About this Research Topic
Gravitational waves have been discovered in recent years. After the first event of gravitational waves from black hole binary merging was caught by LIGO in 2015, nowadays there have been more than 90 gravitational-wave events, whose frequency falls into the detectability range of ground-based gravitational-wave detectors, such as LIGO and VIRGO. Gravitational waves can not only verify the correctness of Einstein’s General Relativity, but also provide us with a steady probe to the early universe, thanks to the fact that it does can only interact with other matter via gravity, therefore can hardly get interfered. Therefore, it has become a hot topic in nowadays physics and has drawn the attention of various scientists and scientific institutes.
On the other hand, the frequency range of gravitational waves spread much larger than the detectability of those ground-based detectors. The lowest frequency that we know reaches $10^{-16}$ Hz, while the highest frequency may get 10 kHz. Therefore, we need different methods in order to detect the gravitational waves all over the range. For example, we need advanced ground-based detectors such as KAGRA and Einstein Telescope, we need space-based detectors such as LISA, Tianqin and Taiji, we need to make use of pulsar timing arrays such as SKA, and we need detectors for primordial gravitational waves such as BICEP/Keck Array, LiteBIRD and AliCPT. Moreover, different gravitational waves may be generated from different mechanisms (such as quantum fluctuations, scalar perturbation inductions, phase transitions, etc.), so in the theoretical side, it is also deserved to investigate problems such as what properties can be brought to gravitational waves by these mechanisms, what rules they will obey and so on. Within 5-10 years, the space-based detectors will be put into use successively. They will bring us more precious data and information, which can help us put new constraints on the gravitational-wave signals. With more precise data, theorists can also perform a more thorough analysis, discovering more interesting and useful properties of gravitational waves, and test many theories about gravity.
This specific issue aims at the inspiring discussion on both theoretical and observational sides of gravitational waves. We hope that through this issue, people can think and understand more deeply about gravitational waves. The specific themes include (but are not confined to):
• primordial gravitational waves generated from quantum fluctuations in the very early universe;
• induced gravitational waves generated from scalar perturbations as well as primordial black holes;
• gravitational waves generated from first-order phase transitions which can form stochastic gravitational-wave background;
• other aspects that can be connected with gravitational waves.
We also welcome reviews on the progress we have reached so far and what more we can achieve within the upcoming 5-10 years.
On the other hand, the frequency range of gravitational waves spread much larger than the detectability of those ground-based detectors. The lowest frequency that we know reaches $10^{-16}$ Hz, while the highest frequency may get 10 kHz. Therefore, we need different methods in order to detect the gravitational waves all over the range. For example, we need advanced ground-based detectors such as KAGRA and Einstein Telescope, we need space-based detectors such as LISA, Tianqin and Taiji, we need to make use of pulsar timing arrays such as SKA, and we need detectors for primordial gravitational waves such as BICEP/Keck Array, LiteBIRD and AliCPT. Moreover, different gravitational waves may be generated from different mechanisms (such as quantum fluctuations, scalar perturbation inductions, phase transitions, etc.), so in the theoretical side, it is also deserved to investigate problems such as what properties can be brought to gravitational waves by these mechanisms, what rules they will obey and so on. Within 5-10 years, the space-based detectors will be put into use successively. They will bring us more precious data and information, which can help us put new constraints on the gravitational-wave signals. With more precise data, theorists can also perform a more thorough analysis, discovering more interesting and useful properties of gravitational waves, and test many theories about gravity.
This specific issue aims at the inspiring discussion on both theoretical and observational sides of gravitational waves. We hope that through this issue, people can think and understand more deeply about gravitational waves. The specific themes include (but are not confined to):
• primordial gravitational waves generated from quantum fluctuations in the very early universe;
• induced gravitational waves generated from scalar perturbations as well as primordial black holes;
• gravitational waves generated from first-order phase transitions which can form stochastic gravitational-wave background;
• other aspects that can be connected with gravitational waves.
We also welcome reviews on the progress we have reached so far and what more we can achieve within the upcoming 5-10 years.
Keywords: gravitational waves, cosmological perturbation, primordial black holes, phase transition, binary systems
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