# Abstracts

### Contributed talks.

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Contributor.   João Luís Figueiredo Rosa (Instituto Superior Técnico – U. Lisboa)

Title.  The phase space of generalised hybrid metric-Palatini theories of gravity.
Abstract.  In this work, we study the cosmological phase space of the recently proposed generalized hybrid metric-Palatini gravity theory using the dynamical system approach. We formulate the propagation equations of the suitable dimentionless variables that describe FLRW universes as an autonomous system. The fixed points are obtained for four different forms of the function $f (R,\mathcal R)$ and the behaviour of the scale factor is computed. We show that due to the structure of the system, no global attractors can be present and also that two different classes of solutions for the scale factor exist. In addition, using a redefinition of the dynamic variables, we also compute solutions for static universes.

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Contributor.   Leonardo Chataignier (Institut für Theoretische Physik – U. Cologne)
Title.   Gauge Fixing and the Semiclassical Interpretation of Quantum Cosmology.
Abstract.   We make a critical review of the semiclassical interpretation of quantum cosmology and emphasise that it is not necessary to consider that time emerges only when the gravitational field is (semi)classical. We show that the usual results of the semiclassical interpretation can be obtained by a gauge fixing, both at the classical and quantum levels. By ‘gauge fixing’ we mean a particular choice of the time coordinate. In the quantum theory, we adopt a tentative definition of the (Klein-Gordon) inner product, which is positive definite for solutions of the quantum constraint equation found via an iterative procedure that corresponds to a weak coupling expansion in powers of the inverse Planck mass. We conclude that the wave function should be interpreted as a state vector for both gravitational and matter degrees of freedom, the dynamics of which is unitary with respect to the chosen inner product and time variable.

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Contributor.   Jan Novák (Technical U. Liberec)
Title.   Nonlinear graviton and plabic graphs.
Abstract.  Construction of a model of Quantum Gravity, which will be some day in concordance with experiments, is one of the most fascinating tasks which we have in modern theoretical physics. There are common features for all of the approaches to quantum gravity, which were developed so far. We mention the non-locality, background independence, dimensional reduction , problem of dark energy and arrow of time. We introduce the concept of nonlinear graviton and we suggest the mathematical apparatus for this paradigm, which is hidden in part of algebraic geometry connected to plabic graphs. We end with possible experimental evidence for our approach and we pose a list of open questions.

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Contributor.   João G. Rosa (U. Aveiro)
Title.   Warm inflation with strong dissipation.
Abstract.  We present the first particle physics model of warm inflation where the dissipative friction that sustains a nearly constant temperature during inflation can be much larger than the Hubble expansion rate for around 60 e-folds of accelerated expansion, consistently with CMB observations. In this scenario, inflation can occur for a simple quadratic potential with sub-Planckian field values and an inflaton mass comparable to the Hubble rate, thus solving the “eta-problem” in a way consistent with the recently proposed swampland conjectures.

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Contributor.   A. Nihan Katirci (İstanbul Technical University)
Title.   Extending ΛCDM model with restrained anisotropy.
Abstract.   We present a cosmological model via generalization the conventional vacuum to a source satisfying zero inertial mass density which yields particular form of shear stress. Doing so shear propagation and Klein Gordon equations becomes mathematically equivalent under some transformations, such that the scale factor replaced by the average scale factor, shear scalar and vacuum energy mimic the kinetic term and potential of a scalar field in Friedmann equations, respectively. When deformation of Λ accompanies the deviation from isotropic expansion, the shear scalar evolves non-trivially, changes very slowly, for instance, can track dark energy. To demonstrate, we apply this model to constant and evolving dark energy parametrizations using the newly revised observational data analysis. (joint with O. Akarsu, Anjan A. Sen and J. Alberto Vazquez).

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Contributor.   Clara Álvarez Luna (U. Complutense Madrid)
Title.   Implications of an SU(2) extension in effective potential models.
Abstract.   n the present work we study an extension of the SM with an additional SU(2) and an scalar. By using the Coleman-Weinberg mechanism, masses for both the gauge bosons and the scalars can be obtained. With this formalism we obtain two sets of gauge bosons, in addition to the two scalars. We analyse the mass ratio of these sectors in order to obtain a hyerarchy between them. With an appropriate choice of parameters we can have one of them in the SM range, while the other has much higher masses. Some potential implications for Cosmology will be discussed.

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Contributor.   Alastair Wickens (King’s College London)
Title.   Gravitational Waves from hot and cold hidden sectors.
Abstract.   We study the spectrum of gravitational waves produced by a first order phase transition in a hidden sector that is colder than the visible sector. In this scenario, bubbles of the hidden sector vacuum can be nucleated through either thermal fluctuations or quantum tunnelling. If a cold hidden sector undergoes a thermally induced transition, the amplitude of the gravitational wave signal produced will be suppressed and its peak frequency shifted compared to if the hidden and visible sector temperatures were equal. This could lead to signals in a frequency range that would otherwise be ruled out by constraints from big bang nucleosynthesis. Alternatively, a sufficiently cold hidden sector could fail to undergo a thermal transition and subsequently transition through the nucleation of bubbles by quantum tunnelling. In this case the bubble walls might accelerate with completely negligible friction. The resulting gravitational wave spectrum has a characteristic frequency dependence, which may allow such cold hidden sectors to be distinguished from models in which the hidden and visible sector temperatures are similar. We compare our results to the sensitivity of the future gravitational wave experimental programme.

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Contributor.   Thomas Helfer (King’s College London)
Title.   Cosmic String Loop Collapse in Full General Relativity.
Abstract.   We present the first fully general relativistic dynamical simulations of Abelian-Higgs cosmic strings using 3+1D numerical relativity. Focusing on cosmic string loops, we show that they collapse due to their tension and can either (i) unwind and disperse or (ii) form a black hole, depending on their tension Gμ and initial radius. We show that these results can be predicted using an approximate formula derived using the hoop conjecture, and argue that it is independent of field interactions. We extract the gravitational waveform produced in the black hole formation case and show that it is dominated by the l=2 and m=0 mode. We also compute the total gravitational wave energy emitted during such a collapse, being 0.5±0.2 % of the initial total cosmic string loop mass, for a string tension of Gμ=1.6×10^−2 and radius R=100 Mpl^-1. Extrapolating these results to the current bound of Gμ∼10^−8, we forecast that such events could be detected by LIGO/VIRGO at about once per decade and hundreds per year by the Einstein Telescope.

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Contributor.   Josu C. Aurrekoetxea (King’s College London)
Title.   Robustness of Inhomogeneous Inflation to Convex/Concave potentials.
Abstract.   In this talk I will compare how initial inhomogeneities in the Inflaton field affect different models. Convex potentials are very robust and support large inhomogeneities whereas Concave ones can fail. Using a prescription to determine whether inflation fails/wins, one can learn about the initial value of the Inflaton field.

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Contributor.   Mark Neyrinck (U. País Vasco)
Title.   Spinning (in) a Cosmic Spiderweb.
Abstract.   Artists and scientists have noted the visual similarity between the cosmic web and other networks in nature. I will mention a rigorous physical correspondence between the cosmic web, in a formalism called the adhesion model, and architectural structures known as spiderwebs (networks of strands that can exist entirely in tension), and also to origami. This relates to new work on how galaxies acquire angular momentum, through angular-momentum conservation’ (in a particular comoving sense) within evolving primordial patches. Many of its predictions are similar to the standard tidal-torque theory (TTT) of galaxy spin-up, which is an approximation to this picture. But it is more accurate and (in my opinion) more conceptually intuitive than the TTT.

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Contributor.   Joanes Lizarraga (UPV/EHU)
Title.   Irreducible gravitational wave emission from a network of cosmic defects.
Abstract.   Independently of the order of the phase transition, topology of the defects, and nature, global or gauge, of the symmetry broken, the defect network emits gravitational waves (GWs), as the network energy-momentum tensor adapts itself to maintain the scaling regime. In this talk I will review how any scaling defect network emits a scale-invariant GW background, which represents an irreducible emission of GWs from any scaling network’s of cosmic defects. I will show results of numerical experiments where we compute, using different techniques, the GW signal generated by the scalar dynamics of a global theory. I will also briefly discuss the ability of direct detection GW observatories to detect this background.

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Contributor.   José Ricardo C.C. de Carvalho Correia (IA – U. Porto)
Title.   On cosmic string network evolution and graphical supercomputing.
Abstract.   Cosmic strings arise in many proposed Grand Unified Theories. The need for higher resolution and extra complexity to model realistic defects, can heavily tax the underlying hardware, to the point where it will be unfeasible to simulate these objects and extract observational constraints. In order to overcome this problem we present a recently developed Abelian-Higgs cosmic string simulation that exploits an architecture capable of higher throughput and bandwidth ceilings: graphics processing units (GPU’s). We discuss briefly the scalability and speed-ups attained when using multiple GPU’s. We then use our simulation to calibrate the canonical semi-analytic model for cosmic string evolution (the Velocity-dependent One-Scale – VOS – model), revealing a few key insights with regards to overall energy losses of the network.

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Contributor.   Imanol Albarran Payo (U. Beira Interior)
Title.   A varying Dark Energy effective speed of sound parameter in the phantom Universe.
Abstract.   We analyse the phenomenological effects of a varying Dark Energy effective speed of sound parameter, $c_{\textrm{sd}}$, on the cosmological perturbations of three phantom Dark Energy models. Each of these models induce a particular abrupt future event known as Big Rip, Little Rip, and Little Sibling of the Big Rip. In these class of abrupt events, all the bound structures in the Universe are ripped apart at a finite cosmic time. We compute the evolution of the perturbations, matter power spectrum and $f\sigma_{8}$ growth rate.

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Contributor.   Elsa Maria Campos Teixeira (IA – U. Lisboa)
Title.   Disformally Coupled Quintessence Models.
Abstract.   Scalar fields have proved to be important entities for the implementation of dark energy models, in search for an explanation of the late time acceleration of the Universe. Armed with the tools of Dynamical Systems, these provide an ideal basis for the development of a theoretical understanding of the evolution of our Universe. There is no fundamental reason to believe that the field should not interact with the matter sector. In this work we consider a canonical scalar field which can couple to the matter species by means of a conformal/disformal transformation of the metric. We present the background equations and perform a detailed characterisation study of the different scenarios. We extend the work present in the literature by assuming that the disformal function in the metric transformation can depend both on the field itself and on its derivatives.

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Contributor.   Miguel Aparicio Resco (U. Complutense Madrid)
Title.   Parametrization of anisotropic modified gravities and their effects in the observable power spectra.
Abstract.   We analyze how parametrize modified gravity theories that include an additional vector field in the sub-Hubble regime within the quasi-static approximation. We consider the most general set of second order equations for metric and vector field perturbations and allow for both temporal and spatial components of the background vector field. When we can neglect dark matter vorticity, we find that four parameters are needed to fully characterize the theory. Additionally to the scale and redshift dependence, these parameters have an angular dependence between the scale and the preferred direction. Finally, we briefly analyze how this parametrization affects the observable power spectra of galaxy distribution and weak lensing.

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Contributor.   Asier Lopez-Eiguren (U. Helsinki)
Title.   Axion string evolution.
Abstract.   The axion is a pseudo-Goldstone boson of a new spontaneously broken symmetry, the Peccei-Quinn symmetry. If the symmetry is broken after inflation, topological strings are formed. As they slowly decay, they emit axion radiation. The axion production analysis is complicated because topological defect structures, global strings, appear in the axionic field and because the only reliable way to analyse their dynamics is through field simulations that require a huge amount of computational resources. In this talk we will investigate the recent claims that, contrary to the usual expectation for cosmic strings, state that the number of Hubble lengths of string per Hubble volume shows a logarithmic increase with time, instead of remaining constant. This would increase the axion dark matter density for a given symmetry-breaking scale and axion mass.

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Contributor.   Mikel Álvarez Urquiola (U. Basque Country)
Title.   Gaussian Random Fields as Models for the String Theory Landscape.
Abstract.   String Theory predicts the existence of a Landscape, a highly non-trivial scalar potential for the low energy 4d theory arising due to dimensional compactification. Many different approaches have been discussed in the last years in order to obtain statistical properties of the Landscape, one of them being its consideration as a Gaussian Random Field. In this talk, I will review some of the most recent results in this area. We will see how, using constraining methods on the probability distributions, Gaussian Random Fields can be used to obtain potentials allowing for inflation and generation of Primordial Black Holes. We will also analyze tunneling processes from metastable vacua in these potentials and how their study can be highly simplified with these constraining techniques.

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Contributor.   Ismael Ayuso (IACE – U. Porto)
Title.   About the sign of the effective gravitational coupling constant.
Abstract.   In this work we seek a cosmological mechanism that may define the sign of the effective gravitational coupling constant, G. To this end, we consider general scalar-tensor gravity theories as they provide the field theory natural framework for the variation of the gravitational coupling. We find that models with a quadratic potential naturally stabilize the value of G into the positive branch of the evolution and further, that de Sitter inflation and a relaxation to General Relativity is easily attained.

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Contributor.   Rita Neves (IA – U. Lisboa)
Title.   Quantum corrections to the flat FLRW model with cosmological constant in Loop Quantum Cosmology.
Abstract.   The dynamics of a flat FLRW cosmological model minimally coupled to a massless scalar field has been intensively studied in the context of Loop Quantum Cosmology. With an appropriate formulation, it is analytically solvable, leading to the resolution of the big-bang singularity in terms of a quantum bounce. However, models with a potential for the scalar field do not generically admit analytical solutions for the quantum dynamics. In this talk, I present the approximate dynamics of a model with constant potential for the scalar field, obtained by applying a procedure that sees the potential as a kind of geometric interaction. Treating the potential as a perturbation of the free model, we obtain corrections to the quantum dynamics and develop the tools to apply this procedure to more interesting models, with non-constant potential.

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Contributor.   Sravan Kumar (U. Groningen)
Title.   Non-singular solutions in IDG gravity.
Abstract.   In this talk, I shall introduce ghost free infinite derivative theory of gravity (IDG) which is inspired from string field theory (SFT). I will discuss the general properties of the theory and discuss UV completeness. I will show that the general solutions of the theory cannot admit singular solutions which are the pathologies of General relativity (GR). I will then describe how big bang and BH singularities can be generically resolved in this theory.

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Contributor.   Carlos García-García (IFF-CSIC)
Title.   Theoretical priors for quintessence. Towards a general parametrization of Horndeski cosmologies.
Abstract.   The late time acceleration of the Universe can be characterized in terms of an extra, time dependent, component of the universe — dark energy. The simplest proposal for dark energy is quintessence, a scalar field, $\phi$, whose dynamics is solely dictated by its potential, $V(\phi)$. We find the time dependence of {$\log(X) \equiv \log(\rho_{DE}{\rho_{DE}^0})$} for a broad family of potentials. Using this information, we propose a new parametrization which is accurate and in terms of which we construct a complete physical prior for quintessence.

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Title.   Chiral symmetry restoration by the Unruh effect.
Abstract.   In this talk we shall consider the possibility of restoring chiral symmetry through the Unruh effect. The Thermalization Theorem formalism and the large $N$ limit (with $N$ being the number of pions) will be employed to solve the lowest-order approximation to QCD at low energies in Rindler spacetime. We shall show that chiral symmetry is restored for accelerations higher than the critical value $a_c=4\pi f_\pi$, with $f_\pi$ being the pion decay constant, in complete analogy with the inertial, finite-temperature case. Due to the formal similarities between Rindler, Schwarzschild and de Sitter spaces, we will also discuss the possible applications of our results to cosmology.

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Contributor.   José Jaime Terente Díaz (U. Complutense de Madrid)
Title.   Dissipation effects in Modified Gravity.
Abstract.   An appealing inflationary scenario that, for some models, resolves the problem of the flatness of the potential is called ‘warm inflation’. In this model, there is a sizeable production of radiation. The inflaton-radiation coupling leads to a significant enhancement of the power spectrum. One needs to consider an imperfect fluid that gives rise to dissipative effects that damp the growth of the fluctuations. However, shear effects can stem from a theory of modified gravity, so they will dominate over the enhancement. In this talk, I present a study of the effects of modified gravity on the power spectrum of primordial perturbations.

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Contributor.   Alexander C. Jenkins (King’s College London)
Title.   Anisotropies in the stochastic gravitational-wave background.
Abstract.   In the new era of gravitational-wave astronomy, one of the most exciting targets for future observations is the stochastic gravitational-wave background (SGWB). While we have yet to detect the SGWB, we expect that by studying the angular power spectrum of its anisotropies, we may learn about the large-scale structure of the Universe (analogous to studies of the CMB). With this in mind, we develop detailed models of the SGWB anisotropies from two important sources of gravitational waves: unresolved compact binary coalescences, and cosmic strings.

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Contributor.   Mindaugas Karčiauskas (U. Complutense de Madrid)
Title.   Inflation, Electroweak Vacuum Stability and Direct Searches at LHC.
Abstract.   The current data favour metastability of the electroweak (EW) vacuum, which poses a number of cosmological challenges. We suggest a novel solution which does not employ any extra fields beyond the inflaton. We show that the Higgs mixing with an inflaton can lead to a stable EW vacuum. A trilinear Higgs–inflaton coupling always results in such a mixing and it is generally present in realistic models describing the reheating stage correctly. We find that cosmological constraints on this coupling are weak and an order one mixing is possible. In this case, the model is effectively described by a single mass scale of the EW size, making it particularly interesting for direct LHC searches.

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Contributor.   Ana Alonso-Serrano (AEI Potsdam)
Title.   Thermodynamics of spacetime: a perspective from the quantum realm.
Abstract.   Classically, the derivation of the Einstein equations from a thermodynamics perspective, taking into account the proportionality between the entropy and the horizon area, has been largely discussed. I will present some preliminary results and ideas of the thermodynamics of spacetime involving the consideration of a quantum corrected entropy.

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Contributor.   Francesco Montanari (IFT UAM-CSIC)
Title.   Friedmann-Lemaître-Robertson-Walker consistency conditions.
Abstract.   If the FLRW metric is a good approximation on large scales, then the distance and the expansion rate, as well different notions of distance, satisfy certain consistency conditions. We fit the JLA SNIa distance data to determine the expected amplitude of the violation of these conditions if accelerated expansion is due to backreaction. Adding cosmic clock and BAO expansion rate data, we also model-independently determine the current observational limits on such violation.

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Contributor.   Héctor Gil Marín (ICC U. Barcelona)
Title.   Testing Dark Energy using spectroscopic galaxy surveys: BAO and RSD.
Abstract.   Cosmic Microwave Background observations, such as WMAP or Planck, have shown the exquisite agreement of the early Universe with the LCDM+GR standard model. However, the information in the late Universe is still to be fully exploited, and surveys such as Euclid, DESI or LSST will do so in the next decade. The baryon acoustic oscillation (BAO) and redshift space distortion (RSD) techniques are going to be the leading observables in the forthcoming years to test potential deviations from the standard LCDM+GR using spectroscopic galaxy surveys. In the same direction, the late Universe is substantially non-Gaussian and therefore the information is not only contained in the 2-point statistics of the galaxy field, but also in its higher order correlators. In this talk I will review the state-of-the-art techniques of galaxy clustering analysis and I will also present the most relevant results from BOSS and eBOSS surveys, and the forecasts for upcoming surveys Euclid and DESI.

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Contributor.  Bence Racskó (University of Szeged)
Title.   Null junction conditions in kinetic braiding theories.
Abstract.   In general relativity, matching two solutions of the Einstein field equations accross a hypersurface separating them is described using the Israel junction conditions, provided the hypersurface is timelike or spacelike. Across a boundary hypersurface, they prescribe the continuity of the induced metric and of the extrinsic curvature. The discontinuity of the extrinsic curvature is also admissible for hypersurfaces referred to as thin shells. The Lanczos equation relates the jump of the extrinsic curvature to the distributional stress-energy tensor of the thin shell. Israel’s procedure breaks down if the hypersurface is null. This is a physically relevant case, as impulsive shock-waves traveling at the speed of light are described by null hypersurfaces, as are event horizons. The generalization of the Israel junction conditions to arbitrary hypersurfaces has been developed by Barrabés and Israel. If the hypersurface is specified to be null, then in Poisson’s reformulation, the junction conditions take a particularily simple form. The most general scalar-tensor theory with second order equations of motion for both the scalar and the metric tensor was given by Horndeski. Recent gravitational wave observations have severely constrained this class. The subclass guaranteeing the propagation of the tensorial modes with the speed of light is often referred to as kinetic braiding theories. Junction conditions accross timelike and spacelike hypersurfaces in the full Horndeski class have been found, but the null case has not been discussed. In this talk I address the junction conditions in the kinetic braiding class of theories accross null hypersurfaces. At a technical level I employ a pseudo-orthonormal basis with two null vectors, one of them being the surface gradient, the other playing the role of the transverse vector, with respect to which a (2+1)+1 decomposition is performed. The continuity of both the metric tensor and scalar are imposed over the hypersurface. Their first derivatives can have jump, and their second order derivatives lead to distributional contributions along the thin shell, providing suitable generalisations of the Lanczos equation and a constraint on the distributional sources. These equations in general contain both the jumps and the averages of the fields and of the functions built from them. I also analyse the subcase, where the averages can be transformed away by a suitable gauge in the tetrad choice.

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Contributor.   Rui Pedro Lopes de Azevedo (IA – U. Lisboa)
Title.   On particle creation/decay in nonminimally coupled models of gravity.
Abstract.   In extended models of gravity a nonminimal coupling to matter has been assumed to lead to irreversible particle creation. We argue that a non-minimal coupling of the matter and gravitational sectors leads instead to a change in particle-momentum on a cosmological timescale, irrespective of particle creation or decay. We further argue that particle creation or decay associated with a non-minimal coupling to gravity could only happen as a result of significant deviations from a homogeneous Friedmann-Robertson-Walker geometry on microscopic scales, and provide a phenomenological description of the impact of particle creation or decay on the cosmological evolution of the density of the matter fields.

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Contributor.   Filipe Mena (IST – U. Lisboa)
Title.   Global dynamics of SO(3) Yang-Mills and perfect fluid FLRW cosmologies.
Abstract.   We apply a new dynamical systems’ formulation to the evolution of flat FLRW cosmologies with matter source given by both Yang-Mills fields (having global SO(3) symmetry) and perfect-fluids (with linear equations of state). We give rigorous mathematical proofs concerning the global dynamics of the models, including their past and future asymptotics.

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Title.   Probing cosmic strings with LISA.
Contributor.   Lara Sousa (IA – U. Porto)
Abstract.   The stochastic gravitational wave background (SGWB) expected from cosmic string loops contains relevant information on the properties of the string network itself. In this talk, we analyze the ability of the Laser Interferometer Space Antenna (LISA) to measure this background under different hypothesis, determining the relevant parameter information that can be extracted from it.

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Contributor.   Catarina Cosme (Carleton University)
Title.   Phenomenology of Self-Interacting Dark Matter in a Matter-Dominated Universe.
Abstract.   We study the production of self-interacting dark matter (DM) during an early matter-dominated phase. As a benchmark scenario, we consider a model where the DM consists of singlet scalar particles coupled to the visible Standard Model (SM) sector via the Higgs portal. We consider scenarios where the initial DM abundance is set by either the usual thermal freeze-out or an alternative freeze-in mechanism, where DM was never in thermal equilibrium with the SM sector. For the first time, we take the effect of self-interactions within the hidden sector into account in determining the DM abundance, reminiscent to the Strongly Interacting Massive Particle (SIMP) scenario. In all cases, the number density of DM may change considerably compared to the standard radiation-dominated case, having important observational and experimental ramifications.

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Contributor.   Sayantan Choudhury (Max Planck Institute)
Title.   Open quantum cosmology from multi entangled atomic lamb shift.
Abstract.
In this paper, our prime objective is to investigate the thermal nature of the de-sitter space generated due to the entanglement between a pair of Unruh-De-Witt detectors in the paradigm of open quantum systems. The Master-equation of pair of a two-level atomic system within a framework of weakly interacting limit in the de-Sitter space is solved. One of the most important phenomena occurring due to the vacuum-fluctuations of a conformally coupled scalar field is the Casimir effect is studied in the framework of open quantum systems with a weakly- interacting environment degrees of freedom. Such vacuum fluctuations enhance the entanglement between the two atoms thereby resulting in the thermalization of the space-time and encodes the Unruh Effect into it. We study how the Resonance Casimir-Polder interaction between the two atoms manifests the curvature of space-time. Another main objective of this paper is to investigate the asymptotic entanglement between the two atoms giving rise to an ensemble of thermal states encoded within the vacuum state of the scalar field. This thermal phenomenon is obtained by the Gibbons-Hawking temperature is widely studied with respect to various toy model Hamiltonian for an entangled pair of atoms weakly conformally coupled to a scalar field

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Contributor.  Jackson Levi Said (Institute of Space Sciences and Astronomy)
Title.   Gravitational Wave in Symmetric Teleparallel Gravity.
Abstract.   Symmetric teleparallel gravity (STG) offers an interesting avenue to formulate a theory of gravitation that relies neither on curvature nor torsion but only on non-metricity. Given the growing number of observations of gravitational waves (GWs), this is a framework in which to study modified gravity and their constraints. STG can reproduce general relativity (GR) precisely at the level of the field equations, which is the so-called symmetric teleparallel equivalent of general relativity (STEGR). However, it can also offer novel modifications that are not available in extensions to GR. In this presentation, the implications of GW observations are used to limit the landscape of potential avenues of modified STEGR so that realistic constraints can be set in further astrophysical and cosmological settings.

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Contributor.   Tom Broadhurst (U. País Vasco).
Title.   TBA.
Abstract.   TBA.

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Contributor.   Manuel Krämer (KU Leuven)
Title.   Testing multiverse models with CMB data
Abstract.   Attempts to find observational tests of theories of quantum gravity face the problem that quantum-gravitational effects are strongly suppressed even in highly energetic scenarios like cosmological inflation. However, measurements of the cosmic microwave background (CMB) point towards a discrepancy between the data and the standard model of cosmology for large-scale anisotropies, which might be due to quantum gravity.
In this talk, I will present how the CMB anisotropy spectrum would be modified if our universe were part of a multiverse consisting of an ensemble of quantum universes described in the context of the so-called third quantization formalism. I will show that this description leads to the appearance of a pre-inflationary phase in the evolution of the individual universes. In particular, introducing an interaction between the universes modifies their evolution and hence their primordial power spectra in a way that can fit the observed large-scale anomaly in the CMB.

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Contributor.  Iván de Martino (Donostia International Physics Center)
Title.   Towards $\psi$-Dark Matter model.
Abstract.   The standard cosmological model has been constrained with unprecedented accuracy. Nevertheless, we are facing off new challenges. The lack of detection of Dark Matter has opened to the “no-WIMP” era. I will introduce a relatively new paradigm for Dark Matter based on ultra-light particles and explain how to probe it with the current and forthcoming dataset.

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Contributor.   Luís Bastos Ventura (U. Aveiro)
Title.   Quintessential Warm Inflation.
Abstract.   We discuss the possibility of the inflaton behaving as dark energy (DE) at late times within the warm inflation paradigm, particularly the “Warm Little Inflaton” scenario, the simplest realization of warm inflation formulated within a concrete quantum field theory framework.
We show that quintessential inflation follows naturally from the model’s symmetries and the absence of reheating, with a smooth transition from the inflationary era to a radiation-dominated phase. We present its dynamics and constraints from both inflationary (scalar spectral index $n_s$ and tensor-to-scalar ratio $r$) and DE observables (slow-roll parameter $\epsilon$ and equation of state $w$), concluding with prospects for future work.

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Contributor.  Francisco José Maldonado Torralba (U. Cape Town/ U. Groningen)
Title.   Non-singular and ghost-free infinite derivative gravity with torsion.
Abstract.   In this talk I will present the most general quadratic curvature action with torsion including infinite covariant derivatives and explain its implications around the Minkowski background. Also, I will show how both the metric and torsion fields can be made ghost and singularity free for a fermionic source.

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Contributor.  Giovanni Acquaviva (ITP, Charles University in Prague)
Title.   Gravitational energy-momentum and thermodynamics.
Abstract.   We present a detailed analysis of the so-called “square root of Bel-Robinson” (SQBR), proposed in the literature as an energy-momentum tensor for the gravitational field. Being constructed exclusively from the Weyl part of the Riemann tensor, the SQBR encapsulates the geometric properties of free gravitational fields in terms of optical scalars of null congruences: making use of the general decomposition of any energy-momentum tensor, we explore the thermodynamic interpretation of such geometric quantities by providing a (tentative) generalized first law of gravitational thermodynamics . While the matter energy-momentum is identically conserved due to Einstein’s field equations, the SQBR is not necessarily conserved and dissipative terms could arise in its vacuum continuity equation. We discuss the possible physical interpretations of such mathematical properties, both in general and in the specific cases of Kerr black holes and pp-waves spacetimes.

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Contributor.  Mar Bastero Gil (CEICO, Prague)
Title.   Vector dark matter production at the end of inflation.
Abstract.   It has been shown that the longitudinal mode of a massive vector boson can be produced by inflationary fluctuations and account for the dark matter content of the Universe. In this work we examine the possibility of instead producing the transverse mode via the coupling ϕFF̃ between the inflaton and the vector field strength. Such a coupling leads to a tachyonic instability and exponential production of one transverse polarization of the vector field, reaching its maximum near the end of inflation. We show that these polarized transverse vectors can account for the observed dark matter relic density in the mass range μeV to hundreds of GeV. We also find that the tachyonic production mechanism of the transverse mode can accommodate larger vector masses and lower Hubble scales of inflation compared to the production mechanism for the longitudinal mode via inflationary fluctuations.

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Contributor.  Gonzalo J. Olmo (U. Valencia)
Title.   New scalar compact objects in Ricci-Based gravity theories.
Abstract.   Spherically symmetric self-gravitating scalar field solutions of the EiBI and quadratic Palatini f(R) theories will be discussed and compared with their corresponding configuration in GR. We will see that in the EiBI model a new class of compact object arises which significantly differs from other proposals currently considered in the literature.

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Contributor.  Rubén Arjona Fernández (Instituto de Física Teórica UAM-CSIC, Madrid)
Title.   Unravelling the effective fluid approach for Modify Gravity and Dark Energy models in the sub-horizon approximation.
Abstract.  Although the Lambda cold dark matter model (ΛCDM) has become the best phenomenological description for the late-time accelerating phase of the Universe, the yet unsolved cosmological constant problem has driven an effort towards alternatives. We will mention two leading approaches which avoid the introduction of a cosmological constant. On the one hand, Dark Energy (DE) models where yet unobserved scalar fields would dominate the energy content at late times, avoiding fine-tuning issues as well as accelerating the Universe. On the other hand, there are Modified Gravity (MG) models that instead modify the current theory of gravity. We will demonstrate how to work out solutions to the perturbations equations in MG and DE models under the sub-horizon approximation. We will see that one can derive analytical solutions for DE perturbations and test them numerically showing that the quasi-static approximation actually performs quite well for this kind of models. Using the latter and simple modifications to the CLASS Boltzmann code, which we call EFCLASS, in conjunction to very accurate analytic approximations for the background evolution, one can obtain competitive results in a much simpler and less error-prone approach. We then use the aforementioned models to derive constraints from the latest cosmological data, including Type Ia supernovae, Baryon Acoustic Oscillations (BAO), Cosmic Microwave Background (CMB), H(z) and growth-rate data, and find they are statistically consistent to the ΛCDM model.

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Contributor.  Lorenzo Reverberi (CEICO, Prague)
Title.   fRevolution – Cosmological simulations of f(R) gravity.
Abstract.   We present the new relativistic N-body code “fRevolution”, for cosmological simulations in metric f(R) gravity theories. We will describe the theoretical framework and the code structure, and show the first results and comparisons with existing modified gravity codes.

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Contributor.  Federico Urban (CEICO, Institute of Physics, Prague)
Title.  Fuzzy Dark Matter and Fifth Forces with Binary Pulsars
Abstract.  I will show how properties such as spin, mass, and couplings of ultra-light (fuzzy) dark matter can be tested with binary pulsars, in particular by searching for secular variations in the orbital parameters. This approach can give the strongest constraints on, for example, vector B-L fifth forces or bi-gravity self-interactions.

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Contributor.  Javier Rubio (Helsinki Institute of Physics)
Title.  Spectator fields on a roller coaster.
Abstract.   I will discuss how the inclusion of non-minimally coupled spectator fields within quintessential inflation leads to the spontaneous symmetry breaking of internal symmetries and its eventual restoration at the onset of radiation domination. The proposed scenario comes together with a rich phenomenology involving the generation of short-lived topological defects that tend to produce gravitational waves until the symmetry is restored. The resulting power spectrum depends on the duration of the heating process and it is potentially detectable, providing a test on the existence of non-minimal couplings to gravity and on the characteristic energy scale of post-inflationary physics.

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Contributor.   Victor I. Afonso (U. Campina Grande)
Title.   Solving scalar field models in Ricci-based gravities.
Abstract.   By considering metric and connection as independent objects, metric-affine formalism allows for several modified gravity theories to enjoy healthy second-order ghosts-free field equations, without propagating any extra degrees of freedom besides the two polarizations of spin-two GWs recently confirmed by observations. On the other hand, the same freedom on the connection components brings in nonlinearities even stronger than in General Relativity, making the direct resolution of the equations in both, analytical or numerical approaches, a practically unattainable task. However, a way out of these difficulties appears when realizing the existence of an Einstein frame’ that recovers the structure of the Einstein equations at the cost of including a more intricate relation with the matter sources. In this talk we present some recent results obtained by implementing such mechanism in the case of Ricci-based gravity theories (RBG’s) coupled to scalar fields sources.

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Contributor.  Claus Kiefer (U. Cologne, Germany)
Title.  Singularity avoidance in anisotropic quantum cosmology
Abstract.   I discuss the fate of classical singularities in quantum cosmological models. After stating criteria of singularity avoidance and reviewing results for Friedmann models, I apply them to the anisotropic case of a Bianchi I universe. We find that the classical singularities are generally avoided.
(Ref.: C. Kiefer, N. Kwidzinski, D. Piontek, arXiv:1903.04391 [gr-qc]).

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Contributor.  Celia Escamilla-Rivera (MCTP/ICTP, Mexico)
Title.  New parametrized equation of state for dark energy survey.Abstract.   We present a new parameterisation for the equation of state (EoS), which can reproduce a f(R)-like evolution with a precision between [0.5%–0.8%] over the numerical solutions. Also, this proposal can render a variety of popular f(R) models that are considered as viable candidates for the cosmic late time acceleration. By using observational data from baryonic acoustic oscillations, supernovae and cosmic chronometers we investigate the constraints on the new EoS parameters. This proposal set an EoS formulation which can be used in an efficient way and makes a good candidate to be implemented in a variety of surveys in order to test the f(R) generic behaviour.

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Contributor. Carlos Martins (U. Porto)
Title.  From ESPRESSO to ELT-HIRES
Abstract.  Progress in fundamental physics and cosmology increasingly relies on high-resolution astrophysical spectroscopy. The first such instrument to have fundamental cosmology as a key science driver is ESPRESSO, whose GTO started in October 2018. Its successor ELT-HIRES will start its PhaseB in late 2019. Here I will highlight some early ESPRESSO results and discuss how the field may evolve in the coming years, also presenting detailed forecasts of the performance of ELT-HIRES and its impact on cosmology.