by jsendak | Jan 15, 2024 | GR & QC Articles
Brane world models have shown to be promising to understand the late cosmic
acceleration, in particular because such acceleration can be naturally derived,
mimicking the dark energy behaviour just with a five dimensional geometry. In
this paper we present a strong lensing joint analysis using a compilation of
early-type galaxies acting as a lenses, united with the power of the well
studied strong lensing galaxy cluster Abell,1689. We use the strong lensing
constraints to investigate a brane model with variable brane tension as a
function of the redshift. In our joint analysis we found a value $n =
7.8^{+0.9}_{-0.5}$, for the exponent related to the brane tension, showing that
$n$ deviates from a Cosmological Constant (CC) scenario (n=6). We obtain a
value for the deceleration parameter, $q(z)$ today, $q(0)=-1.2^{+0.6}_{-0.8}$,
and a transition redshift, $z_t=0.60pm0.06$ (when the Universe change from an
decelerated phase to an accelerated one). These results are in contrast with
previous work that favors CC scenario, nevertheless our lensing analysis is in
agreement with a formerly reported conclusion suggesting that the variable
brane tension model is able to source a late cosmic acceleration without an
extra fluid as in the standard one.
Examining the Conclusions of Brane World Models for Late Cosmic Acceleration
Brane world models have emerged as a promising avenue for understanding the phenomenon of late cosmic acceleration. These models offer an intriguing approach by utilizing a five-dimensional geometry that naturally mimics the behavior of dark energy. In this paper, we present a strong lensing joint analysis that combines the power of well-studied strong lensing galaxy cluster Abell,1689 with a compilation of early-type galaxies acting as lenses. Our goal is to investigate a brane model with variable brane tension as a function of redshift and derive valuable insights from the strong lensing constraints.
Key Findings:
- We have obtained a value of $n = 7.8^{+0.9}_{-0.5}$ for the exponent related to the brane tension in our joint analysis. This deviation from the Cosmological Constant (CC) scenario (where n=6) suggests that the brane model with variable brane tension offers a unique perspective on late cosmic acceleration.
- The deceleration parameter, $q(z)$, evaluated at present day shows a value of $q(0)=-1.2^{+0.6}_{-0.8}$. This indicates an accelerated phase for the universe and further solidifies the viability of the brane model in predicting late cosmic acceleration.
- We have determined a transition redshift, $z_t=0.60pm0.06$, which signifies the point at which the universe transitions from a decelerated phase to an accelerated one. This result strengthens our understanding of the dynamics involved in late cosmic acceleration.
These findings challenge previous work favoring the Cosmological Constant scenario and provide further support for the variable brane tension model as a viable explanation for late cosmic acceleration. Our strong lensing joint analysis showcases the potential of this model in deriving meaningful insights without the need for an extra fluid, as in the standard approach.
Roadmap for the Future:
- Further Observations and Data: To strengthen the conclusions drawn from our analysis, future efforts should focus on obtaining additional observations and data of early-type galaxies and strong lensing galaxy clusters. This will allow for a more comprehensive and robust analysis that can provide even deeper insights into the behavior of late cosmic acceleration.
- Refining the Brane Model: While the variable brane tension model has shown promise in this analysis, future research should focus on refining and expanding upon this model. Exploring additional variations and parameters that may contribute to late cosmic acceleration can help build a more complete understanding of the underlying mechanisms.
- Comparative Analysis: It would be valuable to conduct comparative analyses between the variable brane tension model and other theoretical frameworks for late cosmic acceleration. This can help identify the unique strengths and weaknesses of each model and potentially lead to the development of hybrid models that combine the best aspects of multiple approaches.
- Experimental Testing: As our understanding of the brane model improves, efforts should be made to test its predictions through experimental observations. This could involve studying gravitational lensing, cosmic microwave background, or other astrophysical phenomena that can provide empirical evidence supporting or challenging the predictions of the brane model.
In conclusion, our strong lensing joint analysis utilizing brane world models has provided valuable insights into the nature of late cosmic acceleration. The results challenge previous assumptions, offer new perspectives, and highlight the potential of the variable brane tension model. By addressing the challenges and pursuing the opportunities outlined above, future research can unlock further mysteries surrounding late cosmic acceleration and contribute to our broader understanding of the universe.
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by jsendak | Jan 15, 2024 | GR & QC Articles
This study investigates the formation of primordial black holes (PBHs)
resulting from extremely large amplitudes of initial fluctuations in a
radiation-dominated universe. We find that, for a sufficiently large initial
amplitude, the configuration of trapping horizons shows characteristic
structure due to the existence of bifurcating trapping horizons. We call this
structure of the trapping horizons “Type II PBH”, while the structure without
a bifurcating trapping horizon “Type I PBH”, which is typically generated
from a relatively small amplitude of the initial fluctuation. In Ref.[1], in
the dust-dominated universe, the Type II PBH can be realized by the Type II
initial fluctuation, which is characterized by a non-monotonic areal radius as
a function of the radial coordinate (throat structure) in contrast with the
standard case with a monotonic areal radius (Type I fluctuation). Our research
reveals that a type II fluctuation does not necessarily result in a type II PBH
in the case of the radiation fluid. We also find that for the initial amplitude
well above the threshold value, the resulting PBH mass may either increase or
decrease with the initial amplitude depending on its specific profile rather
than its fluctuation type.
The study examines the formation of primordial black holes (PBHs) that result from large amplitudes of initial fluctuations in a radiation-dominated universe. The researchers identify two types of PBHs based on the configuration of trapping horizons: Type I PBHs generated from relatively small initial fluctuations and Type II PBHs generated from large initial fluctuations with a characteristic structure of bifurcating trapping horizons.
However, the study also finds that the Type II initial fluctuation in a radiation fluid does not necessarily result in a Type II PBH. The researchers discover that for initial amplitudes well above the threshold value, the resulting PBH mass may either increase or decrease depending on its specific profile, rather than its fluctuation type.
Potential Challenges:
- Verification of the study’s findings through experimental or observational evidence.
- Determining the exact threshold value for initial amplitude that leads to the formation of PBHs.
- Understanding the specific profiles of initial fluctuations that lead to an increase or decrease in PBH mass.
Potential Opportunities:
- Further exploration and research into the formation of primordial black holes.
- Investigation into the relationship between initial fluctuation profiles and resulting PBH masses.
- Development of new theoretical models to better understand the behavior of trapping horizons and their connection to PBH formation.
- Possibility of using PBHs as a tool for studying the early universe and its characteristics.
Future Roadmap:
- Conduct further research to validate the study’s findings using observational data or experimental simulations.
- Investigate the physical mechanisms that drive the formation of Type I and Type II PBHs in different types of universes.
- Refine the understanding of the threshold value for initial amplitude and its relationship to PBH formation.
- Explore the specific profiles of initial fluctuations that result in varied PBH masses.
- Continue developing theoretical models and computational tools to study trapping horizons and PBH formation in greater detail.
- Collaborate with experimental physicists and cosmologists to design experiments or observations that can provide further insights into PBH formation.
- Apply the knowledge gained from studying PBHs to enhance our understanding of the early universe and its evolution.
Conclusion:
The study presents new insights into the formation of primordial black holes and identifies two types based on the configuration of trapping horizons. Further investigations are needed to validate the findings and to explore the relationship between initial fluctuation profiles and resulting PBH masses. This research opens up opportunities for deeper exploration into the behavior of trapping horizons and could contribute to our understanding of the early universe.
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by jsendak | Jan 15, 2024 | GR & QC Articles
Short review of the Weyl geometry is given. To describe the phenomenological
particle creation we suggest the modified perfect fluid model taking into
account the back reaction on the geometry of both the already created particles
and the very process of their creation. It is found that the relation for
particle creation is conformal invariant. This requires the creation law
consisting of the source terms as the Weyl Lagrangian plus two quite new terms
depending of the particle number density.
Short review of the Weyl geometry is given. To describe the phenomenological particle creation we suggest the modified perfect fluid model taking into account the back reaction on the geometry of both the already created particles and the very process of their creation. It is found that the relation for particle creation is conformal invariant. This requires the creation law consisting of the source terms as the Weyl Lagrangian plus two quite new terms depending on the particle number density.
Conclusions
- The Weyl geometry provides a framework for understanding particle creation.
- A modified perfect fluid model is proposed to describe particle creation, considering the back reaction on the geometry.
- The relation for particle creation is found to be conformal invariant.
- The creation law consists of source terms including the Weyl Lagrangian and two new terms dependent on the particle number density.
Roadmap for the Future
Challenges
- Further Theoretical Development: One of the challenges going forward will be to develop further theoretical understanding of the modified perfect fluid model and its implications for particle creation.
- Experimental Verification: Experimental verification of the proposed modified perfect fluid model and its conformal invariance will be crucial in validating the findings and expanding our knowledge in this field.
- Predictive Power: Exploring the predictive power of the creation law, including the newly introduced terms dependent on particle number density, will be important to understand their role in real-world scenarios.
Opportunities
- Advancements in Cosmology: Understanding particle creation and its relation to the Weyl geometry can provide valuable insights into cosmological phenomena and the evolution of the universe.
- Technological Applications: Exploring the modified perfect fluid model and conformal invariance in particle creation may open up new avenues for technological advancements, such as energy generation or particle manipulation.
- Theoretical Breakthroughs: Further research in this field has the potential to contribute to broader theoretical breakthroughs in physics and our understanding of fundamental particles and their interactions.
Overall, the study of particle creation within the framework of Weyl geometry presents exciting opportunities for both theoretical advancements and practical applications. However, challenges such as further theoretical development, experimental verification, and exploring the predictive power of the creation law must be overcome to fully grasp the potential of this research.
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by jsendak | Jan 15, 2024 | GR & QC Articles
The model of two dimensional quantum gravity defining the “Virasoro Minimal
String”, presented recently by Collier, Eberhardt, M”{u}hlmann, and Rodriguez,
was also shown to be perturbatively (in topology) equivalent to a random matrix
model. An alternative definition is presented here, in terms of double-scaled
orthogonal polynomials, thereby allowing direct access to non-perturbative
physics. Already at leading order, the defining string equation’s properties
yield valuable information about the non-perturbative fate of the model,
confirming that the case $c{=}25$ (spacelike Liouville) is special, by virtue
of sharing certain key features of the ${cal N}{=}1$ supersymmetric JT gravity
string equation. Solutions of the full string equation are constructed using a
special limit, and the (Cardy) spectral density is complete to all genus and
beyond. The distributions of the underlying discrete spectrum are readily
accessible too, as is the spectral form factor. Some examples of these are
exhibited.
The recent model of two-dimensional quantum gravity, known as the “Virasoro Minimal String,” has been shown to be perturbatively equivalent to a random matrix model. However, this article presents an alternative definition of the model using double-scaled orthogonal polynomials, which allows for direct access to non-perturbative physics. This new definition offers valuable insights into the non-perturbative fate of the model, particularly in the case of $c{=}25$ (spacelike Liouville), which shares important features with the ${cal N}{=}1$ supersymmetric JT gravity string equation.
By constructing solutions of the full string equation using a special limit, it is possible to analyze the (Cardy) spectral density to all genus and beyond. This means that the distributions of the discrete spectrum underlying the model and the spectral form factor can be readily examined. The article provides some examples of these distributions.
Roadmap for Readers:
1. Introduction
Begin by introducing the concept of the Virasoro Minimal String and its perturbative equivalence to a random matrix model. Mention that this article presents an alternative definition using double-scaled orthogonal polynomials.
2. Non-Perturbative Physics
Explain the importance of accessing non-perturbative physics in understanding the model’s behavior. Discuss how the new definition allows for valuable insights into the non-perturbative fate of the model.
3. Case of $c{=}25$ (Spacelike Liouville)
Highlight the special nature of the case $c{=}25$ and its similarities to the ${cal N}{=}1$ supersymmetric JT gravity string equation.
4. Solutions of the Full String Equation
Describe the method of constructing solutions of the full string equation using a special limit. Emphasize that this allows for analysis of the (Cardy) spectral density to all genus and beyond.
5. Distributions of the Discrete Spectrum
Explain how the new definition provides accessibility to the distributions of the discrete spectrum underlying the model. Offer some examples of these distributions.
6. Conclusion
Summarize the main findings and their implications for understanding two-dimensional quantum gravity. Highlight the significance of the alternative definition using double-scaled orthogonal polynomials.
Potential Challenges and Opportunities:
Challenges:
- One potential challenge is the complexity of the mathematical framework involved in the alternative definition using double-scaled orthogonal polynomials. Readers without a strong background in mathematical physics may struggle to fully grasp the concepts presented.
- The article assumes some prior knowledge of the Virasoro Minimal String and its perturbative equivalence to a random matrix model. This may make it difficult for readers who are new to the topic to follow along.
Opportunities:
- The alternative definition using double-scaled orthogonal polynomials opens up new avenues for studying non-perturbative physics in two-dimensional quantum gravity. This presents exciting opportunities for further research and exploration in the field.
- The analysis of the (Cardy) spectral density and the distributions of the discrete spectrum provide rich insights into the behavior of the model. Researchers can leverage this information to make further advances in understanding two-dimensional quantum gravity.
Overall, while there may be challenges in understanding the mathematical framework and prior knowledge required, this article offers a valuable roadmap for readers to explore the alternative definition of the Virasoro Minimal String and its implications for non-perturbative physics in two-dimensional quantum gravity.
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by jsendak | Jan 15, 2024 | GR & QC Articles
We propose a connection between the newly formulated Virasoro minimal string
and the established minimal string by deriving the string equation from the
expansion of the Virasoro minimal string’s density of states in powers of
$E^{m+1/2}$. This string equation is expressed as a power series involving
double-scaled multicritical matrix models, which are dual to $(2,2m-1)$ minimal
strings. This reformulation of Virasoro minimal strings enables us to employ
matrix theory tools for computing $n$-boundary correlators. We analyze the
scaling behavior of these correlators in the JT gravity limit and deduce the
scaling of quantum volumes $V^{(b)}_{0,n}(ell_1,dots,ell_n)$ in this limit.
The article discusses the connection between the newly formulated Virasoro minimal string and the established minimal string. By deriving the string equation from the expansion of the Virasoro minimal string’s density of states, the authors express this equation as a power series involving double-scaled multicritical matrix models that are dual to $(2,2m-1)$ minimal strings. This reformulation allows for the use of matrix theory tools to compute $n$-boundary correlators.
The analysis in the article focuses on the scaling behavior of these correlators in the JT gravity limit. From this analysis, the authors are able to deduce the scaling of quantum volumes $V^{(b)}_{0,n}(ell_1,dots,ell_n)$ in this limit.
Future Roadmap
Potential Challenges
- The use of matrix theory tools for computing $n$-boundary correlators may require further development and refinement. Researchers may encounter challenges in accurately modeling and analyzing these correlators.
- Expanding the analysis beyond the JT gravity limit may present challenges in terms of understanding the scaling behavior in different scenarios and potentially incorporating other gravity theories.
- The reformulation of Virasoro minimal strings and the use of double-scaled multicritical matrix models may require further validation and testing to ensure their accuracy and applicability in various contexts.
Potential Opportunities
- The use of matrix theory tools provides a new approach to computing $n$-boundary correlators, which can potentially lead to advancements in our understanding of string theory and its connections to other areas of physics.
- By analyzing the scaling behavior of correlators in the JT gravity limit, researchers can gain insights into the behavior of quantum volumes in this specific limit. These findings can contribute to our understanding of the interplay between string theory and gravity.
- The reformulation of Virasoro minimal strings opens up possibilities for exploring their connections to other string theories and uncovering new mathematical structures and relationships.
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by jsendak | Jan 15, 2024 | GR & QC Articles
The detectability of the gravitational-wave signal from $r$-modes depends on
the interplay between the amplification of the mode by the CFS instability and
its damping due to dissipative mechanisms present in the stellar matter. The
instability window of $r$-modes describes the region of stellar parameters
(angular velocity, $Omega$, and redshifted stellar temperature, $T^infty$),
for which the mode is unstable. In this study, we reexamine this problem in
nonbarotropic neutron stars, taking into account the previously overlooked
nonanalytic behavior (in $Omega$) of relativistic $r$-modes and enhanced
energy dissipation resulting from diffusion in superconducting stellar matter.
We demonstrate that at slow rotation rates, relativistic $r$-modes exhibit
weaker amplification by the CFS instability compared to Newtonian ones.
However, their dissipation through viscosity and diffusion is significantly
more efficient. In rapidly rotating neutron stars within the framework of
general relativity, the amplification of $r$-modes by the CFS mechanism and
their damping due to shear viscosity become comparable to those predicted by
Newtonian theory. In contrast, the relativistic damping of the mode by
diffusion and bulk viscosity remains significantly stronger than in the
nonrelativistic case. Consequently, account for diffusion and general
relativity leads to a substantial modification of the $r$-mode instability
window compared to the Newtonian prediction. This finding is important for the
interpretation of observations of rotating neutron stars, as well as for
overall understanding of $r$-mode physics.
The Detectability of Gravitational-Wave Signals from $r$-Modes: A Roadmap for the Future
The detectability of gravitational-wave signals from $r$-modes is influenced by a combination of factors including the amplification of the mode by the CFS instability and its damping due to dissipative mechanisms present in the stellar matter. In this study, we reexamine the problem of $r$-mode instability in nonbarotropic neutron stars, taking into account previously overlooked nonanalytic behavior in $Omega$ and enhanced energy dissipation resulting from diffusion in superconducting stellar matter.
We first demonstrate that relativistic $r$-modes exhibit weaker amplification by the CFS instability compared to Newtonian ones at slow rotation rates. However, their dissipation through viscosity and diffusion is significantly more efficient. As neutron stars rotate more rapidly, the amplification of $r$-modes by the CFS mechanism and their damping due to shear viscosity become comparable to those predicted by Newtonian theory within the framework of general relativity. However, the relativistic damping of the mode by diffusion and bulk viscosity remains significantly stronger than in the nonrelativistic case.
As a result, our findings show that considering diffusion and general relativity leads to a substantial modification of the $r$-mode instability window compared to the Newtonian prediction. This has important implications for the interpretation of observations of rotating neutron stars and enhances our overall understanding of $r$-mode physics.
Roadmap for Future Research
- Further investigation into the nonanalytic behavior of relativistic $r$-modes in different stellar environments to fully understand its impact on their amplification and dissipation.
- Explore the influence of different dissipative mechanisms, such as diffusion and bulk viscosity, on the stability and detectability of $r$-modes in neutron stars.
- Conduct detailed observations and measurements of rotating neutron stars to validate the modified instability window predicted by our findings.
- Develop improved theoretical models and computational techniques for better understanding and predicting the behavior of $r$-modes in various astrophysical scenarios.
- Investigate the potential implications of the modified $r$-mode instability window on other astrophysical phenomena, such as gravitational-wave emissions and the evolution of neutron star populations.
Challenges and Opportunities
While our study provides valuable insights into the behavior of $r$-modes in nonbarotropic neutron stars, there are several challenges and opportunities on the horizon:
- Complexity of the problem: The interplay between amplification and damping mechanisms in $r$-modes is highly complex, requiring sophisticated theoretical models and computational techniques for accurate predictions.
- Validation through observations: The modified instability window predicted by our findings needs to be validated through detailed observations and measurements of rotating neutron stars, which may pose observational challenges.
- Improved modeling and simulations: Further advancements in theoretical models and numerical simulations are necessary to gain a deeper understanding of $r$-mode physics in different astrophysical scenarios.
- Interdisciplinary collaborations: Collaboration between astrophysicists, theoretical physicists, and computational scientists is crucial to address the challenges posed by $r$-mode physics and to uncover new opportunities for research and discovery.
In conclusion, our research highlights the importance of considering diffusion and general relativity in the study of $r$-mode instability in neutron stars. It provides a roadmap for future investigations and opens up new possibilities for understanding the behavior of $r$-modes and their detectability through gravitational-wave observations. Through interdisciplinary collaborations and advancements in theory and observation, we can continue to deepen our knowledge of $r$-mode physics, leading to significant advancements in our understanding of astrophysical phenomena.
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