Project

CANTATA - Cosmology and Astrophysics Network for Theoretical Advances and Training Actions - COST Action CA15117.

Goal: Observations of unprecedented quality reveal a Universe that is at tension with the standard, and very successful description of matter and energy in Physics. Around 95% of the substratum of the Universe is of unknown nature, split into an accreting component (dark matter) and a repelling component (dubbed dark energy). There are auspicious prospects that the combination of state-of-the-art experiments, and theoretical advances will provide us with tools to elucidate this fundamental issue. This Action explores the viewpoint that cosmological observations reveal a degree of incongruous with theory not because of mysterious elements, but because of a need to review and extend Einstein Relativity to scales where it has not been properly tested. So this Action “CANTATA” gathers a team of European leading experts in gravitational physics and cosmology around the timely goal of investigating the extension of Einstein’s theory of General Relativity. A program including complementary aspects of theoretical physics, cosmology and astrophysics is put forward which is set to consider, in a coordinated and multidisciplinary way, the build up self-consistent models at the various scales and, in principle, to find out some “crucial feature” capable of confirming or ruling out Extended Theories of Gravity with respect to General Relativity. This Action will enhance already existing collaborations and establish an European network with the goal of developing a synergy between expertise and competences, leverage female gender representation, and foster participation of young researchers.

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General Relativity and the $\Lambda$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. We list the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization is performed in the framework of the COST European Action "Cosmology and Astrophysics Network for Theoretical Advances and Training Actions".
In this letter we reflect on the propagation of gravitational waves in alternative theories of gravity, which are typically formulated using extra gravitational degrees of freedom in comparison to General Relativity. We propose to understand that additional structure as forming a diagravitational medium for gravitational waves characterized by a refractive index. Furthermore, we shall argue that the most general diagravitational medium has associated an anisotropic dispersion relation. In some situations a refractive index tensor, which takes into account both the deflection of gravitational waves due to the curvature of a non-flat spacetime and the modifications of the general relativistic predictions, can be defined. The most general media, however, entail the consideration of at least two independent tensors.
In this letter we investigate graviton-photon oscillation in the presence of an external magnetic field in alternative theories of gravity. Whereas the effect of an effective refractive index for the electromagnetic radiation was already considered in the literature, we develop the first approach to take into account the effect of the modification of the propagation of gravitational waves in alternative theories of gravity in the phenomenon of graviton-photon mixing.
Extended theories of gravity have gathered a lot of attention over the last years, for they not only provide an excellent framework to describe the inflationary era but also yields an alternative to the elusive and mysterious dark energy. Among the different extended theories of gravity, on this work we focus on metric $f(R)$ theories. In addition, it is well known that if the late-time acceleration of the universe is stronger than the one induced by a cosmological constant then some future cosmic singularities might arise, being the Big Rip the most virulent one. Following this reasoning, on this work, we analyse the Big Rip singularity in the framework of $f(R)$ quantum geometrodynamics. Invoking the DeWitt criterium, i. e. that the wave function vanishes at the classical singularity, we proof that a class of solutions to the Wheeler-DeWitt equation fulfilling this condition can be found. Therefore, this result hints towards the avoidance of the Big Rip in metric $f(R)$ theories of gravity.
A new simple expression for the circular velocity of spiral galaxies is proposed and tested against HI Nearby Galaxy Survey (THINGS) data set. Its accuracy is compared with the one coming from MOND.
We investigate the Generalized Uncertainty Principle (GUP) corrections to the entropy content and the information flux of black holes, as well as the corrections to the sparsity of the Hawking radiation at the late stages of evaporation. We find that due to these quantum gravity motivated corrections, the entropy flow per particle reduces its value on the approach to the Planck scale due to a better accuracy in counting the number of microstates. We also show that the radiation flow is no longer sparse when the mass of a black hole approaches Planck mass which is not the case for non-GUP calculations.
A new round of applications for Short Term Scientific Missions (STSM) is now open until the 31st of January, 2018. These STSM should be completed by the end of March, 2018 due to the end of the current financial year. Details on STSM and how to apply appear below.
Short Term Scientific Missions (STSMs) are a networking tool with the purpose to provide additional chances of individual mobility, and to potentiate already established networks and exchanges by letting scientists within CANTATA to visit an institution in either another participating COST Country or an approved NNC institution or an approved IPC institution. A STSM should absolutely contribute to progress toward the objectives of CANTATA. As discussed and agreed at the early meeting and to contribute to this goal we have prepared the following list of hot topics:
• Dark energy from purified gravity.
• Modified gravity signatures in growth , fσ8 cosmological data and in sub-mm experimental data searching for new forces.
• Modified Gravity and the Dynamics of the Local Galaxies.
• Modified Gravity signatures in Gravitational Waves Observables.
• Effective Field Theory of Dark Energy.
• f(T) teleparallel gravity and its extensions.
• Gravitational Waves in teleparallel, metric and Palatini gravity.
• Equivalence principle formulation in teleparallel, metric and Palatini gravity.
• Nonmetricity: definition and signature.
• Exact solutions in teleparallel gravity.
• Exact solutions in higher order gravity.
• Bigravity cosmology.
• Nonlinear GR effects in large scale structure formation in the era of precision cosmology.
• Degeneracies between Modified Gravity, Dark Matter and baryonic physics at cluster of galaxies scales in view of Euclid and other galaxy surveys.
• Stellar structure models in teleparallel, metric, and Palatini gravity.
• Astrophysics and cosmology with matter curvature couplings (such as f(R,T) and the like).
Applicants should normally be employed at, or be a a student at, an institution of a COST participating country having accepted the MoU of CANTATA. Moreover, it is expected that this country has a record of active participation in CANTATA. In the following we outline some useful information for candidates who wish to apply for STSM funding.
• All candidates must apply via e-cost after which they will be informed to submit the following documentation:
1. A letter of invitation from a senior researcher affiliated to the Host institution
2. STSM application form (attached to this email as a pdf document).
3. Motivation letter including a work plan detailing the proposed activities [1 page].
4. Support letter / email from the Home Institution [1 page].
5. A full C.V. (including a list of academic publications – if applicable).
• Candidates should have a quick look at the rules for submitting travel expenses as they contain some specifics. Applicants are encouraged to get familiar with the travel reimbursement instructions
1. Accommodation expenses are paid against a flat per-diem rate of e120 irrespective of the actual cost, this flat rate includes breakfast. No copies of receipts are required.
2. Meals are paid against flat rates only. No invoices or receipts from the meal providers are required. Flat rates can never exceed EUR 20 per eligible meal. A maximum of up to two meals per day are eligible (two meals strictly concern lunch and dinner – breakfast expenses are not eligible expenses). Meals provided by other sources must be deducted from the number of meals that can be claimed by each participant.
3. Economy class air travel only, even if business class happens to be less expensive.
• Any informal requests about STSM can be sent to the STSM coordinator (Christian G Boehmer, UCL, c [dot] boehmer [at] ucl [dot] ac [dot] uk)

A new round of applications for Short Term Scientific Missions (STSM) is now open until the 22nd of December, 2017. These STSM should be completed by the end of March, 2018 due to the end of the current financial year. Details on STSM and how to apply appear below.
Short Term Scientific Missions (STSMs) are a networking tool with the purpose to provide additional chances of individual mobility, and to potentiate already established networks and exchanges by letting scientists within CANTATA to visit an institution in either another participating COST Country or an approved NNC institution or an approved IPC institution. A STSM should absolutely contribute to progress toward the objectives of CANTATA. As discussed and agreed at the early meeting and to contribute to this goal we have prepared the following list of hot topics:
• Dark energy from purified gravity.
• Modified gravity signatures in growth , fσ8 cosmological data and in sub-mm experimental data searching for new forces.
• Modified Gravity and the Dynamics of the Local Galaxies.
• Modified Gravity signatures in Gravitational Waves Observables.
• Effective Field Theory of Dark Energy.
• f(T) teleparallel gravity and its extensions.
• Gravitational Waves in teleparallel, metric and Palatini gravity.
• Equivalence principle formulation in teleparallel, metric and Palatini gravity.
• Nonmetricity: definition and signature.
• Exact solutions in teleparallel gravity.
• Exact solutions in higher order gravity.
• Bigravity cosmology.
• Nonlinear GR effects in large scale structure formation in the era of precision cosmology.
• Degeneracies between Modified Gravity, Dark Matter and baryonic physics at cluster of galaxies scales in view of Euclid and other galaxy surveys.
• Stellar structure models in teleparallel, metric, and Palatini gravity.
• Astrophysics and cosmology with matter curvature couplings (such as f(R,T) and the like).
Further information can be found on the CANTATA website: https://cantata-cost.eu/ and on our Facebook profile: https://www.facebook.com/costcantata/

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We consider gravitational waves from the point of view of both their production and their propagation in doubly coupled bigravity in the metric formalism. In bigravity, the two gravitons are coupled by a non-diagonal mass matrix and show birefrigence. In particular, we find that one of the two gravitons propagates with a speed which differs from one. This deviation is tightly constrained by both the gravitational Cerenkov effect and the energy loss of binary pulsars. When emitted from astrophysical sources, the Jordan frame gravitational wave, which is a linear combination of the two propagating gravitons, has a wave form displaying beats. The best prospect of detecting this phenomenon would come from nano-Hertz interferometric experiments.
Explaining the origin of the acceleration of the expansion of the Universe remains as challenging as ever. In this review, we present different approaches from dark energy to modified gravity. We also emphasize the quantum nature of the problem and the need for an explanation which should violate Weinberg's no go theorem. This might involve a self-tuning mechanism or the acausal sequestering of the vacuum energy. Laboratory tests of the coupling to matter of nearly massless scalar fields, which could be one of the features required to explain the cosmic acceleration, are also reviewed.
Relaxing the Riemannian condition to incorporate geometric quantities such as torsion and non-metricity may allow to explore new physics associated with defects in a hypothetical space-time microstructure. Here we show that non-metricity produces observable effects in quantum fields in the form of 4-fermion contact interactions, thereby allowing us to constrain the scale of non-metricity to be greater than 1 TeV by using results on Bahbah scattering. Our analysis is carried out in the framework of a wide class of theories of gravity in the metric-affine approach. The bound obtained represents an improvement of several orders of magnitude to previous experimental constraints.
We propose a new formula to explain circular velocity profiles of spiral galaxies. It is based on the assumption that the gravity can be described by two conformally related metrics: one of them is responsible for the measurement of distances, while the other so-called dark metric, is responsible for a geodesic equation and therefore can be used for the description of the velocity profile. The formula is tested against HI Nearby Galaxy Survey (THINGS) data set and their accuracy is compared with the one coming from MOND.
General Relativity has shown an outstanding observational success in the scales where it has been directly tested. However, modifications have been intensively explored in the regimes where it seems either incomplete or signals its own limit of validity. In particular, the breakdown of unitarity near the Planck scale strongly suggests that General Relativity needs to be modified at high energies and quantum gravity effects are expected to be important. This is related to the existence of spacetime singularities when the solutions of General Relativity are extrapolated to regimes where curvatures are large. In this sense, Born-Infeld inspired modifications of gravity have shown an extraordinary ability to regularise the gravitational dynamics, leading to non-singular cosmologies and regular black hole spacetimes in a very robust manner and without resorting to quantum gravity effects. This has boosted the interest in these theories in applications to stellar structure, compact objects, inflationary scenarios, cosmological singularities, and black hole and wormhole physics, among others. We review the motivations, various formulations, and main results achieved within these theories, including their observational viability, and provide an overview of current open problems and future research opportunities.
In Stoica (Int. J. Theor. Phys. 55, 71–80, 2016) a regularization procedure is suggested for regularizing Big Bang singularities in Friedmann-Lemaître-Robertson-Walker (FLRW) spacetimes. We argue that this procedure is only appliable to one case of Big Bang singularities and does not affect other types of singularities.
The present accelerated expansion of the universe has enriched the list of possible scenarios for its fate, singular or not. In this paper a unifying framework for analyzing such behaviors is proposed, based on generalized power and asymptotic expansions of the barotropic index $w$, or equivalently of the deceleration parameter $q$, in terms of the time coordinate. Besides well known singular and non-singular future behaviors, other types of strong singularities appear around the phantom divide in flat models, with features similar to those of big rip or big bang/crunch, which we have dubbed grand rip and grand bang/crunch respectively, since energy density and pressure diverge faster than $t^{-2}$ in coordinate time. In addition to this, the scale factor does not admit convergent generalized power series around these singularities with a finite number of terms with negative powers.
We describe a new type of gravity-matter models where gravity couples in a non-conventional way to two distinct scalar fields providing a unified Lagrangian action principle description of: (a) the evolution of both "early" and "late" Universe - by the "inflaton" scalar field; (b) dark energy and dark matter as a unified manifestation of a single material entity - the "darkon" scalar field. The essential non-standard feature of our models is employing the formalism of non-Riemannian space-time volume forms - alternative generally covariant integration measure densities (volume elements) defined in terms of auxiliary antisymmetric tensor gauge fields. Although being (almost) pure-gauge degrees of freedom, the non-Riemannian space-time volume forms trigger a series of important features unavailable in ordinary gravity-matter models. When including in addition interactions with the electro-weak model bosonic sector we obtain a gravity-assisted generation of electro-weak spontaneous gauge symmetry breaking in the post-inflationary "late" Universe, while the Higgs-like scalar remains massless in the "early" Universe.
We consider a non-standard model of gravity coupled to a neutral scalar "inflaton" as well as to SU(2)xU(1) iso-doublet scalar with positive mass squared and without self-interaction, and to SU(2)xU(1) gauge fields. The principal new ingredient is employing two alternative non-Riemannian space-time volume-forms (covariant integration measure densitities) independent of the metric. The latter have a remarkable impact - although not introducing any additional propagating degrees of freedom, their dynamics triggers a series of important features: appearance of infinitely large flat regions of the effective "inflaton" potential as well as dynamical generation of Higgs-like spontaneous symmetry breaking effective potential for the SU(2)xU(1) iso-doublet scalar.
Here we generalize ideas of unified Dark Matter Dark Energy in the context of Two Measure Theories and of Dynamical space time Theories. In Two Measure Theories one uses metric independent volume elements and this allows to construct unified Dark Matter Dark Energy, where the cosmological constant appears as an integration constant associated to the equation of motion of the measure fields. The Dynamical space time Theories generalize the Two Measure Theories by introducing a vector field whose equation of motion guarantees the conservation of a certain Energy Momentum tensor, which may be related, but in general is not the same as the gravitational Energy Momentum tensor. We purpose two formulations of this idea: I - by demanding that this vector field be the gradient of a scalar, II - by considering the dynamical space field appearing in another part of the action. Then the Dynamical space time Theory becomes a theory of Diffusive Unified Dark Energy and Dark Matter. These generalizations produces non conserved energy momentum tensors instead of conserved energy momentum tensors which leads at the end to a formulation of interacting DE-DM dust models in the form of a diffusive type interacting Unified Dark Energy and Dark Matter scenario. We solved analytically the theories for perturbative solution and asymptotic solution, and we show that the $\Lambda$CDM is a fixed point of these theories at large times. Also a preliminary argument about the good behavior of the theory at the quantum level is proposed for both theories.
In this paper, we consider the recently proposed hybrid metric-Palatini gravitational theory, which consists of adding to the Einstein-Hilbert Lagrangian an $f(\mathcal{R})$ term constructed \`a la Palatini. Using the respective dynamically equivalent scalar-tensor representation, we explore the cosmological evolution of a specific model, given by $f(\mathcal{R}) \propto \mathcal{R}^2$, and obtain constraints on the free parameters by using different sources of cosmological data. The viability of the model is analysed by combining the conditions imposed by the Supernovae Ia and Baryonic Acoustic Oscillations data and the results are compared with the local constraints.
We study the role of field redefinitions in general scalar-tensor theories. In particular, we first focus on the class of field redefinitions linear in the spin-2 field and involving derivatives of the spin-0 mode, generically known as disformal transformations. We start by defining the action of a disformal transformation in the tangent space. Then, we take advantage of the great economy of means of the language of differential forms to compute the full transformation of Horndeski's theory under general disformal transformations. We obtain that Horndeski's action maps onto itself modulo a reduced set of non-Horndeski Lagrangians. These new Lagrangians are found to be invariant under disformal transformation that depend only in the first derivatives of the scalar. Moreover, these combinations of Lagrangians precisely appear when expressing in our basis the constraints of the recently proposed Extended Scalar-Tensor (EST) theories. These results allow us to classify the different orbits of scalar-tensor theories invariant under particular disformal transformations, namely the special disformal, kinetic disformal and disformal Horndeski orbits. In addition, we consider generalizations of this framework. We find that there are possible well-defined extended disformal transformations that have not been considered in the literature. However, they generically cannot link Horndeski theory with EST theories. Finally, we study further generalizations in which extra fields with different spin are included. These field redefinitions can be used to connect different gravity theories such as multi-scalar-tensor theories, generalized Proca theories and bi-gravity. We discuss how the formalism of differential forms could be useful for future developments in these lines.
The method of non-Riemannian (metric-independent) spacetime volume-forms (alternative generally-covariant integration measure densities) is applied to construct a modified model of gravity coupled to a single scalar field providing an explicit unification of dark energy (as a dynamically generated cosmological constant) and dust fluid dark matter flowing along geodesics as an exact sum of two separate terms in the scalar field energy-momentum tensor. The fundamental reason for the dark species unification is the presence of a non-Riemannian volume-form in the scalar field action which both triggers the dynamical generation of the cosmological constant as well as gives rise to a hidden nonlinear Noether symmetry underlying the dust dark matter fluid nature. Upon adding appropriate perturbation breaking the hidden “dust” Noether symmetry we preserve the geodesic flow property of the dark matter while we suggest a way to get growing dark energy in the present universe’ epoch free of evolution pathologies. Also, an intrinsic relation between the above modified gravity $$+$$ single scalar field model and a special quadratic purely kinetic “k-essence” model is established as a weak-versus-strong-coupling duality.
Instructions on how to join the CANTATA network appear here: http://cantata-cost.eu/how-to-join/

Dear collaborators,
recall that CANTATA papers must be the result of a collaboration between at least two COST countries (two or more authors in different countries). Single authored papers or papers not involving N>1 COST countries are not eligible for being posted here. Otherwise Working Group leaders will have to do more work than necessary when preparing their respective progress reports.
Also, do not forget that according to the Guidelines for the dissemination of COST Action results and outcomes (http://www.cost.eu/media/dissemination-corporate-identity) the following sentence ensures a standard acknowledgement of COST funding and should appear in your CANTATA papers:
Kind regards,
Gonzalo.

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