Quantum gravity phenomenology at the dawn of the multi-messenger era -- A review (2111.05659v2)

Abstract: The exploration of the universe has recently entered a new era thanks to the multi-messenger paradigm, characterized by a continuous increase in the quantity and quality of experimental data that is obtained by the detection of the various cosmic messengers (photons, neutrinos, cosmic rays and gravitational waves) from numerous origins. They give us information about their sources in the universe and the properties of the intergalactic medium. Moreover, multi-messenger astronomy opens up the possibility to search for phenomenological signatures of quantum gravity. On the one hand, the most energetic events allow us to test our physical theories at energy regimes which are not directly accessible in accelerators; on the other hand, tiny effects in the propagation of very high energy particles could be amplified by cosmological distances. After decades of merely theoretical investigations, the possibility of obtaining phenomenological indications of Planck-scale effects is a revolutionary step in the quest for a quantum theory of gravity, but it requires cooperation between different communities of physicists (both theoretical and experimental). This review is aimed at promoting this cooperation by giving a state-of-the art account of the interdisciplinary expertise that is needed in the effective search of quantum gravity footprints in the production, propagation and detection of cosmic messengers.

• The paper reviews quantum gravity frameworks and their testable predictions by linking theoretical models with cosmic messenger signals.
• It details methodologies like time-of-flight measurements and polarization studies to identify energy-dependent speed variations and cosmic birefringence.
• It underscores the importance of interdisciplinary collaboration and next-generation observatories in overcoming experimental challenges and refining data precision.

Overview of the Paper on Quantum Gravity Phenomenology

The paper "Quantum gravity phenomenology at the dawn of the multi-messenger era A review" serves as a comprehensive account of the current status in the field of quantum gravity (QG) phenomenology. It emphasizes the exploration of QG effects observable in cosmic messengers such as photons, neutrinos, cosmic rays, and gravitational waves. This review discusses both theoretical developments and experimental advancements, aiming to facilitate collaboration among theorists and experimenters to seek phenomenological signatures of QG.

Theoretical Frameworks and Models

The paper outlines various approaches to QG, recognizing the absence of a singular, comprehensive theory. It mentions key frameworks:

1. String Theory: Highlighted for incorporating QG effects, yet criticized for its landscape problem and background dependence.
2. Loop Quantum Gravity (LQG): This theory attempts to quantize gravity using a connection-based approach, facing challenges such as managing higher-order quantum corrections and modifications of the dispersion relations.
3. Asymptotically Safe Gravity: This approach seeks to extend the renormalization properties of QFT to gravity, using quantum scale symmetry.
4. Group Field Theory (GFT): Offers a quantum field theoretical perspective on discrete spacetime structures.

Quantum Gravity Effects in Astrophysics

The paper emphasizes the role of cosmic messengers in testing QG effects, such as:

• Modified Dispersion Relations (MDRs): Key predictions that can manifest as energy-dependent speed of light, testable via time-of-flight measurements.
• Lorentz Invariance Violation (LIV) vs. Deformation (DSR): Distinguishing different symmetry treatments in QG theories—LIV allows for explicit symmetry breaking while DSR modifies the symmetry structure without breaking it.

Phenomenological Implications

The review cautions that while MDRs and LIV are promising observational effects, they may not directly point to a specific QG framework. There is an emphasis on leveraging both existing and future high-energy astrophysical observatories to probe these effects.

• Time-of-Flight Studies: These are crucial for detecting energy-dependent variations in the speed of cosmic messengers.
• Cosmic Birefringence: Observations of polarization can provide insights into possible violations of fundamental symmetries.

Cosmic Messenger Techniques

A significant portion of the discussion is dedicated to the techniques for detecting and analyzing cosmic messengers:

• Gamma Rays: Observed via ground-based and space-based telescopes, with implications for both QG testing and understanding cosmic accelerators.
• Neutrinos and Cosmic Rays: Their propagation and interaction across cosmic distances can reveal QG effects, though they require sophisticated large-volume detectors for observation.
• Gravitational Waves (GWs): As a direct probe of spacetime geometry, GWs can potentially unveil new physics in QG.

Future Outlook and Challenges

The review concludes with a discussion of the challenges and opportunities in QG research:

• Data Quality and Interpretation: Improved precision in astrophysical data is crucial for resolving QG effects from source intrinsic properties.
• Interdisciplinary Collaboration: Encourages blending theoretical insights with experimental advancements to refine models and interpretations.
• Next-Generation Observatories: Innovations in observational technologies are necessary to enhance sensitivity and energy reach, potentially unveiling new QG phenomena.

This review delineates a path forward in the quest to understand QG, focusing on the synergy between theory and experiment in the multi-messenger astronomy era. While significant challenges remain, especially concerning theoretical predictions and experimental uncertainties, the collaborative efforts and technological advancements discussed hold promise for new discoveries in the field of quantum gravity.