The basic idea: when a celestial object (galaxy,
star, quasar) is moving at a high speed away
from Earth, its light waves will be stretched so
that the light appears more red; this is called
‘redshift’. If it is moving toward the Earth the
light waves will be compressed, causing the
light to appear more blue; ‘blueshift.’
Most astronomers believe that when redshift is
observed, it gives them an accurate reading of
‘recessional velocity’ (how fast something is
moving away). So, if they know how fast an
object is moving away from us, they can
calculate when the universe began and how
far away the object is now.
Based on this premise the Big Bang occurred
13.73 billion years ago.
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