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14 pages, 616 KiB

Open AccessArticle

Gravitational Lensing Effects from Models of Loop Quantum Gravity with Rigorous Quantum Parameters

by Haida Li and Xiangdong Zhang

Universe 2024, 10(11), 421; https://doi.org/10.3390/universe10110421 - 8 Nov 2024

Viewed by 438

Abstract

Many previous works have studied gravitational lensing effects from Loop Quantum Gravity. So far, gravitational lensing effects from Loop Quantum Gravity have only been studied by choosing large quantum parameters much larger than the Planck scale. However, by construction, the quantum parameters of [...] Read more.

Many previous works have studied gravitational lensing effects from Loop Quantum Gravity. So far, gravitational lensing effects from Loop Quantum Gravity have only been studied by choosing large quantum parameters much larger than the Planck scale. However, by construction, the quantum parameters of the effective models of Loop Quantum Gravity are usually related to the Planck length and, thus, are extremely small. In this work, by strictly imposing the quantum parameters as initially constructed, we study the true quantum corrections of gravitational lensing effects by five effective black hole models of Loop Quantum Gravity. Our study reveals several interesting results, including the different scales of quantum corrections displayed by each model and the connection between the quantum correction of deflection angles and the quantum correction of the metric. Observables related to the gravitational lensing effect are also obtained for all models in the case of SgrA* and M87*. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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23 pages, 849 KiB

Open AccessArticle

Revisiting Quantum Field Theory in Rindler Spacetime with Superselection Rules

by K. Sravan Kumar and João Marto

Universe 2024, 10(8), 320; https://doi.org/10.3390/universe10080320 - 8 Aug 2024

Cited by 4 | Viewed by 757

Abstract

Quantum field theory (QFT) in Rindler spacetime is a gateway to understanding unitarity and information loss paradoxes in curved spacetime. Rindler coordinates map Minkowski spacetime onto regions with horizons, effectively dividing accelerated observers into causally disconnected sectors. Employing standard quantum field theory techniques [...] Read more.

Quantum field theory (QFT) in Rindler spacetime is a gateway to understanding unitarity and information loss paradoxes in curved spacetime. Rindler coordinates map Minkowski spacetime onto regions with horizons, effectively dividing accelerated observers into causally disconnected sectors. Employing standard quantum field theory techniques and Bogoliubov transformations between Minkowski and Rindler coordinates yields entanglement between states across these causally separated regions of spacetime. This results in a breakdown of unitarity, implying that information regarding the entangled partner may be irretrievably lost beyond the Rindler horizon. As a consequence, one has a situation of pure states evolving into mixed states. In this paper, we introduce a novel framework for comprehending this phenomenon using a recently proposed formulation of direct-sum quantum field theory (DQFT), which is grounded in superselection rules formulated by the parity and time reversal (

P T

) symmetry of Minkowski spacetime. In the context of DQFT applied to Rindler spacetime, we demonstrate that each Rindler observer can, in principle, access pure states within the horizon, thereby restoring unitarity. However, our analysis also reveals the emergence of a thermal spectrum of Unruh radiation. This prompts a reevaluation of entanglement in Rindler spacetime, where we propose a novel perspective on how Rindler observers may reconstruct complementary information beyond the horizon. Furthermore, we revisit the implications of the Reeh-Schlieder theorem within the framework of DQFT. Lastly, we underscore how our findings contribute to ongoing efforts aimed at elucidating the role of unitarity in quantum field theory within the context of de Sitter and black hole spacetimes. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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25 pages, 622 KiB

Open AccessArticle

On Minimal Entanglement Wedge Cross Section for Holographic Entanglement Negativity

by Jaydeep Kumar Basak, Vinay Malvimat, Himanshu Parihar, Boudhayan Paul and Gautam Sengupta

Universe 2024, 10(3), 125; https://doi.org/10.3390/universe10030125 - 5 Mar 2024

Cited by 32 | Viewed by 1292

Abstract

We demonstrate the equivalence of two different conjectures in the literature for the holographic entanglement negativity in AdS₃/CFT₂, modulo certain constants. These proposals involve certain algebraic sums of bulk geodesics homologous to specific combinations of subsystems, and the entanglement [...] Read more.

We demonstrate the equivalence of two different conjectures in the literature for the holographic entanglement negativity in AdS₃/CFT₂, modulo certain constants. These proposals involve certain algebraic sums of bulk geodesics homologous to specific combinations of subsystems, and the entanglement wedge cross section (EWCS) backreacted by a cosmic brane for the conical defect geometry in the bulk gravitational path integral. It is observed that the former conjectures reproduce the field theory replica technique results in the large central charge limit whereas the latter involves constants related to the Markov gap. In this context, we establish an alternative construction for the EWCS of a single interval in a CFT₂ at a finite temperature to resolve an issue for the latter proposal involving thermal entropy elimination for holographic entanglement negativity. Our construction for the EWCS correctly reproduces the corresponding field theory results modulo the Markov gap constant in the large central charge limit. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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18 pages, 432 KiB

Open AccessArticle

Vacuum Energy, the Casimir Effect, and Newton’s Non-Constant

by Benjamin Koch, Christian Käding, Mario Pitschmann and René I. P. Sedmik

Universe 2023, 9(11), 476; https://doi.org/10.3390/universe9110476 - 8 Nov 2023

Cited by 1 | Viewed by 1690

Abstract

The idea of quantum mechanical vacuum energy contributing to the cosmological vacuum energy density is not new. However, despite the persisting cosmological constant problem, few investigations have focused on this subject. We explore the possibility that the quantum vacuum energy density contributes to [...] Read more.

The idea of quantum mechanical vacuum energy contributing to the cosmological vacuum energy density is not new. However, despite the persisting cosmological constant problem, few investigations have focused on this subject. We explore the possibility that the quantum vacuum energy density contributes to the (local) gravitational energy density in the framework of a scale-dependent cosmological constant

Λ

and Newton’s constant G. This hypothesis has several important consequences, ranging from quantum scale-dependence to the hypothetical prospect of novel experimental insight concerning the quantum origin of cosmological energy density. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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29 pages, 20294 KiB

Open AccessArticle

Quantum Black Holes in Conformal Dilaton–Higgs Gravity on Warped Spacetimes

by Reinoud Jan Slagter

Universe 2023, 9(9), 383; https://doi.org/10.3390/universe9090383 - 26 Aug 2023

Viewed by 1752

Abstract

A promising method for understanding the geometric properties of a spacetime in the vicinity of the horizon of a Kerr-like black hole can be developed by applying the antipodal boundary condition on the two opposite regions in the extended Penrose diagram. By considering [...] Read more.

A promising method for understanding the geometric properties of a spacetime in the vicinity of the horizon of a Kerr-like black hole can be developed by applying the antipodal boundary condition on the two opposite regions in the extended Penrose diagram. By considering a conformally invariant Lagrangian on a Randall–Sundrum warped five-dimensional spacetime, an exact vacuum solution is found, which can be interpreted as an instanton solution on the Riemannian counterpart spacetime,

R_{+}^{2} \times R^{1} \times S^{1}

, where

R_{+}^{2}

is conformally flat. The antipodal identification, which comes with a CPT inversion, is par excellence, suitable when quantum mechanical effects, such as the evaporation of a black hole by Hawking radiation, are studied. Moreover, the black hole paradoxes could be solved. By applying the non-orientable Klein surface, embedded in

R^{4}

, there is no need for instantaneous transport of information. Further, the gravitons become “hard” in the bulk, which means that the gravitational backreaction on the brane can be treated without the need for a firewall. By splitting the metric in a product

ω^{2} {\tilde{g}}_{μ ν}

, where

ω

represents a dilaton field and

{\tilde{g}}_{μ ν}

the conformally flat “un-physical” spacetime, one can better construct an effective Lagrangian in a quantum mechanical setting when one approaches the small-scale area. When a scalar field is included in the Lagrangian, a numerical solution is presented, where the interaction between

ω

and

Φ

is manifest. An estimate of the extra dimension could be obtained by measuring the elapsed traversal time of the Hawking particles on the Klein surface in the extra dimension. Close to the Planck scale, both

ω

and

Φ

can be treated as ordinary quantum fields. From the dilaton field equation, we obtain a mass term for the potential term in the Lagrangian, dependent on the size of the extra dimension. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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18 pages, 409 KiB

Open AccessArticle

Newtonian Cosmology and Evolution of κ-Deformed Universe

by E. Harikumar, Harsha Sreekumar and Suman Kumar Panja

Universe 2023, 9(7), 343; https://doi.org/10.3390/universe9070343 - 24 Jul 2023

Cited by 3 | Viewed by 1091

Abstract

Considering space-time to be non-commutative, we study the evolution of the universe employing the approach of Newtonian cosmology. Generalizing the conservation of energy and the first law of thermodynamics to

κ

-deformed space-time, we derive the modified Friedmann equations, valid up to the [...] Read more.

Considering space-time to be non-commutative, we study the evolution of the universe employing the approach of Newtonian cosmology. Generalizing the conservation of energy and the first law of thermodynamics to

κ

-deformed space-time, we derive the modified Friedmann equations, valid up to the first order, in the deformation parameter. Analyzing these deformed equations, we derive the time evolution of the scale factor in cases of radiation-dominated, matter-dominated, and vacuum (energy)-dominated universes. We show that the rate of change of the scale factor in all three situations is modified by the non-commutativity of space-time, and this rate depends on the sign of the deformation parameter, indicating a possible explanation for the observed Hubble tension. We undertake this investigation for two different realizations of non-commutative space-time coordinates. In both cases, we also argue for the existence of bounce in the evolution of the universe. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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16 pages, 512 KiB

Open AccessArticle

Fermionic Greybody Factors in Schwarzschild Acoustic Black Holes

by Sara Kanzi and İzzet Sakallı

Universe 2023, 9(2), 108; https://doi.org/10.3390/universe9020108 - 19 Feb 2023

Cited by 3 | Viewed by 1381

Abstract

In Schwarzschild acoustic black hole (SABH) spacetime, we investigate the wave dynamics for the fermions. To this end, we first take into account the Dirac equation in the SABH by employing a null tetrad in the Newman–Penrose (NP) formalism. Then, we consider the [...] Read more.

In Schwarzschild acoustic black hole (SABH) spacetime, we investigate the wave dynamics for the fermions. To this end, we first take into account the Dirac equation in the SABH by employing a null tetrad in the Newman–Penrose (NP) formalism. Then, we consider the Dirac and Rarita–Schwinger equations, respectively. The field equations are reduced to sets of radial and angular equations. By using the analytical solution of the angular equation set, we decouple the radial wave equations and obtain the one-dimensional Schrödinger-like wave equations with their effective potentials. The obtained effective potentials are graphically depicted and analyzed. Finally, we investigate the fermionic greybody factors (GFs) radiated by the SABH spacetime. A thorough investigation is conducted into how the acoustic tuning parameter affects the GFs of the SABH spacetime. Both the semi-analytic WKB method and bounds for the GFs are used to produce the results, which are shown graphically and discussed. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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Review

Jump to: Research

16 pages, 347 KiB

Open AccessReview

Quantum Imprints on CMBR

by Shreya Banerjee

Universe 2023, 9(9), 405; https://doi.org/10.3390/universe9090405 - 4 Sep 2023

Viewed by 1295

Abstract

Quantum cosmology aims to develop a quantum theory of the universe, attempting to answer open questions of physical cosmology, mainly related to the early epochs of the universe. Such a theory aims to unite relativity theory and quantum theory. Here, the whole universe [...] Read more.

Quantum cosmology aims to develop a quantum theory of the universe, attempting to answer open questions of physical cosmology, mainly related to the early epochs of the universe. Such a theory aims to unite relativity theory and quantum theory. Here, the whole universe is treated as a quantum mechanical system and is described by a wave function rather than by a classical spacetime. In this review, I shall describe the mathematical structure and primary formulations that form the backbone of quantum cosmology. We know that over a period of time, several approaches were developed to form a quantum theory of gravity. However, in order to decide which approach is the best, we need testable predictions, effects that can be observed in cosmic microwave background radiation (CMBR). I shall discuss the methodologies for generating quantum gravitational corrections to inflationary background leading to testable predictions. Another aspect of finding quantum imprints on CMBR results through the application of resolution of the ‘quantum measurement problem’ to early universe physics. In this article, I shall also discuss two such promising models explaining the classicalization of inflationary perturbation and are capable of leaving distinct observational imprints on the observables. Full article

(This article belongs to the Special Issue Quantum Field Theory in Curved Spacetime and Its Implications for Cosmology, Blackholes and Quantum Gravity)

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