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Gravitational Waves
Gravitational waves are distortions (waves) of spacetime that propagate at the speed of light. Their existence was predicted by Einstein’s general theory of relativity, proposed in 1915. Since around the 1980s, observations of the orbital period of binary star systems have shown a perfect match with theoretical predictions based on the assumption that gravitational waves exist, leading scientists to believe in their presence. However, the signals of gravitational waves reaching Earth are extremely weak, and for a long time, direct detection was not achieved. However, in September 2015, a gravitational-wave observatory in U.S., LIGO, finally made the first direct detection of gravitational waves, generated by the merger of a binary black hole system.Celestial objects that generate strong gravitational waves are not limited to binary black hole mergers, which were the first to be detected. Other potential sources include neutron star binary mergers and supernova explosions. Gravitational waves from a neutron star binary merger were already observed in 2017, but gravitational waves from supernova explosions have not yet been detected. Beyond astrophysical sources, gravitational waves are also thought to be generated by high-energy, violent events in the early universe, such as cosmic inflation, cosmic strings, phase transitions, and reheating. Future observational projects are currently being developed to achieve their direct detection.
What We Can Learn from Gravitational-Wave Observations
The observation of gravitational waves is significant not only as a test of general relativity but also for its applications in astronomy and cosmology, as it is expected to reveal aspects of the universe that traditional observations could not. Since gravitational interaction is much weaker than electromagnetic interaction, gravitational waves can reach us from regions that electromagnetic waves cannot penetrate—such as the interiors of stars, dense interstellar matter, non-luminous objects, and even directly from the early universe. In other words, gravitational waves allow us to “see” the dark universe rather than just the bright universe observed through electromagnetic radiation. A prime example of this is the merger of binary black holes. Moreover, observing the universe in a completely new way may lead to the discovery of unknown phenomena that we had never anticipated. For these reasons, research on gravitational wave applications spans a wide range of fields, including gravitational theory, particle physics, nuclear physics, astronomy, and cosmology, and is being actively pursued.Here are some of the research topics currently being pursued.
Testing Gravity with Gravitational Waves
Before the first detection of gravitational waves, tests of gravitational theories have been limited to weak and quasi-static (slow-moving) gravitational fields, such as those in the Solar System. However, since the first detection, it has become possible to study the properties of dynamic spacetimes, such as the strong gravitational fields of black holes and gravitational waves themselves. Many extended theories of gravity have been proposed for various purposes, and in order to uncover the true theory of gravity, it is crucial to investigate its behavior in extreme environments that have not yet been tested.To address this, we have proposed a method that utilizes the polarization modes of gravitational waves. The types and number of polarization modes are unique characteristics of each gravitational theory. In general relativity, there are only two polarization modes, whereas extended theories of gravity can allow for three or more modes. By analyzing gravitational wave data and identifying the types and number of polarization modes, we can narrow down the possible properties of gravitational theories in extreme environments where gravitational waves are generated.
The propagation of gravitational waves is also crucial for testing gravitational theories. The speed of propagation may be related to the possible violation of Lorentz symmetry in gravity or quantum effects in spacetime, while the damping rate of gravitational waves during propagation is linked to time variations in the strength of gravity. By measuring these observational quantities, we aim to test general relativity and further constrain the correct theory of gravity.
Gravitational-Wave Cosmology
The waveform of gravitational waves emitted from binary systems can be precisely calculated using general relativity. By comparing theoretical predictions with observational data, we can use gravitational waves to measure the distance to astrophysical objects. If the distances to multiple objects are determined, we can extract information about the expansion rate of the universe and the large-scale distribution of dark matter, allowing us to measure key cosmological parameters. One of the most significant applications is the measurement of the Hubble constant, which describes the current expansion rate of the universe. Different observational methods have yielded conflicting values for the Hubble constant, and its precise value remains uncertain. This has made gravitational-wave observations a highly anticipated tool for resolving the discrepancy. Our research has explored new measurement methods and sensitivity improvements, demonstrating that gravitational-wave observations are a promising approach for observational cosmology. We are currently working on applying these techniques to real observational data.Primordial gravitational waves, generated in the very early universe (just after its birth), are the ultimate target of gravitational-wave observations. This is because they can only be detected directly through gravitational waves, making them a unique probe of the universe's earliest moments. By observing primordial gravitational waves, we can investigate how the universe was born and evolved before the Big Bang. Moreover, since the early universe was an extremely high-energy environment, detecting these waves may reveal traces of unknown fundamental physical laws or the quantum nature of spacetime. One of the future projects aiming for this detection is the DECIGO mission, in which we are actively involved. Currently, we are developing advanced data analysis techniques to enable the observation of primordial gravitational waves.