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.
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.
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.
Friedmann-like universes with torsion Kranas, Dimitrios; Tsagas, Christos G.; Barrow, John D. ...
The European physical journal. C, Particles and fields,
04/2019, Letnik:
79, Številka:
4
Journal Article
Recenzirano
Odprti dostop
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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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Szekeres universes with homogeneous scalar fields Barrow, John D.; Paliathanasis, Andronikos
The European physical journal. C, Particles and fields,
09/2018, Letnik:
78, Številka:
9
Journal Article
Recenzirano
Odprti dostop
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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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Deviations from the predictions of general relativity due to energy-momentum squared gravity (EMSG) are expected to become pronounced in the high density cores of neutron stars. We derive the ...hydrostatic equilibrium equations in EMSG and solve them numerically to obtain the neutron star mass-radius relations for four different realistic equations of state. We use the existing observational measurements of the masses and radii of neutron stars to constrain the free parameter, α, that characterizes the coupling between matter and spacetime in EMSG. We show that −10−38 cm3/erg<α<+10−37 cm3/erg. Under this constraint, we discuss what contributions EMSG can provide to the physics of neutron stars, in particular, their relevance to the so called hyperon puzzle in neutron stars. We also discuss how EMSG alters the dynamics of the early universe from the predictions of the standard cosmological model. We show that EMSG leaves the standard cosmology safely unaltered back to t∼10−4 seconds at which the energy density of the universe is ∼1034 erg cm−3.
Dynamical analysis in scalar field cosmology Paliathanasis, Andronikos; Tsamparlis, Michael; Basilakos, Spyros ...
Physical review. D, Particles, fields, gravitation, and cosmology,
06/2015, Letnik:
91, Številka:
12
Journal Article
Recenzirano
Odprti dostop
We give a general method to find exact cosmological solutions for scalar-field dark energy in the presence of perfect fluids. We use the existence of invariant transformations for the Wheeler De Witt ...(WdW) equation. We show that the existence of a point transformation under which the WdW equation is invariant is equivalent to the existence of conservation laws for the field equations, which indicates the existence of analytical solutions. We extend previous work by providing exact solutions for the Hubble parameter and the effective dark-energy equation of state parameter for cosmologies containing a combination of perfect fluid and a scalar field whose self-interaction potential is a power of hyperbolic functions. We find solutions explicitly when the perfect fluid is radiation or cold dark matter and determine the effects of nonzero spatial curvature. Using the Planck 2015 data, we determine the evolution of the effective equation of state of the dark energy. Finally, we study the global dynamics using dimensionless variables. We find that if the current cosmological model is Liouville integrable (admits conservation laws) then there is a unique stable point which describes the de-Sitter phase of the universe.
Dynamics of Einstein–Aether scalar field cosmology Paliathanasis, Andronikos; Papagiannopoulos, G.; Basilakos, Spyros ...
The European physical journal. C, Particles and fields,
08/2019, Letnik:
79, Številka:
8
Journal Article
Recenzirano
Odprti dostop
We study the cosmological evolution of the field equations in the context of Einstein–Aether cosmology by including a scalar field in a spatially flat Friedmann–Lemaître–Robertson–Walker spacetime. ...Our analysis is separated into two separate where a pressureless fluid source is included or absent. In particular, we determine the critical points of the field equations and we study the stability of the specific solutions. The limit of general relativity is fully recovered, while the dynamical system admits de Sitter solutions which can describe the past inflationary era and the future late-time attractor. Results for generic scalar field potentials are presented while some numerical behaviours are given for specific potential forms.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Two-fluid solutions of particle-creation cosmologies Pan, Supriya; Barrow, John D.; Paliathanasis, Andronikos
The European physical journal. C, Particles and fields,
02/2019, Letnik:
79, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Cosmological evolution driven incorporating continuous particle creation by the time-varying gravitational field is investigated. We consider a spatially flat, homogeneous and isotropic universe with ...two matter fluids in the context of general relativity. One fluid is endowed with gravitationally induced “adiabatic” particle creation, while the second fluid simply satisfies the conservation of energy. We show that the dynamics of the two fluids is entirely controlled by a single nonlinear differential equation involving the particle creation rate,
Γ
(
t
)
. We consider a very general particle creation rate,
Γ
(
t
)
, that reduces to several special cases of cosmological interest, including
Γ
=
constant,
Γ
∝
1
/
H
n
(
n
∈
N
),
Γ
∝
exp
(
1
/
H
)
. Finally, we present singular algebraic solutions of the gravitational field equations for the two-fluid particle creation models and discuss their stability.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We consider Friedmann-like universes with torsion and take a step towards studying their stability. In so doing, we apply dynamical-system techniques to an autonomous system of differential ...equations, which monitors the evolution of these models via the associated density parameters. Assuming relatively weak torsion, we identify the system’s equilibrium points. These are found to represent homogeneous and isotropic spacetimes with nonzero torsion that undergo accelerated expansion. We then examine the linear stability of the aforementioned fixed points. Our results indicate that Friedmann-like cosmologies with weak torsion are generally stable attractors, either asymptotically or in the Lyapunov sense. In addition, depending on the equation of state of the matter, the equilibrium states can also act as intermediate saddle points, marking a transition from a torsional to a torsion-free universe.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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