| 1. |
- Akrami, Yashar, et al.
(författare)
-
The nature of spacetime in bigravity : Two metrics or none?
- 2015
-
Ingår i: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 47:1, s. 1838-
-
Tidskriftsartikel (refereegranskat)abstract
- The possibility of matter coupling to two metrics at once is considered. This appears natural in the most general ghost-free, bimetric theory of gravity, where it unlocks an additional symmetry with respect to the exchange of the metrics. This double coupling, however, raises the problem of identifying the observables of the theory. It is shown that if the two metrics couple minimally to matter, then there is no physical metric to which all matter would universally couple, and that moreover such an effective metric generically does not exist even for an individual matter species. By studying point particle dynamics, a resolution is suggested in the context of Finsler geometry.
|
|
| 2. |
- Amendola, L., et al.
(författare)
-
Cosmology and fundamental physics with the Euclid satellite
- 2018
-
Ingår i: Living Reviews in Relativity. - : Springer. - 1433-8351 .- 2367-3613. ; 21:1
-
Tidskriftsartikel (refereegranskat)abstract
- Euclid is a European Space Agency medium-class mission selected for launch in 2020 within the cosmic vision 2015–2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and red-shifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid’s Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.
|
|
| 3. |
- Ashoorioon, A., et al.
(författare)
-
Anisotropic non-gaussianity from rotational symmetry breaking excited initial states
- 2016
-
Ingår i: Journal of Cosmology and Astroparticle Physics. - : Institute of Physics Publishing (IOPP). - 1475-7516. ; 2016:12
-
Tidskriftsartikel (refereegranskat)abstract
- If the initial quantum state of the primordial perturbations broke rotational invariance, that would be seen as a statistical anisotropy in the angular correlations of the cosmic microwave background radiation (CMBR) temperature fluctuations. This can be described by a general parameterisation of the initial conditions that takes into account the possible direction-dependence of both the amplitude and the phase of particle creation during inflation. The leading effect in the CMBR two-point function is typically a quadrupole modulation, whose coefficient is analytically constrained here to be |B| <~ 0.06. The CMBR three-point function then acquires enhanced non-gaussianity, especially for the local configurations. In the large occupation number limit, a distinctive prediction is a modulation of the non-gaussianity around a mean value depending on the angle that short and long wavelength modes make with the preferred direction. The maximal variations with respect to the mean value occur for the configurations which are coplanar with the preferred direction and the amplitude of the non-gaussianity increases (decreases) for the short wavelength modes aligned with (perpendicular to) the preferred direction. For a high scale model of inflation with maximally pumped up isotropic occupation and ϵ~ 0.01 the difference between these two configurations is about 0.27, which could be detectable in the future. For purely anisotropic particle creation, the non-Gaussianity can be larger and its anisotropic feature very sharp. The non-gaussianity can then reach fNL ∼ 30 in the preferred direction while disappearing from the correlations in the orthogonal plane.
|
|
| 4. |
- Ashoorioon, Amjad, et al.
(författare)
-
Hemispherical anomaly from asymmetric initial states
- 2016
-
Ingår i: PHYSICAL REVIEW D. - : American Physical Society. - 2470-0010. ; 94:4
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate if the hemispherical asymmetry in the CMB is produced from "asymmetric" excited initial conditions. We show that in the limit where the deviations from the Bunch-Davies vacuum are large and the scale of new physics is maximally separated from the inflationary Hubble parameter, the primordial power spectrum is modulated only by position-dependent dipole and quadrupole terms. Requiring the dipole contribution in the power spectrum to account for the observed power asymmetry, A = 0.07 +/- 0.022, we show that the amount of quadrupole terms is roughly equal to A(2). The mean local bispectrum, which gets enhanced for the excited initial state, is within the 1 sigma bound of Planck 2015 results for a large field model, f(NL) similar or equal to 4.17, but is reachable by future CMB experiments. The amplitude of the local non-Gaussianity modulates around this mean value, depending on the angle that the correlated patches on the 2d CMB surface make with the preferred direction. The amount of variation is minimized for the configuration in which the short and long wavelength modes are around the preferred pole and vertical bar(k(3)) over right arrow vertical bar approximate to vertical bar(k) over right arrow (l approximate to 10)vertical bar << vertical bar(k(1)) over right arrow vertical bar approximate to vertical bar(k(2)) over right arrow vertical bar approximate to vertical bar(k) over right arrow (l approximate to 2500)vertical bar with f(NL)(min) approximate to 3.64. The maximum occurs when these modes are at the antipode of the preferred pole, f(NL)(max) approximate to 4.81. The difference of non-Gaussianity between these two configurations is as large as similar or equal to 1.17, which can be used to distinguish this scenario from other scenarios that try to explain the observed hemispherical asymmetry.
|
|
| 5. |
- Beltran Jimenez, Jose, et al.
(författare)
-
Coincident general relativity
- 2018
-
Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 98:4
-
Tidskriftsartikel (refereegranskat)abstract
- The metric-affine variational principle is applied to generate teleparallel and symmetric teleparallel theories of gravity. From the latter we discover an exceptional class which is consistent with a vanishing affine connection. Based on this remarkable property, this work proposes a simpler geometrical formulation of general relativity that is oblivious to the affine spacetime structure, thus fundamentally depriving gravity of any inertial character. The resulting theory is described by the Hilbert action purged from the boundary term and is more robustly underpinned by the spin-2 field theory, where an extra symmetry is now manifest, possibly related to the double-copy structure of the gravity amplitudes. This construction also provides a novel starting point for modified gravity theories, and the paper presents new and simple generalizations where analytical self-accelerating cosmological solutions arise naturally in the early-and late-time Universe.
|
|
| 6. |
- Beltran Jimenez, Jose, et al.
(författare)
-
Teleparallel Palatini theories
- 2018
-
Ingår i: Journal of Cosmology and Astroparticle Physics. - : Institute of Physics Publishing (IOPP). - 1475-7516. ; :8
-
Tidskriftsartikel (refereegranskat)abstract
- The Palatini formalism, which assumes the metric and the affine connection as independent variables, is developed for gravitational theories in flat geometries. We focus on two particularly interesting scenarios. First, we fix the connection to be metric compatible, as done in the usual teleparallel theories, but we follow a completely covariant approach by imposing the constraints with suitable Lagrange multipliers. For a general quadratic theory we show how torsion naturally propagates and we reproduce the Teleparallel Equivalent of General Relativity as a particular quadratic action that features an additional Lorentz symmetry. We then study the much less explored theories formulated in a geometry with neither curvature nor torsion, so that all the geometrical information is encoded in the non-metricity. We discuss how this geometrical framework leads to a purely inertial connection that can thus be completely removed by a coordinate gauge choice, the coincident gauge. From the quadratic theory we recover a simpler formulation of General Relativity in the form of the Einstein action, which enjoys an enhanced symmetry that reduces to a second linearised diffeomorphism at linear order. More general theories in both geometries can be formulated consistently by taking into account the inertial connection and the associated additional degrees of freedom. As immediate applications, the new cosmological equations and their Newtonian limit are considered, where the role of the lapse in the consistency of the equations is clarified, and the Schwarzschild black hole entropy is computed by evaluating the corresponding Euclidean action. We discuss how the boundary terms in the usual formulation of General Relativity are related to different choices of coordinates in its coincident version and show that in isotropic coordinates the Euclidean action is finite without the need to introduce boundary or normalisation terms. Finally, we discuss the double-copy structure of the gravity amplitudes and the bootstrapping of gravity within the framework of coincident General Relativity.
|
|
| 7. |
- Beltran Jimenez, Jose, et al.
(författare)
-
The canonical frame of purified gravity
- 2019
-
Ingår i: International Journal of Modern Physics D. - : World Scientific Publishing Co. Pte Ltd. - 0218-2718. ; 28:14
-
Tidskriftsartikel (refereegranskat)abstract
- In the recently introduced gauge theory of translations, dubbed Coincident General Relativity (CGR), gravity is described with neither torsion nor curvature in the spacetime affine geometry. The action of the theory enjoys an enhanced symmetry and avoids the second derivatives that appear in the conventional Einstein-Hilbert action. While it implies the equivalent classical dynamics, the improved action principle can make a difference in considerations of energetics, thermodynamics and quantum theory. This paper reports on possible progress in those three aspects of gravity theory. In the so-called purified gravity, (1) energy-momentum is described locally by a conserved, symmetric tensor, (2) the Euclidean path integral is convergent without the addition of boundary or regulating terms and (3) it is possible to identify a canonical frame for quantization.
|
|
| 8. |
- Beltran Jimenez, Jose, et al.
(författare)
-
The Geometrical Trinity of Gravity
- 2019
-
Ingår i: Universe. - : MDPI. - 2218-1997. ; 5:7
-
Tidskriftsartikel (refereegranskat)abstract
- The geometrical nature of gravity emerges from the universality dictated by the equivalence principle. In the usual formulation of General Relativity, the geometrisation of the gravitational interaction is performed in terms of the spacetime curvature, which is now the standard interpretation of gravity. However, this is not the only possibility. In these notes, we discuss two alternative, though equivalent, formulations of General Relativity in flat spacetimes, in which gravity is fully ascribed either to torsion or to non-metricity, thus putting forward the existence of three seemingly unrelated representations of the same underlying theory. Based on these three alternative formulations of General Relativity, we then discuss some extensions.
|
|
| 9. |
- Capozziello, Salvatore, et al.
(författare)
-
Hybrid Metric-Palatini Gravity
- 2015
-
Ingår i: Universe. - : MDPI. - 2218-1997. ; 1:2, s. 199-238
-
Forskningsöversikt (refereegranskat)abstract
- Recently, the phenomenology of f(R) gravity has been scrutinized. This scrutiny has been motivated by the possibility to account for the self-accelerated cosmic expansion without invoking dark energy sources. Besides, this kind of modified gravity is capable of addressing the dynamics of several self-gravitating systems alternatively to the presence of dark matter. It has been established that both metric and Palatini versions of these theories have interesting features but also manifest severe and different downsides. A hybrid combination of theories, containing elements from both these two formalisms, turns out to be also very successful accounting for the observed phenomenology and is able to avoid some drawbacks of the original approaches. This article reviews the formulation of this hybrid metric-Palatini approach and its main achievements in passing the local tests and in applications to astrophysical and cosmological scenarios, where it provides a unified approach to the problems of dark energy and dark matter.
|
|
| 10. |
- Carloni, Sante, et al.
(författare)
-
Dynamical system analysis of hybrid metric-Palatini cosmologies
- 2015
-
Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 92:6
-
Tidskriftsartikel (refereegranskat)abstract
- The so-called f(X) hybrid metric-Palatini gravity, where X = R + T, with T the stress-energy trace and R the Ricci scalar, presents a unique viable generalization of the f(R) theories within the metric-affine formalism. In this paper, the cosmology of the f(X) theories is studied using the dynamical system approach. The method consists of formulating the propagation equation in terms of suitable (expansion-normalized) variables as an autonomous system. The fixed points of the system then represent exact cosmological solutions described by power law or de Sitter expansion. The formalism is applied to two classes of f(X) models, revealing both standard cosmological fixed points and new accelerating solutions that can be attractors in the phase space. In addition, the fixed point with vanishing expansion rate is considered with special care in order to characterize the stability of Einstein static spaces and bouncing solutions.
|
|
| 11. |
- Conroy, Aindriu, et al.
(författare)
-
Generalized quadratic curvature, non-local infrared modifications of gravity and Newtonian potentials
- 2015
-
Ingår i: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 32:1
-
Tidskriftsartikel (refereegranskat)abstract
- Metric theories of gravity are studied, beginning with a general action that is quadratic in curvature and allows arbitrary inverse powers of the d' Alembertian operator, resulting in infrared non-local extensions of general relativity. The field equations are derived in full generality and their consistency is checked by verifying the Bianchi identities. The weak-field limit is computed and a straightforward algorithm is presented to infer the post-Newtonian corrections directly from the action. This is then applied to various infrared gravity models including non-local Rf (R/square) dark energy and non-local massive gravity models. Generically, the Newtonian potentials are not identical and deviate from the 1/r behaviour at large distances. However, the former does not occur in a specific class of theories that does not introduce additional degrees of freedom in flat spacetime. A new non-local model within this class is proposed, defined by the exponential of the inverse d' Alembertian. This model exhibits novel features, such as the weakening of the gravity in the infrared, suggesting de-gravitation of the cosmological constant.
|
|
| 12. |
- Conroy, Aindriu, et al.
(författare)
-
The spectrum of symmetric teleparallel gravity
- 2018
-
Ingår i: European Physical Journal C. - : SPRINGER. - 1434-6044 .- 1434-6052. ; 78:11
-
Tidskriftsartikel (refereegranskat)abstract
- General Relativity and its higher derivative extensions have symmetric teleparallel reformulations in terms of the non-metricity tensor within a torsion-free and flat geometry. These notes present a derivation of the exact propagator for the most general infinite-derivative, even-parity and generally covariant theory in the symmetric teleparallel spacetime. The action made up of the non-metricity tensor and its contractions is decomposed into terms involving the metric and a gauge vector field and is found to complement the previously known non-local ghost- and singularity-free theories.
|
|
| 13. |
- Dialektopoulos, Konstantinos F., et al.
(författare)
-
Noether symmetries in symmetric teleparallel cosmology
- 2019
-
Ingår i: European Physical Journal C. - : Springer. - 1434-6044 .- 1434-6052. ; 79:7
-
Tidskriftsartikel (refereegranskat)abstract
- We consider a general theory of all possible quadratic, first-order derivative terms of the non-metricity tensor in the framework of Symmetric Teleparallel Geometry. We apply the Noether Symmetry Approach to classify those models that are invariant under point transformations in a cosmological background and we use the symmetries of these models to reduce the dynamics of the system in order to find analytical solutions.
|
|
| 14. |
- Golovnev, Alexey, et al.
(författare)
-
Cosmological perturbations in modified teleparallel gravity models
- 2018
-
Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP PUBLISHING LTD. - 1475-7516. ; :11
-
Tidskriftsartikel (refereegranskat)abstract
- Cosmological perturbations are considered in f (T) and in scalar-torsion f (phi) T teleparallel models of gravity. Full sets of linear perturbation equations are accurately derived and analysed at the relevant limits. Interesting features of generalisations to other teleparallel models, spatially curved backgrounds, and rotated tetrads are pointed out.
|
|
| 15. |
- Golovnev, Alexey, et al.
(författare)
-
Effectively nonlocal metric-affine gravity
- 2016
-
Ingår i: PHYSICAL REVIEW D. - : American Physical Society. - 2470-0010. ; 93:6
-
Tidskriftsartikel (refereegranskat)abstract
- In metric-affine theories of gravity such as the C-theories, the spacetime connection is associated to a metric that is nontrivially related to the physical metric. In this article, such theories are rewritten in terms of a single metric, and it is shown that they can be recast as effectively nonlocal gravity. With some assumptions, known ghost-free theories with nonsingular and cosmologically interesting properties may be recovered. Relations between different formulations are analyzed at both perturbative and nonperturbative levels, taking carefully into account subtleties with boundary conditions in the presence of integral operators in the action, and equivalences between theories related by nonlocal redefinitions of the fields are verified at the level of equations of motion. This suggests a possible geometrical interpretation of nonlocal gravity as an emergent property of non-Riemannian spacetime structure.
|
|
| 16. |
- Golovnev, Alexey, et al.
(författare)
-
On the covariance of teleparallel gravity theories
- 2017
-
Ingår i: Classical and quantum gravity. - : Institute of Physics Publishing (IOPP). - 0264-9381 .- 1361-6382. ; 34:14
-
Tidskriftsartikel (refereegranskat)abstract
- The basics of teleparallel gravity and its extensions are reviewed with particular emphasis on the problem of the Lorentz-breaking choice of connection in pure-tetrad versions of the theories. Various possible ways to covariantise such models are discussed. A by-product is a new form of f (T) field equations.
|
|
| 17. |
- Harko, Tiberiu, et al.
(författare)
-
Coupling matter in modified Q gravity
- 2018
-
Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 98:8
-
Tidskriftsartikel (refereegranskat)abstract
- We present a novel theory of gravity by considering an extension of symmetric teleparallel gravity. This is done by introducing, in the framework of the metric-affine formalism, a new class of theories where the nonmetricity Q is nonminimally coupled to the matter Lagrangian. More specifically, we consider a Lagrangian of the form L similar to f(1)(Q) + f(2)(Q)L-M, where f(1) and f(2) are generic functions of Q, and L-M is the matter Lagrangian. This nonminimal coupling entails the nonconservation of the energy-momentum tensor, and consequently the appearance of an extra force. The formulation of the gravity sector in terms of the Q instead of the curvature may result in subtle improvements of the theory. In the context of nonminimal matter couplings, we are therefore motivated to explore whether the new geometrical formulation in terms of the Q, when implemented also in the matter sector, would allow more universally consistent and viable realizations of the nonminimal coupling. Furthermore, we consider several cosmological applications by presenting the evolution equations and imposing specific functional forms of the functions f(1)(Q) and f(2)(Q), such as power-law and exponential dependencies of the nonminimal couplings. Cosmological solutions are considered in two general classes of models, and found to feature accelerating expansion at late times.
|
|
| 18. |
- Harko, T, et al.
(författare)
-
Novel couplings between nonmetricity and matter
- 2018
-
Ingår i: 15th Marcel Grossmann Meeting on Recent Developments in Theoretical and Experimental General Relativity, Astrophysics, and Relativistic Field Theories, MG 2018. - : World Scientific Publishing Co. Pte. Ltd..
-
Konferensbidrag (refereegranskat)abstract
- We present a novel theory of gravity, namely, an extension of symmetric teleparallel gravity. This is done by introducing a new class of theories where the nonmetricity Q is coupled nonminimally to the matter Lagrangian. This nonminimal coupling entails the nonconservation of the energy-momentum tensor, and consequently the appearance of an extra force. We also present several cosmological applications.
|
|
| 19. |
- Högås, Marcus, 1991-
(författare)
-
Gravitational collapse in bimetric gravity
- 2019
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Einstein’s theory of general relativity has been the prevailing theory of classical gravity for over a century. Its observational consistency is unparalleled. There are however a few (arguably) serious short-comings. First, the cosmological standard model, within which general relativity plays a crucial role, implies that the main part of the matter content of the universe is unknown (dark matter). Second, Einstein’s theory cannot be quantized and predicts singularities where the theory breaks down. Hence, there are strong reasons to consider extended theories of gravitation. Bimetric gravity is such a theory. Besides a massless spin-2 field, this theory exhibits a massive spin-2 field.To be competitive, bimetric gravity should describe observational data at least as well as general relativity. Therefore, it is of uttermost importance to analyze the solutions to the equations of motion. In this licentiate thesis, we analyze gravitational collapse in spherical symmetry. We examine an exact solution where the matter content consists of massless and pressureless particles (dust). One of the main conclusions is that the end state is a Schwarzschild space-time which is also a solution to Einstein’s equations. Besides exact solutions, we perform numerical simulations of gravitational collapse in spherical symmetry. Starting with initial data close to a corresponding solution in general relativity, the evolution stays close and there is no evidence of a physical instability. We also investigate the role of symmetries and topology for bimetric gravity.
|
|
| 20. |
- Iosifidis, Damianos, et al.
(författare)
-
Scale Transformations in Metric-Affine Geometry
- 2019
-
Ingår i: Universe. - : MDPI. - 2218-1997. ; 5:3
-
Tidskriftsartikel (refereegranskat)abstract
- This article presents an exhaustive classification of metric-affine theories according to their scale symmetries. First it is clarified that there are three relevant definitions of a scale transformation. These correspond to a projective transformation of the connection, a rescaling of the orthonormal frame, and a combination of the two. The most general second order quadratic metric-affine action, including the parity-violating terms, is constructed in each of the three cases. The results can be straightforwardly generalised by including higher derivatives, and implemented in the general metric-affine, teleparallel, and symmetric teleparallel geometries.
|
|
| 21. |
- Jiménez, J. B., et al.
(författare)
-
Cosmology for quadratic gravity in generalized Weyl geometry
- 2016
-
Ingår i: Journal of Cosmology and Astroparticle Physics. - : Institute of Physics Publishing (IOPP). - 1475-7516. ; 2016:4
-
Tidskriftsartikel (refereegranskat)abstract
- A class of vector-tensor theories arises naturally in the framework of quadratic gravity in spacetimes with linear vector distortion. Requiring the absence of ghosts for the vector field imposes an interesting condition on the allowed connections with vector distortion: the resulting one-parameter family of connections generalises the usual Weyl geometry with polar torsion. The cosmology of this class of theories is studied, focusing on isotropic solutions wherein the vector field is dominated by the temporal component. De Sitter attractors are found and inhomogeneous perturbations around such backgrounds are analysed. In particular, further constraints on the models are imposed by excluding pathologies in the scalar, vector and tensor fluctuations. Various exact background solutions are presented, describing a constant and an evolving dark energy, a bounce and a self-tuning de Sitter phase. However, the latter two scenarios are not viable under a closer scrutiny.
|
|
| 22. |
- Jimenez, Jose Beltran, et al.
(författare)
-
Modified Gravity with Vector Distortion and Cosmological Applications
- 2017
-
Ingår i: Universe. - : MDPI AG. - 2218-1997. ; 3:2
-
Tidskriftsartikel (refereegranskat)abstract
- We briefly review the basics of Weyl geometry and its natural extension by a general linear "distortion" of the metric connection by a vector field. A special class of the connections has torsion but retains the Weyl's semi-metricity condition. We present ghost-free gravitational theories in this geometrical setup and highlight their possible cosmological applications, such as new self-tuning solutions and new bouncing solutions found in the quadratic-curvature theories. The vector distortion can mimic the cosmological effects of dark matter.
|
|
| 23. |
- Jimenez, Jose Beltran, et al.
(författare)
-
Spacetimes with vector distortion : Inflation from generalised Weyl geometry
- 2016
-
Ingår i: Physics Letters B. - : Elsevier. - 0370-2693 .- 1873-2445. ; 756, s. 400-404
-
Tidskriftsartikel (refereegranskat)abstract
- Spacetime with general linear vector distortion is introduced. Thus, the torsion and the nonmetricity of the affine connection are assumed to be proportional to a vector field (and not its derivatives). The resulting two-parameter family of non-Riemannian geometries generalises the conformal Weyl geometry and some other interesting special cases. Taking into account the leading nonlinear correction to the Einstein-Hilbert action results uniquely in the one-parameter extension of the Starobinsky inflation known as the alpha-attractor. The most general quadratic curvature action introduces, in addition to the canonical vector kinetic term, novel ghost-free vector-tensor interactions.
|
|
| 24. |
- Kocic, Mikica
(författare)
-
Geometry of interactions in ghost-free bimetric theory
- 2017
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ghost-free bimetric theory is an extension to general relativity where two metric tensors are used instead of one. A priori, the two metrics may not have compatible notions of space and time, which makes the formulation of the initial-value problem problematic. Moreover, the metrics are coupled through a specific ghost-free interaction term that is nonunique and possibly nonreal. We prove that the reality of the bimetric potential leads to a classification of the allowed configurations of the two metrics in terms of the intersections of their null cones. Then, the equations of motion and general covariance are enough to restrict down the allowed configurations to metrics that admit compatible notions of space and time, and furthermore, lead to a unique definition of the potential. This ensures that the ghost-free bimetric theory can be defined unambiguously. In addition, we apply the results on spherically symmetric spacetimes. First, we explore the behavior of the black hole solutions both at the common Killing horizon and at the large radii. The study leads to a new classification for black holes within the bidiagonal ansatz. Finally, we consider the bimetric field equations in vacuum when the two metrics share a single common null direction. We obtain a class of exact solutions of the generalized Vaidya type parametrized by an arbitrary function. The found solutions are nonstationary and thus nonstatic, which formally disproves an analogous statement to Birkhoff's theorem in the ghost-free bimetric theory.
|
|
| 25. |
- Koivisto, Tomi S., et al.
(författare)
-
A note on viability of nonminimally coupled f(R) theory
- 2016
-
Ingår i: General Relativity and Gravitation. - : Springer Science and Business Media LLC. - 0001-7701 .- 1572-9532. ; 48:7
-
Tidskriftsartikel (refereegranskat)abstract
- Consistency conditions for nonminimally coupled f(R) theories have been derived by requiring the absence of tachyons and instabilities in the scalar fluctuations. This note confirms these results and clarifies a subtlety regarding different definitions of sound speeds.
|
|
| 26. |
- Koivisto, Tomi S.
(författare)
-
An integrable geometrical foundation of gravity
- 2018
-
Ingår i: International Journal of Geometric Methods in Modern Physics (IJGMMP). - : World Scientific Publishing Co. Pte Ltd. - 0219-8878. ; 15
-
Tidskriftsartikel (refereegranskat)abstract
- In a talk at the conference Geometrical Foundations of Gravity at Tartu, 2017, it was suggested that the affine spacetime connection could be associated with purely fictitious forces. This leads to gravitation in a flat and smooth geometry. Fermions are found to nevertheless couple with the metrical connection and a phase gauge field. The theory is reviewed in this proceeding in a Palatini, and in a metric-affine gauge formulation.
|
|
| 27. |
- Koivisto, Tomi Sebastian, et al.
(författare)
-
Disformal vectors and anisotropies on a warped brane\protect Hulluilla on Halvat Huvit
- 2015
-
Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; 2015:3
-
Tidskriftsartikel (refereegranskat)abstract
- The Maxwell action is conformally invariant and classically ignorant of conformally flat metrics. However, if the vector lives in a disformal metric - as it does if residing upon a moving brane - this is no longer true. The disformal coupling is then mediated by a Dirac-Born-Infeld scalar field. Here a systematic dynamical system analysis is developed for anisotropic Bianchi I cosmology with a massive disformally coupled vector field. Several new fixed points are found, including anisotropic scaling solutions. The formalism here presented can be conveniently applied to general scenarios with or without extra dimensional motivations. This is illustrated here by performing a complete analysis with the assumption that both the potentials and the warp factor for the brane are (nearly) exponential. In that case, the anisotropic fixed points are either not attractors, do not describe accelerating expansion or else they feature too large anisotropies to be compatible with observations. Nonetheless, viable classes of models exist where isotropy is retained due to rapid oscillations of the vector field, thus providing a possible realisation of disformally interacting massive dark matter.
|
|
| 28. |
- Koivisto, Tomi S., et al.
(författare)
-
Doubly-boosted vector cosmologies from disformal metrics
- 2015
-
Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 90:9
-
Tidskriftsartikel (refereegranskat)abstract
- A systematic dynamical system approach is applied to study the cosmology of anisotropic Bianchi I universes in which a vector field is assumed to operate on a disformal frame. This study yields a number of new fixed points, among which are anisotropic scaling solutions. Within the simplifying assumption of (nearly) constant-slope potentials these are either not stable attractors, do not describe accelerating expansion or else they feature too large anisotropies to be compatible with observations. Nonetheless, some solutions do have an appeal for cosmological applications in that isotropy is retained due to rapid oscillations of the vector field. Such cosmological scenarios could describe physics beyond standard model such as extra dimensional models that predict disformal couplings between vector and scalar fields.
|
|
| 29. |
- Koivisto, Tomi S., et al.
(författare)
-
Scalar-fluid theories : cosmological perturbations and large-scale structure
- 2015
-
Ingår i: Journal of Cosmology and Astroparticle Physics. - : Institute of Physics (IOP). - 1475-7516. ; :9
-
Tidskriftsartikel (refereegranskat)abstract
- Recently a new Lagrangian framework was introduced to describe interactions between scalar fields and relativistic perfect fluids. This allows two consistent generalizations of coupled quintessence models: non-vanishing pressures and a new type of derivative interaction. The implications of these to the formation of cosmological large-scale structure are uncovered here at the linear order. The full perturbation equations in the two cases are derived in a unified formalism and their Newtonian, quasi-static limit is studied analytically. Requiring the absence of an effective sound speed term in the coupled dark matter fluid restricts the Lagrangian to be a linear function of the matter number density. This leaves new potentially viable classes of both algebraically and derivatively interacting models wherein the coupling may impact the background expansion dynamics and imprint new signatures into the large-scale structure.
|
|
| 30. |
- Koivisto, Tomi S., et al.
(författare)
-
Stability of disformally coupled accretion disks
- 2017
-
Ingår i: Physica Scripta. - : Institute of Physics Publishing (IOPP). - 0031-8949 .- 1402-4896. ; 92:10
-
Tidskriftsartikel (refereegranskat)abstract
- The no-hair theorem postulates that the only externally observable properties of a black hole are its mass, its electric charge, and its angular momentum. In scalar-tensor theories of gravity, a matter distribution around a black hole can lead to the so called 'spontaneous scalarisation' instability that triggers the development of scalar hair. In the Brans-Dicke type theories, this effect can be understood as a result of tachyonic effective mass of the scalar field. Here we consider the instability in the generalised class of scalar-theories that feature non-conformal, i.e. 'disformal', couplings to matter. Such theories have gained considerable interest in the recent years and have been studied in a wide variety of systems, both cosmological and astrophysical. In view of the prospects of gravitational wave astronomy, it is relevant to explore the implications of the theories in the strong-gravity regime. In this article, we concentrate on the spontaneous scalarisation of matter configurations around Schwarzschild and Kerr black holes. We find that in the more generic scalar-tensor theories, the instability of the Brans-Dicke theory can be enhanced, suggesting violations of the no-hair theorem. On the other hand, we find that, especially if the coupling is very strong, or if the gradients in the matter distribution are negligible, the disformal coupling tends to stabilise the system.
|
|
| 31. |
- Koivisto, Tomi S., et al.
(författare)
-
The General Linear Cartan Khronon
- 2019
-
Ingår i: Universe. - : MDPI. - 2218-1997. ; 5:7
-
Tidskriftsartikel (refereegranskat)abstract
- A Cartan geometry of the General Linear symmetry is formulated by dividing out the displacements from the group. The resulting action is quadratic in curvature, polynomial in all the (minimal) variables, and describes an observer space that-in the symmetry-broken phase-reproduces the predictions of General Relativity in the presence of dark matter.
|
|
| 32. |
- Koivisto, Tomi S., et al.
(författare)
-
The Spectrum of Teleparallel Gravity
- 2019
-
Ingår i: Universe. - : MDPI. - 2218-1997. ; 5:3
-
Tidskriftsartikel (refereegranskat)abstract
- The observer's frame is the more elementary description of the gravitational field than the metric. The most general covariant, even-parity quadratic form for the frame field in arbitrary dimension generalises the New General Relativity by nine functions of the d'Alembertian operator. The degrees of freedom are clarified by a covariant derivation of the propagator. The consistent and viable models can incorporate an ultra-violet completion of the gravity theory, an additional polarisation of the gravitational wave, and the dynamics of a magnetic scalar potential.
|
|
| 33. |
- Nersisyan, Henrik, et al.
(författare)
-
Dynamical analysis of R1/square R-2 cosmology : Impact of initial conditions and constraints from supernovae
- 2016
-
Ingår i: PHYSICAL REVIEW D. - : American Physical Society. - 2470-0010. ; 94:4
-
Tidskriftsartikel (refereegranskat)abstract
- We discuss the cosmological implications of the R square R-2 nonlocal modification to standard gravity. We relax the assumption of special initial conditions in the local formulation of the theory, perform a full phase-space analysis of the system, and show that the late-time cosmology of the model exhibits two distinct evolution paths, on which a large range of values for the present equation of state can be reached. We then compare the general solutions to supernovae data and place constraints on the parameters of the model. In particular, we find that the mass parameter of the theory should be smaller than 1.2 in Hubble units.
|
|
| 34. |
- Nersisyan, Henrik, et al.
(författare)
-
Instabilities in tensorial nonlocal gravity
- 2017
-
Ingår i: Physical Review D. - : AMER PHYSICAL SOC. - 2470-0010 .- 2470-0029. ; 95:4
-
Tidskriftsartikel (refereegranskat)abstract
- We discuss the cosmological implications of nonlocal modifications of general relativity containing tensorial structures. Assuming the presence of standard radiation-and matter-dominated eras, we show that, except in very particular cases, the nonlocal terms contribute a rapidly growing energy density. These models therefore generically do not have a stable cosmological evolution.
|
|
| 35. |
- Solomon, A. R., et al.
(författare)
-
Cosmological viability of massive gravity with generalized matter coupling
- 2015
-
Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; 2015:4
-
Tidskriftsartikel (refereegranskat)abstract
- There is a no-go theorem forbidding flat and closed FLRW solutions in massive gravity on a flat reference metric, while open solutions are unstable. Recently it was shown that this no-go theorem can be overcome if at least some matter couples to a hybrid metric composed of both the dynamical and the fixed reference metric. We show that this is not compatible with the standard description of cosmological sources in terms of effective perfect fluids, and the predictions of the theory become sensitive either to the detailed field-theoretical modelling of the matter content or to the presence of additional dark degrees of freedom. This is a serious practical complication. Furthermore, we demonstrate that viable cosmological background evolution with a perfect fluid appears to require the presence of fields with highly contrived properties. This could be improved if the equivalence principle is broken by coupling only some of the fields to the composite metric, but viable self-accelerating solutions due only to the massive graviton are difficult to obtain.
|
|
| 36. |
- Sundell, Peter, et al.
(författare)
-
Anisotropic cosmology and inflation from a tilted Bianchi IX model
- 2015
-
Ingår i: Physical Review D. - : American Physical Society. - 1550-7998 .- 1550-2368. ; 92:12
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamics of the tilted axisymmetric Bianchi IX cosmological models are explored allowing energy flux in the source fluid. The Einstein equations and the continuity equation are presented treating the equation of state w and the tilt angle of the fluid lambda as time-dependent functions, but when analyzing the phase space w and lambda are considered free parameters and the shear, the vorticity and the curvature of the spacetime span a three-dimensional phase space that contains seven fixed points. One of them is an attractor that inflates the universe anisotropically, thus providing a counterexample to the cosmic no-hair conjecture. Also, examples of realistic though fine-tuned cosmologies are presented wherein the rotation can become significant towards the present epoch but the shear stays within the observational bounds. The examples suggest that the model used here can explain the parity-violating anomalies of the cosmic microwave background. The result significantly differs from an earlier study, where a nonaxisymmetric Bianchi IX type model with a tilted perfect dust source was found to induce too much shear for observationally significant vorticity.
|
|
| 37. |
- Van De Bruck, C., et al.
(författare)
-
Disformally coupled inflation
- 2016
-
Ingår i: Journal of Cosmology and Astroparticle Physics. - : Institute of Physics Publishing (IOPP). - 1475-7516. ; 2016:3
-
Tidskriftsartikel (refereegranskat)abstract
- A disformal coupling between two scalar fields is considered in the context of cosmological inflation. The coupling introduces novel derivative interactions mixing the kinetic terms of the fields but without introducing superluminal or unstable propagation of the two scalar fluctuation modes. Though the typical effect of the disformal coupling is to inhibit one of the fields from inflating the universe, the energy density of the other field can drive viable near Sitter -inflation in the presence of nontrivial disformal dynamics, in particular when one assumes exponential instead of power-law form for the couplings. The linear perturbation equations are written for the two-field system, its canonical degrees of freedom are quantised, their spectra are derived and the inflationary predictions are reported for numerically solved exponential models. A generic prediction is low tensor-to-scalar ratio.
|
|
| 38. |
- Van De Bruck, C., et al.
(författare)
-
Non-Gaussianity in multi-sound-speed disformally coupled inflation
- 2017
-
Ingår i: Journal of Cosmology and Astroparticle Physics. - : Institute of Physics Publishing. - 1475-7516. ; 2017:2
-
Tidskriftsartikel (refereegranskat)abstract
- Most, if not all, scalar-tensor theories are equivalent to General Relativity with a disformally coupled matter sector. In extra-dimensional theories such a coupling can be understood as a result of induction of the metric on a brane that matter is confined to. This article presents a first look at the non-Gaussianities in disformally coupled inflation, a simple two-field model that features a novel kinetic interaction. Cases with both canonical and Dirac-Born-Infeld (DBI) kinetic terms are taken into account, the latter motivated by the possible extra-dimensional origin of the disformality. The computations are carried out for the equilateral configuration in the slow-roll regime, wherein it is found that the non-Gaussianity is typically rather small and negative. This is despite the fact that the new kinetic interaction causes the perturbation modes to propagate with different sounds speeds, which may both significantly deviate from unity during inflation.
|
|
| 39. |
- Zlosnik, Tom, et al.
(författare)
-
Spacetime and dark matter from spontaneous breaking of Lorentz symmetry
- 2018
-
Ingår i: Classical and quantum gravity. - : Institute of Physics (IOP). - 0264-9381 .- 1361-6382. ; 35:23
-
Tidskriftsartikel (refereegranskat)abstract
- It is shown that a spontaneously-broken gauge theory of the Lorentz group contains Ashtekar's chiral formulation of general relativity accompanied by dust. From this perspective, gravity is described entirely by a connection omega valued in the Lie algebra of the complexified Lorentz group and a Lorentz-charged scalar field tau. The model is 'pre-geometric' in the sense that the spacetime metric may be constructed only in the symmetry-broken regime. We speculate on the extent to which this dust may account for dark matter and on the behaviour of the theory in regimes where the symmetry is not broken.
|
|
