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Degrees of Freedom in New General Relativity: An Analysis of Type 4, Type 7, and Type 9

Q: How do the findings of this paper impact the broader search for modified theories of gravity that can address outstanding cosmological questions?

This paper significantly contributes to the search for viable modified theories of gravity, particularly within the framework of New General Relativity (NGR). By meticulously analyzing the degrees of freedom (DoF) in less-studied NGR types (Type 4, 7, and 9), the authors provide crucial insights that refine the direction of future research. Here's how the findings impact the broader search: Narrowing down viable candidates: The paper concludes that these specific NGR types are unsuitable for describing gravity due to issues like over-constraint (Type 7) or the absence of propagating gravitational modes (Types 4 and 9). This elimination is crucial as it allows researchers to focus on more promising avenues within NGR and other modified gravity theories. Highlighting limitations of linear perturbation theory: The paper underscores that the identified pathologies, such as strong coupling in some NGR types, might stem from the limitations of linear perturbation theory around the Minkowski background. This encourages exploring alternative background spacetimes or non-linear approaches to uncover potentially viable models. Emphasizing the importance of constraint structure: The detailed analysis of constraint structures in different NGR types emphasizes its critical role in determining a theory's viability. The presence of first-class constraints, particularly those forming a closed algebra, is crucial for local symmetries and the existence of physical propagating modes, including those of gravity. In essence, while the paper rules out certain NGR types for describing gravity, it provides valuable lessons and clarifies the criteria for viable modified gravity theories. This knowledge helps streamline the search for models that can effectively address cosmological puzzles like dark energy, dark matter, and inflation.

Q: Could the analyzed NGR types, despite being unsuitable for describing gravity, still hold relevance in other areas of physics, such as condensed matter physics, where emergent gravity scenarios are explored?

Yes, absolutely. Even though these specific NGR types might not adequately describe gravitational phenomena, their unique properties could be valuable in other areas of physics, particularly in condensed matter systems exhibiting emergent gravity. Here's why these NGR types could be relevant: Effective field theories: NGR types with scalar or vector degrees of freedom could serve as effective field theories describing the collective behavior of condensed matter systems. For instance, the scalar degree of freedom in Type 9 NGR might model density fluctuations or order parameters in specific materials. Analog gravity models: Condensed matter systems can sometimes mimic aspects of gravity, creating "analog gravity" models. The analyzed NGR types, with their specific constraint structures and DoF, could provide new theoretical frameworks for exploring these analogies. For example, the local SO(3) symmetry achievable in Type 7 NGR through gauge fixing might find applications in systems with rotational symmetries. Exploring new theoretical avenues: Even if not directly applicable, studying these NGR types can inspire new theoretical tools and concepts. The semi-first-class constraints encountered in Type 8 NGR, for example, might offer insights into systems with partially broken symmetries, a common theme in condensed matter physics. Therefore, while unsuitable for gravitational physics, these NGR types offer a rich playground for theoretical exploration in condensed matter physics and potentially other areas where emergent phenomena and effective field theories are crucial.

Q: If the limitations of ADM-foliation are considered, could the constraint structure and degrees of freedom in these NGR types change, potentially altering the conclusions drawn in this paper?

Yes, acknowledging the limitations of ADM-foliation, particularly in theories like NGR that break local Lorentz symmetry, could potentially impact the conclusions about constraint structure and DoF. Here's how the limitations of ADM-foliation could matter: Observer dependence of foliation: As highlighted in the paper, the work of [46] suggests that in theories without local Lorentz symmetry, the existence of a global foliation might depend on the observer's vorticity. This observer dependence could lead to different observers identifying different constraint structures and DoF, potentially altering the conclusions drawn from a specific foliation choice. Ambiguities in constraint analysis: The ADM formalism relies on splitting spacetime into spatial slices, and this slicing becomes ambiguous in the absence of a preferred time direction, as is the case when Lorentz symmetry is broken. This ambiguity could propagate into the constraint analysis, leading to different interpretations of constraint classes and ultimately affecting the DoF count. Need for alternative approaches: The paper rightly points out the potential of using differential forms or other covariant approaches to circumvent the limitations of ADM-foliation. These alternative methods could provide a more observer-independent picture of the constraint structure and offer a more robust determination of the true DoF. Therefore, while the paper's analysis within the ADM framework is valuable, further investigation using covariant methods or carefully considering the observer dependence of foliation is crucial. Such investigations might reveal subtle differences in the constraint structure and DoF, potentially changing the conclusions about the suitability of these NGR types for describing gravity or other physical phenomena.

Conceitos Básicos

This paper investigates the degrees of freedom in three less-studied types of New General Relativity (NGR), a modified theory of gravity, finding them unsuitable for describing gravity due to over-constraint or lack of necessary propagating modes.

Resumo

Bibliographic Information:

Tomonari, K. (2024). Degrees of Freedom of New General Relativity 2: Type 4, Type 7, and Type 9. arXiv:2411.11118v1 [gr-qc] 17 Nov 2024

Research Objective:

This paper aims to determine the degrees of freedom (DoF) in Type 4, Type 7, and Type 9 of New General Relativity (NGR) using the Dirac-Bergmann analysis, a method for analyzing constrained Hamiltonian systems. This analysis is crucial for understanding the viability of these NGR types in describing gravity.

Methodology:

The author employs the Dirac-Bergmann analysis to investigate the constraint structure of each NGR type. This involves identifying primary and secondary constraints, classifying them as first-class or second-class, and determining if any bifurcations arise from specific conditions on Lagrange multipliers.

Key Findings:

Type 4: This type has five degrees of freedom and exhibits only second-class constraints.
Type 7: This type has null degrees of freedom due to over-constraint, rendering it unsuitable for describing gravity.
Type 9: This type has three degrees of freedom and, like Type 4, only exhibits second-class constraints.
None of the analyzed types exhibit bifurcations in their constraint structure, unlike Type 8, which was analyzed in a previous work.

Main Conclusions:

The author concludes that Type 4, Type 7, and Type 9 of NGR are not suitable for describing gravity. This is because Type 7 is over-constrained, while Type 4 and Type 9 lack the necessary first-class constraints that would indicate the presence of local symmetries and physically propagating modes, including gravitational waves.

Significance:

This research contributes to the understanding of modified theories of gravity by systematically analyzing the degrees of freedom in different types of NGR. The findings help narrow down the potential candidates within NGR that could provide a viable alternative to General Relativity.

Limitations and Future Research:

The analysis assumes the validity of ADM-foliation, a method for decomposing spacetime, which has recently been questioned for certain theories like NGR. Future research could explore the implications of these concerns for the presented results. Additionally, investigating alternative approaches to analyzing these NGR types, such as using differential forms, could provide further insights.

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Degrees of Freedom of New General Relativity 2: Type 4, Type 7, and Type 9

by Kyosuke Tomo... às arxiv.org 11-19-2024

https://arxiv.org/pdf/2411.11118.pdf

Degrees of Freedom of New General Relativity 2: Type 4, Type 7, and Type 9

Perguntas Mais Profundas

How do the findings of this paper impact the broader search for modified theories of gravity that can address outstanding cosmological questions?

This paper significantly contributes to the search for viable modified theories of gravity, particularly within the framework of New General Relativity (NGR). By meticulously analyzing the degrees of freedom (DoF) in less-studied NGR types (Type 4, 7, and 9), the authors provide crucial insights that refine the direction of future research.
Here's how the findings impact the broader search:

Narrowing down viable candidates: The paper concludes that these specific NGR types are unsuitable for describing gravity due to issues like over-constraint (Type 7) or the absence of propagating gravitational modes (Types 4 and 9). This elimination is crucial as it allows researchers to focus on more promising avenues within NGR and other modified gravity theories.
Highlighting limitations of linear perturbation theory: The paper underscores that the identified pathologies, such as strong coupling in some NGR types, might stem from the limitations of linear perturbation theory around the Minkowski background. This encourages exploring alternative background spacetimes or non-linear approaches to uncover potentially viable models.
Emphasizing the importance of constraint structure: The detailed analysis of constraint structures in different NGR types emphasizes its critical role in determining a theory's viability.  The presence of first-class constraints, particularly those forming a closed algebra, is crucial for local symmetries and the existence of physical propagating modes, including those of gravity.
In essence, while the paper rules out certain NGR types for describing gravity, it provides valuable lessons and clarifies the criteria for viable modified gravity theories. This knowledge helps streamline the search for models that can effectively address cosmological puzzles like dark energy, dark matter, and inflation.

Could the analyzed NGR types, despite being unsuitable for describing gravity, still hold relevance in other areas of physics, such as condensed matter physics, where emergent gravity scenarios are explored?

Yes, absolutely. Even though these specific NGR types might not adequately describe gravitational phenomena, their unique properties could be valuable in other areas of physics, particularly in condensed matter systems exhibiting emergent gravity.
Here's why these NGR types could be relevant:

Effective field theories: NGR types with scalar or vector degrees of freedom could serve as effective field theories describing the collective behavior of condensed matter systems. For instance, the scalar degree of freedom in Type 9 NGR might model density fluctuations or order parameters in specific materials.
Analog gravity models:  Condensed matter systems can sometimes mimic aspects of gravity, creating "analog gravity" models. The analyzed NGR types, with their specific constraint structures and DoF, could provide new theoretical frameworks for exploring these analogies. For example, the local SO(3) symmetry achievable in Type 7 NGR through gauge fixing might find applications in systems with rotational symmetries.
Exploring new theoretical avenues:  Even if not directly applicable, studying these NGR types can inspire new theoretical tools and concepts. The semi-first-class constraints encountered in Type 8 NGR, for example, might offer insights into systems with partially broken symmetries, a common theme in condensed matter physics.
Therefore, while unsuitable for gravitational physics, these NGR types offer a rich playground for theoretical exploration in condensed matter physics and potentially other areas where emergent phenomena and effective field theories are crucial.

If the limitations of ADM-foliation are considered, could the constraint structure and degrees of freedom in these NGR types change, potentially altering the conclusions drawn in this paper?

Yes, acknowledging the limitations of ADM-foliation, particularly in theories like NGR that break local Lorentz symmetry, could potentially impact the conclusions about constraint structure and DoF.
Here's how the limitations of ADM-foliation could matter:

Observer dependence of foliation: As highlighted in the paper, the work of [46] suggests that in theories without local Lorentz symmetry, the existence of a global foliation might depend on the observer's vorticity. This observer dependence could lead to different observers identifying different constraint structures and DoF, potentially altering the conclusions drawn from a specific foliation choice.
Ambiguities in constraint analysis: The ADM formalism relies on splitting spacetime into spatial slices, and this slicing becomes ambiguous in the absence of a preferred time direction, as is the case when Lorentz symmetry is broken. This ambiguity could propagate into the constraint analysis, leading to different interpretations of constraint classes and ultimately affecting the DoF count.
Need for alternative approaches: The paper rightly points out the potential of using differential forms or other covariant approaches to circumvent the limitations of ADM-foliation. These alternative methods could provide a more observer-independent picture of the constraint structure and offer a more robust determination of the true DoF.
Therefore, while the paper's analysis within the ADM framework is valuable, further investigation using covariant methods or carefully considering the observer dependence of foliation is crucial. Such investigations might reveal subtle differences in the constraint structure and DoF, potentially changing the conclusions about the suitability of these NGR types for describing gravity or other physical phenomena.