An examination of the most carefully written scientific treatments of the astronomical evidence, and of the cosmological theory which appears to fit the evidence, will discover that the big bang theory does not posit an absolute beginning of the cosmos — a coming into existence out of nothing — but only an initial event in the development of the cosmos as we now know it, an event that occurred at a time that is estimated as between fifteen and twenty billion years ago.
Our present techniques of observation and measurement, and the technical facilities they employ, do not permit us to penetrate the past beyond the time, some fifteen to twenty billion years ago, when the big bang occurred.
What is being said here is not that past time is limited (finite rather than infinite), but only that our knowledge of past time is limited — limited to a time beyond which our observations and measurements cannot go. Time may extend back infinitely beyond that initial explosion of matter, out of which the present shape of the cosmos has developed, but unless some radical alteration in our techniques and instruments of observation and measurements occurs, we will never be able to penetrate the veil that hides the infinite past from us.
— Mortimer J. Adler