arXiv subjects

Piero Nicolini

Publications and source records attributed to Piero Nicolini.

At least 19 recordsLinked to original sources

arXiv2025

T-Duality Effects in Electrodynamics: The (2+1)-dimensional Case

We investigate the interplay between T-duality and (2+1)- dimensional electrodynamics, revealing a relationship between short and large length scales of the gauge potential. Our findings demonstrate that the electrostatic potential energy between static charges is no longer divergent at short distances in the presence of T-duality effects. It remains logarithmic at large distances, suggesting the possibility of a regulatory role for the T-duality scale $ l_0 $ in the space where the radial coordinate goes into its inverse. We also discuss the potential of T-duality to elucidate fractalization effects in physical systems, paving the way for future research on the implications for superconductors and condensed matter systems in general.

hep-th↗

arXiv2025

Stellar Objects From Quantum Gravity

This paper explores the theoretical implications of quantum gravity by analyzing compact stellar objects, presenting three distinct models that serve as alternatives to traditional black holes. These models are characterized by their extreme compactness and incorporation of a quantum core, successfully avoiding the curvature singularities typically associated with classical general relativity. Central to these models is the noncommutative parameter, which plays a crucial role in determining stellar properties and enables the exploration of various astrophysical regimes. While pure Planckian effects pose significant challenges for observational detection, our findings suggest that lower energy scales may reveal exotic stellar objects with Earth and Sun-like masses that lack classical counterparts, potentially providing the first experimental evidence of non-classical gravity. We propose that the metrics derived in this study can be tested against known neutron stars, offering promising avenues for future research aimed at understanding the interplay between quantum effects in gravity and stellar evolution, ultimately enhancing our comprehension of the universe's fundamental properties.

gr-qc↗

arXiv2025

Quantum gravity black holes as dark matter?

One of the major problems in quantum gravity research is the lack of signals at the reach of present or near-future experimental facilities. In this paper, we show that this is not the case. Contrary to previous claims, the quantum decay of de Sitter space into black hole spacetimes can be significant even after inflation and can be observed on galactic scales. Using the instanton formalism within the no-boundary proposal for a class of short-scale, quantum-gravity-improved black holes, we show that de Sitter space decay would result in the production of $10^{60}$ stable Planck-size black hole remnants within the current Hubble horizon, which is the number required to explain dark matter.

gr-qc↗

arXiv2024

Geodesic completeness from string T-duality

By studying the Raychaudhuri equation for the gravitational force resulting from a string T-duality modified propagator, we present an analysis of the geodesic compression beyond the conventional classical limit. The result is that gravity on short length scales is subject to a screening effect similar to the Debye screening in electrostatics, which prevents the formation of curvature singularities. Using model-independent arguments, we conclude that the conventional attractive nature of gravity is only a low-energy effect.

hep-th↗

arXiv2024

Reconciling microscopic and macroscopic tests of the Compton-Schwarzschild correspondence

We review the experimental constraints on the parameter $\alpha$ associated with the Generalized Uncertainty Principle (GUP) and the parameter $\beta$ associated with the Generalized Event Horizon (GEH). The Compton-Schwarzschild correspondence implies a relationship between $\alpha$ and $\beta$, with both parameters being of order 1. This presents a problem for our previous `$M+1/M$' model since the extra gravitational force at sub-Planckian masses contravenes observations. Various resolutions of this problem are discussed.

gr-qc↗

arXiv2023

Minimal Lengths in 3D via the Generalized Uncertainty Principle

We investigate an extension of the Generalized Uncertainty Principle (GUP) in three dimensions by modifying the three dimensional position and momentum operators in a manner that remains coordinate-independent and retains as much of the standard position-momentum commutators as possible. Moreover, we bound the physical momentum which leads to an effective minimal length in every coordinate direction. The physical consequences of these modified operators are explored in two scenarios: (i) when a spherically-symmetric wave function is `compressed' into the smallest possible volume; (ii) when the momentum is directed in a single direction. In case (ii), we find that the three dimensional GUP exhibits interesting phenomena that do not occur in one dimension: the minimal distance in the direction parallel to a particle's momentum is different from the minimal distance in the orthogonal directions.

quant-ph↗

arXiv2023

How strings can explain regular black holes

This paper reviews the role of black holes in the context of fundamental physics. After recalling some basic results stemming from Planckian string calculations, I present three examples of how stringy effects can improve the curvature singularity of classical black hole geometries.

gr-qc↗

arXiv2022

Charged black holes from T-duality

In this paper, we present a family of regular black hole solutions in the presence of charge and angular momentum. We also discuss the related thermodynamics and we comment about the black hole life cycle during the balding and spin down phases. Interestingly the static solution resembles the Ayón-Beato--García spacetime, provided the T-duality scale is redefined in terms of the electric charge, $l_0\to Q$. The key factor at the basis of our derivation is the employment of Padmanabhan's propagator to calculate static potentials. Such a propagator encodes string T-duality effects. This means that the regularity of the spacetimes here presented can open a new window on string theory phenomenology.

hep-th↗

arXiv2022

Quantum gravity and the zero point length

In this paper, we present an overview of some of the existing issues of the research in quantum gravity. We also introduce the basic ideas that led Padmanabhan to consider a duality property in path integrals. Such a duality is consistent with the T-duality in string theory. More importantly, the path integral duality discloses a universal feature of any quantum geometry, namely the existence of a zero point length $L_0$. We also comment about recent developments aiming to expose effects of the zero point length in strong electrodynamics and black holes. There are reasons to believe that the main characters of the phenomenology of quantum gravity may be described by means of a single parameter like $L_0$.

hep-th↗

arXiv2022

Finite electrodynamics from T-duality

In this paper, we present the repercussions of Padmanabhan's propagator in electrodynamics. This corresponds to implement T-duality effects in a U(1) gauge theory. By formulating a nonlocal action consistent with the above hypothesis, we derive the profile of static potentials between electric charges via a path integral approach. Interestingly, the Coulomb potential results regularized by a length scale proportional to the parameter $(α^\prime)^{1/2}$. Accordingly, fields are vanishing at the origin. We also discuss an array of experimental testbeds to expose the above results. It is interesting to observe that T-duality generates an effect of dimensional fractalization, that resembles similar phenomena in fractional electromagnetism. Finally, our results have also been derived with a gauge-invariant method, as a necessary check of consistency for any non-Maxwellian theory.

hep-th↗

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.

gr-qc↗

arXiv2021

Regularization ambiguity and van der Waals black hole in 2+1 dimensions

Charged black holes in a ($2+1$)-dimensional anti-de Sitter space-time suffer from some limitations such as the ambiguity in the definition of the mass and the bad short distance behavior. In this paper we present a way to resolve such issues. By extending the parameter space of the BTZ geometry, we properly identify the integration constants in order to remove the conical singularity sitting at the origin. In such a way we obtain a well defined Minkowski limit and horizons also in the case of de Sitter background space. On the thermodynamic side, we obtain a proper internal energy, by invoking the consistency with the Area Law, even if the mass parameter does not appear in the metric coefficients. As a further improvement, we show that it is sufficient to assume a finite size of the electric charge to obtain a short scale regular geometry. The resulting solution, generalizing the charged BTZ metric, is dual to a van der Waals gas.

gr-qc↗

arXiv2020

Self-complete and GUP-Modified Charged and Spinning Black Holes

We explore some implications of our previous proposal, motivated in part by the Generalised Uncertainty Principle (GUP) and the possibility that black holes have quantum mechanical hair that the ADM mass of a system has the form $M + βM_\mathrm{Pl}^2/(2M)$, where $M$ is the bare mass, $M_\mathrm{Pl}$ is the Planck mass and $β$ is a positive constant. This also suggests some connection between black holes and elementary particles and supports the suggestion that gravity is self-complete. We extend our model to charged and rotating black holes, since this is clearly relevant to elementary particles. The standard Reissner-Nordström and Kerr solutions include zero-temperature states, representing the smallest possible black holes, and already exhibit features of the GUP-modified Schwarzschild solution. However, interesting new features arise if the charged and rotating solutions are themselves GUP-modified. In particular, there is an interesting transition below some value of $β$ from the GUP solutions (spanning both super-Planckian and sub-Planckian regimes) to separated super-Planckian and sub-Planckian solutions. Equivalently, for a given value of $β$, there is a critical value of the charge and spin above which the solutions bifurcate into sub-Planckian and super-Planckian phases, separated by a mass gap in which no black holes can form.

gr-qc↗

arXiv2020

Primordial black holes in a dimensionally oxidizing Universe

The spontaneous creation of primordial black holes in a violently expanding Universe is a well studied phenomenon. Based on quantum gravity arguments, it has been conjectured that the early Universe might have undergone a lower dimensional phase before relaxing to the current $(3 + 1)$ dimensional state. In this article we combine the above phenomena: we calculate the pair creation rates of black holes nucleated in an expanding Universe, by assuming a dimensional evolution, we term ``oxidation'', from $(1 + 1)$ to $(2 + 1)$ and finally to $(3 + 1)$ dimensions. Our investigation is based on the no boundary proposal that allows for the construction of the required gravitational instantons. If, on the one hand, the existence of a dilaton non-minimally coupled to the metric is necessary for black holes to exist in the $(1 + 1)$ phase, it becomes, on the other hand, trivial in $(2 + 1)$ dimensions. Nevertheless, the dilaton might survive the oxidation and be incorporated in a modified theory of gravity in $(3 + 1)$ dimensions: by assuming that our Universe, in its current state, originates from a lower-dimensional oxidation, one might be led to consider the pair creation rate in a sub-class of the Horndeski action. Our findings for this case show that, for specific values of the Galileon coupling to the metric, the rate can be unsuppressed. This would imply the possibility of compelling parameter bounds for non-Einstein theories of gravity by using the spontaneous black hole creation.

gr-qc↗

arXiv2019

Quantum Corrected Black Holes from String T-Duality

In this paper we present some stringy corrections to black hole spacetimes emerging from string T-duality. As a first step, we derive the static Newtonian potential by exploiting the relation between the T-duality and the path integral duality. We show that the intrinsic non-perturbative nature of stringy corrections introduce an ultraviolet cutoff known as zero-point length in the path integral duality literature. As a result, the static potential is found to be regular. We use this result to derive a consistent black hole metric for the spherically symmetric, electrically neutral case. It turns out that the new spacetime is regular and is formally equivalent to the Bardeen metric, apart from a different ultraviolet regulator. On the thermodynamics side, the Hawking temperature admits a maximum before a cooling down phase towards a thermodynamically stable end of the black hole evaporation process. The findings support the idea of universality of quantum black holes.

gr-qc↗

arXiv2019

Generalized Uncertainty Principle and Black Holes in Higher Dimensional Self-Complete Gravity

In this paper we consider generalized uncertainty principle (GUP) effects in higher dimensional black hole spacetimes via a nonlocal gravity approach. We study three possible modifications of momentum space measure emerging from GUP, including the original Kempf-Mangano-Mann (KMM) proposal. By following the KMM model we derive a family of black hole spacetimes. The case of five spacetime dimensions is a special one. We found an exact black hole solution with a Barriola-Vilenkin monopole at the origin. This object turns out to be the end point of the black hole evaporation. Interestingly for smaller masses, we found a "naked monopole" rather than a generic naked singularity. We also show that the Carr-Lake-Casadio-Scardigli proposal leads to mild modifications of spacetime metrics with respect to the Schwarzschild-Tangherlini solution. Finally, by demanding the same degree of convergence in the ultraviolet regime for any spacetime dimension, we derive a family of black hole solutions that fulfill the gravity self-completeness paradigm. The evaporation of such black holes is characterized by a fluctuating luminosity, which we dub a lighthouse effect.

gr-qc↗

arXiv2018

Primordial black holes in a dimensionally reduced universe

We investigate the spontaneous creation of primordial black holes in a lower-dimensional expanding early universe. We use the no-boundary proposal to construct instanton solutions for both the background and a black hole nucleated inside this background. The resulting creation rate could lead to a significant population of primordial black holes during the lower dimensional phase. We also consider the subsequent evaporation of these dimensionally reduced black holes and find that their temperature increases with mass, whereas it decreases with mass for 4-dimensional black holes. This means that they could leave stable sub-Planckian relics, which might in principle provide the dark matter.

gr-qc↗

arXiv2018

Superradiance in Modified Gravity (MOG)

We consider the case of rotating black holes in a dark-matter-emulating theory of gravity called MOG. The latter introduces a gravitational vector field with an associated gravitational charge proportional to the black hole mass and a scalar field in place of the gravitational constant. The resulting black hole metrics resemble the Kerr-Newman geometry and enjoy superradiant scattering. MOG, however, presents important new features. By studying the scattering of a scalar field, we show that there is a marked reduction of the critical frequency of mode amplification. This corresponds to saying that the superradiance peak frequency is red shifted. Analyses of the reflected energy flux also show that MOG black holes are fainter with respect to the standard ones. The proposed results pave the way for testing MOG against astronomical observations.

gr-qc↗