We study the scalar stochastic gravitational-wave background ( SGWB ) from astrophysical sources , including compact binary mergers and stellar collapses , in the Bras-Dicke theory of gravity .
By contrast to tensor waves , we found the scalar SGWB to predominantly arise from stellar collapses .
These collapses not only take place at higher astrophysical rates , but emit more energy .
This is because , unlike tensor radiation , which mainly starts from quadrupole order , the scalar perturbation can be excited by changes in the monopole moment .
In particular , in the case of stellar collapse into a neutron star or a black hole , the monopole radiation , at frequencies below 100 Hz , is dominated by the memory effect .
At low frequencies , the scalar SGWB spectrum follows a power law of \Omega _ { \text { S } } \propto f ^ { \alpha } , with \alpha = 1 .
We predict that \Omega _ { \text { S } } is inversely proportional to the square of \omega _ { BD } +2 , with ( \omega _ { BD } +2 ) ^ { 2 } \Omega _ { S } ( f = 25 { Hz } ) = 2.8 \times 10 ^ { -6 } .
We also estimate the detectability of the scalar SGWB for current and third-generation detector networks , and the bound on \omega _ { BD } that can be imposed from these observations .