If we consider the course of light to be a rich curve like a magnetic field line, perception that is not infinite is born.
13 billion light-years is also a shorter distance than I thought, maybe the next galaxy.
Considering like this, if the carrier's enterprise goes straight along the straight route as human good, the route itself is considered to be a warp of the space.
If there is gravitational wave at present, it will be the most straightforward and representative of space warp.
In addition, it can not be called warp unless it is at least 1 million kilometers per second.