1,721,165 research outputs found
Lorentz symmetry breaking: phenomenology and constraints
Full text linkIn this talk I shall review several motivations for considering departures from exact Lorentz invariance and the different theoretical frameworks adopted to describe these departures. Among these, I shall focus on an effective field theory approach and discuss the phenomenology and constraints of Lorentz symmetry breaking in the Standard Model as well as in Gravity. In particular I will focus on current constraints on UV breaking inspired by quantum gravity scenarios and briefly discuss the open issues and future perspectives for this field of research
Lorentz Breaking Effective Field Theory and Observational Tests
No full textAnalogue models of gravity have provided an experimentally realizable test field for our ideas on quantum field theory in curved spacetimes but they have also inspired the investigation of possible departures from exact Lorentz invariance at microscopic scales. In this role they have joined, and sometime anticipated, several quantum gravity models characterized by Lorentz breaking phenomenology. A crucial difference between these speculations and other ones associated to quantum gravity scenarios, is the possibility to carry out observational and experimental tests which have nowadays led to a broad range of constraints on departures from Lorentz invariance. We shall review here the effective field theory approach to Lorentz breaking in the matter sector, present the constraints provided by the available observations and finally discuss the implications of the persisting uncertainty on the composition of the ultra high energy cosmic rays for the constraints on the higher order, analogue gravity inspired, Lorentz violations. © Springer International Publishing Switzerland 2013
Modified special relativity on a fluctuating spacetime
No full textIt was recently proposed that deformations of the relativistic symmetry, as those considered in deformed
special relativity (DSR), can be seen as the outcome of a measurement theory in the presence of nonnegligible
(albeit small) quantum gravitational fluctuations [S. Liberati, S. Sonego, and M. Visser, Phys.
Rev. D 71, 045001 (2005).][R. Aloisio, A. Galante, A. Grillo, S. Liberati, E. Luzio, and F. Mendez, Phys.
Rev. D 73, 045020 (2006).]. In this paper we explicitly consider the case of a spacetime described by a flat
metric endowed with stochastic fluctuations and, for a free particle, we show that DSR-like nonlinear
relations between the spaces of the measured and classical momenta, can result from the average of the
stochastic fluctuations over a scale set to be the de Broglie wavelength of the particle. As illustrative
examples we consider explicitly the averaging procedure for some simple stochastic processes and discuss
the physical implications of our results
Lorentz Violation: Motivation and new constraints
No full textWe review the main theoretical motivations and observational constraints on Planck scale–suppressed violations of Lorentz invariance. After introducing the problems related to the phenomenological study of quantum gravitational effects, we discuss the main theoretical frameworks within which possible departures from Lorentz invariance can be described. In particular, we focus on the framework of effective field theory, describing several possible ways of including Lorentz violation therein and discussing their theoretical viability. We review the main low-energy effects that are expected in this framework. We discuss the current observational constraints on such a framework, focusing on those achievable through high-energy astrophysics observations. In this context, we present a summary of the most recent and strongest constraints on quantum electrodynamics with Lorentz-violating nonrenormalizable operators. Finally, we discuss the status of the field and its future perspectives
The gyroscopic frequency of metric f(R) and generalised Brans–Dicke theories: constraints from Gravity Probe–B
No full textWe confront the predicted gyroscopic precession (in particular the geodetic precession) from metric f (R) theory with the data provided by the mission, Gravity Probe-B. We find the constraint, vertical bar a(2)vertical bar < 1.33 x 10(12) m(2), where a(2) is the coefficient assessing the strength of the lowest order correction to the Einstein-Hilbert action for a metric f (R) theory with f analytic. This constraint improves over astrophysical bounds provided by massive black holes and planetary precession which are vertical bar a(2)vertical bar greater than or similar to 10(17) m(2) and vertical bar a(2)vertical bar less than or similar to 1.2 x 10(18) m(2) respectively and it is complementary to the stringent ones provided by lab based experiments, like the Eot-Wash experiment. We also investigate the modification of our result for gyroscopic precession if the oblateness of Earth is taken into account by considering the quadrupole moment of Earth. Finally, we provide a generalisation of our result for the gyroscopic precession in the context of Brans-Dicke theories with a potential (recovering the previously derived results in the appropriate limits)
Testing non-minimally coupled BEC dark matter with gravitational waves
No full textWe study the phenomenology associated to non-minimally coupled dark matter. In particular, we consider the model where the non-minimal coupling arises from the formation of relativistic Bose-Einstein condensates in high density regions of dark matter [1]. This non-minimal coupling is of Horndeski type and leads to a local modification of the speed of gravity with respect to the speed of light. Therefore we can constrain the model by using the joint detection of GW170817 and GRB170817A. We show that the constraints obtained in this way are quite tight, if the dark matter field oscillates freely, whereas they are substantially weakened, if the oscillations are damped by the non-minimal coupling
Gravitoelectromagnetism in metric f(R) and Brans–Dicke theories with a potential
No full textA Gravitoelectromagnetism formalism in the context of metric f(R) theory is presented and the analogue Lorentz force law is derived. Some interesting results such as the dependence of the deviation from General Relativity (GR) on the absolute value of the scalar potential are found, it is also found that the f(R) effects are only relevant at a shorter distance or when the distance is much less than the compton wavelength, and that the effects are attractive in nature. An investigation of gravitational time delay in the context of metric f(R) is also presented showing that the Ricci scalar alone is responsible for the time delay effect which seems to suggest that the extra scalar degree of freedom associated to f(R) does not provide any modification. Also, to generalise our results, the Lorentz force law and gravitational time delay in the case of Brans-Dicke theories with a potential are derived; it is shown that the results are consistent with those obtained in the case of metric f(R) and GR in the appropriate limits
Regular black holes and horizonless ultra-compact objects in Lorentz-violating gravity
Full text linkThere is growing evidence that Hořava gravity may be a viable quantum theory of gravity. It is thus legitimate to expect that gravitational collapse in the full, non-projectable version of the theory should result in geometries that are free of space-time singularities. Previous analyses have shown that such geometries must belong to one of the following classes: simply connected regular black holes with inner horizons; non-connected black holes “hiding” a wormhole mouth (black bounces); simply connected or non-connected horizonless compact objects. Here, we consider a singular black hole in the low-energy limit of non-projectable Hořava gravity, i.e. khronometric theory, and describe examples of its possible “regularisations”, covering all of the viable classes. To our knowledge, these examples constitute the first instances of black holes with inner universal horizons, of black bounces and of stars with a de Sitter core in the context of Lorentz-violating theories of gravity
On the resilience of the gravitational variational principle under renormalization
Full text linkA well-defined variational principle for gravitational actions typically requires to cancel boundary terms produced by the variation of the bulk action with a suitable set of boundary counterterms. This can be achieved by carefully balancing the coefficients multiplying the bulk operators with those multiplying the boundary ones. A typical example of this construction is the Gibbons-Hawking-York boundary action that needs to be added to the Einstein-Hilbert one in order to have a well-defined metric variation for General Relativity with Dirichlet boundary conditions. Quantum fluctuations of matter fields lead to a renormalization of these coefficients which may or may not preserve this balance. Indeed, already at the level of General Relativity, the resilience of the matching between bulk and boundary constants is far from obvious and it is anyway incomplete given that matter generically induces quadratic curvature operators. We investigate here the resilience of the matching of higher-order couplings upon renormalization by a non-minimally coupled scalar field and show that a problem is present. Even though we do not completely solve the latter, we show that it can be greatly ameliorated by a wise splitting between dynamical and topological contributions. Doing so, we find that the bulk-boundary matching is preserved up to a universal term (present for any Weyl invariant matter field content), whose nature and possible cancellation we shall discuss in the end
Painlevé–Gullstrand coordinates versus Kerr spacetime geometry
Full text linkWe discuss the tension between the possible existence of Painlevé–Gullstrand coordinate systems versus the explicit geometrical features of the Kerr spacetime; a subject of interest to Professor Thanu Padmanabhan in the weeks immediately preceding his unexpected death. We shall carefully distinguish strong and weak Painlevé–Gullstrand coordinate systems, and conformal variants thereof, cataloguing what we know can and cannot be done—sometimes we can make explicit global statements, sometimes we must resort to implicit local statements. For the Kerr spacetime the best that seems to be achievable is to set the lapse function to unity and represent the spatial slices with a 3-metric in factorized unimodular form; this arises from considering the Doran version of Kerr spacetime in Cartesian coordinates. We finish by exploring the (limited) extent to which this construction might possibly lead to implementing an “analogue spacetime” model suitable for laboratory simulations of the Kerr spacetime
- …
