One of the greatest challenges that astronomers face is determining distances beyond the reach of current science. For instance, today, we can estimate that the universe has a radius of around 13 billion light years, a figure obtained by multiplying our estimated age of the universe by the speed of light.
The reason why this fairly basic equation is used is that we can only see as far as the distance that light could have reached us since the universe began. Scientists can get a good idea of the age of the universe by estimating the age of the oldest stars we can see, which evolve predictably over billions of years according to the conventions of atomic and nuclear physical theories. They can also estimate the time of the Big Bang by calculating the distances of many galaxies, then establishing the speed that they’re moving at and extrapolating back.
Ultimately, we are limited by what we can observe, because some scientific theories propose that the universe expanded so rapidly after the Big Bang event that only part of it has remained within range of our current methods of detection.