(R) gravity as well as the normal branch of the Dvali–Gabadadz–Porrati braneworld model, hence covering both large-field value and large-derivative screening effects. We compare the non-linear power spectra produced by the parametrized and model-specific approaches over the full range of scales set by the box size and resolution of our simulations, k = (0.05 − 2.5) $h\, \mathrm{Mpc}^{-1}$, and for two redshift slices, z = 0 and z = 1. We find sub-percent to one-percent level recovery of all the power spectra generated with the model-specific codes for the full range of scales. MG-evolution can be used for generalized and accurate tests of gravity and dark energy with the increasing wealth of high-precision cosmological survey data becoming available over the next decade.