119224226
Entropy of an acoustic black hole in Bose-Einstein condensates
We compute the entanglement entropy associated to the Hawking emission of a $(1+1)$-dimensional acoustic black hole in a Bose-Einstein condensate. We use the brick wall model proposed by 't Hooft, adapted to the momentum space, in order to tackle the case when high frequency dispersion is taken in account. As expected, we find that in the hydrodynamic limit the entropy only depends on the size of the box in the near-horizon region, as for gravitational $(1+1)$-dimensional black holes. When dispersion effects are considered, we find a correction that depends on the square of the size of the near-horizon region measured in units of healing length, very similar to the universal correction to the entropy found in the case of spin-1/2 Heisenberg XX chains.
16113420
Self-T-dual brane cosmology and the cosmological constant problem
We consider a codimension-one brane embedded in a gravity-dilaton bulk action, whose symmetries are compatible with T-duality along the space-like directions parallel to the brane, and the bulk time-like direction. The equations of motions in the string frame allow for a smooth background obtained by the union of two symmetric patches of AdS space. The Poincare invariance of the solution appears to hold independently of the value of the brane vacuum energy, through a self-tuning property of the dilaton ground state. Moreover, the effective cosmology displays a bounce, at which the scale factor does not shrink to zero. Finally, by exploiting the T-duality symmetry, we show how to construct an ever-expanding Universe, along the lines of the Pre-Big-Bang scenario.
53531318
PARTICLE PRODUCTION AND TRANSPLANCKIAN PROBLEM ON THE NONCOMMUTATIVE PLANE
We consider the coherent state approach to noncommutativity and we derive from it an effective quantum scalar field theory. We show how the noncommutativity can be taken into account by a suitable modification of the Klein–Gordon product, and of the equal-time commutation relations. We prove that, in curved space, the Bogoliubov coefficients are unchanged, hence the number density of the produced particle is the same as for the commutative case. What changes though is the associated energy density, and this offers a simple solution to the transplanckian problem.
12251695
Self-T-Dual Brane Cosmology
We show how T-duality can be implemented with brane cosmology. As a result, we obtain a smooth bouncing cosmology with features similar to the ones of the pre-Big Bang scenario. Also, by allowing T-duality transformations along the time-like direction, we find a static solution that displays an interesting self tuning property.
119287029
ASPECTS OF QUANTUM GRAVITY IN COSMOLOGY
We review some aspects of quantum gravity in the context of cosmology. In particular, we focus on models with a phenomenology accessible to current and near-future observations, as the early Universe might be our only chance to peep through the quantum gravity realm.
118790490
Observational signatures of pre-inflationary and lower-dimensional effective gravity
Several alternative ways to quantize gravitational interactions seem to indicate that gravity at short distances is effectively two-dimensional. If this were true, and the energy scale at which the dimensional transition occurs is of the order of the inflationary one, it is reasonable to investigate eventual signatures in the primordial perturbation spectra. In this paper we look at this possibility by assuming that the inflationary era was preceded by an effectively two-dimensional evolution of the Universe. We model the transition with a mode matching and we show that, in this case, no observational signatures are expected in the tensor and scalar power spectra nor in their ratio. We also show that assuming modified dispersion relations before the matching between two sets of four-dimensional modes leads to similar results.
119115922
Extracting generalized neutron parton distributions from $^3He$ data
An impulse approximation analysis is described of the generalized parton distributions (GPDs) H and E of the 3He nucleus, quantities which are accessible in hard exclusive processes, such as coherent deeply virtual Compton scattering (DVCS). The calculation is based on the Av18 interaction. The electromagnetic form factors are correctly recovered in the proper limits. The sum of the GPDs H and E of 3He, at low momentum transfer, is largely dominated by the neutron contribution, thanks to the unique spin structure of 3He. This nucleus is therefore very promising for the extraction of the neutron information. By increasing the momentum transfer, however, this conclusion is somehow hindered by the the fast growing proton contribution. Besides, even when the neutron contribution to the GPDs of 3He is largely dominating, the procedure of extracting the neutron GPDs from it could be, in principle, nontrivial. A technique is therefore proposed, independent on both the nuclear potential and the nucleon model used in the calculation, able to take into account the nuclear effects included in the IA analysis and to safely extract the neutron information at values of the momentum transfer large enough to allow the measurements. Thanks to this observation, coherent DVCS should be considered a key experiment to access the neutron GPDs and, in turn, the orbital angular momentum of the partons in the neutron.
118175128
Field theory and strings
118521890
Neutron orbital structure from generalized parton distributions of 3He
The generalized parton distribution H and E of the 3He nucleus, which could be measured in hard exclusive processes, such as coherent deeply virtual Compton scattering, are thoroughly analyzed in impulse approximation, within the Av18 interaction. It is found that their sum is dominated to a large extent by the neutron contribution: The peculiar spin structure of 3He makes this target unique for the extraction of the neutron information. This observation could allow to access for the first time, in dedicated experiments, the orbital angular momentum of the partons in the neutron.
120477826
Spin structures, spinors and the Dirac operator: real versus complex manifolds
Abstract We describe the relation between spin-structures, spinors and the Dirac operator on a (real) manifold and the analogous definitions in complex holomorphic terms. This may be useful for physicists interested in the algebraic geometric approach to superstrings.
121725055
Conformal invariance on a generic Riemann surface in the presence of a non-flat background metric
Abstract By using bases of meromorphic differentials, recently proposed by Krichever and Novikov, we calculate the brackets of the moments of the energy-momentum tensor of a bosonic string theory in the presence of a generic target metric. To do this we expand the target metric in normal coordinates. To the lowest perturbative order we find that the BRST operator is nilpotent if we impose the condition that the Ricci tensor of the target metric vanish.