Daniel C. Guariento

Analytical solutions of accreting black holes immersed in a Lambda-CDM model

Abstract The evolution of the mass of a black hole embedded in a universe filled with dark energy and cold dark matter is calculated in a closed form within a test fluid model in a Schwarzschild metric, taking into account the cosmological evolution of both fluids. The result describes exactly how accretion asymptotically switches from the matter-dominated to the Λ-dominated regime. For early epochs, the black hole mass increases due to dark matter accretion, and on later epochs the increase in mass stops as dark energy accretion takes over. Thus, the unphysical behaviour of previous analyses is improved in this simple exact model.

Evolution of primordial black holes in a radiation and phantom energy environment

In this work we extend previous work on the evolution of a primordial black hole (PBH) to address the presence of a dark energy component with a super-negative equation of state as a background, investigating the competition between the radiation accretion, the Hawking evaporation and the phantom accretion, the latter two causing a decrease on black hole mass. It is found that there is an instant during the matter-dominated era after which the radiation accretion becomes negligible compared to the phantom accretion. The Hawking evaporation may become important again depending on a mass threshold. The evaporation of PBHs is quite modified at late times by these effects, but only if the generalized second law of thermodynamics is violated.

How the expansion of the Universe determines the causal structure of McVittie spacetimes

We present a theorem which allows one to recognize and classify the asymptotic behavior and causal structure of McVittie metrics for different choices of scale factor, establishing whether a black hole or a pair black-white hole appears in the appropriate limit. Incidentally, the theorem also solves an apparent contradiction present in the literature over the causal structure analysis of the McVittie solution. Although the classification we present is not fully complete, we argue that this result covers most if not all physically relevant scenarios.

Separating expansion and collapse in general fluid models with heat flux

The work of M. Le D. has been supported by CSIC (JAEDoc072), CICYT (FPA2006-05807) in Spain and FAPESP (2011/24089-5) in Brazil. F. C. M. thanks CMAT, Universidade Minho, for support through FEDER Funds COMPETE, FCT Projects No. Est-C/MAT/UI0013/2011, No. PTDC/MAT/108921/2008, No. CERN/FP/123609/2011, and also Instituto de Fisica, UERJ, Rio de Janeiro, for hospitality. M. LeD. and J. P. M. acknowledge the CAAULs Project No. PEst-OE/FIS/UI2751/2011. J. P. M. also wishes to thank FCT for Grants No. CERN/FP/123615/2011 and No. CERN/FP/123618/2011. M. F. is supported by FAPESP Grant No. 2011/11365-4, D. C. G. is supported by FAPESP Grant No. 2010/08267-8, and E. A. is supported by FAPESP and CNPq.

Phantom accretion by black holes and the generalized second law of thermodynamics

Abstract The accretion of a phantom fluid with non-zero chemical potential by black holes is discussed with basis on the generalized second law of thermodynamics. For phantom fluids with positive temperature and negative chemical potential we demonstrate that the accretion process is possible, and that the condition guaranteeing the positiveness of the phantom fluid entropy coincides with the one required by the generalized second law.

Realistic fluids as source for dynamically accreting black holes in a cosmological background

We show that a single imperfect fluid can be used as a source to obtain the generalized McVittie metric as an exact solution to Einsteins equations. The mass parameter in this metric varies with time thanks to a mechanism based on the presence of a temperature gradient. This fully dynamical solution is interpreted as an accreting black hole in an expanding universe if the metric asymptotes to Schwarzschild-de Sitter at temporal infinity. We present a simple but instructive example for the mass function and briefly discuss the structure of the apparent horizons and the past singularity.

Horndeski theory meets the McVittie solution: A scalar field theory for accretion onto cosmological black holes

We show that the generalized McVittie spacetime, which represents a black hole with time-dependent mass in an expanding universe, is an exact solution of a subclass of the Horndeski family of actions. The heat-flow term responsible for the energy transfer between the black hole and the cosmological background is generated by the higher-order kinetic gravity braiding term, which generalizes the cuscuton action that yields McVittie with constant mass as a solution. Finally, we show that this generalization can be understood in terms of a duality realized by a disformal transformation, connecting the cuscuton field theory to an extension of the Horndeski action which does not propagate any scalar degrees of freedom. Our finding opens a novel window into studies of nontrivial interactions between dark energy/modified gravity theories and astrophysical black holes.

Cosmological black holes from self-gravitating fields

Both cosmological expansion and black holes are ubiquitous features of our observable Universe, yet exact solutions connecting the two have remained elusive. To this end, we study self-gravitating classical fields within dynamical spherically symmetric solutions that can describe black holes in an expanding universe. After attempting a perturbative approach of a known black-hole solution with scalar hair, we show by exact methods that the unique scalar field action with first-order derivatives that can source shear-free expansion around a black hole requires noncanonical kinetic terms. The resulting action is an incompressible limit of k-essence, otherwise known as the cuscuton theory, and the spacetime it describes is the McVittie metric. We further show that this solution is an exact solution to the vacuum Ho\v{r}ava-Lifshitz gravity with anisotropic Weyl symmetry.

Cosmological black holes and white holes with time-dependent mass

We consider the causal structure of generalized uncharged McVittie spacetimes with increasing central mass

Hamiltonian analysis of the cuscuton

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