Ph.D., 2012, Kyoto
Theoretical Astrophysics, Gravity and Cosmology
My current research goal is to probe fundamental physics using astrophysical compact objects, such as black holes and neutron stars. I am particularly interested in testing strong/dynamical-field gravity and determining the equation of state of nuclear matter with gravitational waves from compact binaries. Such gravitational waves allow us to probe gravity and nuclear physics in the regime that was inaccessible from previous experiments and observations. Deviations away from General Relativity and tidal effects on neutron stars that contain nuclear physics information both affect binary’s orbital dynamics, and such effects are encoded in the gravitational waveform. Having these theoretical predictions at hand, one can carry out a data analysis study to address how well future gravitational wave observations can probe these fundamental physics.
I am also interested in using binary pulsar observations to probe strong-field gravity. Typical gravitational theories beyond General Relativity have extra gravitational degrees of freedom on top of the General Relativity ones, which enhance the amount of radiation from a binary system and change the amount of orbital shrinkage due to such emission. Since pulsars act as very precise “clocks”, one can measure the orbital decay rate of a binary pulsar very accurately, which allows one to probe the presence of the additional gravitational degrees of freedom.
I also work on the theoretical modeling of neutron stars. In particular, I am interested in studying universal relations among neutron star observables that are insensitive to the stellar internal structure. This is in contrast with the famous neutron star mass-radius relation that depends strongly on the equation of state. One example of such universal relations is the “I-Love-Q” relations, those among the moment of inertia (I), the tidal deformability (or the Love number), and the quadrupole moment (Q). Universal relations allow us to probe important physics, including astrophysics, nuclear physics, gravitational physics, and cosmology, with future electromagnetic wave and gravitational wave observations.
Our group has a strong collaboration with researchers within the physics department, such as high energy physicists and nuclear physicists, as well as people in the Astronomy Department and NRAO.
Z. Carson and K. Yagi, "Multi-band gravitational wave tests of general relativity," Class. Quant. Grav. Lett. 37, 02LT01 (2020).
Z. Carson, A. W. Steiner and K. Yagi, "Constraining nuclear matter parameters with GW170817", Phys. Rev. D99, 043010 (2019).
S. Tahura and K. Yagi, "Parameterized Post-Einsteinian Gravitational Waveforms in Various Modified Theories of Gravity," Phys. Rev. D98, 084042 (2018).
N. Yunes, K. Yagi and F. Pretorius, “Theoretical Physics Implications of the Binary Black-Hole Mergers GW150914 and GW151226, Phys. Rev. D94, 084002 (2016) [Editors’ Suggestion].
K. Yagi, D. Blas, N. Yunes and E. Barausse, “Strong Binary Pulsar Constraints on Lorentz Violation in Gravity,” Phys. Rev. Lett. 112, 161101 (2014).
K. Yagi and N. Yunes, “I-Love-Q,'' Science 341, 365 (2013).
K. Yagi, N. Yunes and T. Tanaka, “Gravitational Waves from Quasi-Circular Black Hole Binaries in Dynamical Chern-Simons Gravity,” Phys. Rev. Lett. 109, 251105 (2012).
“For his insightful and broad contributions to the physics of gravitational waves, neutron stars, and experimental gravitation”