Supermassive binary black holes ( SMBBHs ) in galactic centers may radiate gravitational wave ( GW ) in the nano-Hertz frequency band , which are expected to be detected by pulsar timing arrays ( PTAs ) in the near future .
GW signals from individual SMBBHs at cosmic distances , if detected by PTAs , are potentially powerful standard sirens that can be used to independently measure distances and thus put constraints on cosmological parameters .
In this paper , we investigate the constraint that may be obtained on the equation of state ( w ) of dark energy by using those SMBBHs , expected to be detected by the PTAs in the Square Kilometre Array ( SKA ) era .
By considering both the currently available SMBBH candidates and mock SMBBHs in the universe resulting from a simple galaxy major merger model , we find that \sim 200 to 3000 SMBBHs with chirp mass > 10 ^ { 9 } M _ { \odot } are expected to be detected with signal-to-noise ratio > 10 by SKA-PTA with conservative and optimistic settings and they can be used to put a constraint on w to an uncertainty of \Delta w \sim 0.02 - 0.1 .
If further information on the mass and mass ratio of those SMBBHs can be provided by electromagnetic observations ( e.g. , chirp mass uncertainty \lesssim 50 \% ) , the constraint may be further improved to \lesssim 0.01 level , as many more SMBBHs will be detected by SKA-PTA with relatively better distance measurements and can be used as the standard sirens .