Determining the size of the universe is a huge undertaking. Despite centuries of speculation and scientific exploration, the question of how big our universe really is -- or if it is, in fact, infinite -- remains unanswered, and may be unanswerable. Sheer scale presents the greatest challenge to determining the size of the universe, which is simply too large to see in its entirety. Even the most advanced instruments, which allow astronomers to observe galaxies 10 to 12 billion light years away, give no clue to what lies beyond the distance light could have traveled since the birth of the universe, about 14 billion years ago.
These challenges have not stopped astronomers from trying, though. After all, scientific understanding often follows from indirect observations, the formulation of theories and testable hypotheses, and the creation of scientific models. For example, at the opposite end of the scale with regard to size, no one has ever seen an atom. Yet, through experimentation and indirect observations, physicists have developed a clear understanding of this once mysterious particle's basic structure.
The same scientific process, based mostly on indirect observations, has led to some startling discoveries about the size of the universe. In 1929, for example, astronomer Edwin Hubble found that the universe is expanding, rather than collapsing, as the theory of gravity would suggest. More recently, astronomers have found evidence to suggest that the expansion of the universe, rather than slowing, is accelerating in response to an unknown force stronger than gravity. This latest theory is based, in part, on the wavelength of light given off by exploding stars, called supernovae. This indirect evidence allows astronomers to determine both the distance and the rate of acceleration of the supernovae and the galaxies to which they belong.
Ultimately, such diligent use of the scientific process may result in an answer to the question, How big is the universe? Then again, we may never know. Regardless, the search for an answer will undoubtedly provide a better understanding of many other aspects of our universe and may give rise to a whole new set of compelling questions.