Dirichlet baths and the not-so-fine-grained Page curve

We present a doubly holographic prescription for computing entanglement entropy on a gravitating brane. It involves a Ryu-Takayanagi surface with a Dirichlet anchoring condition. In braneworld cosmology, a related approach was used previously in arXiv:2007.06551. There, the prescription naturally computed a co-moving entanglement entropy, and was argued to resolve the information paradox for a black hole living in the cosmology. In this paper, we show that the Dirichlet prescription leads to reasonable results, when applied to a recently studied wedge holography set up with a gravitating bath. The nature of the information paradox and its resolution in our Dirichlet problem have a natural understanding in terms of the strength of gravity on the two branes and at the anchoring location. By sliding the anchor to the defect, we demonstrate that the limit where gravity decouples from the anchor is continuous — in other words, as far as island physics is considered, weak gravity on the anchor is identical to no gravity. The weak and (moderately) strong gravity regions on the brane are separated by a “Dirichlet wall”. We find an intricate interplay between various extremal surfaces, with an island coming to the rescue whenever there is an information paradox. This is despite the presence of massless gravitons in the spectrum. The overall physics is consistent with the slogan that gravity becomes “more holographic”, as it gets stronger. Our observations strengthen the case that the conventional Page curve is indeed of significance, when discussing the information paradox in flat space. We work in high enough dimensions so that the graviton is non-trivial, and our results are in line with the previous discussions on gravitating baths in arXiv:2005.02993 and arXiv:2007.06551.

A note on supergravity inflation in braneworld

We discuss supergravity inflation in braneworld cosmology for the class of potentials [Formula: see text] with [Formula: see text]. These minimal SUGRA models evade the [Formula: see text] problem due to a broken shift symmetry and can easily accommodate the observational constraints. In the high energy regime [Formula: see text], the numerical predictions and approximate analytic formulas are given for the scalar spectral index [Formula: see text] and tensor-to-scalar ratio [Formula: see text]. The models with smaller [Formula: see text] are preferred while the models with larger [Formula: see text] are out of the [Formula: see text] region. Remarkably, the [Formula: see text] correction to the energy density in Friedmann equation results in sub-Planckian inflaton excursions [Formula: see text].

Inflation with a class of concave inflaton potentials in Randall–Sundrum model

We investigate inflation with a class of concave inflaton potentials of the form $$\sim \phi ^n$$∼ϕn$$(0<n<1)$$(0<n<1) in the Randall–Sundrum model with an infinite extra spatial dimension. We show that this class of models is much more in good agreement with observations compared to the standard inflation. We also find the range of the five-dimensional Planck scale ($$M_5$$M5) and show that large tensor-to-scalar ratios do not eliminate small-field inflation in braneworld cosmology.

Nonlinear matter terms in general scalar–tensor braneworld cosmology

A five-dimensional braneworld cosmological model in general scalar–tensor action that is comprised of various Horndeski Lagrangians is considered. The Friedmann equations in the case of strongly and weakly coupled [Formula: see text] Horndeski Lagrangians have been obtained. The strongly coupled [Formula: see text] model produces the Cardassian term [Formula: see text] with [Formula: see text], which can serve as an alternative explanation for the accelerated expansion phase of the universe. Furthermore, the latest combined observational facts from BAO, CMB, SNIa, [Formula: see text] and [Formula: see text] value observation suggest that the [Formula: see text] term lies quite close to the constrained value. On the other hand, the weakly coupled [Formula: see text] case has several new correction terms which are omitted in the braneworld Einstein–Hilbert model, e.g. the cubic [Formula: see text] and the dark radiation–matter interaction term [Formula: see text]. Furthermore, this model provides a cosmological constant constructed from the bulk scalar field, requires no brane tension and supports the big bang nucleosynthesis (BBN) constraint naturally.

Tachyon scalar field in a braneworld cosmology

We analyze a tachyon cosmological model based on the dynamics of a 3-brane in the bulk of the second Randall–Sundrum model. This model contains extended general warp functions, i.e. generalized bulk geometry. We study a power law warp factor in cosmological context. This type of warp factor generates an inverse power law tachyonic potential.

Tachyon inflation in an AdS braneworld with backreaction

We analyze the inflationary scenario based on the tachyon field coupled with the radion of the second Randall–Sundrum model (RSII). The tachyon Lagrangian is derived from the dynamics of a 3-brane moving in the five-dimensional bulk. The AdS5 geometry of the bulk is extended to include the radion. Using the Hamiltonian formalism we find four nonlinear field equations supplemented by the modified Friedmann equations of the RSII braneworld cosmology. After a suitable rescaling we reduce the parameters of our model to only one free parameter related to the brane tension and the AdS5 curvature. We solve the equations numerically assuming a reasonably wide range of initial conditions determined by physical considerations. Varying the free parameter and initial conditions we confront our results with the Planck 2015 data.