Since 1916, general relativity has provided a description of gravity that can explain many observations, including objects in free fall, gravitational lensing by massive objects, and black holes. Despite the success of the theory for nearly 100 years, scientists have been looking at ways to modify general relativity in order for it to better explain certain observations—particularly the accelerated expansion of the universe. Although these modifications can be very different from one another, they generally fall into the category of "modified gravity."
Like any scientific prediction, modified gravity must be experimentally tested in order for scientists to confirm its validity. Although significant progress has been achieved in recent years in designing observational tests of gravity's effects in the universe that might reveal the presence of modified gravity, there is still no conclusive evidence for its existence.
Now in a new paper published in Physical Review Letters, Wojciech A. Hellwing, et al., have proposed a new test of modified gravity that is based on measuring the tendency of well-separated galaxies to approach each other. This movement is called the galaxy pairwise velocity.