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.
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.
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.
The differentiation of bone marrow-derived progenitor cells into monocytes, tissue macrophages and some dendritic cell (DC) subtypes requires the growth factor CSF1 and its receptor, CSF1R. ...Langerhans cells (LCs) and microglia develop from embryonic myeloid precursor cells that populate the epidermis and central nervous system (CNS) before birth. Notably, LCs and microglia are present in CSF1-deficient mice but absent from CSF1R-deficient mice. Here we investigated whether an alternative CSF1R ligand, interleukin 34 (IL-34), is responsible for this discrepancy. Through the use of IL-34-deficient (Il34(LacZ/LacZ)) reporter mice, we found that keratinocytes and neurons were the main sources of IL-34. Il34(LacZ/LacZ) mice selectively lacked LCs and microglia and responded poorly to skin antigens and viral infection of the CNS. Thus, IL-34 specifically directs the differentiation of myeloid cells in the skin epidermis and CNS.
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.
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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