Black Holes In Brans-Dicke Theory Research Articles

Non-singular Brans–Dicke collapse in deformed phase space

We study the collapse process of a homogeneous perfect fluid (in FLRW background) with a barotropic equation of state in Brans–Dicke (BD) theory in the presence of phase space deformation effects. Such a deformation is introduced as a particular type of non-commutativity between phase space coordinates. For the commutative case, it has been shown in the literature (Scheel, 1995), that the dust collapse in BD theory leads to the formation of a spacetime singularity which is covered by an event horizon. In comparison to general relativity (GR), the authors concluded that the final state of black holes in BD theory is identical to the GR case but differs from GR during the dynamical evolution of the collapse process. However, the presence of non-commutative effects influences the dynamics of the collapse scenario and consequently a non-singular evolution is developed in the sense that a bounce emerges at a minimum radius, after which an expanding phase begins. Such a behavior is observed for positive values of the BD coupling parameter. For large positive values of the BD coupling parameter, when non-commutative effects are present, the dynamics of collapse process differs from the GR case. Finally, we show that for negative values of the BD coupling parameter, the singularity is replaced by an oscillatory bounce occurring at a finite time, with the frequency of oscillation and amplitude being damped at late times.

Phase transition of charged Black Holes in Brans–Dicke theory through geometrical thermodynamics

In this paper, we take into account black hole solutions of Brans-Dicke-Maxwell theory and investigate their stability and phase transition points. We apply the concept of geometry in thermodynamics to obtain phase transition points and compare its results with those, calculated in canonical ensemble through heat capacity. We show that these black holes enjoy second order phase transitions. We also show that there is a lower bound for the horizon radius of physical charged black holes in Brans-Dicke theory which is originated from restrictions of positivity of temperature. In addition, we find that employing specific thermodynamical metric in the context of geometrical thermodynamics, yields divergencies for thermodynamical Ricci scalar in places of the phase transitions. It will be pointed out that due to characteristics behavior of thermodynamical Ricci scalar around its divergence points, one is able to distinguish the physical limitation point from the phase transitions. In addition, the free energy of these black holes will be obtained and its behavior will be investigated. It will be shown that the behavior of the free energy in the place where the heat capacity diverges, demonstrates second order phase transition characteristics.

Vacuum Energy and Primordial Black Holes in Brans-Dicke Theory

In our work, we study the evolution of primordial black holes within the context of Brans-Dicke theory by assuming present universe as vacuum dominated. We also consider the accretion of radiation, matter and vacuum energy only during respective dominant periods. From our study, we found that the accretion rate is slower in both radiation and vacuum energy dominated eras in Brans-Dicke theory in comparision with General Theory of Relativity. Thus the PBHs evaporate at a faster rate in Brans-Dicke theory than Standard Cosmology, if we consider the presence of vacuum energy in both cases. We also find that vacuum energy accretion efficiency should be less than 0.61.

Note on nonstationarity and accretion by primordial black holes in Brans-Dicke theory

We consider the evolution of primordial black holes by including non-stationarity in their formation process and accretion of radiation in Brans-Dicke theory. Specifically, we focus on how $\eta$, the fraction of the horizon mass the black hole comprises capturing nonstationarity, affects the lifetimes of these primordial black holes. Our calculation reveals that the primordial black hole dynamics is controlled by the product $f\eta$ where $f$ is the accretion efficiency. We also estimate the impact of $\eta$ through $f\eta$ on the primordial black holes' initial mass fraction constraint obtained from the $\gamma$-ray background limit.

Astrophysical constraints on primordial black holes in Brans-Dicke theory

We consider cosmological evolution in Brans-Dicke theory with a populationof primordial black holes. Hawking radiation from the primordial black holesimpacts various astrophysical processes during the evolution of the Universe.The accretion of radiation by the black holes in the radiation dominated eramay be effective in imparting them a longer lifetime. We present a detailed study of how this affects various standard astrophysical constraints coming from the evaporation of primordial black holes. We analyze constraints arisingfrom the present density of the Universe, the present photonspectrum, the distortion of the cosmic microwave background spectrum and also from processes affecting light element abundances after nucleosynthesis. We find that the constraints on the initial primordialblack hole mass fractions are tightened with increased accretion efficiency.

Effect of accretion on primordial black holes in Brans-Dicke theory

We consider the effect of accretion of radiation in the early Universe on primordial black holes in Brans-Dicke theory. The rate of growth of a primordial black hole due to accretion of radiation in Brans-Dicke theory is considerably smaller than the rate of growth of the cosmological horizon, thus making available sufficient radiation density for the black hole to accrete causally. We show that accretion of radiation by Brans-Dicke black holes overrides the effect of Hawking evaporation during the radiation dominated era. The subsequent evaporation of the black holes in later eras is further modified due to the variable gravitational ``constant,'' and they could survive up to longer times compared to the case of standard cosmology. We estimate the impact of accretion on modification of the constraint on their initial mass fraction obtained from the $\ensuremath{\gamma}$-ray background limit from presently evaporating primordial black holes.

Uniqueness of self-similar asymptotically Friedmann-Robertson-Walker spacetime in Brans-Dicke theory

We investigate spherically symmetric self-similar solutions in Brans-Dicke theory. Assuming a perfect fluid with the equation of state $p=(\gamma-1)\mu (1 \le \gamma<2)$, we show that there are no non-trivial solutions which approach asymptotically to the flat Friedmann-Robertson-Walker spacetime if the energy density is positive. This result suggests that primordial black holes in Brans-Dicke theory cannot grow at the same rate as the size of the cosmological particle horizon.

Non-Abelian black holes in Brans-Dicke theory

We find a black hole solution with non-Abelian field in Brans-Dicke theory. It is an extension of non-Abelian black hole in general relativity. We discuss two non-Abelian fields: "SU(2)" Yang-Mills field with a mass (Proca field) and the SU(2)$\times$SU(2) Skyrme field. In both cases, as in general relativity, there are two branches of solutions, i.e., two black hole solutions with the same horizon radius. Masses of both black holes are always smaller than those in general relativity. A cusp structure in the mass-horizon radius ($M_{g}$-$r_{h}$) diagram, which is a typical symptom of stability change in catastrophe theory, does not appear in the Brans-Dicke frame but is found in the Einstein conformal frame. This suggests that catastrophe theory may be simply applied for a stability analysis as it is if we use the variables in the Einstein frame. We also discuss the effects of the Brans-Dicke scalar field on black hole structure.

Collapse to black holes in Brans-Dicke theory. I. Horizon boundary conditions for dynamical spacetimes.

We present a new numerical code that evolves a spherically symmetric configuration of collisionless matter in the Brans-Dicke theory of gravitation. In this theory the spacetime is dynamical even in spherical symmetry, where it can contain gravitational radiation. Our code is capable of accurately tracking collapse to a black hole in a dynamical spacetime arbitrarily far into the future, without encountering either coordinate pathologies or spacetime singularities. This is accomplished by truncating the spacetime at a spherical surface inside the apparent horizon, and subsequently solving the evolution and constraint equations only in the exterior region. We use our code to address a number of long-standing theoretical questions about collapse to black holes in Brans-Dicke theory.

Collapse to black holes in Brans-Dicke theory. II. Comparison with general relativity

We discuss a number of long-standing theoretical questions about collapse to black holes in the Brans-Dicke theory of gravitation. Using a new numerical code, we show that Oppenheimer-Snyder collapse in this theory produces black holes that are identical to those of general relativity in final equilibrium, but are quite different from those of general relativity during dynamical evolution. We find that there are epochs during which the apparent horizon of such a black hole passes {\it outside\/} the event horizon, and that the surface area of the event horizon {\it decreases\/} with time. This behavior is possible because theorems which prove otherwise assume $R_{ab}l^al^b \ge 0$ for all null vectors $l^a$. We show that dynamical spacetimes in Brans-Dicke theory can violate this inequality, even in vacuum, for any value of $\omega$.

Static black holes in Brans-Dicke theory are Schwarzschild

Static black holes in Brans-Dicke theory are Schwarzschild