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The conference will be held on ZOOM platform. The links to the virtual rooms will be sent, every day, within one hour from the start, to the emails which have been used to register.
In view of the very uncertain situation related to the pandemic of Covid-19, we have decided to hold our conference entirely online on 6th-10th of September of 2021.
More technical details will be provided in due time.
If you have any questions please send us an email to: altecosmo20@gmail.com
Yours faithfully,
Mariusz P. Dąbrowski (Organizing Committee chair)
Vincenzo Salzano (Scientific Committee chair)
The virtual conference “Alternative Gravities and Fundamental Cosmology” will take place in Szczecin, Poland from 14th to 18th of September 2020 from 6th to 10th of September 2021. This is the fifth event in the series of fundamental cosmology conferences organized by the University of Szczecin (previous were Cosmofun'2005, Grasscosmofun'09, Multicosmofun'12, Varcosmofun'16).
This time the task of the conference is to bring together specialists dealing with the problems of alternative gravities who want to exchange the current ideas in this topic. Among the problems being studied will be:
On Thursday 09.09 we will have a special day with a "Doctoral Students Workshop", where selected talks from Ph.D. Students will be allocated special extra time to present their research and discuss actively about it with other participants.
There will be a joint Special Issue published in the MDPI journal ''Universe''.
Szczecin Cosmology Group, Institute of Physics @ University of Szczecin, in collaboration with the Polish Physical Society, the Copernicus Center for Interdisciplinary Studies (Kraków), the National Centre for Nuclear Research (Świerk), the Maritime University of Szczecin, and Fundacja Eureka im. prof. Jerzego Stelmacha.
Most cosmologists today believed that our universe corresponds to a Big Bang homogeneous (FLRW) metric with an age which is only three times larger than the age of Earth. This seems in agreement with most observations and with the cosmological principle which states that spacetime is homogeneous only in space, but not in time. Recent measurements indicate that our cosmic expansion is dominated by a repulsive cosmological constant $\Lambda>0$. This indicates that we are inside a closed hypersphere, $r
Compact astrophysical objects like black holes and neutron stars are excellent tools to test the strong gravity regime of General Relativity and alternative gravity theories by comparing their theoretical predictions with current and future observations, since alternative gravity theories may feature distinctive signatures for these compact objects. While the analysis of the properties of black holes may yield direct insights for the gravity theories, an additional step is required in the case of neutron stars, whose properties depend also on their unknown equation of state. Thus universal relations should be obtained, to have (almost) equation of state independent predictions, that may, however, differ among the various gravity theories.
Unparticles are a hypothetical new form of matter created from fermions in an SU(N) gauge theory. Unparticles provide a wide spectrum of new cosmological applications. In my talk (based on arxiv:2010.02998 and arxiv: 1912.10532), I will show that they can display a cosmological-constant-like behavior, and since then they can be used to generate cosmic inflation or dark energy. I will show realistic bouncing and cyclic Universes filled with unparticles and perfect fluid. I will also discuss constraints on unparticles energy density and their possible role in relaxing the Hubble tension
In the 90s it was shown that the Einstein equation could be understood as an equation of state, general relativity as the equilibrium state of gravity, and f(R) gravity as a non-equilibrium one. In this presentation I discuss how the application of Eckart's first order thermodynamics to the effective dissipative fluid describing scalar-tensor gravity leads to a thermodynamics for the space of theories of gravity. Surprisingly, within this picture one obtains simple expressions for the effective heat flux, "temperature of gravity", shear and bulk viscosity, and entropy density, plus a generalized Fourier law in a consistent Eckart thermodynamical picture. Furthermore, a well-defined notion of the approach to equilibrium, missing in the current thermodynamics of spacetime scenarios, naturally emerges.
Recently the entanglement entropy between universes has been calculated, an entropy which somehow describes the quantumness of a homogeneous multiverse. The third quantization formalism of canonical quantum gravity is used here. Improvements of the results in a more general scenario will be shown, studying what happens at critical points of the evolution of a classical universe. Besides, we infer the relation of that entanglement entropy with the Hubble parameter of a single universes.
The primordial abundance of lithium is still a subject of controversy, given the disagreement between numerical results and observational estimates. We show how this discrepancy can be understood in the context of variation of fundamental constants at the epoch of Big Bang Nucleosynthesis. The variation of Newton's constant plays a crucial role. In particular, its interpretation in terms of additional relativistic degrees of freedom suggests an alleviation to the $H_0$ tension.
Using the logarithmic superfluid model, one can formulate quantum post-relativistic theory of superfluid vacuum, which contains special and general relativity in the “phononic” (low-momenta) limit, but differs at higher momenta. According to the theory, an effective gravitational potential is induced by the quantum wavefunction of physical vacuum in a stationary state, while the vacuum itself is viewed as the superfluid described by the logarithmic quantum wave equation. On a galactic scale, the model explains the non-Keplerian behaviour of galactic rotation curves, as well as why their profiles can vary depending on the galaxy. It also makes a number of predictions about the behaviour of gravity at larger galactic and extragalactic scales, which are expected to be seen in the outer regions of large spiral galaxies. We compare the non-flat asymptotics’ prediction with the furthest data points available for a number of galaxies. Using a two-parameter fit, we do a preliminary estimate; which disregards the combined effect of gas and stellar disc, but is relatively simple and uses minimal assumptions for galactic luminous matter. The data strongly points out at the existence of a crossover transition from flat to non-flat regimes at galactic outskirts and beyond. Another range of applications of the “logarithmic” matter can be found in the astrophysics of cold dense stars. We demonstrate the existence of equilibria in self-gravitating logarithmic fluid, described by spherically symmetric nonsingular finite-mass asymptotically-flat solutions in general relativity. Unlike other boson star models known to date, equilibrium configurations of relativistic logarithmic fluids are shown not to have scale bounds for their gravitational mass or size. Therefore, they can describe large massive dense astronomical objects, such as bosonized superfluid stars or cores of neutron stars.
The accelerated expansion of the Universe implies the existence of an energy contribution known as dark energy. Associated with the cosmological constant in the standard model of cosmology, the nature of this dark energy is still unknown. In this talk I will discuss an alternative gravity model in which this dark energy contribution emerges naturally, as a result of allowing for a time-dependence on the gravitational constant, $G$, in Einstein Field Equations. With this modification, Bianchi identities require an additional tensor field to be introduced so that the usual conservation equation for matter and radiation is satisfied. The equation of state of this tensor field is obtained using additional constraints, coming from the assumption that this tensor field represents the space-time response to the variation of $G$. I will also present the predictions of this model for the late Universe data, and show that the energy contribution of this new tensor is able to explain the accelerated expansion of the Universe without the addition of a cosmological constant. Unlike many other alternative gravities with varying gravitational strength, the predicted $G$ evolution is also consistent with local observations and therefore this model does not require screening. I will finish by discussing possible other implications this approach might have for cosmology and some future prospects.
The principle of finite amplitudes postulates that semi-classical transition amplitudes from the early universe up to current field values should be well defined. We will show in this talk that the application of this simple principle has strong theoretical constraining power for fundamentally motivated alternative theories of gravity and their solutions for the very early universe. In particular, we will present universes that emerge from the big bang in quadratic gravity and show that only inflating backgrounds (both isotropic and anisotropic) are consistent with finite quantum amplitudes. We will also present the analysis for non-singular cosmologies from limiting curvature gravity and fully $\alpha'$-corrected string cosmology, which are shown to be consistent with the principle of finite amplitudes.
If we are so eager to modify gravity, why can´t we modify string theory?, which in turn can give us even more modified gravity theories. For example the string tension does not have to be put in by hand, it can be dynamically
generated, as in the case when we formulate string theory in the modified measure formalism.
For gravity theories, the modified measure formalism gives a dynamical cosmological constant.
Then string tension appears, but as an additional
dynamical degree of freedom . It can be seen however that this string tension is not
universal, but rather each string generates its own string tension, which can have a
different value for each string. We also define a new Tension scalar background field
which change locally the value of the string tension along the world sheets of the strings.
When there are many strings with different string tensions this Tension field can be
determined from the requirement of world sheet conformal invariance and for two types of
string tensions depending on the relative sign of the tensions we obtain non singular
cosmologies and warp space scenarios and when the two string tensions are positive,
we obtain scenarios where the Hagedorn temperature is avoided in the early universe or in
regions of warped space time where the string tensions
become very big. Bubbles and Braneworld scenarios where strings are constrained to be be
between two surfaces where the string tension grows to infinity also appear naturally in
this approach
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references
1. Light Like Segment Compactification and Braneworlds with Dynamical String Tension Eduardo Guendelman, e-Print: 2107.08005 [hep-th]
2. Escaping the Hagedorn Temperature in Cosmology and Warped Spaces with Dynamical Tension Strings E.I. Guendelman, e-Print: 2105.02279 [hep-th] 3. Cosmology and Warped Space Times in Dynamical String Tension Theories Eduardo Guendelman, e-Print: 2104.08875 [hep-th]
We reconsider the dynamical systems approach to analyze inflationary universe in the Jordan frame models of scalar field nonminimally coupled to curvature or torsion. The adopted set of variables allows us to clearly distinguish between different asymptotic states in the phase space, including the kinetic and inflationary regimes. Inflation is realized as a heteroclinic trajectory originating either at infinity from a nonhyperbolic asymptotic de Sitter point or from a regular saddle de Sitter point. We also present a comprehensive picture of possible initial conditions leading to sufficient inflationary expansion and show their extent on the phase diagrams. In addition we determine the correct slow roll conditions applicable in the Jordan frame and show how they approximate the leading inflationary “attractor solution”. To seek the asymptotic fixed points we outline a heuristic method in terms of the “effective potential” and “effective mass”, which can be applied for any nonminimally coupled theories.
Both a metric and a tetrad $3+1$ formulation for a general affine connection is developed while also assuming nonmetricity. By splitting the space-time metric and tetrad into their spatial and temporal parts as well as through finding the Gauss-like equations for any tensor through which gravity is expressed, a general foundation for the formalisms is set up. Based on this foundation the resulting general $3$-tetrad and $3$-metric evolution equations are derived. Finally, through the choice of the two respective connections, the metric $3+1$ formulation for General Relativity is reaffirmed and the tetrad 3+1 formulation of the Teleparallel Equivalent of General Relativity and the metric 3+1 formulation of Symmetric Teleparallel Gravity with the coincident gauge are derived up to the latest state of the research.
In the semi-classical regime, quantum fluctuations embedded in a Riemannian spacetime can be effectively recast as classical back reactions and manifest themselves in the form of non-minimal couplings between matter and curvature. In this work, we exhibit that this semi-classical description can also be applied within the Teleparallel formulation. In the Teleparallel description, quantum fluctuations generically lead to non-minimal matter-torsion couplings. Due to the equivalence between the (classical) Einstein gravity in the Riemannian description and that in the Teleparallel description, some effective models which were constructed using the Riemannian description can be recovered completely with the Teleparallel description. Besides, when the effective quantum correction term is proportional to the torsion scalar T, we obtain a subclass of novel f(T,B,T) gravity, where B is a boundary term, and T is the trace of the energy-momentum tensor. Next, we investigate the cosmological properties in this f(T,B,T) theory. In this work, the matter Lagrangian is solely constructed by a dynamical scalar field. We exhibit some interesting cosmological solutions, such as those with decelerating expansion followed by a late-time accelerating phase.
In this work, we investigate gravitational baryogenesis in the framework of f(P) gravity to understand the applicability of this class of modified gravity in addressing the baryon asymmetry of the Universe. For the analysis, we set f(P)=αP where α is the model parameter. We found that in f(P) gravity, the CP-violating interaction acquires a modification through the addition of the nontopological cubic term P in addition to the Ricci scalar R and the mathematical expression of the baryon-to-entropy ratio depends not only on the time derivative of R but also the time derivative of P. Additionally, we also investigate the consequences of a more complete and generalized CP-violating interaction proportional to f(P) instead of P in addressing the baryon asymmetry of the Universe. For this type of interaction, we report that the baryon-to-entropy ratio is proportional to R˙, P˙ and f′(P). We report that for both of these cases, rational values of α and χ generate acceptable baryon-to-entropy ratios compatible with observations.
Reference: arXiv:2103.15312
Euclid is an ESA medium class astronomy and astrophysics space mission. Euclid was selected by ESA in October 2011 and its launch is planned for 2022. Euclid will explore how the Universe evolved over the past 10 billion years to address questions related to fundamental physics and cosmology..
I will give a general overview of the Euclid satellite and its mission, and describe the main probes and how they observe the evolution of the Universe. Focusing particularly on dark energy and modified gravity models, I will then briefly review what we know now, and how the Euclid observations will help to improve our knowledge.
Abstract:
Recent observations carried out using all 4 VLTs simultaneously have provided new quasar spectra of unprecedented quality. I will describe the very recent analysis of such data, carried out using new AI methods and other statistical tools that permit fully automated and unbiased estimates of the fine structure constant at high redshift.
We introduce a quantum interferometric scheme that uses states that are sharp in frequency and delocalized in position. The states are frequency modes of a quantum field that is trapped at all times in a finite volume potential, such as a small box potential. This allows for significant miniaturization of interferometric devices. We consider a concrete implementation using the ground state and two phononic modes of a trapped Bose-Einstein condensate. We apply this to show that frequency interferometry can improve the sensitivity of phononic gravitational waves detectors by several orders of magnitude, even in the case that squeezing is much smaller than assumed previously and that the system suffers from short phononic lifetimes. Other applications range from magnetometry, gravimetry and gradiometry to dark matter/energy searches.
Where, I venture into how the noise in gravitational wave detectors could tell us about the physics beyond the standard model of particle physics, and the fundamental nature of quantum black holes.
I will present an impact of alternative theories of gravity on low-mass (sub-)stellar objects' evolution and properties. I will also demonstrate how seismic data acquired from earthquakes and marsquakes could be use to test theories of gravity.
CGHS black holes have rightfully garnered much attention over the last few decades as the models are simplified (1+1)-dimensional versions of black hole evaporation. Their solubility has lead to tractable physical insights into the radiative process. Concurrently, moving mirrors are well-known simplified (1+1)-dimensional models for black hole evaporation. We synthesize the two by finding an exact correspondence between the CGHS black hole and exponentially accelerated moving mirror. The equivalence of these two models can be seen from several matching quantities such as trajectory of the moving mirror that, in turn, corresponds to the center of the black hole; spectrum of the particle radiation; the event horizon locations and the temperatures.
Furthermore, a novel derivation and understanding of the mirror power and self-force are applied to this particular moving mirror, CGHS mirror.
It is an observationally established fact that dark matter forms large scale structures in the intergalactic space. However it is not fully known if any structures can emerge on the stellar scale and if so, what would they look like.
In this short talk I will discuss the possibilities of the emergence of axionlike particle (ALP) clouds around compact objects, such as black holes. Using Einstein-Maxwell-ALP theory I will present how the geometrical structure of the clouds depends on the kind of the hosting object and its parameters. By virtue of the system’s free energy I will indicate the most probable scenarios for ALP clouds formation.
This work explores the effects of charge on a peculiar stellar object, recognized as gravastar, under the influence curvature-matter coupling gravity. The gravastar is also known as an alternative to a black hole and is expressed by three distinct domains named as (i) the interior domain, (ii) the intermediate shell and (iii) the exterior domain. We analyze these domains for a specific modified gravity model conceding the conformal Killing vectors. In the interior domain, we assume that pressure is equal to negative energy density which leads to the existence of repulsive force on the spherical shell. The intermediate shell consists of ultra-relativistic plasma and pressure which shows a direct relation with energy density and counterbalances the repulsive force applied by the interior domain. The exterior vacuum spherical domain is taken to be the de Sitter spacetime illustrated by the Reissner-Nordstrom metric.
The fate of matter forming a black hole is still an open problem, although models of quantum gravity corrected black holes are available. In loop quantum gravity (LQG) models were presented, which resolve the classical singularity in the centre of the black hole by means of a black-to-white hole transition, but neglect the collapse process. The situation is similar in other quantum gravity approaches, where eternal non-singular models are available. A strategy is presented to generalise eternal models to dynamical collapse models by surface matching. Assuming 1) the validity of a static quantum black hole spacetime outside the collapsing matter, 2) homogeneity of the collapsing matter, and 3) differentiability at the surface of the matter fixes the dynamics of the spacetime uniquely. It is argued that these assumptions resemble a collapse of pressure-less dust and thus generalises the Oppenheimer-Snyder-Datt model. The junction conditions and the spacetime dynamics are discussed generically for bouncing black hole spacetimes, as proposed by LQG, although the scheme is approach independent. A global spacetime picture of the collapse for a specific LQG inspired model is discussed.
Amplitude methods have shown to be a promising technique to perform Post-Minkowskian calculations used as inputs to construct gravitational waveforms. In this talk, I will show how to extend these methods beyond GR. As proof of principle, I will consider spinless particles conformally coupled to a gravitational helicity-0 mode. This setup leads to subtleties in the matching procedure used to construct the potential for conformally coupled matter. I will show how to tackle these subtleties when computing the potential and scattering angle for the binary system, and how the result involves a non-trivial dependence on the momentum of the scattered particles.
During the last years many inspired Loop Quantum Gravity (LQG) models for homogeneous cosmology were carefully studied, however all these models required extra input to be self consistent. In this talk I will briefly present a gauge fixed version of LQG adapted to cosmological systems. The interesting feature of this model is the resulting cosmological dynamics: by using the full structure of LQG the usual bouncing scenario is replaced by the so called emergent bouncing universe.
We will review our previous work on precanonical quantization of GR and the recent work on precanonical quantization of the teleparallel equivalent of GR. Both approaches are based on Palatini formulations in vielbein variables and the analysis of constraints within the De Donder-Weyl Hamiltonian formulation which treats space and time variables on equal footing. In both theories, we obtain the generalized Dirac brackets of fundamental variables represented by differential forms. Their quantization leads to two different descriptions of quantum space-time: quantum connection dynamics in the case of GR and the quantum frames dynamics in the case of TEGR. In both cases, we present the corresponding covariant precanonical Schroedinger equations and briefly discuss the classical limit, the quantum-gravitational avoidance of singularities, the emergence of the cosmological constant, and compare the simplest quantum cosmological solutions and their potentially observable consequences.
In non-smooth and discrete metric spaces of some models of quantum gravity, e.g., those based on Ricci calculus, it is a nontrivial task to introduce a notion of curvature that works at any length scale down to the cutoff scale and in the continuum limit converges to a curvature defined in terms of the Riemann tensor. The recently introduced quantum Ricci curvature has those properties. In the talk I will present this quantity and the results of calculating it in discrete spaces of several kinds, including the newest results in the most physically relevant four-dimesional model of Causal Dynamical Triangulations with the toroidal spatial topology.
Higher order extensions of Einstein gravity play important roles in various areas such as cosmology, the early universe or quantum gravity. In this talk, I will take a look into quantum properties of general higher order extensions of gravity provided that they depend on the Riemann tensor and the inverse metric. Using the functional renormalisation group, a flow equation for such theories is derived and its implications for a UV completion of gravity and gravitational fixed points are discussed.
There is an increasing interest in cosmological models with scalar fields that present kinetically dominated phases in their evolution, since these may have played a relevant role in the very early stages of the Universe and lead to modifications in observable quantities, e.g. the cosmic microwave background. The departures of this scenario from standard slow-roll inflation prevent one for employing the approximate analytical formulas for the power spectrum that are valid in slow roll, complicating the calculations, that, in most cases, have to be done numerically. Moreover, the complexity of these calculations increases if the model takes into account the quantum behavior of the background, incorporating it by means of expectation values on the background geometry, as it happens in hybrid quantum cosmology. In this situation, an interesting possibility consists in approximating our description of the perturbations around the free evolution without potential, so that only the knowledge of the dynamics of this particular case is required in full detail. In order to consider the influence of the potential, it is necessary to include the corrections that its presence produces on this free dynamics. We analyze these corrections at dominant order. In principle, the analysis that we present can be extended to cover higher-order corrections as well. In particular, our results facilitate the study of the quantum geometry effects on the primordial perturbations, which, in models as those of LQC, occur in kinematically dominated regimes.
The exact one-loop beta functions for the four-derivative terms (Weyl tensor squared, Ricci scalar
squared, and the Gauss-Bonnet) are derived for the minimal six-derivative quantum gravity (QG)
theory in four spacetime dimensions. The calculation is performed by means of the Barvinsky and
Vilkovisky generalized Schwinger-DeWitt technique. With this result we gain, for the first time, the
full set of the relevant beta functions in a super-renormalizable model of QG. The complete set of
renormalization group (RG) equations, including also these for the Newton and the cosmological
constant, is solved explicitly in the general case and for the six-derivative Lee-Wick (LW) quantum
gravity proposed in a previous paper by two of the authors. In the ultraviolet regime, the minimal
theory is shown to be asymptotically free and describes free gravitons in Minkowski or (anti-) de
Sitter ((A)dS) backgrounds, depending on the initial conditions for the RG equations. The ghost-like
states appear in complex conjugate pairs at any energy scale consistently with the LW prescription.
However, owing to the running, these ghosts may become tachyons. We argue that an extension
of the theory that involves operators cubic in Riemann tensor may change the beta functions and
hence be capable of overcoming this problem.
The Cosmic Microwave Background temperature and polarization anisotropy measurements have provided strong confirmation of the LCDM model of structure formation. Even if this model can explain incredibly well the observations in a vast range of scales and epochs, with the increase of the experimental sensitivity, a few interesting tensions between the cosmological probes, and anomalies in the CMB data, have emerged with different statistical significance. While some portion of these discrepancies may be due to systematic errors, their persistence across probes strongly hints at cracks in the standard LCDM cosmological scenario. The most statistically significant are the Hubble constant puzzle, the S8 parameter tensions, the Alens anomaly and a curvature of the Universe. I will review these tensions, showing some interesting extended cosmological scenarios that can alleviate them.
The standard cosmological model, LCDM, is based on General relativity and assumes the Universe is made of a Dark energy component in the form of a cosmological constant (L). Although LCDM gives an astonishing description of the Universe, the model shows some shortcomings: the so-called cosmological constant problems.Furthermore, some mild observational tensions among different datasets emerge in this model, for instance, on the value of the Hubble constant and amplitude of the matter power spectrum at present time and scale of 8 h/Mpc. This picture summarizes the motivations at the basis of speculations on the validity of the LCDM model and the search for new physics beyond the standard model.
I will present the phenomenology and cosmological bounds on alternative cosmological models compatible with the stringent constraint on the speed of propagation of gravitational waves from GW170817 and GRB170817A and which either offer a better fit to data than LCDM or alleviate the cosmic tensions.
In the previous works, using the SALT measurements of three luminous quasars, we confirmed the presence of the Broad Line Region radius-luminosity relation for the ultraviolet line of MgII. Together with SDSS-RM as well as Oz-DES datasets, we studied the classical as well as extended versions of the radius-luminosity (RL) relation. Using 78 sources, we simultaneously fitted the parameters of the RL relation as well as the cosmological parameters of six cosmological models (both flat and spatially curved). We found that regardless of the cosmological model, the RL relation is consistent and robust with the scatter of ~0.3 dex, which makes it possible to use MgII quasars as standardizable candles. The obtained cosmological constraints are consistent with the BAO+H(z) sample, favouring spatially flat $\Lambda$CDM model. However, the current dataset of MgII quasars, when used jointly with the BAO+H(z) sample, does not exclude cosmological models with mild dark energy and a little spatial curvature.
We analyse the emergent cosmological dynamics corresponding to the mean field hydrodynamics of quantum gravity condensates, in the tensorial group field theory formalism. We focus in particular on the cosmological effects of fundamental interactions, and on the contributions from different quantum geometric modes. The general consequence of such interactions is to produce an accelerated expansion of the universe, which can happen both at early times, after the quantum bounce predicted by the model, and at late times. Our main result is that, while this fails to give a compelling inflationary scenario in the early universe, it produces naturally a phantom-like dark energy dynamics at late times, compatible with cosmological observations. By recasting the emergent cosmological dynamics in terms of an effective equation of state, we show that it can generically cross the phantom divide, purely out of quantum gravity effects without the need of any additional phantom matter. Furthermore, we show that the dynamics avoids any Big Rip singularity, approaching instead a de Sitter universe asymptotically.
ΛCDM model to date remains the best observationally fitting model for late time cosmology. However, this model suffers from the theoretical issue that the quantum vacuum energy, which is the only known candidate for Λ, gives from QFT calculation a value that mismatches with the observed value of Λ by orders of magnitude. This theoretical issue motivated the search for alternative late-time cosmological models. Among various alternative models, a broad class of models incorporate modified gravity, within which a significant subclass is f(R) gravity models. A very pertinent question to ask is whether there are some f(R) gravity models that can exactly mimic the ΛCDM evolution history. This question is of interest because if there are indeed such f(R) gravity models, then one need not worry about the theoretical issue on Λ. This problem can be approached with the reconstruction method of f(R) gravity, although the form is too complicated for further analytical consideration. We approach this problem with a new model-independent dynamical systems formulation of f(R) that we recently introduced in 2103.02274. We show that there is an inherent issue in trying to reproduce ΛCDM cosmology in f(R) gravity.
In the first part of this talk, I will review the Hubble tension and then describe some theoretical efforts to alleviate it---as well as the discrepancy with the BAO Lyman-α data — via the dark energy models that yield negative density values in the past. I will then discuss a recent work with two minimal extensions of the ΛCDM model, together or separately, can realize such a scenario: (i) The spatial curvature, which, in the case of spatially closed universe, mimics a negative density source, (ii) Simple-graduated dark energy, which promotes the null inertial mass density of the usual vacuum energy to an arbitrary constant — if negative, the corresponding energy density decreases with redshift similar to the phantom models, but unlike them crosses below zero at a certain redshift. I will close the talk by presenting the results when these are constrained using the latest observational data.
I will motivate how moving beyond the FLRW paradigm may be the only way to resolve Hubble tension.
In this paper we have investigated a bulk viscous universe in $f(R,T)$ gravity where $R$ and $T$ are the Ricci scalar and trace of energy momentum tensor respectively. We have obtained explicit solutions of field equations in modified gravity by considering the power law form of scale factor. The Hubble parameter and deceleration
parameter are derived in terms of cosmic time and redshift both. We have estimated the present values of these parameters with observational Hubble data and SN Ia data sets. At 1$\sigma$ level, the estimated values of $q_{0}$ and $m$ are obtained as $q_{0}=-0.30 \pm 0.05$ \& $ m = 0.70 \pm 0.02 $ where $q_{0}$ is the present value of deceleration parameter and $m$ is the model parameter. The energy conditions and Om(z) analysis for the anisotropic LRS Bianchi type I model are also discussed.
Cosmological tensions in recent measurements of both Hubble expansion and the growth of structure in the Universe has led to a reconsideration of certain aspects of the concordance model of standard cosmology. One part of this comes from the growing tension between observations that are independent of cosmological models against others that are dependent on $\Lambda$CDM. To this end, the ability of reconstruction techniques to provide efficient and effective extractions of cosmological data has become ever more pressing. In this talk, some recent approaches to the problem are explored such as the use of Gaussian processes and the Locally weighted Scatterplot Smoothing together with Simulation and extrapolation method (LOESS-Simex) together with their advantages and disadvantages. The talk will also cover how genetic algorithmics may help improve the performance of these approaches. We close with a look at how deep learning may help improve the ability of these approaches to produce reconstructions of cosmic data
The recent gravitational wave observations of the collision of black holes and neutron stars have allowed us to pierce into the extreme gravity regime, where gravity is simultaneously unfathomably large and wildly dynamical. These waves encode a trove of information about physics that is prime for the taking, including potential revelations about the validity of Einstein's theory. In this talk, I will describe some of the physics inferences we have made from the data and what comes next when gravity waves.
Gravitational lensing of light is a well established test of gravity. However, little is known about how gravitational waves (GW) propagate beyond the simplest space-times in theories beyond Einstein’s General Relativity (GR). I will present a framework for GW lensing beyond GR at leading order in frequency. The modified causal structure and kinetic mixing between metric and additional degrees of freedom leads to new phenomena, providing clear-cut tests that do not require an electromagnetic counterpart. I will present detailed predictions for static, spherically symmetric lenses in an quartic Horndeski theory in which novel GW lensing effects can provide tests far more stringent than the multi-messenger event GW170817. The next terms in the frequency expansion will further enrich the phenomenology of GW lensing and enable new precision tests of gravity.
We study the effects of cosmological vector fields on the propagation of gravitational waves (GWs). The so-called dark sector in Cosmology remains unexplained, even though it makes up most of the content of the Universe. This fact has led to the proposal of several models of dark matter, dark energy or dark radiation. Among them, we can find some based upon vector fields (such as ultralight vector fields, which contribute to the matter content). Vector fields generically contribute with a non-zero anisotropic stress, thus affecting GWs, which are becoming increasingly important in observational astrophysics and cosmology. In this talk we focus on the effect of vectors on GW propagation. We present some phenomenological features, which include suppression, anisotropy and linear polarization of GWs, and show results for some specific models.
Gravitational waves (GWs) have opened a new window of fundamental physics in a number of important ways. The next generation of GW detectors may reveal more information about the polarization structure of GWs. Additionally, there is growing interest in theories of gravity beyond GR. One such theory which remains viable within the context of recent measurements of the speed of propagation of GWs is the teleparallel analogue of Horndeski gravity. In this work, we explore the polarization structure of this newly proposed formulation of Horndeski theory. In curvature-based gravity, Horndeski theory is almost synonymous with extensions to GR since it spans a large portion of these possible extensions. We perform this calculation by taking perturbations about a Minkowski background and consider which mode propagates. The result is that the polarization structure depends on the choice of model parameters in the teleparallel Horndeski Lagrangian with a maximum of seven propagating degrees of freedom. While the curvature-based Horndeski results follows as a particular limit within this setup, we find a much richer structure of both massive and massless cases which produce scalar--vector--tensor propagating degrees of freedom. We also find that the GW polarization that emerges from the teleparallel analogue of Horndeski gravity results in analogous massive and massless modes which take on at most four polarizations in the massless sector and two scalar ones in the massive sector. In none of the cases do we find vector polarizations.
The GRAVITY Collaboration achieved the remarkable detection of the orbital precession of the S2 star around the Galactic Centre supermassive black hole, providing yet another proof of the validity of the General Relativity. The departure from the Schwarzschild precession is encoded in the parameter $f_{\rm SP}$ which multiplies the predicted general relativistic precession. Such a parameter results to be $f_{\rm SP}=1.10\pm0.19$, which is consistent with General Relativity ($f_{\rm SP}=1$) at 1$\sigma$ level. Nevertheless, this parameter may also hide an effect of modified theories of gravity. We used the Schwarzschild-like metric of Scalar-Tensor-Vector-Gravity to predict the orbital motion of S2-star, and to compare it with the publicly available astrometric data, which include 145 measurements of the positions, 44 measurements of the radial velocities of the S2 star along its orbit, and the recent measurement of the orbital precession. We employed a Monte Carlo Markov Chain algorithm to explore the parameter space, and constrained the only one additional parameter of Scalar-Tensor-Vector-Gravity to $\alpha \leq 0.410$ at $99,7\%$ confidence level, where $\alpha=0$ reduces this modified theory of gravity to General Relativity.
The late universe contains a wealth of information about fundamental physics and gravity, wrapped up in non-Gaussian fields. To make use of as much information as possible it is necessary to go beyond power spectra. Rather than going to higher order N-point correlation functions, this talk will demonstrate that the probability distribution function (PDF) of spheres in the matter field (a 1-point function) already contains a large fraction of this non-Gaussian information. The matter PDF dissects different density environments which are lumped together in 2 point statistics, making it particularly useful for probing modifications of gravity or expansion history.
With an analytic model for the matter PDF we extend this formalism into cosmologies beyond ΛCDM, including $f(R)$ and DGP modified gravity and evolving dark energy. In all cases, the matter PDF provides an excellent complement to the matter power spectrum. Combining weakly non-linear power spectrum information with the matter PDF yields $5\sigma$ detections of both modified gravity theories, and increases the Figure of Merit for dark energy by a factor of 5 beyond power spectrum information alone.
We reconstruct the Hubble function using late-time cosmological data sets and use it to draw out Horndeski theories that are fully anchored on the expansion history. We discuss various formalisms for the inversion of the modified Friedmann equations and complement this with the reconstructed Hubble data to obtain predictive constraints on the Horndeski potentials and the dark energy equation of state.
Screening mechanisms in Extended Theories of Gravity (ETGs) are essential to make theories able to pass Solar System constraints and, at the same time, possibly driving the accelerated expansion of the Universe at large scales (thus behaving as dark energy). In our work, we have considered an ETG belonging to the family of Degenerate High-Order Scalar-Tensor theories (DHOST) and characterized by a partial breaking of such a screening mechanism. We test this theory on galaxy cluster scales, using strong and weak lensing data, X-ray observations, and a multi-component approach. We investigate the consistency of this model with data in two different scenarios: as a dark energy candidate; and, through the breaking of the screening mechanism, we assume and test the possibility it might even mimic dark matter. Final results show that the DHOST model, when acting as dark energy-only model, might be statistically preferred (by Bayesian evidence) in most of the cases with respect to General Relativity. Instead, when the DHOST is assumed to mimic also dark matter, it is generally disfavored.
Adding corrections quadratic in the curvature, like $R^2$ or $Q=R_{\mu \nu}R^{\mu \nu}$, to the Gravity Lagrangian can make the theory perturbatively renormalizable as a quantum field theory or even produce early time inflation. Testing such modifications to gravity is challenging, but can be done by astrophysical observations. Based on our findings in [1], this talk will focus on observable traces of modifications to gravity in the mass-radius relation of neutron stars. Focusing on $f(R)=R+\alpha R^2$ and $f(R,Q)=R+\alpha R^2 + \beta Q$ theories in the Palatini formalism, where $\alpha$ and $\beta$ control the strength of the modification, we show that the influence on the properties of a neutron star can be sizeable for certain combinations of $\alpha$, $\beta$ and some equations of state (EoS). Furthermore, we show that the main factors that influence the deviation from the GR result are, apart from $\alpha$ and $\beta$, the first and second derivative of the EoS, which go into the stellar structure equations. As a consequence, knowledge of the exact neutron star EoS is required to discriminate between GR and modified gravity theories. However, as soon as the neutron star EoS is known, observations of the mass and radius of neutron stars can be used to test modifications to GR of the Palatini $f(R)$ and $f(R,Q)$ type.
[1] https://arxiv.org/abs/2102.05722
The non-product spectral geometry may lead to models that possess features characteristic to bimetric gravity theories. Starting from the pair of Friedmann–Lemaître–Robertson–Walker metrics on the product geometry and mildly modifying the Dirac operator we end up with a class of models that have a nontrivial interacting potential term, and their solutions are stable for several cosmological scenarios. The resulting doubled FLRW geometries can be thought of as the generalization of the family of bimetric models with non-polynomial potential. Based on a joint work with Andrzej Sitarz.
General relativity has been very successful in describing gravity. However, cosmological observations such as the dark sector of the universe, the value of the cosmological constant, and the Hubble constant give indications to new physics. This might be explained by modified theories of gravity. What has often been overlooked is that general relativity has different equivalent descriptions. One of those is generally called teleparallel equivalent to general relativity, with an action formulation which only differs from the Einstein-Hilbert action by a boundary term. Starting from this action it is possible to formulate modified theories of gravity different from those based on the Einstein-Hilbert formulation. These theories are called teleparallel theories of gravity. I will present the present understanding of the viability of those theories based on the Hamiltonian analysis arxiv:2012.09180. I will also mention conclusions drawn from perturbation theory of teleparallel gravity in order to make stricter bounds on the viability.
The polynomial affine gravity is an alternative model to describe gravitational interactions using the affine connection as the sole mediator. The action is built using a sort of dimensional analysis technique and preserving the invariance under diffeomorphisms. Interestingly, the coupling constants are dimensionless, which is desirable from a quantum field stand point. In $3+1$ dimensions the field equations in the torsion free sector contain Einstein's vacuum equations, moreover, it is possible interpret the symmetric part of the Ricci tensor or a special combination of the product of two torsion tensors as an emergent metric in this model. Similar analysis can be done in $2+1$ dimensions. Therefore, starting from a purely affine geometrical model, we can obtain a metric tensor, and consequently define physical quantities such as the redshift, classification of space-null-time like self-parallel curves, providing a way to differentiate trajectories of massive and massless particles.
From the study of relativistic dynamics of fluids out of equilibrium in a curved background, a new cosmological framework, dubbed Ricci Cosmology, has emerged in which linear terms in Ricci scalar and Ricci tensor lead to modifications of the equilibrium pressure in the energy-momentum tensor in the fluids filling the Universe. The coefficients in front of such terms are called second order transport coefficients and parametrise the fluids response to the pressure terms arising from the spacetime curvature.
Under the assumption of constant coefficients, we find the simplest solution in which the presence of such terms causes a departure from the perfect fluid redshift scaling for matter components in the Universe. By using the second law of thermodynamics, theoretical bounds on the transport coefficients are imposed. In order to test the viability of this solution, we make four different ansätze on the transport coefficients, giving rise to four different cases of our model. The observational bounds on the second order transport coefficients obtained by testing each case against cosmological data are compatible with the thermodynamical bounds and indicate that Ricci Cosmology is compatible with ΛCDM cosmology for all the ansätze.
Abstract
After the recent detection of GW170817, the most interesting terms of Horndeski theory were severely
constrained. Nevertheless, the analog of Horndeski theory in the Teleparallel Gravity framework is far richer in structure since the extra term in the Lagrangian, Ltele emerges.
As a result, the terms that were eliminated in standard Horndeski theory could, in this case, survive through the Lagrangian contribution leading to a varied phenomenology.
In order to determine the unknown functions Gi(φ; x) of the Horndeski analog in the Teleparallel framework, we adopt Noether point symmetries as a classification criterion. The existence of such symmetry not only selects the form of the Gi(φ; x) but also could lead to valid cosmological models for future research
and study.
Scalar Tensor Vector Gravity (STVG) is a metric theory of gravity with dynamical scalar fields and a massive vector field introduced in addition to the metric tensor. In the weak field approximation STVG modifies the Newtonian acceleration with a Yukawa like repulsive term due to Maxwell-Proca type Lagrangian. This associates matter with a fifth force and a modified equation of motion.STVG has been successful in explaining galaxy rotation curves, gravitational lensing, cosmological observations and all other solar system observation without the need of dark matter. In this talk we present the key concepts of STVG theory. Then I will discuss existing observational bounds on STVG parameters.In particular I will present our original results obtained from X-COP sample of
galaxy clusters.
CDT is a numerical approach to quantum gravity which attempts to describe our Universe with the help of Regge Calculus and Path Integral formalism. The study of the past years revealed the rich phase-diagram of the model, which contains a physical de Sitter phase with higher order phase-transitions on its boarders. Recently we added scalar fields to the model. The classical fields were used as coordinates, which revealed the structure of the CDT Universes which resembled the cosmological voids and webs. When the dynamical / quantum fields were used they triggered a phase-transition which effectively changed the space-time topology. During my talk I will show the most recent results related to scalar-fields in the model of Causal Dynamical Triangulations.
In the context of the late time cosmic acceleration phenomenon, many geometrically
modified theories of gravity have been proposed in recent times. In this paper, we have investigated the role of a recently proposed extension of symmetric teleparallel gravity dubbed as f(Q,T) gravity in getting viable cosmological models, where Q and T respectively denote the non-metricity and the trace of energy momentum tensor. We stress upon the mathematical simplification of the formalism in the f(Q,T) gravity and derived the dynamical parameters in more general form in terms of the Hubble parameter. We considered two different cosmological models mimicking non-singular matter bounce scenario. Since energy conditions play a vital role in providing bouncing scenario, we have analyzed different possible energy conditions to show that strong energy condition and null energy condition be violated in this theory. The models considered in the work are validated through certain cosmographic tests and stability analysis.
Flat space theories of spinning particles must obey nontrivial consistency conditions that follow from locality, unitarity and gauge invariance. These conditions are especially powerful in the case of Lorentz invariant theories of massless particles, where they have been used to derive the gravitational equivalence principle, among other interesting results. In a recent paper, we have shown that even if we drop the assumption of boost invariance, as long as all particles are assumed to propagate at the speed of light, similar conclusions still apply and the gravitational interactions pass consistency tests only if they are Lorentz invariant. This result should be kept in mind when considering Lorentz violations in theories defined on the Minkowski background, and perhaps also in the study of the early universe, where boost invariance is broken by the expanding space.
Recently, based on swampland considerations in string theory, the (no) eternal inflation principle has been put forward. The natural question arises whether similar conditions hold in other approaches to quantum gravity. In this talk I will discuss the asymptotic safety hypothesis in the context of eternal inflation. As exemplary inflation- ary models the SU(N) Yang-Mills in the Veneziano limit and various RG-improvements of the gravitational action will be discussed. I will also discuss the finite action principle in the context of initial conditions for the Universe and (eternal) inflation.
Taking the minimalistic approach, within MSSM, we propose the model of inflation in which the inflaton field is a scalar component of the MSSM state(s).
The proposed model turns out to be very predictive. The inflationary phase is fully governed by the MSSM Yukawa superpotential couplings. The values of the scalar spectral index and the tensor-to-scalar ratio are predicted to be ns≃0.966 and r=0.00118. The post-inflation reheating of the Universe proceeds by the inflaton’s decay with the reheating temperature around 10 thousands TeV.
Some phenomenological implication will be also outlined and discussed.
Based on connections between gravity and thermodynamics, interpreting the dynamics of the universe as a quest for achieving holographic equipartition is a novel concept proposed by Padmanabhan. However, the generalization of Padmanabhan's conjecture to the non-flat universe had resulted in uncertainty about the choice of volume. We have shown that the exact mathematical formulation of the conjecture is impossible with the proper invariant volume (Volume term derived from the FRW metric) for a non-flat universe. The deep connection between the first law of thermodynamics and the law of emergence motivated us to also explore the status of the first law in a non-flat universe when one uses proper invariant volume. We have shown that the first law of thermodynamics, $dE=TdS+WdV$, cannot be formulated properly for a non-flat universe using proper invariant volume. We can also show that the energy change within the horizon is not equivalent to the outward energy flux in the non-flat universe if one used the proper invariant volume. We further point out that the consistency between the above two forms of the first law will hold only with the use of areal volume, which hints us why our universe appears to be spatially flat.
By proposing an appropriate dynamical deformation between the momenta associated with the scalar field (of the Sáez–Ballester theory) and scale factor of the spatially flat FLRW metric, we establish a modified cosmological model. Subsequently, for some particular cases, by focusing on the early epoch of the universe, we show that our model provides a more successful description for evolution of the universe with respect to the corresponding standard models.
I discuss in this talk a new formulation of dark-matter (DM) coupling to gravity. Unlike the Standard Model (SM) sector which couples to the metric, DM couples to the spacetime affine connection through a $Z_2$-symmetry breaking term. I will show that such a structure allows DM to be only scalar particles (unlike the other alternative gravities). I discuss the different decay modes of DM in this framework, and comment on bounds from observational data. Furthermore, I will discuss the possible signatures at present and future colliders with an emphasis on light DM masses, i.e. $m_\phi \simeq \mathcal{O}(10)~{\rm GeV}$.
We will review the Scale Invariant Vacuum idea as related to Weyl Integrable Geometry. Main results related to SIV and inflation 1, the growth of the density fluctuations 2, and application of the SIV to scale-invariant dynamics of Galaxies, MOND, Dark Matter, and the Dwarf Spheroidals 3 will be summarized. If time permits, a potential connection of the weak field SIV results to the un-proper time parametrization within the reparametrization paradigm, will be discussed as well 4.
1 Maeder, A., Gueorguiev, V. G., Scale invariance, horizons, and inflation. MNRAS 504, 4005 (2021). arXiv: 2104.09314 [gr-qc].
2 Maeder, A., Gueorguiev, V., G., The growth of the density fluctuations in the scale-invariant vacuum theory, Phys. Dark Univ. 25, 100315 (2019). arXiv: 1811.03495 [astro-ph.CO]
3 Maeder, A.; Gueorguiev, V.G. Scale-invariant dynamics of galaxies, MOND, dark matter, and the dwarf spheroidals, MNRAS 492, 2698 (2019). arXiv: 2001.04978 [gr-qc]
4 Gueorguiev, V. G., Maeder, A., Geometric Justification of the Fundamental Interaction Fields for the Classical Long-Range Forces. Symmetry 13, 379 (2021). arXiv: 1907.05248 [math-ph].
We present a simple quantum description of the Universe in which the effective de Sitter spacetime geometry emerges from a coherent state of background gravitons. Once localised baryonic matter is added consistently, this quantum state is shown to contain the necessary components to describe MoND phenomenology at galactic scales and possibly explain the tension between values of the Hubble parameter measured from the CMB and supernovae data.
We study the relation between quasinormal modes and geodesic quantities recently brought back due to the black hole shadow observation by Event Horizon Telescope. With the help of WKB method we found an analytical relation between the real part of quasinormal frequencies at the eikonal limit and shadow radius of the same black hole. Some examples fulfilling the correspondence are provided.
Testing strong gravity regimes such as the vicinity of black holes is likely to be attainable with the future developments of observing technology. In this talk, adopting a theory-agnostic approach, we first propose a class of Kerr-like rotating black holes, whose Z_2 symmetry is generically broken. We focus on the possibility that such a violation of Z_2 symmetry is induced by the spin of the black hole. This class of Kerr-like spacetimes could be a good approximation to general black hole solutions in effective low-energy theories of a fundamental quantum theory of gravity. In the model, the violation of the Z_2 symmetry can be parametrized by a single parameter. Then, we discuss how the Z_2 asymmetry in the spacetime could give interesting astrophysical consequences which may be observable.
One of the consequences of Einstein’s general theory of relativity is bending of light as it passes through a gravitational field. Examining the path of light in a very strong gravitational field of a black hole can provide a huge amount of information about the geometry and characteristics of the surrounding space.
On the other hand, the path of light rays, extent, and shape of gravitational lensing, are directly related to the type of background geometry in which light is emitted. Since the theory of general relativity in very high energies and very strong gravitational fields is expected to be corrected, researchers have been looking at the phenomenon of gravitational lensing in the context of alternative theories for general relativity to find out the needed corrections for the results of general relativity and these corrections are likely to be more significant in a very strong gravitational field of a black hole.
Among the various theories that have been proposed for correcting the gravity in high energies, gauge theories of gravity have great importance. One of the important results of these theories is changing the geometry for the background in general relativity, Riemannian space-time, to a non-Riemannian geometry in which, in addition to curvature, there is also torsion. In these theories, the presence of torsion coupled to spin of a matter can affect the path of light rays and correct the results of gravitational lensing.
In this work, we want to study the effects of non-Riemannian geometry on the gravitational lensing of a black hole, and in particular the effects of torsion and spin in this context.
The IceCube neutrino observatory is a neutrino telescope situated near the South Pole in Antarctica. A cubic kilometer of ice is instrumented with optical modules sensitive to photons. When high energetic particles produce light in interactions with the ice, the signature can be recorded and used for reconstruction of the primary particle.
The design of IceCube not only facilitates the detection of astrophysical neutrinos up to PeV energies but also the direct and indirect probe of physics beyond the Standard Model with leading sensitivities. Exotic particles which can penetrate through the ice sheet or even the entire Earth can be measured directly, these include magnetic monopoles, Q-Balls, or partially charged particles. Dark matter is indirectly searched for by investigating its effect on neutrino spectra as well as arrival directions.
The discovery of astrophysical neutrinos enables the measurement of neutrino interactions at unprecedented energies where new physics might emerge such as Lorentz Invariance Violation. The recent achievements of IceCube in the search for beyond Standard Model physics will be presented.
In recent years there have appeared several constructions of traversable wormholes, in four and other dimensions, which only involve physically acceptable, controllable ingredients. They connect in deep ways many aspects of gravity, quantum field theory, and quantum information. I will discuss several features of these constructions, with a focus on traversability, connectivity between multiple mouths, and the (im)possibility of time travel.
Over the last years some interest has been gathered by f(Q) theories, which are new candidates to replace Einstein’s prescription for gravity. The nonmetricity tensor Q allows to put forward the assumption of a free torsionless connection and, consequently, new degrees of freedom in the action are taken into account. This work focuses on a class of f(Q) theories, characterized by the presence of a general power-law term which adds up to the standard (linear in) Q term in the action, and on new cosmological scenarios arising from them. Using the Markov chain Monte Carlo method, we carry out statistical tests relying upon background data such as Type Ia supernovae luminosities and direct Hubble data (from cosmic clocks), along with cosmic microwave background shift and baryon acoustic oscillations data. This allows us to perform a multifaceted comparison between these new cosmologies and the (concordance) ΛCDM setup. We conclude that, at the current precision level, the best fits of our f(Q) models correspond to values of their specific parameters which make them hardly distinguishable from our general relativity “échantillon,” that is, ΛCDM.
We perform observational tests on the $f(T) $ gravity using the Cosmic Chronometer data, SNIa data and BAO data together with three different independent measurements of the current value of $H_0$. In this work, we investigate the impact of these priors on five core models in $f(T)$ gravity. In addition, we perform background studies on these models to better distinguish the impacts of the priors and $f(T)$ models. To do so, the Markov chain Monte Carlo (MCMC) technique was used in order to constrain the varying parameters of the models, including the Hubble constant $H_0$. These models, in turn, are compared to the $\Lambda$CDM model which allows us to investigate the $H_0$ tension.
Cyclic universes with bouncing solutions are candidates for solving the big bang initial singularity problem. Here I will look for bouncing solutions in the context of modified theories of gravity whose field equations contain up to fourth-order derivatives of the metric tensor. In finding such bouncing solutions I will resort to an order reduction technique that reduces the order of the differential equations of the theory to second-order and thus enables one to find solutions which are perturbatively close to general relativity. I will also build the covariant effective actions of the resulting order reduced theories.
Based on: arXiv:1904.00260, arXiv:1907.11732, arXiv:2107.07777.
Galaxy clusters constitute a powerful tool to investigate modification of gravity at cosmological scales; in particular, with the combination of cluster’s mass profiles derived with lensing and internal kinematic analyses, it is possible to constrain departures from General Relativity in a complementary way with respect to other cosmological and astrophysical probes. In this context, I will present MG-MAMPOSSt, a code to constrain modified gravity models by reconstructing the mass profile with the kinematics of cluster's member galaxies. I will focus in particular on recent results and forecasts on two classes of models characterized by different screening mechanisms, namely chameleon and Vainshtein screening. I will show the capability of the method when combined with lensing data as well as some criteria to control possible systematics in view of the application to the data of upcoming imaging and spectroscopic surveys.
We have shown the potential of next-generation astrometric satellites for distinguishing between a cusp and a core in the dark matter density profile. This goal can be achieved with the measure of the proper motions of at least 6000 stars within a nearby dwarf galaxy with an accuracy of 1 km~s$^{-1}$ at most. We have built mock star catalogues similar to those expected in future astrometric missions like Theia. Our mocks include celestial coordinates, radial velocity, and proper motion of the stars, while density and velocity fields of the stars are sampled from an extended Navarro-Frank-White (eNWF) spherical model. Employing a Monte Carlo Markov Chain algorithm, we have shown that the eNFW parameters with a relative uncertainty of 20\%, on average can be recovered, and thus we can distinguish between a core and a cusp at $3\sigma$. Our result shows that the measure of the proper motions of stars can provide a fundamental contribution to understanding the nature and the properties of dark matter particles.
In this talk I will review the recent results of a numerical study of chameleon gravity in the context of galaxy clusters and cosmic voids. In this study we solved the chameleon field equation for NFW halos and cosmic void density profiles for the currently observationally viable chameleon models. The obtained results shine light on the non-trivial relationship between the NFW halo parameters and the chameleon acceleration and have important implications for the future observational searches for the fifth force.
The quasi-static approximation (QSA) is a useful tool to get a quick and clear physical understanding of the phenomenology of modified gravity which is encoded in two functions (of scale and time): the effective gravitational constant (describing the modified evolution of matter perturbations) and the slip (parametrizing the relations between the two gravitational potentials). This approximation is often used to put constraints on cosmological models using phenomenological expressions. In this talk I will consider three different formulations based on the QSA for Horndeski models and assess their performance on some cosmological observables and assess the range of validity of this approximation. I will also highlight why different schemes lead to different expressions on very large scales and how we can improve them.
With the advent of surveys such as the Legacy Survey of Space and Time (LSST), there will be opportunities in the near future to study nonlinear aspects of modified gravity (MG) theories through weak lensing and galaxy clustering measurements. These will be important in constraining the theory space for MG theories with screening effects, which are manifestly nonlinear. As the typical method for studying nonlinear effects, N-body simulation, is expensive computationally and temporally, an alternative method is desirable. In this talk, I will show our current progress in being able to solve the background equations of motion for any model in the reduced-Horndeski framework and produce its power spectrum, valid on mildly nonlinear scales, using the Co-moving Lagrangian Acceleration (COLA) numerical scheme.
Schemes of gravitationally induced decoherence are being actively investigated as possible mechanisms for the quantum-to-classical transition. In this talk, I introduce a decoherence process attributable to quantum gravity effects. In particular, I assume a foamy quantum spacetime with a fluctuating minimal length coinciding on average with the Planck scale. Considering deformed canonical commutation relations with a fluctuating deformation parameter, it is possible to derive a Lindblad master equation that yields localization in energy space and decoherence times consistent with the currently available observational evidence. Compared to other schemes of gravitational decoherence, one can see that the decoherence rate predicted by this model is extremal, being minimal in the deep quantum regime below the Planck scale and maximal in the mesoscopic regime beyond it. Finally, I briefly discuss possible experimental tests of the model based on cavity optomechanics setups with ultracold massive molecular oscillators and provide preliminary estimates on the values of the physical parameters needed for actual laboratory implementations.
The tools of spectral geometry lead to the derivation of the action functionals both for gauge theories and gravity. The simplest, mildly noncommutative models with a product geometry give the standard Yang-Mills-Higgs models and the General Relativity action with a cosmological constant. An interesting situation occurs when the geometry is not of the product type, thus allowing the metric to be dynamical in the discrete degrees of freedom. The resulting model resembles bimetric gravity, and demonstrates stability for a class of typical cosmological solutions. Based on a joint work with Arkadiusz Bochniak.
The idea of massive graviton plays a fundamental role in modern physics as a landmark of most scenarios related to the modification of the theory of gravity. Limits on graviton mass can be obtained with capabilities of multi-messenger astronomy. In particular, non-zero graviton mass would modify estimates of the total cluster mass (Yukawa term influences Newtonian potential). This can be measured through the X-ray surface brightness of the intracluster medium combined with a characteristic distortion in the cosmic microwave bacground spectrum observed in the cluster direction, known as thermal Sunyaev-Zel’dovich (SZ) effect. Using X-COP galaxy cluster sample, where total masses up to certain radii were measured by using X-ray data from XMM-Newton telescope combined with SZ data from Planck satellite, we obtaned that m_g < (4.99 − 6.79) × 10^(−29) eV (at 95% C.L.) which is one of the stringest available. On the other hand, modified relativistic dispersion relation of massive graviton may lead to changes in travel time of gravitational waves (GWs) emitted from a distant astrophysical objects. Strong gravitational lensing of signals emitted from a carefully selected class of extra-galactic sources like compact object binaries (in particular, binary neutron stars) is predicted to play an important role in this context. Particularly, comparing time delays between images of the lensed GW signal and its electromagnetic counterpart may be a new model-independent strategy, especially promising in the time of successful operating runs of LIGO/Virgo GW detectors (recently joined by KAGRA observatory) resulting in numerous records of GW signals from coalescing compact object binaries. In this talk I will discuss the above ideas in more details.
The phenomena of standing waves are mostly studied in the context of mechanical or electromagnetic waves. In the context of General Relativity, the issue of how to define standing gravitational waves was addressed by Bondi and later by Stefani. We investigate an expanding universe filled with standing gravitational waves. We study how freely falling particles in this spacetime behave, namely, we investigate the geodesic equation and the geodesic deviation equation. We show that antinodes attract freely falling particles and we trace the velocity memory effect.