RECENT DEVELOPMENTS IN GRAVITATION AND BEC’S PHENOMENOLOGY: IV Mexican Meeting on Experimental and Theoretical Physics: Symposium on Gravitation BEC’s Phenomenology

Quantum‐Gravity Phenomenology of soft ultraviolet/infrared mixing
View Description Hide DescriptionWe reexamine the motivation for ultraviolet/infrared mixing in quantum gravity and some of the quantum‐spacetime formalizations where it has been found. We then focus on cases in which the infrared manifestations of the mixing are relatively soft, arguing that they can motivate a particularly appealing phenomenology. Among the possible implications for the large‐distance behavior of gravity one intriguingly finds a correction with logarithmic dependence on distance. And one can explain in terms of soft ultraviolet/infrared mixing a four‐standard‐deviation discrepancy that was recently established in the context of studies of cold neutrons.

Solar‐system tests of relativistic gravity
View Description Hide DescriptionA number of recent experiments have successfully tested general theory of relativity to a remarkable precision. Various experimental techniques were used for this purpose, including spacecraft tracking, planetary ranging, lunar laser ranging, dedicated gravity experiments in space and many ground‐based efforts. We discuss the progress in the tests of relativistic gravity, present some proposed space‐based experiments and elaborate on their contribution to fundamental physics.

Lessons in quantum gravity from quantum field theory
View Description Hide DescriptionThis paper reviews advances in the understanding of quantum gravity based on field theory calculations in the AdS/CFT correspondence.

Delta‐gravity and the Accelerating Universe
View Description Hide DescriptionWe present a model of the gravitational field based on two symmetric tensors. Outside matter, the predictions of the model coincide exactly with General Relativity, so all classical tests are satisfied. In Cosmology, we get accelerated expansion without a cosmological constant.

An extended Einstein‐Cartan formulation of Chern‐Simons gravity
View Description Hide DescriptionWe consider the non‐dynamical Chern‐Simons modification to general relativity in the framework of the Einstein‐Cartan formulation, as providing a source for torsion. Since the experimental and observational bounds on torsion are very stringent, we propose a new iterative procedure to look for vacuum solutions of the system by expanding the tetrad, connection and embedding parameter, together with all derived quantities, in terms of a dimensionless small parameter β which codifies the Chern‐Simons coupling. Careful consideration is given to the Bianchi identities together with the consistency conditions they impose via the equations of motion. Starting from a torsionless zeroth‐order vacuum solution we derive the second order differential equation for the O(β) corrections to the metric, for an arbitrary embedding parameter. Subsequent specialization to either the canonical or the axial embedding allows us to show that a slowly rotating Kerr metric is an O(β) solution of the system.

Alternative explanations of “dark energy” in cosmology
View Description Hide DescriptionI consider alternative explanations for the data suggesting detection of dark energy (or a cosmological constant Λ). Apart from alternative gravity theories and some form of ‘quintessence’, one proposal to explain a small non‐zero value of Λ is a multiverse, but this is not testable in the usual sense. Other possibilities are that it may at least in part be due to the backreaction from and observational effects of small scale inhomogeneity, or it might be completely explained by large scale spatial inhomogeneity with zero Λ. The latter proposal can be observationally tested in a number of ways.

Dark Energy Phenomena as Gigaparsec Voids: Constraints due to Spectral Distortion
View Description Hide DescriptionCosmological spacetimes constructed from the Lemaitre‐Tolman‐Bondi metric are implemented to explain the distance‐redshift relationship inferred from type 1a supernovae and other tracers of cosmic evolution, in place of dark energy. The observer is required to be located at or very near the center of this spherically‐symmetric but radially‐inhomogeneous spacetime, in order to match the degree of isotropy exhibited by the cosmic microwave background (CMB). As viewed from near the center, radial lines‐of‐sight traversed by light rays cannot be used to distinguish between cosmological constant‐like dark energy and a Copernican Principle‐violating void at the center of the Universe. Scattered light, however, affords a view that can help determine whether we occupy a privileged location. Specifically, CMB photons that scatter off reionized gas in a radially‐inhomogeneous Universe contribute to a distortion of the blackbody spectrum that is distnct from the predictions in the case of radial homogeneity. This observational test of the Copernican Principle is shown to rule out a family of dark energy‐inspired Lemaitre‐Tolman‐Bondi spacetimes.

Bose‐Einstein condensates from scalar field dark matter
View Description Hide DescriptionWe review the properties of astrophysical and cosmological relevance that may arise from the bosonic nature of scalar field dark matter models. The key property is the formation of Bose‐Einstein condensates, but we also consider the presence of non‐empty excited states that may be relevant for the description of scalar field galaxy halos and the properties of rotation curves.

Collision of BEC dark matter structures and comparison with the collision of ideal gas structures
View Description Hide DescriptionIn this work we present an important feature of the Bose Einstein Condensate (BEC) dark matter model, that is, the head‐on collision of BEC dark matter virialized structures. This model of dark matter is assumed to be ruled by the Schrödinger‐Poisson system of equations, which is interpreted as the Gross‐Pitaevskii equation with a gravitational potential sourced by the density of probability. It has been shown recently that during the collision of two structures a pattern formation in the density of probability appears. We explore the pattern formation for various initial dynamical conditions during the collision. In order to know whether or not the pattern formation is a particular property of the BEC dark matter, we compare with the collision of two structures of virialized ideal gas under similar dynamical initial conditions, which is a model more consistent with usual models of dark matter. In order to do so, we also solve Euler's equations using a smoothed particle hydrodynamics approach. We found that the collision of the ideal gas structures does not show interference patterns, which in turn implies that the pattern formation is a property of the BEC dark matter.

General analytic results on averaging Lemaître‐Tolman‐Bondi models
View Description Hide DescriptionAn effective acceleration, which mimics the effect of dark energy, may arise in the context of Buchert’s scalar averaging formalism. We examine the conditions for such an acceleration to occur in the asymptotic radial range in generic spherically symmetric Lemaître‐Tolman‐Bondi (LTB) dust models. By looking at the behavior of covariant scalars along space slices orthogonal to the 4‐velocity, we show that this effective acceleration occurs in a class of models with negative spatial curvature that are asymptotically convergent to sections of Minkowski spacetime. As a consequence, the boundary conditions that favor LTB models with an effective acceleration are not a void inhomogeneity embedded in a homogeneous FLRW background (Swiss cheese models), but a local void or clump embedded in a large cosmic void region represented by asymptotically Minkowski conditions.

Dark‐energy thermodynamic models
View Description Hide DescriptionWe study cosmological consequences of dark‐energy thermodynamic models. The assumption that dark energy is conformed of quanta, and an extensivity argument generalize its equation of state. This implies that dark energy and another key component exchange energy. The energy densities of dark energy and the other component then tend asymptotically to a constant, thus explaining the coincidence of dark matter and dark energy today. On the other hand, a model of non‐relativistic particles in a Bose‐Einstein condensate, with a short‐range attractive interaction, produces acceleration. It is shown that the phantom‐acceleration regime, at the beginning of the universe, solves the horizon problem.

A warning on the determination of the halo mass
View Description Hide DescriptionDuring this talk, I will summarize our studies from the determination of the mass of the dark matter halo, based on the observations of the rotation curves of test particles or of the gravitational lensing. As I will show you, it is not uncommon that some studies on the nature of dark matter include extra assumption, some even on the very nature of the dark matter, what we want to determine!, that bais the studies and the results obtained from the observation and, in some cases, imply an inconsistent system altogether.

Black Holes and Trapped Surfaces
View Description Hide DescriptionThe relation between the presence of closed trapped surfaces and the existence of black holes is reviewed paying special attention to the possibility of defining the surface of the latter. Closed future‐trapped surfaces are believed to signal the formation of black holes and its event horizon. Trapped surfaces are, however, easier to handle, as they can be defined locally—contrary to event horizons, which must be defined globally. This is especially important in numerical relativity, and has led to the quasi‐local definitions of dynamical and trapping horizons. Nevertheless, it has become clear recently that closed trapped surfaces may extend outside any dynamical horizon associated to a black hole in evolution. Actually, closed trapped surfaces can even intersect flat portions of spacetime. These surprising results open new questions concerning the definition and characterization of dynamical black holes, and of what can be considered their external surface.

The two faces of sound in BECs
View Description Hide DescriptionFluctuations around a Bose‐Einstein condensate can be described by means of Bogolubov theory leading to the notion of quasiparticle and antiquasiparticle familiar to non‐relativistic condensed matter practitioners. On the other hand, we already know that these perturbations evolve according to a relativistic Klein‐Gordon equation in the long wavelength approximation. For shorter wavelengths, we show that this equation acquires nontrivial corrections which modify the Klein‐Gordon product. In this approach, quasiparticles can also be defined (up to the standard ambiguities due to observer‐dependence). We demonstrate that—in the low as well as in the high energy regimes—both concepts of quasiparticle are actually the same, regardless of the formalism (Bogolubov or Klein‐Gordon) used to describe them. These results also apply to any barotropic, inviscid, irrotational fluid, with or without quantum potential.

Gravitational waves from complexified Myers‐Perry black hole
View Description Hide DescriptionIn this contribution we complexify five dimensional stationary solutions to obtain time‐dependent ones. In particular the analytic continuation of the Myers‐Perry black hole is obtained; remarkably, the corresponding time‐dependent solution represents a gravitational wave spacetime with boost and rotational symmetries. We also address the complexification of the Emparan‐Reall black ring solution, in this case a 5D inhomogeneous closed cosmology is obtained.

Static binary systems of extreme charged black holes
View Description Hide DescriptionThe extreme limit of the double‐Reissner‐Nordström spacetime results in two particular solutions. The first one is the Majumdar‐Papapetrou solution which describes two charged non‐rotating extreme black holes in neutral equilibrium, the individual charges being equal to the respective masses. The second one is identified as the Bonnor solution whose constituents cannot be in equilibrium and are separated by a strut, their charges having opposite signs and exceeding the respective masses in absolute value.

Inhomogeneous Cosmology with Exact Solutions of General Relativity
View Description Hide DescriptionIt is commonly stated that we have entered the era of precision cosmology in which a number of important observations have reached a degree of precision, and a level of agreement with theory, that is comparable with many Earth‐based physics experiments. One of the consequences is the need to examine at what point our usual, well‐worn assumption of homogeneity associated to the use of perturbation theory begins to compromise the accuracy of our models. It is now a widely accepted fact that the effect of the inhomogeneities observed in the Universe cannot be ignored when one wants to construct an accurate cosmological model. Well‐established physics can explain several of the observed phenomena without introducing highly speculative elements, like dark matter, dark energy, exponential expansion at densities never attained in any experiment (i. e. inflation), and the like. Two main classes of methods are currently used to deal with these issues. Averaging, sometimes linked to fitting procedures à la Stoeger and Ellis, provide us with one promising way of solving the problem. Another approach is to use exact inhomogeneous solutions of General Relativity. This last method will be shortly developed and discussed here.

The “Schwarzschild‐Kerr” Equilibrium Configurations
View Description Hide DescriptionWe discuss the possibility of equilibrium between a Schwarzschild black hole possessing zero intrinsic angular momentum and a hyperextreme Kerr source. The balance occurs due to frame‐dragging exerted by the latter source on the black‐hole constituent, thus giving rise to a non‐zero horizon’s angular velocity parallel to the angular momentum of the Kerr object.

Supersymmetric solutions of 4‐dimensional supergravities
View Description Hide DescriptionWe review general and recent results on the characterization and construction of timelike supersymmetric solutions of 4‐dimensional supergravity theories.