A Positive Cosmological Constant in String Theory Through Ads/CFT Wormholes

There are two important examples of physical systems which violate the strong energy condition : Universes (like, it would seem, our own) with a positive cosmological constant, and wormholes. We suggest that a positive cosmological constant can be reconciled with string theory by considering wormholes in string backgrounds. This is argued in two directions : first, we show that brane-worlds with positive cosmological constants give rise to bulk singularities which are best resolved by embedding the brane-world in an AdS/CFT wormhole; and second, for the simplest kind of wormhole in an asymptotically AdS space, we show that the IR stability of the matter needed to keep the wormhole open requires the presence of a brane-world. UV stability conditions then forbid a negative cosmological constant on the brane-world.Comment: 21 pages, references added, clarification of claim that string theory on a wormhole background requires a positive observed cosmological constant; NPB versio

On the Relativistic Description of the Nucleus

We present here a formalism able to generalise to a relativistically covariant scheme the standard nuclear shell model. We show that, using some generalised nuclear Green's functions and their Lehmann representation we can define the relativistic equivalent of the non relativistic single particle wave function (not loosing, however, the physical contribution of other degrees of freedom, like mesons and antinucleons). It is shown that the mass operator associated to the nuclear Green's function can be approximated with the equivalent of a shell-model potential and that the corresponding ``single particle wave functions'' can be easily derived in a specified frame of reference and then boosted to any other system, thus fully restoring the Lorentz covarianc

The Electron-Phonon Interaction in the Presence of Strong Correlations

We investigate the effect of strong electron-electron repulsion on the electron-phonon interaction from a Fermi-liquid point of view: the strong interaction is responsible for vertex corrections, which are strongly dependent on the $v_Fq/\omega$ ratio. These corrections generically lead to a strong suppression of the effective coupling between quasiparticles mediated by a single phonon exchange in the $v_Fq/\omega \gg 1$ limit. However, such effect is not present when $v_Fq/\omega \ll 1$. Analyzing the Landau stability criterion, we show that a sizable electron-phonon interaction can push the system towards a phase-separation instability. A detailed analysis is then carried out using a slave-boson approach for the infinite-U three-band Hubbard model. In the presence of a coupling between the local hole density and a dispersionless optical phonon, we explicitly confirm the strong dependence of the hole-phonon coupling on the transferred momentum versus frequency ratio. We also find that the exchange of phonons leads to an unstable phase with negative compressibility already at small values of the bare hole-phonon coupling. Close to the unstable region, we detect Cooper instabilities both in s- and d-wave channels supporting a possible connection between phase separation and superconductivity in strongly correlated systems.Comment: LateX 3.14, 04.11.1994 Preprint no.101

GEM: a Distributed Goal Evaluation Algorithm for Trust Management

Trust management is an approach to access control in distributed systems where access decisions are based on policy statements issued by multiple principals and stored in a distributed manner. In trust management, the policy statements of a principal can refer to other principals' statements; thus, the process of evaluating an access request (i.e., a goal) consists of finding a "chain" of policy statements that allows the access to the requested resource. Most existing goal evaluation algorithms for trust management either rely on a centralized evaluation strategy, which consists of collecting all the relevant policy statements in a single location (and therefore they do not guarantee the confidentiality of intensional policies), or do not detect the termination of the computation (i.e., when all the answers of a goal are computed). In this paper we present GEM, a distributed goal evaluation algorithm for trust management systems that relies on function-free logic programming for the specification of policy statements. GEM detects termination in a completely distributed way without disclosing intensional policies, thereby preserving their confidentiality. We demonstrate that the algorithm terminates and is sound and complete with respect to the standard semantics for logic programs.Comment: To appear in Theory and Practice of Logic Programming (TPLP

Love thy neighbour? Coronavirus politics and their impact on EU freedoms and rule of law in the Schengen Area. CEPS Paper in Liberty and Security in Europe No. 2020-04, April 2020

Restrictions on international and intra-EU traffic of persons have been at the heart of the political responses to the coronavirus pandemic. Border controls and suspensions of entry and exist have been presented as key policy priorities to prevent the spread of the virus in the EU. These measures pose however fundamental questions as to the raison d’être of the Union, and the foundations of the Single Market, the Schengen system and European citizenship. They are also profoundly intrusive regarding the fundamental rights of individuals and in many cases derogate domestic and EU rule of law checks and balances over executive decisions. This Paper examines the legality of cross-border mobility restrictions introduced in the name of COVID-19. It provides an in-depth typology and comprehensive assessment of measures including the reintroduction of internal border controls, restrictions of specific international traffic modes and intra-EU and international ‘travel bans’. Many of these have been adopted in combination with declarations of a ‘state of emergency’

Finite-size corrections vs. relaxation after a sudden quench

We consider the time evolution after sudden quenches of global parameters in translational invariant Hamiltonians and study the time average expectation values and entanglement entropies in finite chains. We show that in noninteracting models the time average of spin correlation functions is asymptotically equal to the infinite time limit in the infinite chain, which is known to be described by a generalized Gibbs ensemble. The equivalence breaks down considering nonlocal operators, and we establish that this can be traced back to the existence of conservation laws common to the Hamiltonian before and after the quench. We develop a method to compute the leading finite-size correction for time average correlation functions and entanglement entropies. We find that large corrections are generally associated to observables with slow relaxation dynamics.Comment: 12 pages, 2 figures; V2: minor changes and reference adde