Gravitational waves do not directly cause the expansion of the universe. The expansion of the universe is primarily driven by a phenomenon called cosmic expansion, which is described by the theory of general relativity. The expansion of the universe is the result of the overall distribution of matter and energy, as well as the properties of space itself.
Gravitational waves are ripples in the fabric of spacetime that propagate through space at the speed of light. They are generated by certain astrophysical events, such as the merging of black holes or the explosion of massive stars. While gravitational waves can affect the motion of matter and energy as they pass through, they do not drive the overall expansion of the universe.
The rate of expansion of the universe is determined by a parameter called the Hubble constant, denoted by the symbol "H0." The current best estimate for the value of the Hubble constant is approximately 67.4 kilometers per second per megaparsec (km/s/Mpc). This means that for every megaparsec (a unit of distance equal to about 3.26 million light-years), a galaxy appears to recede from us at a speed of 67.4 kilometers per second due to the expansion of the universe.
So, gravitational waves themselves do not play a direct role in the expansion of the universe, and there is no specific speed requirement for them to have an effect on cosmic expansion.