arXiv subjects

Remo Garattini

Publications and source records attributed to Remo Garattini.

At least 19 recordsLinked to original sources

arXiv2026

Yukawa modification of Traversable Wormholes supported by Holographic Dark Energy

Inspired by holographic dark energy models, we extend the analysis performed in Ref.\cite{RGPC} taking into account Yukawa deformations on the different energy density (HDE) profiles. The profiles we have explored are: the Bekenstein-Hawking HDE, the Moradpour energy density, the Standard Renyi HDE and the Mixed Energy Density. With the help of an inhomogeneous Equation of State of the form $p_{r}(r)=\omega _{r}\left( r\right) \rho (r)$, we have considered Zero Tidal Forces. Differently from the original case, only a particular case of the family of the Bekenstein-Hawking HDE profiles distorted by the Yukawa term has developed a divergent $\omega _{r}\left( r\right) $ for $r\rightarrow \infty $. To cure such a divergence we have introduced a modification at large distances in order to have a finite result. This means that the Zero Tidal Forces can be imposed in a very large but limited region of the spacetime. Such modification did not change the behavior of the Equation of State close to the throat. For every proposal we have computed the components of the Stress-Energy Tensor. What we have found is that the Yukawa-Bekenstein-Hawking HDE is the only positive profile, while the Yukawa-Moradpour and the Yukawa-Renyi have a limited region where the energy density is positive. The remaining Yukawa-Mixed Energy Density is always negative. This means that the Yukawa distortion introduces a kind of negative energy density.

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arXiv2026

Polytropic wormholes

Traversable wormholes in general relativity require non-standard matter sources, making the identification of physically motivated equations of state particularly important. We investigate wormholes supported by a polytropic equation of state, considering homogeneous and inhomogeneous configurations within a unified framework. We derive the corresponding solutions and analyze the effects of the polytropic parameters on the geometry and energy conditions. In the homogeneous case, the polytropic construction yields a consistent wormhole interior whose geometry and matter content are governed by the constant polytropic parameters. For the inhomogeneous case, we obtain a general analytical expression showing that the geometry is completely determined by the radial polytropic coefficient $\omega(r)$. For positive $\omega(r)$, the requirement for physically meaningful solutions naturally restricts the polytropic exponent to odd integer values. Using a power-law profile, we construct explicit classes of solutions exhibiting distinct parameter regimes and finite radial support. Interestingly enough, for an exponent $\alpha=2\gamma-3$, a generalized absurdly benign traversable wormhole-like configuration emerges naturally. Although the flare-out condition implies null-energy-condition violation at the throat, the inhomogeneous framework allows its radial distribution to be controlled. Our results establish a systematic connection between polytropic matter and wormhole geometry, providing a flexible framework for constructing compact wormholes with localized exotic matter.

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arXiv2026

Traversable Wormholes induced by polytropic $n=1$ energy density

We investigate spherically symmetric and static traversable wormholes supported by exotic matter, focusing on solutions sourced by physically motivated dark matter energy density profiles. Considering the polytropic $n=1$ (Lane-Emden) distribution, we construct explicit forms of the shape function $b(r)$ and analyze the resulting radial and tangential pressures, carefully addressing the requirements of the flare-out condition at the throat and the absence of horizons. We explore zero-tidal-force configurations as well as inhomogeneous equations of state, demonstrating how appropriate choices of the radial pressure allow for finite and well-behaved redshift functions throughout the spacetime. Boundary conditions at a finite radius are implemented to ensure vanishing energy density and pressures, and asymptotic expansions are derived to characterize the behavior of the metric and matter content near the edge of the dark matter halo. Additionally, we reformulate the Einstein field equations entirely in terms of the energy density, radial and tangential pressures, and their derivatives, providing a framework to analyze the matter distribution independently of the explicit metric functions. Our results offer a systematic methodology to construct physically consistent wormhole geometries supported by realistic dark matter halos, highlighting the intricate interplay between matter profiles, equations of state, and geometric constraints.

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arXiv2025

Charged rotating Casimir wormholes

We investigate the conditions under which a rotating traversable wormhole can be supported by a Casimir source in the presence of an external electric field. Extending previous studies of static Casimir wormholes and neutral rotating configurations, we construct an electrically charged rotating Casimir wormhole solution and determine the thermal stress-energy tensor required to consistently satisfy the Einstein field equations. A particularly simple configuration arises when the rotation is constant and coincides with that measured by a zero-angular-momentum observer (ZAMO). In this case, the rotating wormhole preserves the same redshift and shape functions as the well-known static charged Casimir case, provided that the angular velocity and thermal components satisfy specific constraints imposed by the field equations. We also examine a configuration in which the angular velocity depends on the radial coordinate and decreases exponentially away from the throat. This damping mechanism removes the unrealistic persistence of frame dragging at large distances, while still allowing a consistent solution supported by Casimir, electromagnetic and thermal contributions.

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arXiv2025

Short-range approximation to Casimir wormholes inspired by scalar and electric fields

We investigate a static traversable wormhole sustained by a combination of a minimally coupled scalar field and an electric field, with exotic matter sourced by Casimir energy. Considering two scenarios, where the Casimir plate separation is either radially variable or fixed, we derive analytical near-throat solutions for both massless and massive scalar fields. To ensure consistency of the field equations, a thermal tensor is also incorporated, consisting solely of pressure terms that vanish at the throat. In all cases, we obtain well-behaved wormhole manifolds with throat sizes that scale proportionally with the number of elementary charges the wormhole can support.

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arXiv2025

Rotating Casimir Wormholes

A Casimir Wormhole is a Traversable Wormhole powered by a Casimir energy source within a static reference frame. A natural extension of this system is the inclusion of rotation. We will explore two basic configurations: one with radially varying Casimir plates and another with parametrically fixed plates. In both cases, we will show that rotations do not alter the structure of a Casimir wormhole, and the behavior observed in a static frame is reaffirmed. Since the case with radially varying plates predicts a constant angular velocity as a solution, we must introduce an exponential cut-off and an additional scale to prevent rotations at infinity. This adjustment is not necessary when the plates are kept parametrically fixed. Moreover, the consistency of the Einstein Field Equations is ensured with the help of an additional source without an accompanying energy density, which we interpret as a thermal stress tensor.

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arXiv2025

Warp Drive in a De Sitter Universe

Generalizing the result of H. Ellis who embedded a warp bubble in the background of a black hole, we introduce a warp bubble in a de Sitter universe. We show that under certain conditions (namely, that the bubble is moving in the radial direction at a velocity equal to the speed of the expansion of the universe), it is possible for the bubble to have strictly non--negative energy density, with the weak and null energy satisfied up to a total divergence term that averages to zero. We discuss the implications of this result and its possible applications to models of dark energy like "dark fluid" and quintessence, as well as to physical systems like Casimir cavities and analogue gravity setups.

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arXiv2024

Effects of additional sources on Casimir Wormholes

In this contribution we explore the consequences of including additional sources to the original Casimir energy Stress-Energy Tensor. In particular, we will discuss the effects of an additional electromagnetic field, the modification induced by non-zero temperature effects on the energy density obtained by a Casimir device and finally the effect obtained by including a massless scalar field. For each of these examples, we have introduced an auxiliary stress tensor which we have interpreted as a thermal tensor. Consequences on the size of the throat are also discussed. We will show that these additional extra fields do not destroy the traversability of the wormhole.

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arXiv2024

Black Holes, Warp Drives, and Energy Conditions

Following the work of H. Ellis, we study warp drives in the gravitational field of a Schwarzschild black hole. We find that as long as the warp drive crosses the black hole horizon at a subluminal speed, the horizon would be effectively absent inside the warp bubble. Moreover, we discover that the black hole's gravitational field can alleviate the violations of the weak energy condition (WEC) and the null energy condition (NEC) and therefore decrease the amount of negative energy required to sustain a warp drive, which may be instrumental for creating microscopic warp drives in lab experiments. We also consider the thermodynamics of a warp bubble interacting with a black hole and point out some paradoxes that may indicate a gap in our understanding of them from the thermodynamic point of view.

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arXiv2024

Hot Casimir Wormholes

In this paper, we have for the first time considered the consequences of thermal fluctuations to the Casimir effect on a traversable wormhole. This was done by using finite temperature generalization of the Casimir effect as a source of a hot traversable wormhole. Thus, we have considered a more physical scenario, where the effects of thermal fluctuations are also considered as a source of a traversable wormhole. To obtain a dependence on such a thermal Casimir effect, consider the plates positioned at a distance either parametrically fixed or radially varying. In both cases, the temperature effects are investigated. We demonstrate that thermal fluctuations modify the throat of the wormhole. Such results have been obtained in both regimes, i.e. high temperature and low temperature. We explicitly investigate the effect of such finite temperature effects on the size of a wormhole.

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arXiv2023

Black Holes and Warp Drive

We study the generalizations of the original Alcubierre warp drive metric to the case of curved spacetime background. We find that the presence of a horizon is essential when one moves from spherical coordinates to Cartesian coordinates in order to avoid additional singularities. For the specific case of Schwarzschild black hole, the horizon would be effectively absent for the observers inside the warp bubble, implying that warp drives may provide a safe route to cross horizons. Moreover, we discover that the black hole's gravitational field can decrease the amount of negative energy required to sustain a warp drive, which may be instrumental for creating microscopic warp drives in lab experiments. A BEC model is also introduced to propose possible test in the Analogue Gravity framework.

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arXiv2023

Traversable Wormholes supported by Holographic Dark Energy with a modified Equation of State

Inspired by holographic dark energy models, we consider different energy density profiles as possible sources needed to have traversable wormholes solutions. Since such energy densities are all positive, we are forced to introduce an equation of state of the form $p_{r}% (r)=\omega_{r}\left( r\right) \rho(r)$. We will find that Zero Tidal Forces can be imposed at the price of having the function $\omega_{r}\left( r\right) $ divergent for $r\rightarrow\infty$. To overcome this inconvenient, we abandon the request of having Zero Tidal Forces, by introducing appropriate modifications on the function $\omega_{r}\left( r\right) $ in such a way to obtain a finite result everywhere. We will find that such modifications will leave the behavior of the Equation of State close to the throat invariant. Moreover, despite of the initial assumption, we will find that every dark energy profile will be moved into the phantom region. Among the different energy density proposals, only one profile will not require a modification of the original $\omega _{r}\left(r\right) $ to have Zero Tidal Forces. Such an energy density profile will be consistent with the appearance of a Global Monopole.

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arXiv2023

Effects of an Electric Charge on Casimir Wormholes: Changing the Throat Size

In this paper we continue the investigation about the connection between Casimir energy and the traversability of a wormhole. In addition to the negative energy density obtained by a Casimir device, we include the effect of an electromagnetic field generated by an electric charge. This combination defines an electrovacuum source which has an extra parameter related to the size of the throat. Even if the electromagnetic energy density is positive the Null Energy Condition is still violated. The main reason is that the electromagnetic field satisfies the property $\rho=-p_{r}$. As a consequence the Traversable Wormhole throat can be changed as a function of the electric charge. This means that the throat is no more Planckian and the traversability is little less in principle but little more in practice.

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arXiv2021

Yukawa-Casimir Wormholes

In this work, we consider a Yukawa modification of the Casimir wormhole. With the help of an Equation of State, we impose Zero Tidal Forces. We will examine two different approaches: in a first approach, we will fix the form of the shape function of the Casimir wormholes modified by a Yukawa term in three different ways and finally a superposition of different profiles. In the second approach, we will consider the original Casimir source modified by a Yukawa term in three different ways and we will deduce the form of the shape function In both the approaches the reference energy density will be that of the Casimir source. Connection with the Absurdly Benign Traversable Wormhole are also discussed.

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arXiv2021

Modified Gravity and Cosmology: An Update by the CANTATA Network

General Relativity and the $\Lambda$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. This work is a Review of the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization was performed in the framework of the COST European Action ``Cosmology and Astrophysics Network for Theoretical Advances and Training Actions''.

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arXiv2021

On the Lichnerowicz operator in traversable wormhole spacetimes

The evaluation of Casimir energies in curved background spacetimes is an essential ingredient to study the stability of traversable wormholes. In practice one has to calculate the contribution of the transverse-traceless component of the metric perturbation on a curved spacetime background. This implies the study of an eigenvalue equation involving a modified form of the Lichnerowicz operator. For arbitrary background spacetimes, however, such an operator does not display transverse-traceless properties, a fact that impedes the determination of the eigenvalues. Against this background, we show that the problem can be circumvented. Casimir energies can be calculated by gauging the original form of the modified Lichnerowicz operator into a transverse-traceless one.

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arXiv2020

Generalized Absurdly Benign Traversable Wormholes powered by Casimir Energy

In this work, we explore the connection between Casimir energy and an Absurdly Benign Traversable Wormhole, which in the literature has been considered only in the pioneering paper of Morris and Thorne. To have consistency with the Casimir source, we need to generalize the idea of an Absurdly Benign Traversable Wormhole into a Generalized Absurdly Benign Traversable Wormhole. With this generalization, we have found that the wormhole throat is not more Planckian, but huge. Three profiles have been studied: one of them is directly connected with the Casimir source, while the other two have been obtained approximating the first one close to the throat. In all profiles the wormhole throat size is predicted to be of the order of $10^{17}m$. This huge size can be fine tuned by modulating the original Casimir energy source size. We have also found that the traceless and divergenceless property of the original Casimir stress energy tensor is here partially reproduced.

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arXiv2019

Black Hole Thermodynamics and Gravity's Rainbow

We consider the effects of rotations on the calculation of some thermodynamical quantities like the free energy, internal energy and entropy. In ordinary gravity, when we evaluate the density of states of a scalar field close to a black hole horizon, we obtain a divergent result which can be kept under control with the help of some standard regularization and renormalization processes. We show that when we use the Gravity's Rainbow approach such regularization/renormalization processes can be avoided. A comparison between the calculation done in an inertial frame and in a comoving frame is presented.

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