A novel method for extending the frequency frontier in gravitational wave observations is proposed. It is shown that gravitational waves can excite a magnon. Thus, gravitational waves can be probed by a graviton-magnon detector which measures resonance fluorescence of magnons. Searching for gravitational waves with a wave length λ by using a ferromagnetic sample with a dimension l, the sensitivity of the graviton-magnon detector reaches spectral densities, around 5.4 × 10 −22 × ( l λ/2π ) −2 [Hz −1/2 ] at 14 GHz and 8.6 × 10 −21 × ( l λ/2π ) −2 [Hz −1/2 ] at 8.2 GHz, respectively.

We study constant-roll inflation in the presence of a gauge field coupled to an inflaton. By imposing the constant anisotropy condition, we find new exact anisotropic constantroll inflationary solutions which include anisotropic powerlaw inflation as a special case. We also numerically show that the new anisotropic solutions are attractors in the phase space.

We study inflation with anisotropic hair induced by form fields. In four dimensions, the relevant form fields are gauge (one-form) fields and two-form fields. Assuming the exponential form of potential and gauge kinetic functions, we find new exact power-law solutions endowed with anisotropic hair. We also explore the phase space of anisotropic inflation and find fixed points corresponding to the exact power-law solutions. Moreover, we perform the stability analysis around the fixed points to reveal the structure of the phase space. It turns out that one of the fixed points becomes an attractor and others (if any) are saddle points. In particular, the one corresponding to anisotropic inflation becomes an attractor when it exists. We also argue that various anisotropic inflation models can be designed by choosing coupling constants.

We reveal the universality of short-term anisotropic inflation. As a demonstration, we study inflation with an exponential type gauge kinetic function which is ubiquitous in models obtained by dimensional reduction from higher dimensional fundamental theory. It turns out that an anisotropic inflation universally takes place in the later stage of conventional inflation. Remarkably, we find that primordial gravitational waves with a peak amplitude around 10 −26 ∼ 10 −27 are copiously produced in high-frequency bands 10MHz∼100MHz. If we could detect such gravitational waves in future, we would be able to probe higher dimensional fundamental theory.

We study the ultralight vector dark matter with a mass around $$10^{-23}\,\mathrm {eV}$$10-23eV. The vector field oscillating coherently on galactic scales induces oscillations of the spacetime metric with a frequency around nHz, which is detectable by pulsar timing arrays. We find that the pulsar timing signal due to the vector dark matter has nontrivial angular dependence unlike the scalar dark matter and the maximal amplitude is three times larger than that of the scalar dark matter.