By Big Bang, I mean simply the point of this universe's creation.
Cosmologists of previous centuries understandably supposed that the universe was static, timeless or self-sustaining, and these are all valid hypotheses considering the evidence of the times. Yet aside from the necessary requirement of being plausible against all current evidence, hypotheses must also stand the test of time and as new data pours in we must constantly reevaluate our theories.
Much like the stars in the heavenly array appear to be fixed in their respective order, a static universe would be a suspended one where planets and stars simply hung motionless in space. Some proposed a hypothetical repulsive force that could precisely counterbalance the effects of gravity, thus canceling out the universe’s expansion. Such a model explains how massive bodies that should normally attract minor bodies could remain static. From the vantage point of an observer on Earth there doesn’t appear to be a whole lot of motion occurring in the cosmos, and this static universe theory met its first (and last) serious opposition around WWI when American astronomer Vesto Slipher and others observed that distant nebulae recede from Earth at very high velocities.
Electromagnetic energies such as light or infrared radiation travel in waves of differing frequencies, and by studying electromagnetic emissions from distant galaxies over time, scientists discovered that wavelengths from our remotest galactic neighbors are steadily elongating. These consistent fluctuations, called Doppler shifts, led to the conclusion that the universe was actually growing. This strongly suggests that the universe could not be static, but before the new evidence it seemed a stock observation to even the sharpest of analytical minds.
Building on Slipher’s observations, subsequent discoveries of Edwin Hubble in 1927 confirmed that our universe is indeed expanding as distant galaxies hurtle away from ours at rates exceeding thousands of miles per second. Over the thousands of years of human observation that took place before the advent of science, one could easily see why cultures around the world assumed the universe was eternal and had simply been here forever, but when combined with the evidence for an expanding universe, the fact that we can still see light from other galaxies strongly implies that the universe had a beginning in time. If the universe were both eternal and expanding, then stars, quasars and galaxies would have separated to remote distances long ago. Clearly the universe could not be both eternal and expanding, and the eventual conclusion was that all the matter in the universe was once at an ultra-dense, ultra-hot singularity that defies the laws the physics.
From a religious standpoint, there is nothing in scripture which contradicts the idea that all the matter in the universe was once at an ultra-dense, ultra-hot singularity that defies the laws the physics, and this, in hasty paraphrase, is the sequence of events that led to acceptance of the Big Bang.