Steady states of holographic interfaces

We find stationary thin-brane geometries that are dual to far-from-equilibrium steady states of two-dimensional holographic interfaces. The flow of heat at the boundary agrees with the result of CFT and the known energy-transport coefficients of the thin-brane model. We argue that by entangling outgoing excitations the interface produces thermodynamic entropy at a maximal rate, and point out similarities and differences with double-sided black funnels. The non-compact, non-Killing and far-from-equilibrium event horizon of our solutions coincides with the local (apparent) horizon on the colder side, but lies behind it on the hotter side of the interface. We also show that the thermal conductivity of a pair of interfaces jumps at the Hawking-Page phase transition from a regime described by classical scatterers to a quantum regime in which heat flows unobstructed.

Covariance group for null geodesic expansion calculations, and its application to the apparent horizon

We show that the recipe for computing the expansions [Formula: see text] and [Formula: see text] of outgoing and ingoing null geodesics normal to a surface admits a covariance group with nonconstant scalar [Formula: see text], corresponding to the mapping [Formula: see text], [Formula: see text]. Under this mapping, the product [Formula: see text] is invariant, and thus the marginal surface computed from the vanishing of [Formula: see text], which is used to define the apparent horizon, is invariant. This covariance group naturally appears in comparing the expansions computed with different choices of coordinate system.

Revisiting geodesic observers in cosmology

Geodesic observers in cosmology are revisited. The coordinates based on freely falling observers introduced by Gautreau in de Sitter and Einstein-de Sitter spaces (and, previously, by Gautreau and Hoffmann in Schwarzschild space) are extended to general FLRW universes. We identify situations in which the relation between geodesic and comoving coordinates can be expressed explicitly in terms of elementary functions. In general, geodesic coordinates in cosmology turn out to be rather cumbersome and limited to the region below the apparent horizon.

Qualitative aspects of Rastall gravity with barotropic fluid

We investigate the dynamical evolution of homogeneous and isotropic flat-FRW universe filled with a barotropic fluid satisfying linear equation of state in Rastall gravity. Using dynamical system approach, we find the fixed points of the system and study their stability. We further explore the thermodynamic aspects at the apparent horizon by investigating the validity of generalized second law of thermodynamics with equilibrium description.

Estimation of temperature of cosmological apparent horizons: a new approach

We consider radiation from cosmological apparent horizon in Friedmann–Lemaitre–Robertson–Walker (FLRW) model in a double-null coordinate setting. As the spacetime is dynamic, there is no timelike Killing vector, instead we have Kodama vector which acts as dynamical time. We construct the positive frequency modes of the Kodama vector across the horizon. The conditional probability that a signal reaches the central observer when it is crossing from the outside gives the temperature associated with the horizon.

The generalized second law of thermodynamics with Barrow entropy

We investigate the validity of the generalized second law of thermodynamics, applying Barrow entropy for the horizon entropy. The former arises from the fact that the black-hole surface may be deformed due to quantum-gravitational effects, quantified by a new exponent $$\Delta $$ Δ . We calculate the entropy time-variation in a universe filled with the matter and dark energy fluids, as well as the corresponding quantity for the apparent horizon. We show that although in the case $$\Delta =0$$ Δ = 0 , which corresponds to usual entropy, the sum of the entropy enclosed by the apparent horizon plus the entropy of the horizon itself is always a non-decreasing function of time and thus the generalized second law of thermodynamics is valid, in the case of Barrow entropy this is not true anymore, and the generalized second law of thermodynamics may be violated, depending on the universe evolution. Hence, in order not to have violation, the deformation from standard Bekenstein–Hawking expression should be small as expected.

Information recovery from evaporating black holes

We show that the apparent horizon and the region near [Formula: see text] of an evaporating charged, rotating black hole are timelike. It then follows that black holes in nature, which invariably have some rotation, have a channel, via which classical or quantum information can escape to the outside, while the black hole shrinks in size. We discuss implications for the information loss problem.