  Hedgehog and antihedgehog spin textures in magnets behave as emergent
monopoles and antimonopoles, which give rise to astonishing transport and
electromagnetic phenomena. Using the Kondo-lattice model in three dimensions,
we theoretically study collective spin-wave excitation modes of magnetic
hedgehog lattices which have recently been discovered in itinerant magnets such
as MnSi1-xGex and SrFeO3. It is revealed that the spin-wave modes, which appear
in the sub-terahertz regime, have dominant amplitudes localized at Dirac
strings connecting hedgehog-antihedgehog pairs and are characterized by their
translational oscillations. It is found that their spectral features
sensitively depend on the number and configuration of the Dirac strings and,
thus, can be exploited for identifying the topological phase transitions
associated with the monopole-antimonopole pair annihilations.
