reaches the sensitivity of SKA. In a more extreme case α ≲ 10 − $^{13}$, the sensitivity band of space-borne detectors, BBO [24] and DECIGO [25] are open for detection of kink-induced GWs, and Ω$_{gw}$ exceeds the sensitivity of correlated analysis of BBO for Gµ ∼ 10 − $^{7}$, and that of ultimate-DECIGO for Gµ ≳ 10 − $^{9}$.$^{7}$In even more extreme case, α ≲ 10 − $^{16}$, it may be possible to detect the inflection point in Figure 4 for Gµ ≳ 10 − 10 and determine the reheating temperature as T$_{r}$ ∼ 10 MeV. 

Moreover, GWs from kinks may be detected through CMB observations. As opposed to GWs from loops, kinks can emit GWs with wavelength comparable to the horizon scale. These GWs induce B-mode polarizations, which is a target of on-going and future CMB surveys. The spectrum of GWs from kinks is quite different from inflationary GWs and hence its effect on CMB is also expected to be distinguished from that of inflationary origin. We will study this issue elsewhere. 

## VI. CONCLUSIONS 

In this paper, we have considered gravitational waves emitted by kinks on infinite cosmic strings. We have calculated the spectrum of the stochastic background of such gravitational waves and discussed their detectability by pulsar timing experiments and space-borne detec- tors. It is found that if the size of cosmic string loops is much smaller than that of Hubble horizon, some frequency bands are open for detection of GWs originating from kinks. It can be detected by pulsar timing experiments for Gµ ≳ 10 − $^{7}$, and by space-borne gravita- tional wave detectors for much smaller Gµ , although the latter may be hidden by the loop contribution unless the typical loop size is extremely small. If it is detected, it will pro- vide information on the physics of the early Universe, such as phase transition and inflation models. Moreover, the spectrum shape depends on the thermal history of the Universe, and hence GWs from cosmic strings can be used as a direct probe into the early evolution of the Universe. Notice that the inflationary GWs also carry information on the thermal history of the Universe [29–31]. Although the inflationary GWs are completely hindered by GWs from cosmic strings if the value of Gµ is sizable and GWs from kinks come to dominate in low-frequency region, GWs from cosmic strings also have rich information on the physics of 

