Exploring the small mass limit of stationary black holes in theories with Gauss-Bonnet terms

In this work we examine the small mass limit of black holes (BHs), with and without spin, in theories where a scalar field is non-minimally coupled to a Gauss–Bonnet (GB) term. First, we provide an analytical example for a theory where a static closed-form solution with a small mass limit is known, and later use analytical and numerical techniques to explore this limit in standard scalar-GB theories with dilatonic, linear and quadratic-exponential couplings. In most cases studied here, we find an inner singularity that overlaps with the event horizon of the static BH as the small mass limit is reached. Moreover, since solutions in this limit possess a non-vanishing Hawking temperature, a naked singularity is expected to be reached through evaporation, raising questions concerning the consistency of these theories altogether. On the other hand, we provide for the first time in this context an example of a coupling where the small mass limit is never reached, thus preferred from the point of view of cosmic censorship. Finally, we consider BHs with spin and numerically investigate how this changes the picture, using these to place the tightest upper bounds to date on the coupling constant for the dilatonic and linear theories, with α‾<1 km

Derivation of regularized field equations for the Einstein-Gauss-Bonnet theory in four dimensions

6 pages + 2 pages with reference

Observational constraints on the regularized 4D Einstein-Gauss-Bonnet theory of gravity

In this paper we study the observational constraints that can be imposed on the coupling parameter, $\hat \alpha$, of the regularized version of the 4-dimensional Einstein-Gauss-Bonnet theory of gravity. We use the scalar-tensor field equations of this theory to perform a thorough investigation of its slow-motion and weak-field limit, and apply our results to observations of a wide array of physical systems that admit such a description. We find that the LAGEOS satellites are the most constraining, requiring $| \hat \alpha | \lesssim 10^{10} \,{\rm m}^2$. This constraint suggests that the possibility of large deviations from general relativity is small in all systems except the very early universe ($t<10^{-3}\, {\rm s}$), or the immediate vicinity of stellar-mass black holes ($M\lesssim100\, M_{\odot}$). We then consider constraints that can be imposed on this theory from cosmology, black hole systems, and table-top experiments. It is found that early universe inflation prohibits all but the smallest negative values of $\hat \alpha$, while observations of binary black hole systems are likely to offer the tightest constraints on positive values, leading to overall bounds $0 \lesssim \hat \alpha \lesssim 10^8 \, {\rm m}^2$.Comment: 17 page

The 4D Einstein-Gauss-Bonnet theory of gravity: a review

57 pages, 2 figures57 pages, 2 figure

Black Holes in the Scalar-Tensor Formulation of 4D Einstein-Gauss-Bonnet Gravity: Uniqueness of Solutions, and a New Candidate for Dark Matter

In this work we study static black holes in the regularized 4D Einstein-Gauss-Bonnet theory of gravity; a shift-symmetric scalar-tensor theory that belongs to the Horndeski class. This theory features a simple black hole solution that can be written in closed form, and which we show is the unique static, spherically-symmetric and asymptotically-flat black hole vacuum solution of the theory. We further show that no asymptotically-flat, time-dependent, spherically-symmetric perturbations to this geometry are allowed, which suggests that it may be the only spherically-symmetric vacuum solution that this theory admits (a result analogous to Birkhoff's theorem). Finally, we consider the thermodynamic properties of these black holes, and find that their final state after evaporation is a remnant with a size determined by the coupling constant of the theory. We speculate that remnants of this kind from primordial black holes could act as dark matter, and we constrain the parameter space for their formation mass, as well as the coupling constant of the theory

Awareness of School Learning Environments

Now, and in the times that follow, student education should focus on developing inclusive skills such as problem-solving and decision-making, where the role of the learning environment plays a crucial part, i.e., it is a process where the screen of the universe of discourse is accomplished in order to consider not only the complex relationships that flow among the objects that populate it, but also its inner structure, co-existing incomplete/unknown or even self-contradictory information or knowledge. As a result, we will focus on the development of an Intelligent Social Machine to assess Learning Environments in high schools, based on factors like School and Disciplinary Climates as well as Parental Involvement. The formal background will be to use Logic Programming to define its architecture based on a Deep Learning-Big Data approach to Knowledge Representation and Reasoning, complemented by an Evolutionary approach to Computing grounded on Virtual Intellects

Determinants for the success of software startups: Insights from a regional cluster

In recent years, we have seen a growing interest in technology-based companies and intensive knowledge. Several regional clusters have appeared supported in dynamic entrepreneurial ecosystems which, alongside intrinsic aspects of the business, are important determinants of the success of new companies. However, most startups created in these innovation-oriented spaces do not survive the first years of life, due to the high competitiveness of the technological market, due to deficiencies in the business model, due to the support conditions provided by the surrounding ecosystem, and finally due to a weak adjustment between all these dimensions. Among several models available, the Early-Life Decision Model (ELDM) presents itself as an interesting framework for studying the development and success conditions of software companies. This article discusses the application of the ELDM based on a series of interviews conducted to 15 Portuguese software startups installed in a technological cluster located in the northeast of Portugal. Based on the results obtained, it was appropriate to add a new dimension to the ELDM model (learning) and complementing it with the perspectives of the business type and internal versus external determinants.This work was supported by COMPETE: POCI-01-0145-FEDER-007043 and FCT (Fundação para a Ciência e a Tecnologia) within the Project Scope: UID/CEC/00319/2013