arXiv:2502.18560v1 Announce Type: new
Abstract: The quantum nature of gravity remains an open question in fundamental physics, lacking experimental verification. Gravitational waves (GWs) provide a potential avenue for detecting gravitons, the hypothetical quantum carriers of gravity. However, by analogy with quantum optics, distinguishing gravitons from classical GWs requires the preservation of quantum coherence, which may be lost due to interactions with the cosmic environment causing decoherence. We investigate whether GWs retain their quantum state by deriving the reduced density matrix and evaluating decoherence, using an environmental model where a scalar field is conformally coupled to gravity. Our results show that quantum decoherence of GWs is stronger at lower frequencies and higher reheating temperatures. We identify a model-independent amplitude threshold below which decoherence is negligible, providing a fundamental limit for directly probing the quantum nature of gravity. In the standard cosmological scenario, the low energy density of the universe at the end of inflation leads to complete decoherence at the classical amplitude level of inflationary GWs. However, for higher energy densities, decoherence is negligible within a frequency window in the range $100 {rm Hz} text{-} 10^8 {rm Hz}$, which depends on the reheating temperature. In a kinetic-dominated scenario, the dependence on reheating temperature weakens, allowing GWs to maintain quantum coherence above $10^7 {rm Hz}$.
The Quantum Nature of Gravity and the Detectability of Gravitons
Gravity, one of the fundamental forces of nature, is still not fully understood in the framework of quantum physics. While we have theories like general relativity to describe gravity, there is no experimental evidence for the existence of gravitons, the hypothetical quantum particles carrying gravity. In this study, we explore the potential of gravitational waves (GWs) to provide a way to detect gravitons. However, distinguishing gravitons from classical GWs is a challenging task due to the loss of quantum coherence caused by interactions with the cosmic environment.
Investigating Quantum Decoherence in Gravitational Waves
To evaluate the preservation of quantum coherence in GWs, we analyze the decoherence effect using an environmental model where a scalar field is conformally coupled to gravity. By deriving the reduced density matrix, we are able to quantify the level of decoherence in GWs. Our findings reveal that the degree of quantum decoherence in GWs is dependent on both the frequency of the waves and the reheating temperature of the universe.
Frequency and Reheating Temperature Dependencies
We establish a key observation that quantum decoherence of GWs is stronger at lower frequencies and higher reheating temperatures. At these conditions, the interaction with the cosmic environment causes stronger decoherence effects and makes it more difficult to maintain the quantum state of the GWs.
Fundamental Limit for Probing the Quantum Nature of Gravity
Our research leads to an important conclusion: there exists an amplitude threshold below which the decoherence of GWs becomes negligible. This threshold serves as a fundamental limit for directly investigating the quantum nature of gravity.
Implications in Cosmological Scenarios
In the standard cosmological scenario, where the energy density of the universe is low at the end of inflation, the decoherence effects in GWs reach a complete level at the classical amplitude of inflationary GWs. However, for higher energy densities, the decoherence becomes negligible within a specific frequency range of 0 {rm Hz} text{-} 10^8 {rm Hz}$, which is dependent on the reheating temperature.
In the context of a kinetic-dominated scenario, the dependence on the reheating temperature weakens, allowing GWs to maintain quantum coherence even at frequencies above ^7 {rm Hz}$. This opens up opportunities to study the quantum nature of gravity in different cosmological scenarios.
Future Roadmap: Challenges and Opportunities
Challenges
- The preservation of quantum coherence in GWs is a challenging task due to the strong interactions with the cosmic environment.
- Differentiating gravitons from classical GWs requires addressing the issue of decoherence.
- Understanding the impact of lower frequencies and higher reheating temperatures on quantum decoherence in GWs.
Opportunities
- The identification of an amplitude threshold for negligible decoherence provides a fundamental limit for directly probing the quantum nature of gravity.
- Exploring the specific frequency range and reheating temperature dependencies allows for the investigation of quantum coherence in different cosmological scenarios.
- Advancing our understanding of the quantum nature of gravity through experimental verification of gravitons using GW detection methods.
Note: This study contributes to the ongoing quest to unify quantum mechanics and gravity, shedding light on the quantum nature of gravity and its potential experimental detection through GWs.