by jsendak | Dec 29, 2023 | GR & QC Articles
Gravitational waves (GW) from the inspiral of binary compact objects offers a
one-step measurement of the luminosity distance to the event, which is
essential for the measurement of the Hubble constant, $H_0$, that characterizes
the expansion rate of the Universe. However, unlike binary neutron stars, the
inspiral of binary black holes is not expected to be accompanied by
electromagnetic radiation and a subsequent determination of its redshift.
Consequently, independent redshift measurements of such GW events are necessary
to measure $H_0$. In this study, we present a novel Bayesian approach to infer
$H_0$ from the cross-correlation between galaxies with known redshifts and
individual binary black hole merger events. We demonstrate the efficacy of our
method with $250$ simulated GW events distributed within $1$ Gpc in colored
Gaussian noise of Advanced LIGO and Advanced Virgo detectors operating at O4
sensitivity. We show that such measurements can constrain the Hubble constant
with a precision of $lesssim 15 %$ ($90%$ highest density interval). We
highlight the potential improvements that need to be accounted for in further
studies before the method can be applied to real data.
The study discusses the importance of measuring the Hubble constant, which characterizes the expansion rate of the Universe. It highlights that gravitational waves (GW) from the inspiral of binary black holes can provide a measurement of the luminosity distance to the event, but additional redshift measurements are necessary to determine the Hubble constant. The researchers present a Bayesian approach to infer the Hubble constant using the cross-correlation between galaxies with known redshifts and individual binary black hole merger events. They demonstrate the effectiveness of their method using simulated GW events.
To further improve this method and apply it to real data, the researchers outline several potential improvements:
- Increased Sample Size: The study used 250 simulated GW events, but a larger sample size would enhance the precision of the Hubble constant measurement.
- Redshift Measurement Accuracy: Accurate and precise redshift measurements are crucial for determining the Hubble constant. Improvements in redshift estimation techniques could enhance the accuracy of the method.
- Effective Signal Filtering: The researchers used colored Gaussian noise in their simulations. To apply the method to real data, effective signal filtering techniques must be developed to minimize noise and enhance signal detection.
- Detector Sensitivity: The study used data from Advanced LIGO and Advanced Virgo detectors at O4 sensitivity. Future improvements in detector sensitivity would allow for the detection of weaker gravitational wave signals, thereby expanding the sample size and improving measurement precision.
Overall, this study presents a promising Bayesian approach to infer the Hubble constant using cross-correlation between galaxies and binary black hole merger events. Future advancements in sample size, redshift measurement accuracy, signal filtering methods, and detector sensitivity will be instrumental in refining this method and applying it to real data, ultimately providing a more precise measurement of the Hubble constant and deepening our understanding of the expansion rate of the Universe.
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by jsendak | Dec 29, 2023 | GR & QC Articles
We develop a novel framework for describing quantum fluctuations in field
theory, with a focus on cosmological applications. Our method uniquely
circumvents the use of operator/Hilbert-space formalism, instead relying on a
systematic treatment of classical variables, quantum fluctuations, and an
effective Hamiltonian. Our framework not only aligns with standard formalisms
in flat and de Sitter spacetimes, which assumes no backreaction, demonstrated
through the $varphi^3$-model, but also adeptly handles time-dependent
backreaction in more general cases. The uncertainty principle and spatial
symmetry emerge as critical tools for selecting initial conditions and
understanding effective potentials. We discover that modes inside the Hubble
horizon emph{do not} necessarily feel an initial Minkowski vacuum, as is
commonly assumed. Our findings offer fresh insights into the early universe’s
quantum fluctuations and potential explanations to large-scale CMB anomalies.
We have developed a novel framework for describing quantum fluctuations in field theory, with a particular focus on cosmological applications. Unlike traditional approaches that rely on operator/Hilbert-space formalism, our method utilizes classical variables, quantum fluctuations, and an effective Hamiltonian. This framework aligns with standard formalisms in flat and de Sitter spacetimes, even without assuming no backreaction, as demonstrated through the $varphi^3$-model. Additionally, it handles time-dependent backreaction in more general cases.
In our research, we have found that the uncertainty principle and spatial symmetry play crucial roles in selecting initial conditions and understanding effective potentials. Contrary to common assumptions, we have discovered that modes inside the Hubble horizon do not necessarily experience an initial Minkowski vacuum. These findings offer valuable insights into quantum fluctuations in the early universe and could provide potential explanations for large-scale cosmic microwave background (CMB) anomalies.
Future Roadmap
Potential Challenges
- Testing the Framework: The first challenge will be to further test and validate the proposed framework. It is essential to compare its predictions with existing theories and empirical observations to ensure its accuracy and reliability.
- Models with Time-Dependent Backreaction: While our framework adeptly handles time-dependent backreaction, exploring and modeling specific cases with this characteristic may present additional challenges. Developing mathematical techniques and computational methods to study these cases will be crucial.
- Addressing CMB Anomalies: Our findings suggest a potential link between quantum fluctuations and large-scale CMB anomalies. To fully understand and explain these anomalies, further investigations, data analysis, and collaborations with observational cosmologists will be necessary.
Potential Opportunities
- Improved Cosmological Models: Our framework opens up new avenues for improving cosmological models by incorporating more realistic treatments of quantum fluctuations. This can lead to more accurate predictions and a better understanding of the early universe.
- Exploring Alternative Initial Conditions: The discovery that modes inside the Hubble horizon do not necessarily start with a Minkowski vacuum opens up possibilities for exploring alternative initial conditions. By considering different starting points, we can gain insights into the dynamics of the early universe and its impact on cosmic structures.
- Bridging Quantum Field Theory and Cosmology: Our method offers a unique approach to bridging the gap between quantum field theory and cosmology. By providing a framework that connects classical variables, quantum fluctuations, and an effective Hamiltonian, we can further our understanding of the fundamental nature of the universe.
Overall, our research presents a promising new framework for describing quantum fluctuations in cosmological field theory. While further testing, modeling, and investigations are needed, this approach offers exciting opportunities for advancing our knowledge of the early universe and cosmic anomalies.
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by jsendak | Dec 29, 2023 | GR & QC Articles
Gravitational waves (GWs) provide a new avenue to test Einstein’s General
Relativity (GR) using the ongoing and upcoming GW detectors by measuring the
redshift evolution of the effective Planck mass proposed by several modified
theories of gravity. We propose a model-independent, data-driven approach to
measure any deviation from GR in the GW propagation effect by combining
multi-messenger observations of GW sources accompanied by EM counterparts,
commonly known as bright sirens (Binary Neutron Star(BNS) and Neutron Star
Black Hole systems(NSBH)). We show that by combining the GW luminosity distance
measurements from bright sirens with the Baryon Acoustic Oscillation (BAO)
measurements derived from galaxy clustering, and the sound horizon measurements
from the Cosmic Microwave Background (CMB), we can make a data-driven
reconstruction of deviation of the variation of the effective Planck mass
(jointly with the Hubble constant) as a function of cosmic redshift. Using this
technique, we achieve a precise measurement of GR with redshift (z) with a
precision of approximately $7.9%$ for BNSs at redshift $z=0.075$ and $10%$
for NSBHs at redshift $z=0.225$ with 5 years of observation from LVK network of
detectors. Employing $CE&ET$ for just 1 year yields the best precision of
about $1.62%$ for BNSs and $2%$ for NSBHs at redshift $z=0.5$ on the
evolution of the frictional term, and a similar precision up to $z=1$. This
measurement can discover potential deviation from any kind of model that
impacts GW propagation with ongoing and upcoming observations.
The Future of Testing Einstein’s General Relativity Using Gravitational Waves
Gravitational waves (GWs) offer a promising avenue to test Einstein’s General Relativity (GR) through the measurement of the redshift evolution of the effective Planck mass, as proposed by modified theories of gravity. In this article, we present a model-independent, data-driven approach to detect any deviations from GR in the propagation of GWs by combining multi-messenger observations of GW sources with electromagnetic (EM) counterparts, known as bright sirens. Specifically, we focus on Binary Neutron Star (BNS) and Neutron Star Black Hole (NSBH) systems.
We demonstrate that by combining measurements of the GW luminosity distance from bright sirens with Baryon Acoustic Oscillation (BAO) measurements obtained from galaxy clustering and sound horizon measurements from the Cosmic Microwave Background (CMB), we can reconstruct the variation of the effective Planck mass (alongside the Hubble constant) as a function of cosmic redshift in a data-driven manner. This approach allows us to achieve a precise measurement of GR with redshift.
Potential Challenges and Opportunities
- Challenge: The precision of the measurement is contingent upon the duration of observation from the LVK network of detectors. Five years of observation yields a precision of approximately 7.9% for BNSs at redshift z=0.075 and 10% for NSBHs at redshift z=0.225.
- Opportunity: Increasing the observation time to just 1 year using $CE&ET$ can significantly enhance precision. In this scenario, a precision of approximately 1.62% for BNSs and 2% for NSBHs at redshift z=0.5, and a similar precision up to z=1, can be achieved.
- Potential Challenge: The measurement aims to discover potential deviations from any model that impacts GW propagation. As such, it may encounter theoretical models or phenomena that were not previously considered, potentially leading to unexpected outcomes or new areas of investigation.
- Opportunity: Ongoing and upcoming observations provide opportunities to uncover new physics and refine our understanding of gravity by detecting and characterizing any deviations from GR.
In conclusion, this data-driven approach offers a promising roadmap for testing Einstein’s General Relativity using gravitational waves. By combining multi-messenger observations, including bright sirens, with BAO and CMB measurements, we can reconstruct the variation of the effective Planck mass with redshift and achieve precise measurements of GR at different cosmic redshifts. While challenges may arise in terms of observation duration and potential theoretical deviations, the opportunities for enhancing precision, discovering new physics, and refining our understanding of gravity outweigh these challenges.
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by jsendak | Dec 29, 2023 | GR & QC Articles
We present a new subgrid model for neutrino quantum kinetics, which is
primarily designed to incorporate effects of collective neutrino oscillations
into neutrino-radiation-hydrodynamic simulations for core-collapse supernovae
and mergers of compact objects. We approximate the neutrino oscillation term in
quantum kinetic equation by Bhatnagar-Gross-Krook (BGK) relaxation-time
prescription, and the transport equation is directly applicable for classical
neutrino transport schemes. The BGK model is motivated by recent theoretical
indications that non-linear phases of collective neutrino oscillations settle
into quasi-steady structures. We explicitly provide basic equations of the BGK
subgrid model for both multi-angle and moment-based neutrino transport to
facilitate the implementation of the subgrid model in the existing neutrino
transport schemes. We also show the capability of our BGK subgrid model by
comparing to fully quantum kinetic simulations for fast neutrino-flavor
conversion. We find that the overall properties can be well reproduced in the
subgrid model; the error of angular-averaged survival probability of neutrinos
is within $sim 20 %$. By identifying the source of error, we also discuss
perspectives to improve the accuracy of the subgrid model.
Introduction
In this article, we present a new subgrid model for neutrino quantum kinetics. The model is designed to incorporate the effects of collective neutrino oscillations into neutrino-radiation-hydrodynamic simulations for core-collapse supernovae and mergers of compact objects.
Neutrino Oscillation Term
We approximate the neutrino oscillation term in the quantum kinetic equation using the Bhatnagar-Gross-Krook (BGK) relaxation-time prescription. This approximation allows the transport equation to be directly applicable for classical neutrino transport schemes. The BGK model is motivated by recent theoretical indications that non-linear phases of collective neutrino oscillations settle into quasi-steady structures.
Basic Equations of the BGK Subgrid Model
We explicitly provide the basic equations of the BGK subgrid model for both multi-angle and moment-based neutrino transport. This will facilitate the implementation of the subgrid model in existing neutrino transport schemes.
Validation of the BGK Subgrid Model
We demonstrate the capability of our BGK subgrid model by comparing it to fully quantum kinetic simulations for fast neutrino-flavor conversion. We find that the overall properties can be well reproduced in the subgrid model, with an error of the angular-averaged survival probability of neutrinos within approximately 20%.
Potential Challenges
- Improving accuracy: Although our subgrid model already shows promising results, there is room for improvement to reduce the error in the survival probability of neutrinos. Future research should focus on identifying the sources of error and finding ways to enhance the accuracy of the model.
- Complex implementation: Implementing the subgrid model in existing neutrino transport schemes may be challenging due to the complexity of the equations and the need for computational resources. This requires collaboration between scientists and developers to ensure a seamless integration.
Opportunities on the Horizon
- Advancements in simulation technology: As computational power continues to improve, simulations incorporating the BGK subgrid model can be performed with higher resolution and accuracy. This opens up new possibilities for studying core-collapse supernovae and compact object mergers.
- Further understanding of neutrino oscillations: Continued research into the behavior of collective neutrino oscillations will provide insights into the dynamics that can be incorporated into the subgrid model. This will lead to more accurate simulations and a deeper understanding of astrophysical phenomena.
Conclusion
The introduction of our BGK subgrid model for neutrino quantum kinetics provides a valuable tool for enhancing the realism of neutrino-radiation-hydrodynamic simulations. While there are still challenges to overcome and opportunities to explore, this model represents a significant step forward in our ability to study core-collapse supernovae and compact object mergers. By improving the accuracy of the subgrid model and leveraging advancements in simulation technology and our understanding of neutrino oscillations, we can unlock new insights into these fascinating astrophysical phenomena.
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by jsendak | Dec 29, 2023 | GR & QC Articles
The ultraviolet cutoff on a quantum field theory can be interpreted as a
condensate of the affine curvature such that while the maximum of the affine
action gives the power-law corrections, its minimum leads to the emergence of
gravity. This mechanism applies also to fundamental strings as their spinless
unstable ground levels can be represented by the scalar affine curvature such
that open strings (D-branes) decay to closed strings and closed strings to
finite minima with emergent gravity. Affine curvature is less sensitive to
massive string levels than the tachyon, and the field-theoretic and stringy
emergent gravities take the same form. It may be that affine condensation
provides an additional link between the string theory and the known physics at
low energies.
According to the article, the ultraviolet cutoff on a quantum field theory can be interpreted as a condensate of the affine curvature. The maximum of the affine action gives power-law corrections, while its minimum leads to the emergence of gravity. This mechanism can also be applied to fundamental strings, with their spinless unstable ground levels represented by the scalar affine curvature. Open strings (D-branes) decay to closed strings and closed strings eventually reach finite minima with emergent gravity.
The use of affine curvature is advantageous compared to the tachyon as it is less sensitive to massive string levels. Both field-theoretic and stringy emergent gravities exhibit the same form. It is possible that the condensation of affine curvature provides a link between string theory and known physics at low energies.
Future Roadmap
Potential Challenges
- Experimental Verification: The proposed mechanism of affine curvature leading to the emergence of gravity and linking string theory with known physics needs experimental verification. Conducting experiments to validate these concepts may pose significant challenges.
- Theoretical Complexity: Further research and mathematical developments are required to fully understand the implications of affine condensation and its role in linking fundamental strings and quantum field theories.
- Integration with Existing Theories: Integrating this new perspective with existing theories, such as general relativity, may present challenges as it would require reconciling different mathematical frameworks.
Potential Opportunities
- Unified Theory of Gravity: If experimental validation is achieved, the emergence of gravity through affine condensation could potentially lead to the development of a unified theory of gravity, merging quantum field theory and string theory.
- Advancements in Cosmology: Understanding the role of affine curvature in the emergence of gravity may provide new insights into cosmological phenomena, such as the expansion of the universe and the behavior of black holes.
- Technological Applications: Research on affine condensation and its connection to known physics could potentially lead to technological advancements in fields such as quantum computing or high-energy physics.
Conclusion
The concept of affine condensation and its implications for the emergence of gravity provide an intriguing avenue for further exploration. While there are challenges to overcome, such as experimental verification and integration with existing theories, the potential opportunities, including a unified theory of gravity and advancements in cosmology and technology, make this a promising area of research.
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by jsendak | Dec 29, 2023 | GR & QC Articles
During the ringdown phase of a gravitational signal emitted by a black hole,
the least damped quasinormal frequency dominates. If modifications to
Einstein’s theory induce noticeable deformations of the black-hole geometry
only near the event horizon, the fundamental mode remains largely unaffected.
However, even a small change near the event horizon can significantly impact
the first few overtones, providing a means to probe the geometry of the event
horizon. Overtones are stable against small deformations of spacetime at a
distance from the black hole, allowing the event horizon to be distinguished
from the surrounding environment. In contrast to echoes, overtones make a much
larger energy contribution. These findings open up new avenues for future
observations.
Conclusions:
Based on the findings discussed in the text, the following conclusions can be drawn:
- The quasinormal frequency dominates during the ringdown phase of a gravitational signal emitted by a black hole.
- Modifications to Einstein’s theory can cause deformations near the event horizon, but the fundamental mode remains largely unaffected.
- Small changes near the event horizon can have a significant impact on the first few overtones, providing a way to study and probe the geometry of the event horizon.
- The overtones are stable against small deformations of spacetime away from the black hole, allowing for the identification of the event horizon amidst its surroundings.
- Compared to echoes, overtones contribute a much larger amount of energy.
- These findings create new possibilities for future observations and investigations.
Future Roadmap:
In light of the above conclusions, here is a potential roadmap for readers interested in this topic:
1. Further Study of Quasinormal Frequencies:
To gain a deeper understanding of gravitational signals emitted during the ringdown phase, researchers should continue to study the properties and behaviors of quasinormal frequencies. This will involve exploring various black hole scenarios and investigating how different factors can influence these frequencies.
2. Examining Modifications to Einstein’s Theory:
An important area for future research is the study of potential modifications to Einstein’s theory of gravity. By investigating and simulating these modifications, scientists can better understand how they affect the geometry of black holes and their event horizons. This will enable a more comprehensive analysis of the first few overtones and their relationship to deformations near the event horizon.
3. Development of Advanced Observational Techniques:
With the knowledge gained from studying quasinormal frequencies and modifications to Einstein’s theory, researchers should focus on developing advanced observational techniques. This may include improving gravitational wave detectors and designing experiments specifically aimed at detecting and analyzing the overtones emitted by black holes. These techniques should aim to distinguish between the energy contributions of overtones and echoes.
4. Collaborative Efforts and Interdisciplinary Research:
Given the complexity of the subject matter, collaboration between experts in different fields such as astrophysics, theoretical physics, and instrumentation will be crucial. Interdisciplinary research should be encouraged to foster innovative approaches and accelerate progress in understanding and utilizing the information provided by the overtones of black holes.
Potential Challenges and Opportunities:
Challenges:
- Understanding the implications of modifications to Einstein’s theory and their impact on black hole geometry.
- Designing experiments or observations that can effectively isolate and measure the distinct energy contributions of overtones.
- Developing advanced detection technologies capable of capturing and analyzing faint gravitational signals emitted during the ringdown phase.
Opportunities:
- Unraveling the mysteries surrounding black hole properties, such as their event horizons, through the analysis of overtones.
- Advancing our understanding of gravity and potentially uncovering new physics beyond Einstein’s theory.
- Opening up possibilities for groundbreaking discoveries and insights into the nature of spacetime.
In summary, continued research into the dominating quasinormal frequencies, modifications to Einstein’s theory, advanced observational techniques, and interdisciplinary collaborations will pave the way for significant advancements in our understanding and utilization of the information provided by the overtones of black holes.
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