Wormholes in spacetimes with cosmological horizons

A generalisation of the asymptotic wormhole boundary condition for the case of spacetimes with a cosmological horizon is proposed. In particular, we consider de Sitter spacetime with small cosmological constant. The wave functions selected by this proposal are exponentially damped in WKB approximation when the scale factor is large but still much smaller than the horizon size. In addition, they only include outgoing gravitational modes in the region beyond the horizon. We argue that these wave functions represent quantum wormholes and compute the local effective interactions induced by them in low-energy field theory. These effective interactions differ from those for flat spacetime in terms that explicitly depend on the cosmological constant.Comment: 10 pages, LaTeX 2.O9, no figure

Scientific collaboration networks: how little differences can matter a lot

Empirical studies such as Goyal, van der Leij and Moraga (2006) or Newman (2004) show that scientific collaboration networks present a highly unequal and hierarchical distribution of links. This implies that some researchers can be much more active and productive than others and, consequently, they can enjoy a much better scientific eputation. One may think that big intrinsical differences among researchers can constitute the main driving force behind these huge inequalities. We propose a model that show how almost identical individuals self-organize themselves in a very unequal and hierarchical structure as is observed in the real-world co-authorship networks. In consequence, this model provides an incentives-based explanation of that empirical evidence.network formation game, scientific collaboration, co-authroship networks, inequality

Structural holes and densely connected communities

It has been empirically shown that structural holes in social networks enable potential large benefits to those individuals who bridge them (Burt, 2004). The pioneering paper Goyal and Vega-Redondo (2007) offers a new incentives based explanation of this phenomenon. But the main equilibrium network of their model does not display a basic empirical regularity: the architecture of social networks is characterized by the existence of densely linked communities loosely connected to one another (Granovetter, 1983). This paper analyzes the conditions under which agents who benefit from bridging structural holes can be sustained in equilibrium networks constituted by densely linked groups.network formation, personal income distribution, structural holes, communities

Frau informàtic i ètica professional

Peer Reviewe

A Dynamic Model of Final Service Competition in fixed Electronic Communications under a Capacity Interconnection Regime

Recientemente el regulador español ha implementado un nuevo modelo de interconexión en redes fijas de telecomunicaciones basado en la compra de capacidad que podrá ser utilizado en combinación con el modelo habitual de interconexión por tiempo. Se propone un modelo dinámico de competencia en duopolio en el cual el entrante puede comprar ex ante un cierto nivel de capacidad al incumbente paraluego ambos operadores competir en precios. Se introduce la posibilidad de que el entrante pueda asignar eficientemente el tráfico entre distintas franjas horarias. Los resultados muestran que el uso simultáneo de ambos modelos genera una competencia más agresiva en precios y puede llegar a producir significativas ganancias de eficiencia (asignativa y otras internas a la firma). De cualquier forma, el entrante necesita alcanzar una masa crítica antes de que el nuevo modelo pueda representar una alternativa para él. Además, una vez alcanzado este nivel, su conveniencia para el entrante dependerá de determinadas condiciones.The Spanish regulatory authority recently implemented a new interconnection regime based on capacity (and not per time) payments in fixed telecommunications. We propose a dynamic duopolistic model of final service competition in which the entrant first acquires a certain capacity at the local loop (at a fixed payment) from the incumbent and then both operators compete in prices for the final services. We introduce the entrant’s possibility to assign efficiently the traffic he offer between different hours along the day as well as the possibility for him to use the capacity based model as well as the per-unit-oftime interconnection regime vis à vis the incumbent. The results show that, the simultaneous use of both interconnection models (instead of only the one based on time) leads always to tougher competition (more aggressive pricing) in the final service market and efficiency (internal to the firm and allocative) gains. Nevertheless, the entrant needs a minimum scale before this new model can be a viable alternative. In addition, once reached this scale, its convenience for the entrant will depend on certain conditions.Interconnection per time, Interconnection by capacity, Dynamics of the competition, Regulation, Network dimension.

On a boundary-localized Higgs boson in 5D theories

In the context of a simple five-dimensional (5D) model with bulk matter coupled to a brane-localized Higgs boson, we point out a new non-commutativity in the 4D calculation of the mass spectrum for excited fermion towers: the obtained expression depends on the choice in ordering the limits, N->infinity (infinite Kaluza-Klein tower) and epsilon->0 (epsilon being the parameter introduced for regularizing the Higgs Dirac peak). This introduces the physical question of which one is the correct order; we then show that the two possible orders of regularization (called I and II) are physically equivalent, as both can typically reproduce the measured observables, but that the one with less degrees of freedom (I) could be uniquely excluded by future experimental constraints. This conclusion is based on the exact matching between the 4D and 5D analytical calculations of the mass spectrum - via the regularizations of type I and II. Beyond a deeper insight into the Higgs peak regularizations, this matching also allows us to confirm the validity of the usual 5D mixed-formalism and to clarify the UV cut-off procedure. All the conclusions, deduced from regularizing the Higgs peak through a brane shift or a smoothed square profile, are expected to remain similar in realistic models with a warped extra-dimension.Comment: 29 pages, 2 table

Algunas activitats extracurriculars a la FIB

Peer Reviewe

Entropy bounds in terms of the w parameter

In a pair of recent articles [PRL 105 (2010) 041302 - arXiv:1005.1132; JHEP 1103 (2011) 056 - arXiv:1012.2867] two of the current authors have developed an entropy bound for equilibrium uncollapsed matter using only classical general relativity, basic thermodynamics, and the Unruh effect. An odd feature of that bound, S <= A/2, was that the proportionality constant, 1/2, was weaker than that expected from black hole thermodynamics, 1/4. In the current article we strengthen the previous results by obtaining a bound involving the (suitably averaged) w parameter. Simple causality arguments restrict this averaged parameter to be <= 1. When equality holds, the entropy bound saturates at the value expected based on black hole thermodynamics. We also add some clarifying comments regarding the (net) positivity of the chemical potential. Overall, we find that even in the absence of any black hole region, we can nevertheless get arbitrarily close to the Bekenstein entropy.Comment: V1: 14 pages. V2: One reference added. V3: This version accepted for publication in JHE