by jsendak | Dec 31, 2023 | GR & QC Articles
We further investigate novel features of the $T-$vacuum state, originally
defined in the context of quantum field theory in a (1+1) dimensional radiation
dominated universe [Modak, JHEP 12, 031 (2020)]. Here we extend the previous
work to a realistic (3+1) dimensional set up and show that $T-$vacuum causes an
emph{anisotropic particle creation} in the radiation dominated early universe.
Unlike the Hawking or Unruh effect, where the particle content is thermal and
asymptotically defined, here it is non-thermal and time dependent. This novel
example of particle creation is interesting because these particles are
detected in the frame of physical/cosmological observers, who envision the
$T-$vacuum as a particle excited state, and therefore may eventually be
compared with observations.
The article explores the concept of the $T-$vacuum state in quantum field theory and its implications for particle creation in the early universe. Building upon previous research conducted in a (1+1) dimensional radiation dominated universe, the authors extend the study to a more realistic (3+1) dimensional setup and demonstrate that the $T-$vacuum leads to anisotropic particle creation.
Unlike the Hawking or Unruh effect, where the particle content is thermal and consistently defined, the particle creation in the $T-$vacuum is non-thermal and varies with time. This unique characteristic makes it particularly intriguing, as these particles can be observed by physical and cosmological observers who perceive the $T-$vacuum as an excited state of particles. Consequently, there is potential for a comparison between these detected particles and actual observations.
Roadmap for the Future
1. Further Experimental Investigations
- Experimental studies should be carried out to validate the predicted anisotropic particle creation induced by the $T-$vacuum in a (3+1) dimensional universe.
- Developing advanced detectors capable of measuring and observing these non-thermal particles is crucial.
2. Comparison with Observations
- Physicists and cosmologists should analyze the detected particles and compare them with observational data to determine the compatibility of the $T-$vacuum model with real-world observations.
- This comparison could shed light on the accuracy and feasibility of the $T-$vacuum as an explanation for early universe phenomena.
3. Theoretical Developments
- Further exploration and theoretical developments are essential to better understand the nature of the $T-$vacuum and its implications.
- Investigating the potential cosmological consequences of the $T-$vacuum and its role in the evolution of the early universe could unveil new insights into quantum field theory and cosmology.
Challenges and Opportunities
Challenges:
- Developing experiments with sufficient sensitivity to detect non-thermal particles created by the $T-$vacuum.
- Ensuring the accuracy of observational data and minimizing potential sources of error during the comparison process.
- Dealing with complex mathematical models and calculations involved in analyzing the $T-$vacuum and its impact on particle creation.
Opportunities:
- Successful experimental verification of anisotropic particle creation could confirm the existence of the $T-$vacuum and promote further exploration of its properties.
- Discovery of a non-thermal particle signature consistent with the $T-$vacuum would provide support for the model and open avenues for developing new theories and expanding our understanding of the early universe.
- Improvements in detector technologies driven by the need to observe non-thermal particles may have broader applications in other fields of research.
In summary, the extension of the $T-$vacuum concept to a (3+1) dimensional universe reveals an intriguing phenomenon of anisotropic particle creation in the radiation dominated early universe. The non-thermal nature and observability of these particles offer opportunities for experimental investigations, as well as comparisons with observational data. While there are challenges to overcome, such as developing sensitive detectors and ensuring accuracy in analysis, successful validation of the $T-$vacuum model could lead to theoretical advancements and a deeper understanding of quantum field theory and cosmology.
Read the original article
by jsendak | Dec 31, 2023 | GR & QC Articles
An attempt to directly use the synchronous gauge ($g_{0 lambda} = –
delta_{0 lambda}$) in perturbative gravity leads to a singularity at $p_0 =
0$ in the graviton propagator. This is similar to the singularity in the
propagator for Yang-Mills fields $A^a_lambda$ in the temporal gauge ($A^a_0 =
0$). There the singularity was softened, obtaining this gauge as the limit at
$varepsilon to 0$ of the gauge $n^lambda A^a_lambda = 0$, $n^lambda = (1,
– varepsilon (partial^j partial_j )^{- 1} partial^k ) $. Then the
singularities at $p_0 = 0$ are replaced by negative powers of $p_0 pm i
varepsilon$, and thus we bypass these poles in a certain way.
Now consider a similar condition on $n^lambda g_{lambda mu}$ in
perturbative gravity, which becomes the synchronous gauge at $varepsilon to
0$. Unlike the Yang-Mills case, the contribution of the Faddeev-Popov ghosts to
the effective action is nonzero, and we calculate it. In this calculation, an
intermediate regularization is needed, and we assume the discrete structure of
the theory at short distances for that. The effect of this contribution is, in
particular, to add non-pole terms to the propagator. In overall, this
contribution vanishes at $varepsilon to 0$. Thus, we effectively have the
synchronous gauge with the resolved singularities at $p_0 = 0$, where only the
physical components $g_{j k}$ are active and there is no need to calculate the
ghost contribution.
Conclusion:
The conclusions of the text are that using the synchronous gauge in perturbative gravity leads to a singularity at p_0=0 in the graviton propagator. However, by applying a similar condition as in the Yang-Mills case and by calculating the contribution of the Faddeev-Popov ghosts, the singularities can be bypassed and resolved. This results in the effective synchronous gauge where only the physical components g_jk are active.
Future Roadmap:
In order to move forward with the research in perturbative gravity and the use of the synchronous gauge, the following roadmap can be outlined:
1. Further investigation of the similarities with Yang-Mills fields:
It is important to continue exploring the similarities between perturbative gravity and Yang-Mills fields in terms of their respective gauges and singularities. This can help to gain a deeper understanding of the nature of the singularities in graviton propagators and how they can be resolved.
2. Development of regularization techniques:
Since an intermediate regularization is needed in the calculation of the Faddeev-Popov ghost contribution, it is crucial to develop precise and reliable regularization techniques. This will ensure accurate calculations and a better understanding of the effects of the ghost contributions on the propagator.
3. Study of the discrete structure at short distances:
Assuming a discrete structure of the theory at short distances is an essential step in the calculation of the Faddeev-Popov ghost contribution. It is necessary to investigate this discrete structure further to validate its accuracy and applicability to perturbative gravity.
4. Exploration of non-pole terms in the propagator:
The addition of non-pole terms to the propagator due to the Faddeev-Popov ghost contribution opens up new avenues for research. It is important to explore the implications of these non-pole terms and how they affect the behavior of the propagator in different scenarios.
5. Verification of the vanishing of contribution at ε→0:
The claim that the contribution of the Faddeev-Popov ghosts vanishes at ε→0 needs to be further verified and validated through rigorous calculations and experiments. This will provide more confidence in the effectiveness of the resolved synchronous gauge in perturbative gravity.
Challenges and Opportunities:
While there are potential challenges in terms of developing accurate regularization techniques and validating assumptions about the discrete structure at short distances, there are also exciting opportunities for further exploration and understanding of perturbative gravity. Resolving the singularities in the graviton propagator opens up possibilities for more precise calculations and predictions in gravitational theories.
Overall, with continued research and investigation, the resolved synchronous gauge in perturbative gravity can lead to significant advancements in our understanding of gravity and its behavior at both small and large scales.
Read the original article
by jsendak | Dec 31, 2023 | GR & QC Articles
In this paper we study the shear free spherical symmetric gravitational
collapse of charged radiating star. All the physical quantities including
pressure, density are regular. Energy conditions are satisfied throughout the
interior of the matter configuration. The luminosity is time independent and
mass is radiated linearly. The causal and non causal temperature remains
greater than that of the uncharged collapsing scenario.
Conclusions:
The study focuses on the shear-free spherically symmetric gravitational collapse of a charged radiating star. The physical quantities, such as pressure and density, are found to be regular and satisfy energy conditions within the interior of the star. The luminosity remains time independent, and the mass is radiated linearly. Additionally, the temperature, both causal and non-causal, is found to be greater than that of an uncharged collapsing scenario.
Future Roadmap:
1. Further Investigation on Charged Radiating Stars:
The study opens up avenues for additional research on charged radiating stars. Since the physical quantities are regular and energy conditions are satisfied, it would be interesting to explore the behavior of other variables under different conditions or assumptions. For example, studying the effect of different charge densities on the collapse could yield valuable insights into the stability and evolution of these stars.
2. Comparative Analysis with Uncharged Collapsing Stars:
The finding that the temperature of the charged radiating star remains higher than that of an uncharged collapsing scenario invites a comparative study. Examining the differences in collapse dynamics, evolution of mass and luminosity between charged and uncharged stars could provide a better understanding of the role of charge in gravitational collapse.
3. Cosmological Applications:
Considering that this study focuses on gravitational collapse, exploring the cosmological implications of charged radiating stars might be worthwhile. Investigating how the presence of charge affects the formation and evolution of galaxies, black holes, or other cosmic structures could have significant implications for our understanding of the universe.
Potential Challenges:
- The complexity of modeling charged radiating stars may pose challenges in accurately predicting the behavior of various physical quantities.
- Obtaining observational data to validate the theoretical findings could be challenging due to the limited number of known charged radiating stars.
- Understanding the interplay between charge and other factors impacting gravitational collapse requires sophisticated mathematical models and computational resources.
Potential Opportunities:
- The unique behavior of charged radiating stars provides an opportunity for novel discoveries and advancements in our understanding of astrophysics.
- Further research on charged radiating stars can contribute to the broader knowledge of general relativity, gravity, and the properties of matter under extreme conditions.
- Advancements in modeling and computational techniques can be made as a result of the challenges faced, benefiting not only the study of charged radiating stars but also other areas of scientific research.
Conclusion:
The study of shear-free spherical symmetric gravitational collapse of charged radiating stars has presented promising results. Building on these conclusions, future research should focus on investigating other aspects of charged radiating stars, conducting comparative analyses with uncharged collapsing scenarios, and exploring cosmological implications. While challenges exist, such opportunities have the potential to significantly expand our knowledge of astrophysics, general relativity, and the universe as a whole.
Read the original article
by jsendak | Dec 31, 2023 | GR & QC Articles
We investigate the effects of an early cosmological period, dominated by
primordial 2-2-holes, on axion dark matter. 2-2-holes emerge in quadratic
gravity, a candidate theory of quantum gravity, as a new family of classical
solutions for ultracompact matter distributions. These objects have the black
hole exterior without an event horizon and hence, as a probable endpoint of
gravitational collapse, they do not suffer from the information loss problem.
Thermal 2-2-holes exhibit Hawking-like classical radiation and satisfy the
entropy-area law. Moreover, these objects, unlike BHs, have a minimum allowed
mass and hence naturally give rise to stable remnants. In this paper, we
consider the remnant contribution to dark matter (DM) small and adopt the axion
DM scenario by the misalignment mechanism. We show that a 2-2-hole domination
phase in the evolution of the universe changes the axion mass window from the
dark matter abundance constraints. The biggest effect occurs when the remnants
have the Planck mass, which is the case for a strongly coupled quantum gravity.
The change in abundance constraints for the Planck mass 2-2-hole remnants
amounts to that of the Primordial Black Hole (PBH) counterpart. Therefore;
since we use the revised constraints on the initial fraction of 2-2-holes from
GWs, the results here can also be considered as the updated version of the PBH
case. As a result, the lower limit on the axion mass is found as $m_a sim
10^{-9}$ eV. Furthermore, the domination scenario itself constrains the remnant
mass $M_{mathrm{min}}$ considerably. Given that we focus on the pre-BBN
domination scenario in order not to interfere with BBN (Big Bang
Nucleosynthesis) constraints, the remnant mass window becomes $m_{mathrm{Pl}}
lesssim M_{mathrm{min}} lesssim 0.1;mathrm{g}$.
The article examines the effects of an early cosmological period, dominated by primordial 2-2-holes, on axion dark matter. These 2-2-holes are classical solutions in quadratic gravity and have a black hole exterior without an event horizon. They exhibit classical radiation and satisfy the entropy-area law. Unlike black holes, they have a minimum allowed mass, leading to stable remnants.
The paper focuses on the remnant contribution to dark matter and adopts the axion dark matter scenario by the misalignment mechanism. It shows that a domination phase of 2-2-holes in the evolution of the universe changes the axion mass window. The biggest effect occurs when the remnants have the Planck mass, which is the case for strongly coupled quantum gravity.
The article states that the revised constraints on the initial fraction of 2-2-holes from gravitational waves can be considered as the updated version of the primordial black hole case. As a result, it finds a lower limit on the axion mass of approximately ^{-9}$ eV.
The domination scenario itself constrains the remnant mass considerably, with a window of approximately $m_{mathrm{Pl}} lesssim M_{mathrm{min}} lesssim 0.1;mathrm{g}$. The focus is on the pre-BBN (Big Bang Nucleosynthesis) domination scenario in order to avoid interference with BBN constraints.
Future Roadmap
Potential Challenges
- Validation of the existence and properties of 2-2-holes in quadratic gravity through further research and experimentation.
- Confirmation of the dominance of 2-2-holes during a specific cosmological period through observational evidence.
- Refinement of gravitational wave constraints on the initial fraction of 2-2-holes.
- Verification of the revised constraints on the axion mass and remnant mass.
- Investigation of the implications of the domination scenario on other cosmological phenomena.
Potential Opportunities
- Further exploration of the role of 2-2-holes in the early universe and their impact on dark matter.
- Development of new experimental methods and technologies to probe the properties of 2-2-holes.
- Improvement of observational techniques to detect signatures or effects of 2-2-hole domination in the cosmic microwave background radiation or other cosmological observations.
- Potential discovery or confirmation of the existence of axion dark matter and its mass range.
- Opportunity to refine our understanding of quantum gravity and its implications for cosmology.
Conclusion
The article highlights the effects of primordial 2-2-holes on axion dark matter and presents a new perspective on the axion mass window. It suggests that 2-2-hole remnants can have a significant contribution to dark matter and sets a lower limit on the axion mass based on the domination scenario. The findings encourage further research and open up new opportunities for studying the properties of 2-2-holes and their implications for cosmology and quantum gravity.
Read the original article
by jsendak | Dec 31, 2023 | GR & QC Articles
We use galaxy-galaxy lensing data to test general relativity and $f(T)$
gravity at galaxies scales. We consider an exact spherically symmetric solution
of $f(T)$ theory which is obtained from an approximate quadratic correction,
and thus it is expected to hold for every realistic deviation from general
relativity. Quantifying the deviation by a single parameter $Q$, and following
the post-Newtonian approximation, we obtain the corresponding deviation in the
gravitational potential, shear component, and effective surface density (ESD)
profile. We used five stellar mass samples and divided them into blue and red
to test the model dependence on galaxy color, and we modeled ESD profiles using
Navarro-Frenk-White (NFW) profiles. Based on the group catalog from the Sloan
Digital Sky Survey Data Release 7 (SDSS DR7) we finally extract
$Q=2.138^{+0.952}_{-0.516}times 10^{-5},$Mpc$^{-2}$ at $1sigma$ confidence.
This result indicates that $f(T)$ corrections on top of general relativity are
favored. Finally, we apply information criteria, such as the AIC and BIC ones,
and although the dependence of $f(T)$ gravity on the off-center effect implies
that its optimality needs to be carefully studied, our analysis shows that
$f(T)$ gravity is more efficient in fitting the data comparing to general
relativity and $Lambda$CDM paradigm, and thus it offers a challenge to the
latter.
Based on the analysis of galaxy-galaxy lensing data, this study examines the validity of general relativity and $f(T)$ gravity at the scale of galaxies. The $f(T)$ theory is a spherically symmetric solution obtained from a quadratic correction, which is expected to hold for realistic deviations from general relativity. By quantifying the deviation using a parameter $Q$ and employing the post-Newtonian approximation, the study investigates the effects of $f(T)$ theory on the gravitational potential, shear component, and effective surface density profiles.
To test the model’s dependence on galaxy color, the study divides the samples into blue and red categories and models the effective surface density profiles using Navarro-Frenk-White (NFW) profiles. Using the group catalog from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), the study extracts a value of $Q=2.138^{+0.952}_{-0.516}times 10^{-5},$Mpc$^{-2}$ at sigma$ confidence. This result suggests that $f(T)$ corrections on top of general relativity are preferred.
The study further applies information criteria such as the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC). While acknowledging that the optimality of $f(T)$ gravity requires careful examination due to its dependence on the off-center effect, the analysis demonstrates that $f(T)$ gravity provides a better fit to the data compared to both general relativity and the $Lambda$CDM paradigm. Therefore, it poses a challenge to the latter.
Future Roadmap: Challenges and Opportunities
The results of this study open up several avenues for future research in the field of gravity theories and cosmology. Here is a roadmap that outlines potential challenges and opportunities:
1. Further Investigation of $f(T)$ Theory
The $f(T)$ theory, with its quadratic correction, has shown promise in explaining deviations from general relativity. However, its optimality needs to be thoroughly examined, especially in terms of the off-center effect. Researchers should conduct in-depth studies to understand the limitations and potential improvements of $f(T)$ gravity.
2. Testing the Model across Different Galaxy Types
While this study considered the dependence of $f(T)$ gravity on galaxy color by dividing the samples into blue and red, future research should explore the applicability of the model to other galaxy types as well. Investigating the effects of $f(T)$ gravity on a wider range of galaxies can provide valuable insights into its universality and suitability for different astrophysical environments.
3. Refining the Measurement of $Q$ Parameter
Improving the accuracy and precision of the measurement of the $Q$ parameter is crucial for a more robust evaluation of $f(T)$ gravity. Researchers should develop innovative observational techniques and data analysis methods to obtain more precise estimates of this parameter.
4. Comparison with Other Gravity Theories
While $f(T)$ gravity has shown advantages over general relativity and $Lambda$CDM paradigm based on the current analysis, it is important to compare it with other alternative gravity theories as well. Investigating how $f(T)$ gravity fares against competing theories can provide a comprehensive understanding of its strengths and weaknesses.
5. Incorporating Cosmological Observations
Expanding the scope of research to include cosmological observations can enhance our understanding of how $f(T)$ gravity operates on larger scales. By investigating the compatibility of $f(T)$ theory with cosmological data, researchers can assess its validity in a broader cosmological context.
In conclusion, the results of this study indicate that $f(T)$ gravity provides a more efficient fit to galaxy-galaxy lensing data compared to general relativity and $Lambda$CDM paradigm. However, further investigations are needed to fully understand the limitations and potential applications of $f(T)$ gravity. Through ongoing research and the exploration of new avenues, scientists can continue to push the boundaries of our understanding of gravity and its implications for our universe.
Read the original article
by jsendak | Dec 31, 2023 | GR & QC Articles
We herein investigate the universal relation proposed by Goon and Penco in de
Sitter black holes with electric charge or angular momentum. Our analysis
focuses on the cosmological horizon, which only exists in de Sitter and Nariai
spacetimes. Because the relation is given in a general case, the overall
relationship may be valid. However, we elucidate the details of the relation,
highlighting distinctions from those of (anti-)de Sitter black holes while
affirming the validity of the relation. Furthermore, based on our analysis of
Schwarzschild–de Sitter, Reissner–Nordstr”om–de Sitter, and Kerr–de Sitter
black holes, we demonstrate the universality of the thermodynamic relation in
de Sitter black holes.
Examining the Conclusions
In this article, Goon and Penco propose a universal relation in de Sitter black holes with electric charge or angular momentum. The authors focus on the cosmological horizon, which is only present in de Sitter and Nariai spacetimes. They argue that the overall relationship may be valid in the general case, but they also emphasize the need to understand the specific details of the relation and distinguish it from those of (anti-)de Sitter black holes. However, their analysis of various types of de Sitter black holes, including Schwarzschild–de Sitter, Reissner–Nordstr”om–de Sitter, and Kerr–de Sitter, leads them to conclude that the proposed thermodynamic relation is indeed universal in these types of black holes.
Future Roadmap
Based on the conclusions of this study, readers can expect further investigations and developments in the understanding of thermodynamic relations in de Sitter black holes. Here is a potential roadmap for future research:
1. Further Refinement of the Proposed Universal Relation
While the authors argue for the universality of the thermodynamic relation in de Sitter black holes, there may be room for further refinement and clarification. Future studies could delve deeper into the specific details of the relation, examining its implications and potential extensions. This would involve rigorous mathematical and theoretical analysis to strengthen the understanding of the proposed relation.
2. Comparison with (Anti-)de Sitter Black Holes
The authors highlight distinctions between the thermodynamic relation in de Sitter black holes and that of (anti-)de Sitter black holes. Exploring these differences and understanding their origins would be an interesting avenue for future research. By comparing and contrasting the thermodynamics of these different types of black holes, researchers can gain deeper insights into the fundamental nature of black hole thermodynamics.
3. Experimental Confirmation
Experimental verification of the proposed thermodynamic relation in de Sitter black holes would be a significant milestone. Future experiments using advanced observational techniques or particle accelerators could provide valuable data to validate or challenge the universality of the relation. Additionally, numerical simulations and modelling could also contribute to the understanding and confirmation of the proposed relation.
4. Exploration of Additional Parameters
While Goon and Penco focus on electric charge and angular momentum, future research could investigate the thermodynamics of de Sitter black holes with other parameters. For example, the inclusion of other quantum numbers or additional fields could provide a more comprehensive understanding of the relation. Exploring the effects of these parameters on the universal thermodynamic relation would expand our knowledge of black hole physics.
Challenges and Opportunities
As researchers delve deeper into the thermodynamics of de Sitter black holes, they will face several challenges and encounter new opportunities:
- Theoretical Challenges: Theoretical analysis in general relativity and quantum field theory will be necessary to fully understand and develop the proposed universal thermodynamic relation. Overcoming mathematical complexities, resolving theoretical inconsistencies, and incorporating quantum effects are among the challenges that lie ahead.
- Experimental Constraints: Experimental confirmation of the thermodynamic relation will require advanced technological capabilities. Designing experiments that can observe or probe de Sitter black holes will present technical challenges, but successful validation of the relation would lead to a deeper understanding of these enigmatic objects.
- Collaborative Research: Collaboration among experts in different fields, such as theoretical physicists, mathematicians, and observational astronomers, will be crucial for progressing research on the thermodynamics of de Sitter black holes. Interdisciplinary approaches can yield innovative insights and novel solutions to complex challenges.
- Applications and Implications: Understanding the thermodynamics of de Sitter black holes has implications beyond fundamental physics. It could have implications for the study of the early universe, cosmology, and the nature of spacetime itself. Exploring these potential applications and broader implications will open up new avenues of research.
Conclusion
Goon and Penco’s investigation into the thermodynamics of de Sitter black holes provides valuable insights and sets the stage for further research. By proposing a universal thermodynamic relation and demonstrating its applicability in various types of de Sitter black holes, they contribute to our understanding of these fascinating objects. However, future research must refine the proposed relation, compare it with (anti-)de Sitter black holes, seek experimental confirmation, and explore additional parameters. Addressing these challenges and seizing the opportunities presented will pave the way for a deeper understanding of the thermodynamics of de Sitter black holes and their implications in theoretical physics and cosmology.
Read the original article
