by jsendak | Mar 20, 2024 | GR & QC Articles
arXiv:2403.12136v1 Announce Type: new
Abstract: One of the foremost concern in the analysis of quantum gravity is whether the locations of classical horizons are stable under a full quantum analysis. In principle, any classical description, when interpolated to the microscopic level, can become prone to fluctuations. The curious question in that case is if there indeed are such fluctuations at the Planck scale, do they have any significance for physics taking place at scales much away from the Planck scale? In this work, we try to attempt the question of small scales and address whether there are definitive signatures of Planck scale shifts in the horizon structure. In a recent work (arXiv:2107.03406), it was suggested that in a nested sequence of Rindler causal wedges, the vacua of preceding Rindler frames appear thermally populated to a shifted Rindler frame. The Bogoliubov analysis used relies on the global notion of the quantum field theory and might be unable to see the local character of such horizon shifts. We investigate this system by means of the Unruh-DeWitt detector and see if this local probe of the quantum field theory is sensitive enough to the shift parameters to reveal any microscopic effects. For the case of infinite-time response, we recover the thermal spectrum, thus reaffirming that the infinite-time response probes the global properties of the field. On the other hand, the finite-time response turns out to be sensitive to the shift parameter in a peculiar way that any detector with energy gap $Omega c/a sim 1$ and is operational for time scale $T a/c sim 1$ has a measurably different response for a macroscopic and microscopic shift of the horizon, giving us direct probe to the tiniest separation between the causal domains of such Rindler wedges. Thus, this study provides an operational method to identify Planck scale effects which can be generalized to various other interesting gravitational settings.
Quantum Gravity and the Stability of Classical Horizons
In the analysis of quantum gravity, one of the key concerns is whether the locations of classical horizons remain stable under a full quantum analysis. When a classical description is extrapolated to the microscopic level, it becomes susceptible to fluctuations. Therefore, it is important to investigate whether these fluctuations at the Planck scale have any significant impact on physics at scales far removed from the Planck scale.
Searching for Signatures of Planck Scale Shifts in Horizon Structure
In a recent work (arXiv:2107.03406), it was proposed that in a nested sequence of Rindler causal wedges, the vacua of preceding Rindler frames appear thermally populated to a shifted Rindler frame. However, the analysis used in this work relies on the global notion of quantum field theory and may overlook the local character of such horizon shifts. Therefore, it is necessary to investigate this system using a local probe, such as the Unruh-DeWitt detector, to determine if it is sensitive enough to the shift parameters to reveal any microscopic effects.
Investigating the Sensitivity of the Unruh-DeWitt Detector
We conducted a study using the Unruh-DeWitt detector to examine the sensitivity of this local probe to the shift parameters of the horizon structure. The results showed that for the case of infinite-time response, the detector recovered the thermal spectrum, confirming that it probes the global properties of the field. However, the finite-time response exhibited a peculiar sensitivity to the shift parameter. We observed that any detector with an energy gap of $Omega c/a sim 1$ and operational for a time scale of $T a/c sim 1$ had a measurably different response for both macroscopic and microscopic shifts of the horizon.
Identifying Planck Scale Effects and Generalization
Our study provides a practical method to identify Planck scale effects using the Unruh-DeWitt detector. This method can be extended to various other interesting gravitational settings and allows for the detection of the tiniest separation between the causal domains of Rindler wedges. By investigating these microscopic effects, we can gain a deeper understanding of the stability and behavior of classical horizons under quantum analysis.
Future Roadmap
- Further refine the Unruh-DeWitt detector methodology for enhanced sensitivity and accuracy.
- Explore other gravitational settings and test the applicability of the method in different scenarios.
- Investigate the implications of Planck scale shifts in horizon structure for various physical phenomena.
- Collaborate with experimentalists to design and conduct experiments to validate the findings.
- Integrate the findings into the broader framework of quantum gravity and continue the quest to understand the fundamental nature of the universe.
Challenges and Opportunities
Challenges:
- Developing experimental setups that can measure the tiniest separation between causal domains.
- Overcoming technical limitations and noise in detector measurements.
- Understanding the implications of Planck scale shifts for different gravitational settings and phenomena.
Opportunities:
- Understanding the stability and behavior of classical horizons under quantum analysis.
- Gaining insights into the fundamental nature of the universe through the exploration of quantum gravity.
- Opening up new avenues for experimental verification and validation of theoretical predictions.
- Potential applications in other areas of physics beyond quantum gravity.
Overall, the study of Planck scale shifts in horizon structure and the detection of associated microscopic effects offer exciting prospects for advancing our understanding of quantum gravity and its implications for the broader framework of physics.
Read the original article
by jsendak | Feb 12, 2024 | GR & QC Articles
We propose an explicit spin-foam amplitude for Lorentzian gravity in three dimensions. The model is based on two main requirements: that it should be structurally similar to its well-known Euclidean analog, and that geometricity should be recovered in the semiclassical regime. To this end we introduce new coherent states for space-like 1-dimensional boundaries, derived from the continuous series of unitary $mathrm{SU}(1,1)$ representations. We show that the relevant objects in the amplitude can be written in terms of the defining representation of the group, just as so happens in the Euclidean case. We derive an expression for the semiclassical amplitude at large spins, showing that it relates to the Lorentzian Regge action.
Future Roadmap for Readers
Overview
In this article, we present an explicit spin-foam amplitude for Lorentzian gravity in three dimensions. Our model satisfies two important requirements: it is structurally similar to its well-known Euclidean analog, and it recovers geometricity in the semiclassical regime. We achieve this by introducing new coherent states for space-like 1-dimensional boundaries, which are derived from the continuous series of unitary $mathrm{SU}(1,1)$ representations. In addition, we demonstrate that the relevant objects in the amplitude can be expressed in terms of the defining representation of the group, just like in the Euclidean case. Lastly, we derive an expression for the semiclassical amplitude at large spins, revealing its relationship to the Lorentzian Regge action.
Roadmap
- Introduction: We provide an overview of the article, discussing the motivation behind our research and the goals we aim to achieve.
- Lorentzian Spin-Foam Amplitude: We present our explicit spin-foam amplitude for Lorentzian gravity in three dimensions. We explain how it satisfies the structural requirements and recovers geometricity in the semiclassical regime.
- New Coherent States: We introduce the new coherent states for space-like 1-dimensional boundaries. These coherent states are derived from the continuous series of unitary $mathrm{SU}(1,1)$ representations.
- Relevant Objects in the Amplitude: We demonstrate that the relevant objects in the amplitude can be expressed in terms of the defining representation of $mathrm{SU}(1,1)$, similar to the Euclidean case. This similarity allows us to maintain the structural similarity between the Lorentzian and Euclidean amplitudes.
- Semiclassical Amplitude at Large Spins: We derive an expression for the semiclassical amplitude at large spins and establish its relationship to the Lorentzian Regge action. This further validates the effectiveness of our spin-foam amplitude model.
Challenges and Opportunities
While our proposed spin-foam amplitude for Lorentzian gravity in three dimensions shows significant promise, there are challenges and opportunities that lie ahead:
- Validation and Testing: The model needs to be thoroughly tested and validated through simulations or comparisons with existing theories and experimental data. This will help ensure its accuracy and reliability.
- Extension to Higher Dimensions: Our current model is limited to three dimensions. Extending it to higher dimensions could open up new possibilities and applications in the field of gravity.
- Integration with Quantum Field Theory: Investigating the integration of our spin-foam amplitude with quantum field theory could lead to a more comprehensive understanding of the quantum nature of gravity.
- Practical Implementation: Developing practical algorithms and computational techniques for implementing the spin-foam amplitude in real-world scenarios is crucial for its practical applications in areas like cosmology, black holes, and quantum gravity.
Conclusion
Our explicit spin-foam amplitude for Lorentzian gravity in three dimensions, which satisfies structural requirements and recovers geometricity in the semiclassical regime, holds great potential for advancing our understanding of gravity at the quantum level. However, further research and development are necessary to validate the model, extend it to higher dimensions, integrate it with quantum field theory, and ensure its practical implementation. By addressing these challenges and capitalizing on the opportunities, we can make significant strides in the field of quantum gravity.
Read the original article
by jsendak | Feb 2, 2024 | GR & QC Articles
In this paper, $F(nu)$ cosmology is proposed for the accelerating universe with asymptotic de Sitter expansion in terms of Hankel function index $nu$. To some extent, both the initial expansion during early inflation and the current accelerated expansion can be studied with a vacuum cosmic fluid i.e. $Lambda$ in the pure de Sitter phase. Observational data further support the notion of a quasi-vacuum fluid, rather than a pure vacuum, contributing to the quasi-de Sitter acceleration in both the early and late universe. By examining the asymptotic expansion of the Henkel function as an approximate solution of the Mukhanov-Sasaki equation, we seek a more detailed study of quasi-de Sitter solutions in cosmology containing vacuum-like fluid.
Recent cosmological observations have provided compelling evidence for the accelerating expansion of the universe. To explain this phenomenon, a new cosmological model called $F(nu)$ cosmology has been proposed. This model incorporates the concept of asymptotic de Sitter expansion, where the universe approaches a state of steady expansion similar to the de Sitter space.
Understanding the Accelerating Expansion
In order to comprehend the accelerating expansion, it is crucial to examine both the initial expansion during early inflation and the current accelerated expansion. The $F(nu)$ cosmology suggests that a vacuum cosmic fluid, often represented by $Lambda$, plays a significant role in both phases.
Traditionally, the concept of vacuum implies the absence of any matter or energy. However, observational data indicates that a quasi-vacuum fluid, which is not purely empty but contains some residual energy, contributes to the quasi-de Sitter acceleration observed in both the early and late universe.
The Role of Hankel Function Index
In this study, we explore the role of the Hankel function index $nu$ in describing the asymptotic expansion of the universe. By examining the asymptotic expansion of the Hankel function as an approximate solution of the Mukhanov-Sasaki equation, we aim to gain a more detailed understanding of quasi-de Sitter solutions in cosmology containing vacuum-like fluids.
Roadmap for Future Research
- Further Investigation: Future research should delve deeper into the $F(nu)$ cosmology and its implications for understanding the accelerating expansion of the universe. This could involve refining the mathematical models and conducting more extensive observational studies.
- Challenges in Observation: One major challenge lies in distinguishing between a pure vacuum and a quasi-vacuum fluid. Robust observational techniques and data analysis methods need to be developed to accurately measure the properties of the cosmic fluid.
- Testing with Future Missions: The upcoming space missions, such as the James Webb Space Telescope and the Euclid mission, provide exciting opportunities to gather new data and test the $F(nu)$ cosmology. These missions can help validate the theoretical predictions and offer insight into the nature of vacuum-like fluids.
- Implications for Fundamental Physics: Understanding the nature of vacuum-like fluids and their role in cosmology can have profound implications for fundamental physics. Exploring the connection between cosmological expansion and quantum field theory may uncover new insights into the nature of space, time, and energy.
As we continue to investigate $F(nu)$ cosmology and its relation to vacuum-like fluids, we move closer to unraveling the mysteries surrounding the accelerating expansion of the universe. The challenges and opportunities on the horizon pave the way for exciting discoveries and a deeper understanding of our cosmic existence.
Disclaimer: The $F(nu)$ cosmology is still a subject of ongoing research and should be considered as a theoretical framework awaiting further empirical validation.
Read the original article
by jsendak | Jan 28, 2024 | GR & QC Articles
In this paper, we investigate the quantum dynamics of scalar and oscillator
fields in a topological defect space-time background under the influence of
rainbow gravity’s. The rainbow gravity’s are introduced into the considered
cosmological space-time geometry by replacing the temporal part $dt to
frac{dt}{mathcal{F}(chi)}$ and the spatial part $dx^i to
frac{dx^i}{mathcal{H} (chi)}$, where $mathcal{F}, mathcal{H}$ are the
rainbow functions and $chi=E/E_p$. We derived the radial equation of the
Klein-Gordon equation and its oscillator equation under rainbow gravity’s in
topological space-time. To obtain eigenvalue of the quantum systems under
investigations, we set the rainbow functions $mathcal{F}(chi)=1$ and
$mathcal{H}(chi)=sqrt{1-beta,chi^p}$, where $p=1,2$. We solve the radial
equations through special functions using these rainbow functions and analyze
the results. In fact, it is shown that the presence of cosmological constant,
the topological defect parameter $alpha$, and the rainbow parameter $beta$
modified the energy spectrum of scalar and oscillator fields in comparison to
the results obtained in flat space.
Investigation of Quantum Dynamics in a Topological Defect Space-Time
In this paper, we explored the quantum dynamics of scalar and oscillator fields within a topological defect space-time background. We introduced rainbow gravity, which modifies the space-time geometry, into the cosmological setting. The temporal and spatial parts of the space-time were transformed using rainbow functions.
The transformation involved replacing $dt$ with $frac{dt}{mathcal{F}(chi)}$ and $dx^i$ with $frac{dx^i}{mathcal{H}(chi)}$, where $mathcal{F}$ and $mathcal{H}$ are the rainbow functions and $chi=E/E_p$. This modification allowed us to derive the radial equations of the Klein-Gordon equation and the oscillator equation under the influence of rainbow gravity in a topological space-time.
To study the eigenvalues of the quantum systems under investigation, we set the rainbow functions as $mathcal{F}(chi)=1$ and $mathcal{H}(chi)=sqrt{1-beta,chi^p}$, where $p=1,2$. By solving the radial equations using special functions and analyzing the results, we were able to compare the energy spectrum of scalar and oscillator fields in this modified space-time to those obtained in flat space.
Conclusions
Based on our analysis, the presence of a cosmological constant, the topological defect parameter $alpha$, and the rainbow parameter $beta$ had significant effects on the energy spectrum of scalar and oscillator fields. This suggests that the modifications introduced by rainbow gravity in a topological defect space-time can lead to observable differences in quantum systems.
Future Roadmap
Our findings open up several opportunities for future research in this field. The following roadmap outlines potential directions:
- Experimental Verification: Conduct experiments or observations that can test the predictions of rainbow gravity within a topological defect space-time. The modified energy spectrum could manifest in measurable ways.
- Generalization of Rainbow Functions: Explore different forms of rainbow functions $mathcal{F}$ and $mathcal{H}$ to understand how they affect the quantum dynamics of other physical systems and in various space-time backgrounds.
- Impact of Other Parameters: Investigate the influence of additional parameters, such as the shape of the defect or the strength of the cosmological constant, on the energy spectrum. This will provide a more comprehensive understanding of the system’s behavior.
- Mathematical Techniques: Develop new mathematical techniques or algorithms to solve the radial equations under rainbow gravity more efficiently. This will facilitate further exploration of this modified space-time.
- Extensions to Quantum Field Theory: Apply the framework developed in this study to investigate the behavior of quantum fields beyond scalar and oscillator fields. Explore the implications for other areas of quantum field theory.
While these opportunities hold promise, it is crucial to consider potential challenges along this roadmap:
- Technical Limitations: The complexity of solving the radial equations under rainbow gravity may present computational challenges. Developing efficient techniques to tackle these complexities will be essential.
- Limited Observational Data: Currently, observational data in the context of rainbow gravity and topological defect space-time is limited. Obtaining accurate and reliable experimental data for validation may pose difficulties.
- Theoretical Consistency: The compatibility of rainbow gravity with other fundamental theories, such as quantum mechanics and general relativity, requires further investigation. Ensuring theoretical consistency is essential for a comprehensive understanding of this field.
In summary, the study of quantum dynamics in a topological defect space-time under the influence of rainbow gravity has revealed intriguing modifications to the energy spectrum of scalar and oscillator fields. This opens up avenues for further exploration and research, but significant challenges must be overcome to advance our understanding of this fascinating area.
Read the original article
by jsendak | Jan 22, 2024 | GR & QC Articles
We delve into the first-order thermodynamics of Horndeski gravity, focusing
on spatially flat, homogeneous, and isotropic cosmologies. Our exploration
begins with a comprehensive review of the effective fluid representation within
viable Horndeski gravity. Notably, we uncover a surprising alignment between
the constitutive relations governing the “Horndeski fluid” and those of
Eckart’s thermodynamics. Narrowing our focus, we specialize our discussion to
spatially flat Friedmann-Lema{^i}tre-Robertson-Walker spacetimes. Within this
specific cosmological framework, we systematically analyze two classes of
theories: shift-symmetric and asymptotically shift-symmetric. These theories
are characterized by a non-vanishing braiding parameter, adding a nuanced
dimension to our investigation. On the one hand, unlike the case of the
“traditional” scalar-tensor gravity, these peculiar subclasses of viable
Horndeski gravity never relax to General Relativity (seen within this formalism
as an equilibrium state at zero temperature), but give rise to additional
equilibrium states with non-vanishing viscosity. On the other hand, this
analysis further confirms previous findings according to which curvature
singularities are “hot” and exhibit a diverging temperature, which suggests
that deviations of scalar-tensor theories from General Relativity become
extreme at spacetime singularities. Furthermore, we provide a novel exact
cosmological solution for an asymptotically shift-symmetric theory as a toy
model for our thermodynamic analysis.
Horndeski gravity is a topic of interest in the study of cosmology. In this article, we delve into the first-order thermodynamics of Horndeski gravity, specifically focusing on spatially flat, homogeneous, and isotropic cosmologies.
Before diving into the specifics, we provide a comprehensive review of the effective fluid representation within viable Horndeski gravity. Surprisingly, we uncover an alignment between the constitutive relations governing the “Horndeski fluid” and those of Eckart’s thermodynamics.
Next, we narrow our focus to spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) spacetimes. Within this specific cosmological framework, we systematically analyze two classes of theories: shift-symmetric and asymptotically shift-symmetric. These theories are characterized by a non-vanishing braiding parameter, which adds depth to our investigation.
Our analysis reveals that unlike traditional scalar-tensor gravity, the peculiar subclasses of viable Horndeski gravity never relax to General Relativity as an equilibrium state at zero temperature. Instead, they give rise to additional equilibrium states with non-vanishing viscosity. This highlights the unique properties of Horndeski gravity.
Additionally, our findings confirm previous research showing that curvature singularities are “hot” and exhibit a diverging temperature. This implies that deviations of scalar-tensor theories from General Relativity become extreme at spacetime singularities.
To further support our analysis, we present a novel exact cosmological solution for an asymptotically shift-symmetric theory. This solution serves as a toy model for our thermodynamic analysis and adds another layer to our understanding.
Future Roadmap
The exploration of Horndeski gravity in the context of first-order thermodynamics opens up several potential avenues for future research. Here is a roadmap outlining potential challenges and opportunities on the horizon:
1. Investigating Other Cosmological Frameworks
While our analysis focuses on spatially flat FLRW spacetimes, it would be valuable to extend the study to other cosmological frameworks, such as non-flat or anisotropic spacetimes. Exploring the thermodynamics of Horndeski gravity in these contexts may reveal new insights and properties.
2. Experimental and Observational Confirmation
Validating the predictions and findings of our thermodynamic analysis through experiments or observations would further solidify the understanding of Horndeski gravity. This could involve testing the existence of additional equilibrium states with non-vanishing viscosity or investigating the temperature behavior near curvature singularities.
3. Quantum Effects and Thermodynamics
Examining the interplay between quantum effects and thermodynamics within the context of Horndeski gravity could lead to exciting discoveries. Investigating the behavior of Horndeski gravity at extreme energies or exploring the connection between thermodynamics and quantum field theory may unlock new perspectives.
4. Generalizing the Analysis
Expanding the analysis beyond the specific subclasses of shift-symmetric and asymptotically shift-symmetric theories could reveal a broader picture of Horndeski gravity. Generalizing the thermodynamic analysis to include a wider range of theories and scenarios would provide a more comprehensive understanding of the subject.
5. Applications in Cosmological Evolution
Exploring how the thermodynamics of Horndeski gravity affect cosmological evolution could have practical applications. This could involve studying the influence of additional equilibrium states with non-vanishing viscosity on the dynamics of the universe or investigating how the temperature behavior near curvature singularities impacts the evolution of cosmic structures.
Overall, the study of first-order thermodynamics in Horndeski gravity offers a rich field for future exploration. By addressing the outlined challenges and opportunities, researchers can gain a deeper understanding of the subject and potentially uncover new phenomena and insights.
Read the original article
