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A three-dimensional slowly rotating black hole solution in the presence of negative cosmological constant in the Einstein-power-Maxwell theory is studied. It is shown that in the small rotation limit the electric field, diagonal metric function and thermodynamic properties in the small rotation limit are the same as for static case, whereas the small rotation gives in addition a non-diagonal metric function and magnetic field which are also small. For these functions cased by rotation of black hole it was obtained exact integral solution and analytic asymptotic solution.

We consider black hole solutions with electric and magnetic sources in the four-dimensional Einstein-Born-Infeld-AdS theory with spherical, planar and hyperbolic horizon geometries. Exact analytical solutions for the metric function, electric and magnetic fields were obtained and they recover the RN-AdS black hole in the limit $$\beta \rightarrow +\infty $$ for spherical horizon in the absence of the magnetic charge. Expressions for temperature, electric and magnetic potential were obtained and they satisfy the first law of the extended black hole thermodynamics, where a negative cosmological constant is associated with thermodynamic pressure. Also, the Born-Infeld vacuum polarization term $$Bd\beta $$ was included into the first law in order to satisfy the Smarr relation. Critical behavior of the black hole was examined and condition on electric and magnetic charges were obtained when phase transition appears. Also, the critical ratio and capacity at constant pressure were calculated. Electric and magnetic charges enter into the metric function and thermodynamic quantities symmetrically and thus the presence of the magnetic charge does not bring very significant new features. Finally, we examine the Joule-Thomson expansion if the black hole mass is fixed. The inversion and isenthalpic curves were plotted and the cooling and heating regions were demonstrated. These results recover the Joule-Thomson expansion recently considered for the RN-AdS black hole in the corresponding limit.

Static black hole with the Power Maxwell invariant (PMI), Born–Infeld (BI), logarithmic (LN), exponential (EN) electromagnetic fields in three-dimensional spacetime with cosmological constant was studied. It was shown that the LN and EN fields represent the Born–Infeld type of nonlinear electrodynamics. It the framework of General Relativity the exact solutions of the field equations were obtained, corresponding thermodynamic functions were calculated and the [Formula: see text] criticality of the black holes in the extended phase-space thermodynamics was investigated.

In this paper, we study the scattering and absorption cross sections of a Schwarzschild–anti-de Sitter black hole surrounded by quintessence. The critical values of the cosmological constant and the normalization factor are obtained. We describe the event horizons and the extremal condition of the black hole surrounded by quintessence. The effects of quintessence on the classical and semi-classical scattering cross sections have been estimated. Additionally, the absorption cross section is studied with the sinc approximation in the eikonal limit. We consider the quintessence state parameter in the particular cases ω = –2/3 and –1/2.

In this work, we present a Big Rip scenario within the framework of the generalized Brans-Dicke (GBD) theory. In the GBD theory, we consider an evolving BD parameter along with a self-interacting potential. An anisotropic background is considered to have a more general view of the cosmic expansion. The GBD theory with a cosmological constant is presented as an effective cosmic fluid within general relativity which favours a phantom field dominated phase. The model parameters are constrained so that the model provides reasonable estimates of the Hubble parameter and other recent observational aspects at the present epoch. The dynamical aspects of the BD parameter and the BD scalar field have been analysed. It is found that the present model witnesses a finite time doomsday at a time of tBR≃16.14Gyr, and for this scenario, the model requires a large negative value of the Brans-Dicke parameter.

International audience; The DBI-Galileon model is a tensor-scalar theory of gravity which finds its foundation as the most general theory of the dynamics of a 4D brane embedded in a 5D bulk. It is of particular interest as it provides a few free parameters with a physical meaning, such as the cosmological constant which is there related to the brane tension. Most studies of this model have been performed assuming a maximally symmetric geometry for the 5D bulk, in which it has been shown that the theory reduces to various types of Galileon. In contrast, the general case for the geometry of the bulk provides a different covariantization of the Galileon model than the covariant Galileon: the DBI-Galileon. From the tight constraints on the gravitational waves speed, we are naturally led to consider the non-relativistic limit of the model where the kinetic energy of the brane is small compared to its tension, that we study in the context of late-time cosmology. The DBI-Galileon in the non-relativistic limit is simply an expansion around General Relativity (GR) which can be expressed as a shift-symmetric Horndeski theory. We developed the description of this theory at the background and perturbation level. However, by studying the scalar and tensor perturbations around a flat FLRW background, we found that they contain a ghost degree of freedom leading to fatal instability of the vacuum for every combination of the free parameters. As a lesson, we emphasized which of the Horndeski terms competes to avoid this instability in more general cases.

We show that the dynamics of Schwarzschild-(A)dS black holes admit a symmetry under the 2d Schr\"{o}dinger group, whatever the sign or value of the cosmological constant. This is achieved by reformulating the spherically-symmetric reduction of general relativity as a 2d mechanical system with a non-trivial potential controlled by the cosmological constant, and explicitly identifying the conserved charges for black hole mechanics. We expect the Schr\"{o}dinger symmetry to drive the dynamics of quantum Schwarzschild-(A)dS black holes. This suggests that Schr\"{o}dinger-preserving non-linear deformations (of the Gross-Piteavskii type) should capture universal quantum gravity corrections to the black hole geometry. Such scenario could be realized in condensed matter analogue models., Comment: 9 pages

We study black-hole thermodynamics in theories that contain dimensionful constants such as the cosmological constant or coupling constants in Wald's formalism. The most natural way to deal with these constants is to promote them to scalar fields introducing a (d-1)-form Lagrange multiplier that forces them to be constant on-shell. These (d-1)-form potentials provide a dual description of them and, in the context of superstring/supergravity theories, a higher-dimensional origin/explanation. In the context of gauged supergravity theories, all these constants can be collected in the embedding tensor. We show in an explicit 4-dimensional example that the embedding tensor can also be understood as a thermodynamical variable that occurs in the Smarr formula in a duality-invariant fashion. This establishes an interesting link between black-hole thermodynamics, gaugings and compactifications in the context of superstring/supergravity theories., References added, 43 pages, no figures

The standard formulation of General Relativity Theory, in the absence of a cosmological constant, is unable to explain the responsible mechanism for the observed late-time cosmic acceleration. On the other hand, by inserting the cosmological constant in Einstein's field equations it is possible to describe the cosmic acceleration, but the cosmological constant suffers from an unprecedented fine-tunning problem. This motivates one to modify Einstein's space-time geometry of General Relativity. The $f(Q)$ modified theory of gravity is an alternative theory to General Relativity, where the non-metricity scalar $Q$ is the responsible candidate for gravitational interactions. In the present work we consider a Friedmann-Lem\^aitre-Robertson-Walker cosmological model dominated by bulk viscous cosmic fluid in $f(Q)$ gravity with the functional form $f(Q)=\alpha Q^n$, where $\alpha$ and $n$ are free parameters of the model. We constrain our model with the recent Pantheon supernovae data set of 1048 data points, Hubble data set of 31 data points and baryon acoustic oscillations data set consisting of six points. For higher values of redshift, it is clear that the $f(Q)$ cosmology better fits data than standard cosmology. We present the evolution of our deceleration parameter with redshift and it properly predicts a transition from decelerated to accelerated phases of the universe expansion. Also, we present the evolution of density, bulk viscous pressure and the effective equation of state parameter with redshift. Those show that bulk viscosity in a cosmic fluid is a valid candidate to acquire the negative pressure to drive the cosmic expansion efficiently.We also examine the behavior of different energy conditions to test the viability of our cosmological $f(Q)$ model. Furthermore, the statefinder diagnostics are also investigated in order to distinguish among different dark energy models., Comment: Comments are welcome

The thermodynamics and the weak cosmic censorship conjecture (WCCC) in a high dimensional RN − AdSd+ 1 black hole with energy-momentum relation are investigated by absorbing a charged particle in the phase space. In the RN − AdSd+ 1 space-time, the cosmological constant Λ is treated as a thermodynamic pressure and its conjugate quantity as a thermodynamic volume. We use the energy-momentum relation of the absorbed particle to discuss the thermodynamics of the RN − AdSd+ 1 black hole and to prove the WCCC in the phase space. Based on this assumption, we find that the first law and the second law of thermodynamics are satisfied in normal phase space. On the other hand, in the extend phase space, the first law is satisfied and the second law is violated. Then we study the WCCC in the phase space, we find that the WCCC is satisfied for an extreme black and a near-extreme black hole in the normal phase space. In the extend phase space, the WCCC is satisfied for an extreme black hole and unidentified for a near-extreme black hole.

Cosmological evolution is examined in a model with a conformally-coupled scalar field and with a cosmological constant. Models of this type arise in two types of gravitational theory: in the general theory of relativity and in scalar-tensor theories with a cosmological function. Exact solutions are found in the absence of nongravitational matter and a qualitative analysis is made of the corresponding dynamical system. It is shown that in later stages of evolution the expansion is accelerated, with a de Sitter solution as a future attractor for the general solution. There is also a special solution with exponential expansion, for which the energy density of the scalar field equals zero. In the early stages, the general solution describes a decelerated expansion similar to the radiation-dominated expansion in the general theory of relativity.

This work is devoted to the analysis of photon surfaces and photon regions in the stationary Plebanski–Demianski space–time, including black holes, naked singularities, and wormholes, with rotation, the electric or magnetic charge, the NUT parameter, the acceleration parameter, and the cosmological constant taken into account. We construct a theory of partially umbilic hypersurfaces, which allows us to describe the photon structure of space–time without resorting to the analysis of geodesic equations. We completely classify photon surfaces for various values of the parameters, including critical ones at which a qualitative change in the photon structure occurs. The results are visualized in the form of diagrams, which allows compactly presenting the information needed to understand strong gravitational lensing and gravitational shadows.

In Loop Quantum Gravity mathematically rigorous models of full quantum gravity were proposed. In this paper we study a cosmological sector of one of the models describing quantum gravity with positive cosmological constant coupled to massless scalar field. In our previous research we introduced a method to reduce the model to homogeneous-isotropic sector at the quantum level. In this paper we propose a method to restrict to the spatially flat sector. After this restriction the number of degrees of freedom gets substantially reduced. This allows us to make numerical calculations. Remarkably, the resulting model shares some structural similarities with the Loop Quantum Cosmological models and therefore sheds some new light on the relation between Loop Quantum Gravity and Loop Quantum Cosmology. According to our model the evolution of the Universe is periodic. The quantum gravity effects resolve the Big Bang singularity leading to a Big Bounce and cause the Universe to contract after a classical expansion phase (Big Crunch)., Comment: 44 pages, 8 figures

Barrow proposed that the area law of the entropy associated with a horizon might receive a “fractal correction” due to quantum gravitational effects—in place of S∝A, we have instead S∝A1+δ/2, where 0⩽δ⩽1 measures the deviation from the standard area law (δ=0). Based on black hole thermodynamics, we argue that the Barrow entropy should run (i.e., energy scale dependent), which is reasonable given that quantum gravitational corrections are expected to be important only in the high-energy regime. When applied to the Friedmann equation, we demonstrate the possibility that such a running Barrow entropy index could give rise to a dynamical effective dark energy, which is asymptotically positive and vanishing, but negative at the Big Bang. Such a sign switching dark energy could help to alleviate the Hubble tension. Other cosmological implications are discussed.

We apply a recently developed 2+1+1 decomposition of spacetime, based on a nonorthogonal double foliation for the study of spherically symmetric, static black hole solutions of Horndeski scalar-tensor theory. Our discussion proceeds in an effective field theory (EFT) of modified gravity approach, with the action depending on metric and embedding scalars adapted to the nonorthogonal 2+1+1 decomposition. We prove that the most generic class of Horndeski Lagrangians compatible with observations can be expressed in this EFT form. By studying the first order perturbation of the EFT action we derive three equations of motion, which reduce to those derived earlier in an orthogonal 2+1+1 decomposition, and a fourth equation for the metric parameter N related to the nonorthogonality of the foliation. For the Horndeski class of theories with vanishing $G_3$ and $G_5$, but generic functions $G_2(\phi,X)$ (k-essence) and $G_4(\phi)$ (nonminimal coupling to the metric) we prove the unicity theorem that no action beyond Einstein--Hilbert allows for the Schwarzschild solution. Next we integrate the EFT field equations for the case with only one independent metric function obtaining new solutions characterized by a parameter interpreted as either mass or tidal charge, the cosmological constant and a third parameter. These solutions represent naked singularities, black holes with scalar hair or have the double horizon structure of the Schwarzschild--de Sitter spacetime. Solutions with homogeneous Kantowski--Sachs type regions also emerge. Finally, one of the solutions obtained for the function $G_4$ linear in the curvature coordinate, in certain parameter range exhibits an intriguing logarithmic singularity lying outside the horizon. The newly derived hairy black hole solutions evade previously known unicity theorems by being asymptotically nonflat, even in the absence of the cosmological constant., Comment: 20 pages, 4 figures

International audience; We introduce a new logotropic model based on a complex scalar field with a logarithmic potential that unifies dark matter and dark energy. The scalar field satisfies a nonlinear wave equation generalizing the Klein-Gordon equation in the relativistic regime and the Schrödinger equation in the nonrelativistic regime. This model has an intrinsically quantum nature and returns the Λ cold dark matter (CDM) model in the classical limit ℏ→0. It involves a new fundamental constant of physics A/c2=2.10×10-26 g m-3 responsible for the late accelerating expansion of the Universe and superseding the Einstein cosmological constant Λ. The logotropic model is almost indistinguishable from the ΛCDM model at large (cosmological) scales but solves the CDM crisis at small (galactic) scales. It also solves the problems of the fuzzy dark matter model. Indeed, it leads to cored dark matter halos with a universal surface density Σ0th=5.85(A/4πG)1/2=133 M⊙/pc2. This universal surface density is predicted from the logotropic model without adjustable parameter and turns out to be close to the observed value Σ0obs=141-52+83 M⊙/pc2. We also argue that the quantities Ωdm,0 and Ωde,0, which are usually interpreted as the present proportions of dark matter and dark energy in the ΛCDM model, are equal to Ωdm,0th=11+e(1-Ωb,0)=0.2559 and Ωde,0th=e1+e(1-Ωb,0)=0.6955 in very good agreement with the measured values Ωdm,0obs=0.2589 and Ωde,0obs=0.6911 (their empirical ratio 2.669 is close to the Euler number e=2.71828…). We point out, however, important difficulties with the logotropic model, similar to those encountered by the generalized Chaplygin gas model. These problems are related to the difficulty of forming large-scale structures due to an increasing speed of sound as the Universe expands. We discuss potential solutions to these problems, stressing in particular the importance to perform a nonlinear study of structure formation.

Different theories of gravity can admit the same black hole solution, but the parameters usually have different physical interpretations. In this work we study in depth the linear term $\beta r$ in the redshift function of black holes, which arises in conformal gravity, de Rham-Gabadadze-Tolley (dRGT) massive gravity, $f(R)$ gravity (as approximate solution) and general relativity. Geometrically we quantify the parameter $\beta$ in terms of the curvature invariants. Astrophysically we found that $\beta$ can be expressed in terms of the cosmological constant, the photon orbit radius and the innermost stable circular orbit (ISCO) radius. The metric degeneracy can be broken once black hole thermodynamics is taken into account. Notably, we show that under Hawking evaporation, different physical theories with the same black hole solution (at the level of the metric) can lead to black hole remnants with different values of their physical masses with direct consequences on their viability as dark matter candidates. In particular, the mass of the graviton in massive gravity can be expressed in terms of the cosmological constant and of the formation epoch of the remnant. Furthermore the upper bound of remnant mass can be estimated to be around $0.5 \times 10^{27}$ kg., Comment: Matches published version

We investigate the effect of particle creation on the evolution of n-dimensional Friedmann–Robertson–Walker (FRW) cosmological model in the framework of interacting variable gravitational fields and cosmological constants. An open thermodynamical system of the Universe is considered where particle creation leads to supplementary negative creation pressure. This pressure is found to be negligible in this derived model. These cosmological findings have been investigated geometrically and related observational constraints are investigated.

Following recent works on corner charges we investigate the boundary structure in the case of the theory of gravity formulated as a constrained BF theory. This allows us not only to introduce the cosmological constant, but also explore the influence of the topological terms present in the action of this theory. Established formulas for charges resemble previously obtained ones, but we show that they are affected by the presence of the cosmological constant and topological terms. As an example we discuss the charges in the case of the AdS--Schwarzschild solution and we find that the charges give correct values., Comment: 24 pages

The article considers tidal forces in the vicinity of the Kottler black hole. We find a solution of the geodesic deviation equation for radially falling bodies, which is determined by elliptic integrals. And also the asymptotic behavior of all spatial geodesic deviation vector components were found. We demonstrate that the radial component of the tidal force changes sign outside the single event horizon for any negative values of the cosmological constant, in contrast to the Schwarzschild black hole, where all the components of the tidal force are sign-constant. We also find the similarity between the Kottler black hole and the Reissner-Nordstrom black hole, because we indicate the value of the cosmological constant, which ensures the existence of two horizons of the black hole, between which the angular components of the tidal force change sign. It was possible to detect non-analytical behavior of geodesic deviation vector components in Anti-de Sitter spacetime and to describe it locally., Comment: 14 pages, 10 figures

In the framework of the Einstein-Dirac-axion-aether theory we consider the quartet of self-interacting cosmic fields, which includes the dynamic aether, presented by the unit timelike vector field, the axionic dark mater, described by the pseudoscalar field, the spinor field associated with fermion particles, and the gravity field. The key, associated with the mechanism of self-interaction, is installed into the modified periodic potential of the pseudoscalar (axion) field constructed on the base of a guiding function, which depends on one invariant, one pseudo-invariant and two cross-invariants containing the spinor and vector fields. The total system of the field equations related to the isotropic homogeneous cosmological model is solved; we have found the exact solutions for the guiding function for three cases: nonzero, vanishing and critical values of the cosmological constant. Based on these solutions, we obtained the expressions for the effective mass of spinor particles, interacting with the axionic dark matter and dynamic aether. This effective mass is shown to bear imprints of the cosmological epoch and of the state of the cosmic dark fluid in that epoch., 12 pages, 0 figures; revised version published in The European Physical Journal C

Last year, Chung and Hassanabadi proposed a higher order general uncertainty principle (GUP$^\ast$) that predicts a minimal length as well as possesses a upper bound momentum limit. In this article, we have discussed an ideal gas system and its thermal properties using that deformed canonical algebra introduced by them. Moreover, we examined blackbody radiation spectrum and the cosmological constant in the presence of the GUP$^\ast$. After a comparison with the existing literature, we concluded that the given formalism of Chung and Hassanabadi yields more accurate results., 12 pages, 7 pages

We present a new viable modified theory of gravity in which the matter sector is characterized by a logarithmic Lagrangian density. The modified Einstein’s field equations are derived, and they are characterized by the emergence of an effective gravitational coupling constant and an effective negative cosmological constant both coupled to the Lagrangian of matter. Applied to the Friedmann-Robertson-Walker cosmological framework, it leads to motivating phenomenology. In particular, the universe is free from the initial singularity, and a unified description of the early inflationary phase, the succeeding non-accelerating, matter-dominated expansion, and then the transition to a late-time accelerating phase is obtained as well. Moreover, the effective dark energy sector can be quintessence, yet the universe starts contracting if the phantom divide-line is crossed. The model is confronted with observations that agree with recent astronomical data. Moreover, the model gives rise to Einstein’s field equations which resemble the equations obtained in unimodular gravity.

In the late 1990's, observations of 93 Type Ia supernovae were analysed in the framework of the FLRW cosmology assuming these to be `standard(isable) candles'. It was thus inferred that the Hubble expansion rate is accelerating as if driven by a positive Cosmological Constant $\Lambda$. This is still the only direct evidence for the `dark energy' that is the dominant component of the standard $\Lambda$CDM cosmological model. Other data such as BAO, CMB anisotropies, stellar ages, the rate of structure growth, etc are all `concordant' with this model but do not provide independent evidence for accelerated expansion. Analysis of a larger sample of 740 SNe Ia shows that these are not quite standard candles, and highlights the "corrections" applied to analyse the data in the FLRW framework. The latter holds in the reference frame in which the CMB is isotropic, whereas observations are made in our heliocentric frame in which the CMB has a large dipole anisotropy. This is assumed to be of kinematic origin i.e. due to our non-Hubble motion driven by local inhomogeneity in the matter distribution. The $\Lambda$CDM model predicts how this peculiar velocity should fall off as the averaging scale is raised and the universe becomes sensibly homogeneous. However observations of the local `bulk flow' are inconsistent with this expectation and convergence to the CMB frame is not seen. Moreover the kinematic interpretation implies a corresponding dipole in the sky distribution of high redshift quasars, which is rejected by observations at 4.9$\sigma$. The acceleration of the Hubble expansion rate is also anisotropic at 3.9$\sigma$ and aligned with the bulk flow. Thus dark energy may be an artefact of analysing data assuming that we are idealised observers in an FLRW universe, when in fact the real universe is inhomogeneous and anisotropic out to distances large enough to impact on cosmological analyses., Comment: 14 pages, 4 figures, 4 tables

The underlying geometri of spacetime algebra allows one to derive a force by contracting the relativistic generalization of angular momentum, M, with the mass-current, mw, where w is a proper 4-vector velocity. By applying this force to a cosmological object, a repulsive inverse distance-square law is found, which is proportional to the velocity dispersion squared of that structure. It is speculated if this finding may be relevant to the recent suggestion, that such a force may accelerate the expanding universe with no need for a cosmological constant., Comment: 4 pages, no figure, submitted to MNRAS

We used modified holographic Ricci dark energy to find anisotropic LRS Bianchi type I cosmological model in five-dimensions based on Lyra geometry. The exact solutions of the Einstein field equations were obtained by using hybrid expansion law (HEL). We have investigated the interacting and noninteracting dark energy and dark matter. It is found that at late times the equation of state parameter (EOS) for non-interacting case behaves like a cosmological constant whereas it behaves like phantom dark energy for interacting model. Some cosmological parameters and stability of the models are discussed. The physical and geometric aspects of the models have been analysed. They are consistent with the recent observational results.

Journal of high energy physics 08(8), 167 (2022). doi:10.1007/JHEP08(2022)167, We compare the path integral for transition functions in unimodular gravity and in general relativity. In unimodular gravity the cosmological constant is a property of states that are specified at the boundaries whereas in general relativity the cosmological constant is a parameter of the action. Unimodular gravity with a nondynamical background spacetime volume element has a time variable that is canonically conjugate to the cosmological constant. Wave functions depend on time and satisfy a Schrödinger equation. On the contrary, in the covariant version of unimodular gravity with a 3-form gauge field, proposed by Henneaux and Teitelboim, wave functions are time independent and satisfy a Wheeler-DeWitt equation, as in general relativity. The 3-form gauge field integrated over spacelike hypersurfaces becomes a “cosmic time” only in the semiclassical approximation. In unimodular gravity the smallness of the observed cosmological constant has to be explained as a property of the initial state., Published by SISSA, [Trieste]

In this paper, I will review an obstruction for theories of the beginning of the universe which can be formulated as semiclassical path integrals. Hartle and Hawking’s no boundary proposal and Vilenkin’s tunneling proposal are examples of such theories. Each may be formulated as the quantum amplitude for obtaining a final 3-geometry by integrating over 4-geometries. The result is obtained using a new mathematical tool — Picard–Lefschetz theory — for defining the semiclassical path integral for gravity. The Lorentzian path integral for quantum cosmology with a positive cosmological constant is mathematically meaningful in this approach, but the Euclidean version is not. Framed in this way, the resulting framework and predictions are unique. Unfortunately, the outcome is that primordial gravitational wave fluctuations are unsuppressed.

We use Pantheon Type Ia supernova (SN Ia) apparent magnitude, DES-3yr binned SN Ia apparent magnitude, Hubble parameter, and baryon acoustic oscillation measurements to constrain six spatially flat and non-flat cosmological models. These sets of data provide mutually consistent cosmological constraints in the six cosmological models we study. A joint analysis of these data sets provides model-independent estimates of the Hubble constant, $H_0=68.8\pm1.8\ \rm{km \ s^{-1} \ Mpc^{-1}}$, and the non-relativistic matter density parameter, $\Omega_{\rm m_0}=0.294\pm0.020$. Although the joint constraints prefer mild dark energy dynamics and a little spatial curvature, they do not rule out dark energy being a cosmological constant and flat spatial hypersurfaces. We also add quasar angular size and H II starburst galaxy measurements to the combined data set and find more restrictive constraints., Comment: 13 pages, 6 figures, autours.xml file added. arXiv admin note: text overlap with arXiv:2009.12953 MNRAS accepted version

Locally rotationally symmetric Bianchi type-I viscous and nonviscous cosmological models are explored in general relativity (GR) and in $$f(R,T)$$ gravity. Solutions are obtained by assuming that the expansion scalar is proportional to the shear scalar, which yields a constant value for the deceleration parameter ( $$q=2$$ ). Constraints are obtained by requiring physical viability of the solutions. A comparison is made between viscous and nonviscous models, and between the models in GR and in $$f(R,T)$$ gravity. The metric potentials remain the same in GR and in $$f(R,T)$$ gravity. Consequently, the geometrical behavior of the $$f(R,T)$$ gravity models remains the same as in GR. It is found that $$f(R,T)$$ gravity or bulk viscosity do not affect the behavior of effective matter which acts as a stiff fluid in all models. The individual fluids have a very rich behavior. In one of the viscous models, the matter either follows a semi-realistic EoS or exhibits a transition from stiff matter to phantom, depending on the values of the parameter. In another model, the matter describes radiation, dust, quintessence, phantom, and the cosmological constant for different values of the parameter. In general, $$f(R,T)$$ gravity diminishes the effect of bulk viscosity.

In this paper we consider that dark energy could be described solely by a complex scalar field with a Bose-Einstein condensate-like potential (denoted as CSFDE), that is, with a self-interaction and a mass term. In particular, we analyse a solution which in a fast oscillation regime at late-times behaves as a Cosmological Constant. Our proposal adequately describes the standard homogeneous and flat Fridman dynamics, furthermore, in this quintessence--complex scalar field scenario it is possible to mimic the dynamics related to dark energy. However, when the precision cosmological tests are implemented in this landscape, the generic Equation-of-State derived for this model in a restricted regime of a_i (which corresponds to the scale factor at which the scalar field turns on), cannot be constrained by late-time current observations, since the analysis constraints solely the scalar field parameters within values ruled out by the theoretical model. This result is a clear hint to consider future CSFDE models with, for instance, two scalar fields in order to study the early-time dynamics of the Universe., Comment: 9 pages; accepted by Monthly Notices of the Royal Astronomical Society

This paper explores the stable and dynamical configuration of thin-shell gravastars developed from the matching of exterior (Hayward and Hayward anti-de Sitter black holes) and interior (de Sitter) spacetimes. These spacetimes are connected at a thin-shell by considering Visser cut and paste technique. The matter surface energy density and pressure are formulated from the Lanczos equations. We consider the shaded regions to discuss the stability of gravastars by using radial perturbation at the equilibrium shell radius. We then observe thin-shell dynamics with massless as well as massive scalar field through equations of motion and Klein-Gordon equations. The respective scalar thin-shell represents collapse and expansion of the developed structure with the shell radius, velocity and effective potential. We conclude that stable regions of gravastar shell decrease and dynamical configuration (collapse and expansion) increase with cosmological constant.

We classify all supersymmetric solutions of minimal D=4 gauged supergravity with (2,2) signature and a positive cosmological constant which admit exactly one Killing spinor. This classification produces a geometric structure which is more general than that found for previous classifications of N=2 supersymmetric solutions of this theory. We illustrate how the N=2 solutions which consist of a fibration over a 3-dimensional Lorentzian Gauduchon-Tod base space can be written in terms of this more generic geometric structure., 20 pages, latex

We use the replica method to compute the entanglement entropy of a universe without gravity entangled in a thermofield-double-like state with a disjoint gravitating universe. Including wormholes between replicas of the latter gives an entropy functional which includes an "island" on the gravitating universe. We solve the back-reaction equations when the cosmological constant is negative to show that this island coincides with a causal shadow region that is created by the entanglement in the gravitating geometry. At high entanglement temperatures, the island contribution to the entropy functional leads to a bound on entanglement entropy, analogous to the Page behavior of evaporating black holes. We demonstrate that the entanglement wedge of the non-gravitating universe grows with the entanglement temperature until, eventually, the gravitating universe can be entirely reconstructed from the non-gravitating one., 30 pages, 7 figures, references added, improved discussion

We combine 23 Hubble constant measurements based on Cepheids-SN Ia, TRGB-SN Ia, Miras-SN Ia, Masers, Tully Fisher, Surface Brightness Fluctuations, SN II, Time-delay Lensing, Standard Sirens and $\gamma$-ray Attenuation, obtaining our best {\it optimistic} $H_0$ estimate, that is $H_0=72.94\pm0.75$ km/s/Mpc at 68\% CL. This is in $5.9\sigma$ tension with the $\Lambda$CDM model, therefore we evaluate its impact on the extended Dark Energy cosmological models that can alleviate the tension. We find more than $4.9\sigma$ evidence for a phantom Dark Energy equation of state in the $w$CDM scenario, the cosmological constant ruled out at more than $3\sigma$ in a $w_0w_a$CDM model and more than $5.7\sigma$ evidence for a coupling between Dark Matter and Dark Energy in the IDE scenario. Finally, we check the robustness of our results, and we quote two additional combinations of the Hubble constant. The {\it ultra-conservative} estimate, $H_0=72.7\pm 1.1$ km/s/Mpc at 68\% CL, is obtained removing the Cepheids-SN Ia and the Time-Delay Lensing based measurements, and confirms the evidence for new physics., Comment: 9 pages, 4 figures, Version accepted for publication in MNRAS

The following is a rendition of what was presented by the author, September 11, 2020 in the DE section of that conference. The topics, while not original, are in strict fidelity with the topics the author was allowed to present in ICRANET Zeldovich 4, 2020. We present a history of the evolution of the cosmological constant “issue” starting with its introduction by Einstein for a static universe, which did not work out because his static universe solution to the Ricci Scalar problem, and GR was and is UNSTABLE. Another model of the cosmological constant has a radius of the Universe specified which is proportional to one over the square root of the cosmological constant, whereas our idea is to use the matching of two spacetime first integrals, for isolating a nonperturbative cosmological constant solution right at the surface of the start of expansion of the universe, i.e. a phenomenological solution to the cosmological constant involves scaling of a radius of the PRESENT universe. Our presented idea is to instead solve the Cosmological constant at the surface of the initial space-time bubble, using the initially derived time step, delta t, as input for the Cosmological constant. As it is, the Zeldovich 4 Section I was in was for Dark Energy, so in solving the initial value of the Cosmological constant, I am giving backing to one of the models of DE as to why the Universe reaccelerates one billion years ago. We conclude as to a reference to a multiverse generalization of Penrose Cyclic Conformal Cosmology as input into the initial nonsingular space-time bubble.

We establish the future non-linear stability of Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) solutions to the Einstein-Euler equations of the universe filled with a large class of perfect fluids (the equations of state are allowed to be certain nonlinear or linear types both). Several previous results as specific examples can be covered in the results of this article. We emphasize that the future stability of FLRW metric for polytropic fluids with positive cosmological constant has been a difficult problem and can not be directly generalized from the previous known results. Our result in this article has not only covered this difficult case for the polytropic fluids, but also unified more types of fluids in a same scheme of proofs., Comment: 47 pages. Agrees with published version

We investigate static cylindrically symmetric solutions of the Weyl and Godel space–times in the framework of modified $$f(R)$$ gravity. With this aim, we consider the modified higher-order theory of gravity based on nonconformal invariant gravitational waves. From the modified Einstein equations, we derive two exact solutions of the Weyl space–time and find one exact and one numerical solution of the Godel space–time. In particular, we obtain a family of exact solutions with a constant scalar curvature $$R$$ depending on arbitrary constants for both space–times. It is interesting that the second solution for the Weyl metric has a nonconstant Ricci scalar. We find that the result obtained by solving the higher-order theory of gravity is similar to the result for the Einstein field equations with a cosmological constant. Moreover, we graphically study the role of the metric coefficients in both space–times.

We are using the book “Towards Quantum Gravity” with an article by Claus Kiefer as to a quantum gravity interpretation of the density matrix in the early universe. The density matrix we are using is a one loop approximation, with inflaton value and potential terms, like V (phi) using the Padmanabhan values one can expect if the scale factor is a ~a (Initial) times t ^ gamma. In doing so, we identify two time steps and presume a very small initial time step candidates initial time values which are from a polynomial for time values. A gravity wave analysis concludes our article with inflaton decay, which is finally linked to BHs. And then finally we show using work done by Hawking, et al. how this may give us Planck Sized Black Holes, in the onset of Inflation, with resulting consequences so outlined. A vastly simplified proof of BH masses of Planck mass is presented which ties in directly with issues of the mass of the inflaton initially generated by the 2nd derivative of the effective potential V (phi) at a time t ~4 times Planck time. And we include at the close questions as to DE, and data sets which may give credence to speculation as to different time flow rates at the start and then the conclusion later on, of expansion of our universe. The DE would be created by the breakup of the black holes due to a mechanism brought up by Dr. Freeze in 2012, and we also are using the future works section 8 to define the contours of our DE model which builds upon quite directly the sequence of material from pages 1 to 9 which are cited as to making connection between early universe conditions and the ideas of primordial DE models.

We have studied scattering of a probe particle by a four dimensional AdS-Schwarzschild black hole at large impact factor. Our analysis is consistent perturbatively to leading order in the AdS radius and black hole mass parameter. Next we define a proper "soft limit" of the radiation and extract out the "soft factor" from it. We find the correction to the well known flat space Classical Soft graviton theorem due to the presence of an AdS background., Comment: 35 pages, 1 figure, 2 new references added, minor changes incorporated

What we are doing is three-fold. First, we examine the gist of the Penrose suggestion as to signals from a prior universe showing up in the CMBR. i.e. , this shows up as data in the CMBR. Second, we give a suggestion as to how super massive black holes could be broken up s of a prior Universe cycle by pre big bang conditions, with say millions of pre-Planck black holes coming up out of a breakup of prior universe black holes. Three, we utilize a discussion as to Bose Einstein Condensates set as Gravitons as to composing the early universe black holes. The BEC formulation gives a number N of gravitons, linked to entropy, per black hole, which could lead to contributions to the alleged CMBR perturbations, which were identified by Penrose et al.

In previous work, black hole vortex solutions in Einstein gravity with AdS$_3$ background were found where the scalar matter profile had a singularity at the origin $r=0$. In this paper, we find numerically static vortex solutions where the scalar and gauge fields have a non-singular profile under Einstein gravity in an AdS$_3$ background. Vortices with different winding numbers $n$, VEV $v$ and cosmological constant $\Lambda$ are obtained. These vortices have positive mass and are not BTZ black holes as they have no event horizon. The mass is determined in two ways: by subtracting the numerical values of two separate asymptotic metrics and via an integral that is purely over the matter fields. The mass of the vortex increases as the cosmological constant becomes more negative and this coincides with the core of the vortex becoming smaller (compressed). We then consider the vortex with gravity in asymptotically flat spacetime for different values of the coupling $\alpha=1/(16 \pi G)$. At the origin, the spacetime has its highest curvature and there is no singularity. It transitions to an asymptotic conical spacetime with angular deficit that increases significantly as $\alpha$ decreases. For comparison, we also consider the vortex without gravity in flat spacetime. For this case, one cannot obtain the mass by the first method (subtracting two metrics) but remarkably, via a limiting procedure, one can obtain an integral mass formula. In the absence of gauge fields, there is a well-known logarithmic divergence in the energy of the vortex. With gravity, we present this divergence in a new light. We show that the metric acquires a logarithmic term which is the $2+1$ dimensional realization of the Newtonian gravitational potential when General Relativity is supplemented with a scalar field. This opens up novel possibilities which we discuss in the conclusion., Comment: 30 pages, 9 figures, 2 tables. v2: added footnotes, published in JHEP

We take the results where we reduplicate the Book “Dark Energy” by M. Li, X-D. Li, and Y. Wang, zero-point energy calculation, as folded in with the Klauder methodology, as given in a prior publication. From there we first access the Rosen solution to a mini universe energy to ascertain an energy value of t, the pre-inflationary near singularity, then access what would be needed as to inject information into our universe. We then close with an argument by Narilkar as to a quantum bound on the Einstein-Hilbert action integral, so as to obtain quantum Gravity. Narlikar omits the cosmological constant. We keep it in, for our overall conclusion about the cosmological constant and its relevance to Quantum gravity.

We study some aspects of the de Sitter version of Jackiw-Teitelboim gravity. Though we do not have propagating gravitons, we have a boundary mode when we compute observables with a fixed dilaton and metric at the boundary. We compute the no-boundary wavefunctions and probability measures to all orders in perturbation theory. We also discuss contributions from different topologies, borrowing recent results by Saad, Shenker and Stanford. We discuss how the boundary mode leads to gravitational corrections to cosmological observables when we add matter. Finally, starting from a four dimensional gravity theory with a positive cosmological constant, we consider a nearly extremal black hole and argue that some observables are dominated by the two dimensional nearly de Sitter gravity dynamics., Comment: 66 pp, 15 figs; v2: ref added

The cosmological implications of the Covariant Canonical Gauge Theory of Gravity (CCGG) are investigated. CCGG is a Palatini theory derived from first principles using the canonical transformation formalism in the covariant Hamiltonian formulation. The Einstein-Hilbert theory is thereby extended by a quadratic Riemann-Cartan term in the Lagrangian. Moreover, the requirement of covariant conservation of the stress-energy tensor leads to necessary presence of torsion. In the Friedman universe that promotes the cosmological constant to a time-dependent function, and gives rise to a geometrical correction with the EOS of dark radiation. The resulting cosmology, compatible with the $\Lambda$CDM parameter set, encompasses bounce and bang scenarios with graceful exits into the late dark energy era. Testing those scenarios against low-z observations shows that CCGG is a viable theory., Comment: Contribution to the International Workshop IWARA on Astronomy and Relativistic Astrophysics, September 2020. Accepted for publication in the Astronomische Nachrichten 2021; 8 pages, 6 figures

We study quadratic gravity $R^2+R_{[\mu\nu]}^2$ in the Palatini formalism where the connection and the metric are independent. This action has a {\it gauged} scale symmetry (also known as Weyl gauge symmetry) of Weyl gauge field $v_\mu= (\tilde\Gamma_\mu-\Gamma_\mu)/2$, with $\tilde\Gamma_\mu$ ($\Gamma_\mu$) the trace of the Palatini (Levi-Civita) connection, respectively. The underlying geometry is non-metric due to the $R_{[\mu\nu]}^2$ term acting as a gauge kinetic term for $v_\mu$. We show that this theory has an elegant spontaneous breaking of gauged scale symmetry and mass generation in the absence of matter, where the necessary scalar field ($\phi$) is not added ad-hoc to this purpose but is "extracted" from the $R^2$ term. The gauge field becomes massive by absorbing the derivative term $\partial_\mu\ln\phi$ of the Stueckelberg field ("dilaton"). In the broken phase one finds the Einstein-Proca action of $v_\mu$ of mass proportional to the Planck scale $M\sim \langle\phi\rangle$, and a positive cosmological constant. Below this scale $v_\mu$ decouples, the connection becomes Levi-Civita and metricity and Einstein gravity are recovered. These results remain valid in the presence of non-minimally coupled scalar field (Higgs-like) with Palatini connection and the potential is computed. In this case the theory gives successful inflation and a specific prediction for the tensor-to-scalar ratio $0.007\leq r \leq 0.01$ for current spectral index $n_s$ (at $95\%$CL) and N=60 efolds. This value of $r$ is mildly larger than in inflation in Weyl quadratic gravity of similar symmetry, due to different non-metricity. This establishes a connection between non-metricity and inflation predictions and enables us to test such theories by future CMB experiments., Comment: 22 pages, 2 figures, LaTeX

We propose a novel explanation for the smallness of the observed cosmological constant (CC). Regions of space with a large CC are short lived and are dynamically driven to crunch soon after the end of inflation. Conversely, regions with a small CC are metastable and long lived and are the only ones to survive until late times. While the mechanism assumes many domains with different CC values, it does not result in eternal inflation nor does it require a long period of inflation to populate them. We present a concrete dynamical model, based on a super-cooled first order phase transition in a hidden conformal sector, that may successfully implement such a crunching mechanism. We find that the mechanism can only solve the CC problem up to the weak scale, above which new physics, such as supersymmetry, is needed to solve the CC problem all the way to the UV cutoff scale. The absence of experimental evidence for such new physics already implies a mild little hierarchy problem for the CC. Curiously, in this approach the weak scale arises as the geometric mean of the temperature in our universe today and the Planck scale, hinting at a new “CC miracle”, motivating new physics at the weak scale independent of electroweak physics. We further predict the presence of new relativistic degrees of freedom in the CFT that should be visible in the next round of CMB experiments. Our mechanism is therefore predictive and experimentally falsifiable.

In this paper, we present novel non-relativistic superalgebras which correspond to supersymmetric extensions of the enlarged extended Bargmann algebra. The three-dimensional non-relativistic Chern-Simons supergravity actions invariant under the aforementioned superalgebras are constructed. The new non-relativistic superalgebras allow to accommodate a cosmological constant in a non-relativistic supergravity theory. Interestingly, we show that one of the non-relativistic supergravity theories presented here leads to the recently introduced Maxwellian exotic Bargmann supergravity when the flat limit $\ell \rightarrow\infty$ is considered. Besides, we show that both descriptions can be written in terms of a supersymmetric extension of the Nappi-Witten algebra or the extended Newton-Hooke superalgebra., Comment: 34 pages

A theory of gravity without masses can be constructed starting from Minkowski [Formula: see text] spaces. The corresponding adapted (p, q) Minkowski potentials, gradients and Laplacians built on each signature lead to a field equation similar to the Newton-Laplace one. In this framework, the anti-de Sitter spacetime is a hypersurface described by a constant potential of Minkowski gravitational force in absence of matter. We show that the cosmological constant of the anti-de Sitter spacetimes is related to a geometric property of Minkowski spheres: the centro-affine conservation of volumes defined from the so-called Minkowski–Tzizteica affine spheres. It is possible to show the connection between the anti-de Sitter AdS(2, 3) space, the non-Euclidean geometry and the difference between Minkowski [Formula: see text] sphere and the pseudosphere seen as a surface of the 3-Euclidean space. According to a suitable parameterization, the relation between de Sitter and anti-de Sitter spacetimes in any dimension is fixed. As a consequence, the light can travel in any anti-de Sitter spacetime defining the geodesic structure. These considerations can be recast in terms of Einstein field equations with cosmological constant and extended to f(R) gravity.

Euclid Consortium: S. Nesseris et al., [Context] The standard cosmological model is based on the fundamental assumptions of a spatially homogeneous and isotropic universe on large scales. An observational detection of a violation of these assumptions at any redshift would immediately indicate the presence of new physics., [Aims] We quantify the ability of the Euclid mission, together with contemporary surveys, to improve the current sensitivity of null tests of the canonical cosmological constant Λ and the cold dark matter (ΛCDM) model in the redshift range 0, [Methods] We considered both currently available data and simulated Euclid and external data products based on a ΛCDM fiducial model, an evolving dark energy model assuming the Chevallier-Polarski-Linder parameterization or an inhomogeneous Lemaître-Tolman-Bondi model with a cosmological constant Λ, and carried out two separate but complementary analyses: a machine learning reconstruction of the null tests based on genetic algorithms, and a theory-agnostic parametric approach based on Taylor expansion and binning of the data, in order to avoid assumptions about any particular model., [Results] We find that in combination with external probes, Euclid can improve current constraints on null tests of the ΛCDM by approximately a factor of three when using the machine learning approach and by a further factor of two in the case of the parametric approach. However, we also find that in certain cases, the parametric approach may be biased against or missing some features of models far from ΛCDM., [Conclusions] Our analysis highlights the importance of synergies between Euclid and other surveys. These synergies are crucial for providing tighter constraints over an extended redshift range for a plethora of different consistency tests of some of the main assumptions of the current cosmological paradigm., J.G.B., M.M. and S.N. acknowledge support from the research project PGC2018-094773-B-C32, and the Centro de Excelencia Severo Ochoa Program SEV-2016-059. S.N. also acknowledges support from the Ramón y Cajal program through Grant No. RYC-2014-15843. M.M. also received support from “la Caixa” Foundation (ID 100010434), with fellowship code LCF/BQ/PI19/11690015. D.S. acknowledges financial support from the Fondecyt Regular project number 1200171. The work of CJM was financed by FEDER – Fundo Europeu de Desenvolvimento Regional funds through the COMPETE 2020 – Operational Programme for Competitiveness and Internationalisation (POCI), and by Portuguese funds through FCT – Fundação para a Ciência e a Tecnologia in the framework of the project POCI-01-0145-FEDER-028987. D.C. thanks CAPES for financial support. V.M. thanks CNPq and FAPES for partial financial support. L.L. was supported by a Swiss National Science Foundation (SNSF) Professorship grant (No. 170547). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 888258. C.C. is supported by the UK Science and Technology Facilities Council (STFC) Consolidated Grant ST/P000592/1. A.dS. acknowledges support from the Fundação para a Ciência e a Tecnologia (FCT) through the Investigador FCT Contract No. IF/01135/2015 and POCH/FSE (EC) and in the form of an exploratory project with the same reference. J.P.M. and A.dS. acknowledge support from FCT Projects with references EXPL/FIS-AST/1368/2021, PTDC/FIS-AST/0054/2021, UIDB/04434/2020, UIDP/04434/2020, CERN/FIS-PAR/0037/2019, PTDC/FIS-OUT/29048/2017. Z.S. acknowledges support from the IRAP and IN2P3 Lyon computing centers. P.F. received the support of a fellowship from “la Caixa” Foundation (ID 100010434). The fellowship code is LCF/BQ/PI19/11690018. I.T. acknowledges support from the Spanish Ministry of Science, Innovation and Universities through grant ESP2017-89838, and the H2020 programme of the European Commission through grant 776247. The Euclid Consortium acknowledges the European Space Agency and a number of agencies and institutes that have supported the development of Euclid, in particular the Academy of Finland, the Agenzia Spaziale Italiana, the Belgian Science Policy, the Canadian Euclid Consortium, the Centre National d’Etudes Spatiales, the Deutsches Zentrum für Luft-und Raumfahrt, the Danish Space Research Institute, the Fundação para a Ciência e a Tecnologia, the Ministerio de Economia y Competitividad, the National Aeronautics and Space Administration, the Netherlandse Onderzoekschool Voor Astronomie, the Norwegian Space Agency, the Romanian Space Agency, the State Secretariat for Education, Research and Innovation (SERI) at the Swiss Space Office (SSO), and the United Kingdom Space Agency.