You might have come across articles talking about galaxies that are 30 billion light years away from us. If our universe is only 13.7 billion years old, how can we have galaxies so far away? Einstein has shown nothing can travel faster than light, right?
Overcoming Einstein’s ‘speed limit’
Hubble’s law implies that galaxies will move away from us faster if their distance from us exceeds a particular value. This violates Einstein’s special relativity theory from 1905 but not his general relativity theory from 1915, and the latter is Einstein’s final word on the subject, according to cosmologist Max Tegmark
“Whereas special relativity says that no two objects can move faster than light relative to one another under any circumstances, general relativity merely insists that they can’t move faster than light relative to one another when they’re in the same place … Nothing is allowed to move faster than light through space, but space itself is free to stretch however fast it wants to,” he writes in his book Our Mathematical Universe
1 light year
is equal to the distance travelled by light in vacuum in one Julian year (365.25 days). As can be seen, it is an unit of length and one light-year = 9,460,730,472,580.8 km. It is used because the distances in astronomy are so great that the usual units become cumbersome