I'm working on various experiments to answer the fundamental question, what is gravity?
Gravity is familiar to us in our everyday life, but its behavior is actually puzzling in many ways. For example, gravity is somehow very weak compared with other fundamental forces, and it somehow affect all objects equally. The standard model of particle physics can explain all the other interactions such as electromagnetic force, strong force, and weak force, based on quantum mechanics. However, gravity is only explained by general relativity. Mysteries in gravity is related to other mysteries such as cosmic inflation, accelerating expansion of the universe, and dark matter. These mysteries imply new laws of physics beyond our understanding of the universe.
To test alternative theories of gravity and cosmology by gravitational wave observations, I'm working on the development of gravitational wave telescopes. Using the interferometer techniques developed for gravitational wave detection, I'm also working on experiments to probe gravity with various approaches, such as dark matter searches, test of quantum mechanics at macroscopic scales, and test of relativity.
Gravitational Waves
Gravitational waves are ripples in spacetime. Since Einstein's prediction a century ago, many people tried to detect gravitational waves but ended up unsuccessful. This was because the effects of gravitational waves are so tiny.
In 2015, the two detectors of Advanced LIGO in the United States observed gravitational waves for the first time. The gravitational waves were from the coalescence of black holes which have about 30 solar masses. Also, in 2017, with Advanced Virgo in Italy, gravitational waves from binary neutron star was detected, and observatories around the globe did the follow-up observations. The new era of gravitational wave astronomy has finally begun.
To further enhance the field of gravitational wave astronomy, we need more and better signals from multiple detectors. In Japan, construction of the gravitational wave telescope KAGRA is underway. KAGRA is the only interferometer in the world which was built in a quiet underground site, and is the only interferometer which will cool down the mirrors to reduce thermal disturbances. I have lead the research and development of the interferometer as a chief of the Main Interferometer group of KAGRA. I have also been working on quantum noise reduction experiments and experiments with mirrors made with new materials for future upgrade of the detectors.
My research also involves the development of a low frequency gravitational wave telescope. Especially, space gravitational wave detector DECIGO focuses on 0.1-10 Hz region, and can detect primordial gravitational waves from the inflationary period. My ultimate goal is to reveal the mechanism of cosmic inflation by the observation of primordial gravitational waves.
Dark matter search
Various astrophysical observations have revealed that our Universe is filled with dark matter. Dark matter is an invisible mass which has almost no interaction with ordinary matter other than gravitational interaction. It's existence is strongly implied from observations such as rotation velocity of galaxies and gravitational lens. Recent observations of temperature fluctuations in the cosmic microwave background have revealed that about 80% of whole matter in our Universe is dark matter, and the standard model of cosmology derived from this result is amazingly consistent with other observations.
That said, the nature of dark matter has not been identified at all. Especially, WIMPs (Weakly Interacting Massive Particles) and primordial black holes are long thought to be promising candidates of dark matter. However, despite the large scale observations and experiments performed over many years to search for them, no clue has yet to be found. Therefore, there is an emerging motivation to comprehensively search for various dark matter candidates with novel ideas.
We are performing experiments to search for ultralight dark matter by focusing on tiny interactions between laser interferometer and dark matter. Especially, we proposed a new method to search for axion dark matter by searching for polarization modulation of laser beams. Ultralight dark matter search can also be done by searching for non-standard forces acting on mirrors of interferometers. With these new methods, we are aiming to search for dark matter with unprecedented sensitivity.
Macroscopic quantum mechanics
Quantum mechanics fundamentally allows superposition of states. Schrodinger's cat is a thought experiment of superposition of states; alive and dead are superposed in this well known example. Quantum superpositions are actually observed in the microscopic scale such as atoms and molecules. However, in the macroscopic scale such as cats, quantum superpositions have never been observed. Since quantum mechanics is scale-independent, we should observe quantum superpositions even in the macroscopic scale, if quantum mechanics is totally correct.
This problem has been at the heart of profound mysteries in present physics. There are roughly two ways to solve this problem. Some think that quantum mechanics is correct and we simply don't have enough technology to observe these at macroscopic scales. Some other think we have to modify quantum mechanics at macroscopic scales. For example, superposition of position states of a massive object imply that the gravitational field is also superposed. There are suggestions that gravity might play a role in destroying macroscopic quantum superposition states.
Recent advances in precision measurements enabled us to experimentally look into this problem. Especially, we are using optomechanical systems to test macroscopic quantum mechanics. In macroscopic scales, environmental disturbances are usually too large to see the quantum nature. So, we proposed a new method to isolate a mg-scale mirror by levitating it with radiation pressure of a laser beam. This optical levitation provides a promising tool to test macroscopic quantum mechanics in almost untouched scale of mg.
Lorentz violation search
According to Einstein's special relativity, the speed of light is isotropic, meaning that it is independent on the direction of propagation. The idea that the laws of physics are independent of the direction is called Lorentz invariance. A wide variety of precise experimental tests of Lorentz invariance have been carried out, but so far no violation has been found. As a consequence, Lorentz invariance underlies all the theories of fundamental physics.
However, theoretical works towards the unification of gravity and quantum theory have revealed the possibility of Lorentz violation. If violated, the speed of light should be slightly anisotropic.
We are performing searches for anisotropy in the speed of light. Especially, we are focusing on measuring the difference between the speed of light propagating in opposite directions, which is difficult to measure with conventional interferometers. We have developed a new type of optical ring cavity to measure the difference. So far, we have succeeded in proving that the relative difference is less than 10-15 level, at the world best limit. Upgrade of the apparatus is ongoing to perform search with increasing precision.