We investigate the consequences for the black hole area of introducing fractal structure for the horizon geometry. We create a three-dimensional spherical analogue of a ‘Koch Snowflake’ using a ...infinite diminishing hierarchy of touching spheres around the Schwarzschild event horizon. We can create a fractal structure for the horizon with finite volume and infinite (or finite) area. This is a toy model for the possible effects of quantum gravitational spacetime foam, with significant implications for assessments of the entropy of black holes and the universe, which is generally larger than in standard picture of black hole structure and thermodynamics, potentially by very considerable factors. The entropy of the observable universe today becomes S≈10120(1+Δ/2), where 0≤Δ≤1, with Δ=0 for a smooth spacetime structure and Δ=1 for the most intricate. The Hawking lifetime of black holes is also reduced.
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We study large-scale inhomogeneous perturbations and instabilities of interacting dark-energy models. Past analysis of large-scale perturbative instabilities has shown that we can only test ...interacting dark-energy models with observational data when their parameter ranges are either wx≥−1 and ξ≥0, or wx≤−1 and ξ≤0, where wx is the dark-energy equation of state and ξ is a coupling parameter governing the strength and direction of the energy transfer. We show that by adding a factor (1+wx) to the background energy transfer, the whole parameter space can be tested against all the data, and thus, the instabilities in such interaction models can be removed. We test three classes of interaction models using the latest astronomical data from the CMB, supernovae, baryon acoustic oscillations, redshift-space distortions, weak lensing, cosmic chronometers, and the local Hubble constant. Precise constraints are found. Our analysis shows that a very small but nonzero deviation from pure Λ-cosmology is suggested by the observational data, while the no-interaction scenario can be recovered at the 68.3% confidence level. In particular, for three interacting dark-energy (IDE) models, identified as IDE 1, IDE 2, and IDE 3, the 68.3% confidence-level constraints on the interaction coupling strengths are ξ=0.0360−0.0360+0.0091 (IDE 1), ξ=0.0433−0.0433+0.0062 (IDE 2), ξ=0.1064−0.1064+0.0437 (IDE 3). In addition, we find that the dark-energy equation of state tends towards the phantom region. Taking the 68.3% confidence-level constraints, wx=−1.0230−0.0257+0.0329 (IDE 1), wx=−1.0247−0.0302+0.0289 (IDE 2), and wx=−1.0275−0.0318+0.0228 (IDE 3). However, the possibility of wx>−1 is also not rejected by the astronomical data employed in this analysis. Moreover, we find in all interaction models that, as the value of the Hubble constant decreases, the behavior of the dark-energy equation of state shifts from a phantom to a quintessence type with its equation of state very close to that of a simple cosmological constant at the present time. Finally, we compare the observational estimations of the coupling strength imposed on some interaction models studied in this work with the past constraints obtained on them for different regions of the dark-energy equation of state.
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We study the cosmological constant (Λ) in the standard Λ cold dark matter model by introducing the graduated dark energy (gDE) characterized by a minimal dynamical deviation from the null inertial ...mass density of the Λ in the form ρinert ∝ ρλ < 0 with λ < 1 being a ratio of two odd integers, for which its energy density ρ dynamically takes negative values in the finite past. For large negative values of λ, it creates a phenomenological model described by a smooth function that approximately describes the Λ spontaneously switching sign in the late Universe to become positive today. We confront the model with the latest combined observational datasets of Planck + baryon acoustic oscillations + supernova + H. It is striking that the data predict bimodal posterior probability distributions for the parameters of the model along with large negative λ values; the new maximum significantly excludes the Λ, and the old maximum contains the Λ. The improvement in the goodness of fit for the Λ reaches highly significant levels, Δχ2min = 6.4, for the new maxima, while it remains at insignificant levels, Δχ2min ≲ 0.02, for the old maxima. We show that, in contrast to the old maxima, which do not distinguish from the Λ, the new maxima agree with the model-independent H0 measurements, high-precision Ly−α data, and model-independent Omh2 diagnostic estimates. Our results provide strong hints of a spontaneous sign switch in the cosmological constant and lead us to conjecture that the Universe has transitioned from anti-de Sitter vacua to de Sitter vacua, at a redshift z ≈ 2.32, and triggered the late-time acceleration, and suggests looking for such mechanisms in string theory constructions.
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We reconsider and extend the cosmological predictions that can be made under the assumption that the total action of the Universe is finite. When initial and final singularities in curvature ...invariants are avoided, it leads to singularities in the gravitational action of the Universe. The following properties are required of a universe with finite action: Compact spatial sections (i.e., a closed universe) giving a finite total lifetime for the Universe. Compactification of flat and open universes is excluded. The Universe can contain perfect fluids with −1<p/ρ<2 on approach to singularities. The Universe cannot display a bounce or indefinite cyclic behavior to the past or the future. Here, we establish new consequences of imposing finite action: the Universe cannot be dominated by massless scalar fields or the kinetic energy of self-interacting scalar fields or a p=ρ perfect fluid on approach to the initial or final singularity. The ekpyrotic scenario with an effective fluid obeying p/ρ>2 in a closed, flat or open universe is excluded. Any bouncing loop quantum gravity model with indefinite past or future evolution is ruled out. The Einstein static and steady-state universes are ruled out along with past or future eternal inflating universes anisotropies of Kasner or Mixmaster type cannot dominate the dynamics on approach to singularities. This excludes density inhomogeneity spectra versus mass, of the form δρ/ρ∝M−q, with q>2/3. Higher-order Lagrangian theories of gravity are significantly constrained. Quadratic Lagrangians are excluded with fluids satisfying p/ρ>−1/3. Lagrangians with Lg=R1+δ have infinite actions on approach to a singularity when 2δ(1−3γ)+2−3γ<0, where p=(γ−1)ρ for the fluid. As shown by Barrow and Tipler, the Gauss-Bonnet quadratic combination causes a cosmological action singularity even though it does not contribute terms to the field equations. Scalar-tensor theories like Brans-Dicke dominated by the scalar field on approach to singularities have action singularities. Dark energy cannot be a simple cosmological constant, as it would create an action singularity to the future: the Universe cannot be asymptotically de Sitter as t→∞. The dark energy must be an evolving energy density in a closed universe that produces collapse to a future singularity and cannot be dominated by the kinetic energy of the scalar field.
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We use Big Bang Nucleosynthesis (BBN) data in order to impose constraints on the exponent of Barrow entropy. The latter is an extended entropy relation arising from the incorporation of ...quantum-gravitational effects on the black-hole structure, parameterized effectively by the new parameter Δ. When considered in a cosmological framework and under the light of the gravity-thermodynamics conjecture, Barrow entropy leads to modified cosmological scenarios whose Friedmann equations contain extra terms. We perform a detailed analysis of the BBN era and we calculate the deviation of the freeze-out temperature comparing to the result of standard cosmology. We use the observationally determined bound on |δTfTf| in order to extract the upper bound on Δ. As we find, the Barrow exponent should be inside the bound Δ≲1.4×10−4 in order not to spoil the BBN epoch, which shows that the deformation from standard Bekenstein-Hawking expression should be small as expected.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
An algorithm is used to generate new solutions of the scalar-field equations in homogeneous and isotropic universes. Solutions can be found for pure scalar fields with various potentials in the ...absence and presence of spatial curvature and other perfect fluids. A series of generalizations of the Chaplygin gas and bulk viscous cosmological solutions for inflationary universes are found. Furthermore other closed-form solutions which provide inflationary universes are presented. We also show how the Hubble slow-roll parameters can be calculated using the solution algorithm and we compare these inflationary solutions with the observational data provided by the Planck 2015 collaboration in order to constrain and rule out some of these models.
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Friedmann-like universes with torsion Kranas, Dimitrios; Tsagas, Christos G.; Barrow, John D. ...
The European physical journal. C, Particles and fields,
04/2019, Volume:
79, Issue:
4
Journal Article
Peer reviewed
Open access
We consider spatially homogeneous and isotropic cosmologies with non-vanishing torsion, which assumes a specific form due to the high symmetry of these universes. Using covariant and metric-based ...techniques, we derive the torsional versions of the continuity, the Friedmann and the Raychaudhuri equations. These show how torsion can drastically change the standard evolution of the Friedmann models, by playing the role of the spatial curvature or that of the cosmological constant. We find, for example, that torsion alone can lead to exponential expansion and thus make the Einstein–de Sitter universe look like the de Sitter cosmos. Also, by modifying the expansion rate of the early universe, torsion could have affected the primordial abundance of helium-4. We show, in particular, that torsion can
reduce
the production of primordial helium-4, unlike other changes to the standard thermal history of the universe. These theoretical results allow us to impose strong observational bounds on the relative strength of the associated torsion field, confining its ratio to the Hubble rate within the narrow interval (
-
0.005813
,
+
0.019370
) around zero. Finally, turning to static spacetimes, we demonstrate that there exist torsional analogues of the Einstein static universe with all three types of spatial geometry. These models can be stable when the torsion field and the universe’s spatial curvature have the appropriate profiles.
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We introduce consideration of a new factor, synchronization of spacetime mixmaster oscillations, that may play a simplifying role in understanding the nature of the general inhomogeneous cosmological ...solution to Einstein's equations. We conjecture that, on approach to a singularity, the interaction of spacetime mixmaster oscillations in different regions of an inhomogeneous universe can produce a synchronization of these oscillations through a coupling to their mean field in the way first demonstrated by the Kuramoto coupled oscillator model.
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We formulate and solve the problem of Newtonian cosmology under the assumption that the absolute space of Newton is non-Euclidean. In particular, we focus on the negatively-curved hyperbolic space, ...H3. We point out the inequivalence between the curvature term that arises in the Friedmann equation in Newtonian cosmology in Euclidean space and the role of curvature in the H3 space. We find the generalisation of the inverse-square law and the solutions of the Newtonian cosmology that follow from it. We find the generalisations of the Euclidean Michell 'black hole' in H3 and show that it leads to different maximum force and area results to those we have found in general relativity. We show how to add the counterpart of the cosmological constant to the gravitational potential in H3 and explore the solutions and asymptotes of the cosmological models that result. We also discuss the problems of introducing compact topologies in Newtonian cosmologies with non-negative spatial curvature.
We consider the existence of an “inflaton” described by an homogeneous scalar field in the Szekeres cosmological metric. The gravitational field equations are reduced to two families of solutions ...which describe the homogeneous Kantowski–Sachs spacetime and an inhomogeneous FLRW(-like) spacetime with spatial curvature a constant. The main differences with the original Szekeres spacetimes containing only pressure-free matter are discussed. We investigate the stability of the two families of solution by studying the critical points of the field equations. We find that there exist stable solutions which describe accelerating spatially-flat FLRW geometries.
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