### Conveners

#### Wormholes, Energy Conditions and Time Machines: Block 1

- Francisco Lobo (Science Faculty of the University of Lisbon)
- Diego Rubiera-Garcia (Complutense University of Madrid)

#### Wormholes, Energy Conditions and Time Machines: Block 2

- Diego Rubiera-Garcia (Complutense University of Madrid)
- Francisco Lobo (Science Faculty of the University of Lisbon)

### Description

The General Theory of Relativity has been an extremely successful theory, with a well established experimental footing, at least for weak gravitational fields. Its predictions range from the existence of black holes, gravitational radiation to the cosmological models, predicting a primordial beginning, namely the big-bang. All these solutions have been obtained by first considering a plausible distribution of matter, and through the Einstein field equation, the spacetime metric of the geometry is determined. However, one may solve the Einstein field equation in the reverse direction, namely, one first considers an interesting and exotic spacetime metric, then finds the matter source responsible for the respective geometry. In this manner, it was found that some of these solutions possess a peculiar property, namely 'exotic matter,' involving a stress-energy tensor that violates the null energy condition. These geometries also allow closed timelike curves, with the respective causality violations. These solutions are primarily useful as 'gedanken-experiments' and as a theoretician's probe of the foundations of general relativity, and include traversable wormholes and superluminal 'warp drive' spacetimes. In this parallel session, in addition to extensively exploring interesting features, in particular, the physical properties and characteristics of these 'exotic spacetimes,' we also explore other non-trivial general relativistic geometries that generate closed timelike curves.

considering the asymmetric thin-shell wormhole (ATSW) model, we find that the impact parameter of the null geodesics is discontinuous through the wormhole in general and hence we identify novel shadows whose sizes are dependent of the photon sphere in the other side of the spacetime. Furthermore, we shoe evident additional photon rings from the ATSW spacetime. Moreover, a potential lensing...

Lensing by wormholes has been explored by several authors and the demagnification effect has been found as a distinctive signature which is not present in lensing from ordinary matter.

We study compact objects made up by ordinary and exotic matter in binary systems with $1/r^n$ potentials, where $n=2$ corresponds to the Ellis wormhole, both in the symmetric case, where the two lenses have...

We discuss the stability and appearance of an asymmetric thin-shell wormhole supported by positive energy sources within the Palatini f(R) gravity. Such object is build using a matching procedure of two Reissner-Nordström space-times with different masses and charges via suitable junction conditions.

I will discuss the observational appearance of wormholes if they were observed by a very long baseline interferometry (VLBI) array such as the Event Horizon Telescope, or some more powerful future VLBI array. Certain properties, like change of the diameter of the critical curve with respect to the Kerr black hole of the same mass are difficult to interpret given typically poor constraints we...

The current interests in the universe motivate us to go beyond Einstein's General theory of relativity. One of the interesting proposals comes from a new class of teleparallel gravity named symmetric teleparallel gravity, i.e., $f(Q)$ gravity, where the non-metricity term $Q$ is accountable for fundamental interaction. These alternative modified theories of gravity's vital role are to deal...

A recently proposed interesting class of black hole mimickers are the so-called "black-bounce" spacetimes. In static spherical symmetry, a candidate spacetime was explored which neatly interpolates between a classical Schwarzschild black hole, a regular black hole, and a traversable wormhole depending on the value of an additional scalar metric parameter. Since this analysis, the discourse...

In this work, we explore the existence of traversable wormhole solutions supported by double-layer thin-shells and satisfying the Null Energy Condition (NEC) throughout the whole spacetime, in a quadratic form of the generalized hybrid metric-Palatini gravity. We start by showing that for a particular quadratic form of the action, some of the junction conditions of the theory can be discarded...

In this talk, the gravitational wave generation by a slowly rotating wormhole with radially pulsating throat is considered. Two types of rotating wormholes are used as the model of the wave generation: the slightly rotating Ellis wormhole and the thin-shell wormhole. The later was made from two Kerr black hole solutions. To treat the problem, the assumption of the slightly rotating is...

Taking advantage of the simplicity of the quantum field theory of a free massless scalar field in 1+1 dimensions, a toy model that explores the effects of the interaction of wormholes and quantum fields is constructed. The conditions for the existence of quantum particle production are discussed in terms of the symmetries of the two-point function of the scalar field theory.

Magnetic fields observed in galaxies have the toroidal component. We present the hypothesis that such fields maybe remnants of relic magnetic torus - shaped wormholes. Such magnetic wormholes produce the two important effects. The first effect is that in the primordial plasma before the recombination magnetic fields of wormholes trap baryons whose energy is smaller than a threshold energy....

Within the framework of F(R) theories of gravity with (2+1)-dimensions and constant scalar curvature R, we construct a family of thin-shell wormholes with circular symmetry and we analyze the stability of the static configurations under radial perturbations. We show an example of asymptotically anti-de Sitter thin-shell wormholes with charge, finding that stable configurations with normal...

In this work, we explore wormhole geometries in a recently proposed modified gravity theory arising from a non-conservative gravitational theory, tentatively denoted action-dependent Lagrangian theories. The generalized gravitational field equation essentially depends on a background four-vector $\lambda^\mu$, that plays the role of a coupling parameter associated with the dependence of the...

In this talk I will give a brief historical review of the original warp bubble geometry, as well as the main problems with it, namely the need for negative energy and the horizon problem.

Both wormholes and warp drives, concepts originally developed within the context of science fiction, have now (for some 30 odd years) been studied and carefully analyzed within the framework of general relativity. An overarching theme of the general relativistic analysis is unavoidable violations of the classical energy conditions. Another science fiction trope, now over 80 years old, is the...

Solitons in space–time capable of transporting time-like observers at superluminal speeds have long been tied to violations of the weak, strong, and dominant energy conditions of general relativity. This talk presents an approach to identify soliton solutions capable of superluminal travel that are sourced by purely positive energy densities. This is the first example of hyper-fast solitons...

Alcubierre warp drive spacetimes allow timelike observers to travel superluminally relative to other timelike observers through the use of negative energy densities. In our study, we develop a general framework for describing warp drives. We show that any such object may be thought of as a shell of inertially moving material that modifies the properties of spacetime enclosed by the shell....

This is the fourth paper of a series where we examine solutions of the Einstein equations with the Alcubierre warp drive geometry having different matter and field sources (Santos-Pereira et al. 2020, 2021; Refs. [1–3]). The Alcubierre metric describes a spacetime geometry that allows a massive particle inside a spacetime distortion, called warp bubble, to travel with superluminal global...

In this work we study the problem of linear stability of gravitational perturbations in stationary and spherically symmetric wormholes. For this purpose, we employ the Newman-Penrose formalism which is well-suited for treating gravitational radiation in General Relativity, as well as the geometrical aspect of this theory. With this method we obtain a "master equation" that describes the...

We investigate a family of ultra-static, Lorentzian wormholes for which the wormhole shapes and sizes are controlled by two metric parameters –b_0 (the throat radius) and n (an even integer). The well-known Ellis- Bronnikov wormhole is a special case (n= 2) in this family. The n>2 spacetimes are however, distinctly different geometrically, a fact evident from their embedding...

We study the absorption of massless scalar waves in a geometry that interpolates between the Schwarzschild solution and a wormhole that belongs to the Morris-Thorne class of solutions. In the middle of the interpolation branch, this geometry describes a regular black hole. We use the partial wave approach to compute the scalar absorption cross section in this geometry. Our results show that...

The classical singularity theorems of General Relativity rely on energy conditions that are easily violated by quantum fields. In this talk I will provide motivation for an energy condition obeyed by semiclassical gravity: the smeared null energy condition (SNEC), a proposed bound on the weighted average of the null energy along a finite portion of a null geodesic. I will then then present the...

We utilize how Weber in 1961 initiated the process of quantization of early universe fields to the problem of what may be emitted at the mouth of a wormhole. While the wormhole models are well developed, there is as of yet no consensus as to how, say GW or other signals from a wormhole mouth could be quantized, or made to be in adherence to a procedure Weber cribbed from Feynman, in 1961.

In...

In this talk the geometrical methods and symmetry principles in gravitation are explored motivating a new perspective into the spacetime paradigm. The effects of post-Riemann spacetime geometries with torsion are briefly studied in applications to fundamental fermionic and bosonic fields, cosmology, astrophysics and gravitational waves. The physical implications and related phenomenological...