Some unusual wormholes in general relativity

In this short review, we present some recently obtained traversable wormhole models in the framework of general relativity (GR) in four and six dimensions that somehow widen our common ideas on wormhole existence and properties. These are, first, rotating cylindrical wormholes, asymptotically flat in the radial direction and existing without exotic matter. The topological censorship theorems are not violated due to lack of asymptotic flatness in all spatial directions. Second, these are cosmological wormholes constructed on the basis of the Lemaître-Tolman-Bondi solution. They connect two copies of a closed Friedmann world filled with dust, or two otherwise distant parts of the same Friedmann world. Third, these are wormholes obtained in six-dimensional GR, whose one entrance is located in 'our' asymptotically flat world with very small extra dimensions while the other 'end' belongs to a universe with large extra dimensions and therefore different physical properties. The possible observable features of such wormholes are briefly discussed. This article is part of the theme issue 'The future of mathematical cosmology, Volume 1'.

Thermochemical Laser-Induced Periodic Surface Structures Formation by Femtosecond Laser on Hf Thin Films in Air and Vacuum

Thermochemical laser-induced periodic surface structures (TLIPSS) are a relatively new type of periodic structures formed in the focal area of linear polarized laser radiation by the thermally stimulated reaction of oxidation. The high regularity of the structures and the possibility of forming high-ordered structures over a large area open up possibilities for the practical application for changing the optical and physical properties of materials surface. Since the mechanism of formation of these structures is based on a chemical oxidation reaction, an intriguing question involves the influence of air pressure on the quality of structure formation. This paper presents the results on the TLIPSS formation on a thin hafnium film with fs IR laser radiation at various ambient air pressures from 4 Torr to 760 Torr. Despite the decrease in the oxygen content in the ambient environment by two orders of magnitude, the formation of high-ordered TLIPSS (dispersion in the LIPSS orientation angle δθ < 5°) with a period of ≈700 nm occurs within a wide range of parameters variation (laser power, scanning speed). This behavior of TLIPSS formation is in agreement with experimental data obtained earlier on the study of the kinetics of high-temperature oxidation of hafnium at various oxygen pressures.

Oxide composition and period variation of thermochemical LIPSS on chromium films with different thickness

In this paper, we present the results of thermochemical LIPSS formation on a chromium film with a thickness in the range of 28-350 nm induced by femtosecond laser radiation (λ = 1026 nm, ν = 200 kHz, τ = 232 fs). The period, height, morphology and chemical composition of TLIPSS as a function of the metal film thickness and focusing configuration are investigated. The growth of TLIPSS period from 678 nm to 950 nm with increasing thickness of the film has been explained by a formation of oxides with different stoichiometry composition. So, the CrO₂ oxide prevails in the composition for the case of TLIPSS formed on thin films which have the minimal period, whereas Cr₂O₃ oxide is dominant in the case of TLIPSS formed on thick chromium films which have the maximal period value. The results obtained are in agreement with numerical modeling of a period defined by the interference between an incident radiation and a scattered one from a single oxide ridge with a different chemical composition.

Regular phantom black holes

We study self-gravitating, static, spherically symmetric phantom scalar fields with arbitrary potentials (favored by cosmological observations) and single out 16 classes of possible regular configurations with flat, de Sitter, and anti-de Sitter asymptotics. Among them are traversable wormholes, bouncing Kantowski-Sachs (KS) cosmologies, and asymptotically flat black holes (BHs). A regular BH has a Schwarzschild-like causal structure, but the singularity is replaced by a de Sitter infinity, giving a hypothetic BH explorer a chance to survive. It also looks possible that our Universe has originated in a phantom-dominated collapse in another universe, with KS expansion and isotropization after crossing the horizon. Explicit examples of regular solutions are built and discussed. Possible generalizations include k-essence type scalar fields (with a potential) and scalar-tensor gravity.