gravitational wave spectrum

The gravitational wave spectrum (GW spectrum), the range of frequencies or wavelengths of gravitational waves, can be divided into regimes based upon types of triggering events and/or methods of GW detection. The waves travel at the speed of light in a vacuum (c) so the same frequency/wavelength relation holds as for electromagnetic radiation. The waves are caused by reconfiguration of the relative positions of masses, such as when massive objects move between positions closer to and further from you. Orbiting bodies produce such waves, on the order of a wave per orbit (for circular orbits of equal sized bodies, actually two waves per orbit: there are two times within the orbit when one of the identically-massed bodies is closer to you). In consequence, such waves, like orbits, can have periods ranging from seconds to centuries and beyond, and you see mention of GW frequencies such as 10^-10 Hz (which corresponds to a period of about three centuries).

Due to the need to sense extremely small distance-changes, sensing a particular wave (e.g., by an instrument) requires that the wave have sufficient amplitude to stand out among the background of waves from every orbit in the universe, i.e., to distinguish it from the gravitational wave background (GWB). The amplitude (gravitational wave strain) is related to the masses of the orbiting objects, and orbiting black holes/neutron stars produce waves more likely to show above the noise. Orbiting binary SMBHs have the most mass but there may be few or none to observe, given their numbers and the final parsec problem, and many would have the very long periods, a challenge to identify. The GW frequency regimes:

Some GWs are expected to be left from early universe activities (primordial gravitational waves or primordial GWs), which potentially span all these regimes. Supernovae under certain conditions may create detectable waves similar to that of merging stellar mass black holes. Ground GW detectors include LIGO, Virgo, and KAGRA, and PTAs include NANOGrav, Parkes Pulsar Timing Array (PPTA), European Pulsar Timing Array (EPTA), Chinese Pulsar Timing Array (CPTA), MeerKAT Pulsar Timing Array (MPTA), Indian Pulsar Timing Array (InPTA), and the International Pulsar Timing Array (IPTA). There are no space-based detectors now, but plans/concepts include LISA, the New Gravitational Wave Observatory (NGO), DECIGO, and TianQin. The planned space-based arm-lengths vary from a thousand to five million km (as compared to ground-based detectors, which have up to 4 km arms, with future plans up to 40 km).