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DECIGO/BBO as a probe to constrain alternative theories of gravity

We calculate how strongly one can constrain the alternative theories of gravity with deci-Hz gravitational wave interferometers such as DECIGO and BBO. Here we discuss Brans-Dicke theory and massive graviton theories as typical examples. We consider the inspiral of compact binaries composed of a neutron star (NS) and an intermediate mass black hole (IMBH) for Brans-Dicke (BD) theory and those composed of a super massive black hole (SMBH) and a black hole (SMBH) for massive graviton theories. Using the restricted 2PN waveforms including spin effects and taking the spin precession into account, we perform the Monte Carlo simulations of $10^4$ binaries to estimate the determination accuracy of binary parameters including the Brans-Dicke parameter $ω_{\mathrm{BD}}$ and the graviton Compton length $λ_g$. Assuming a $(1.4, 10)M_{\odot}$ NS/BH binary of SNR=$\sqrt{200}$, the constraint on $ω_{\mathrm{BD}}$ is obtained as $ω_{\mathrm{BD}}>2.32\times 10^6$, which is 300 times stronger than the estimated constraint from LISA observation. Furthermore, we find that, due to the expected large merger rate of NS/BH binaries of $O(10^4)$ yr$^{-1}$, a statistical analysis yields $ω_{\mathrm{BD}}>3.77\times10^8$, which is 4 orders of magnitude stronger than the current strongest bound obtained from the solar system experiment. For massive graviton theories, assuming a $(10^6, 10^5)M_{\odot}$ BH/BH binary at 3Gpc, one can put a constraint $λ_g>3.35\times10^{20}$cm, on average. This is three orders of magnitude stronger than the one obtained from the solar system experiment. From these results, it is understood that DECIGO/BBO is a very powerful tool for constraining alternative theories of gravity, too.