How do we “time travel” while observing the universe?
Light is fast; however, its speed
is not unlimited. Therefore, lights from other objects take some time to reach
us. Hence, the events that happened a long time ago become visible to us only
after the light rays from those events have reached us. Consider a supernova
explosion. If the explosion happened about 6 trillion miles away (about the
distance of a light year), then we would see the explosion happening only a
year after it has actually happened (lookback time). Thus, distant light is
equivalent to old light for they present us events that happened mostly a long
while ago after we first see them.
The Evidence for Big Bang
There is copious evidence for that the universe began in a hot, dense state 13.8 billion years ago. Here are two of them:
1.
Scientists know that the universe is expanding.
If we trace back time, then there must have been a time when all the universe
was squeezed into a single point.
2.
Microwave background is present wherever we look
distant at the observable universe. As the universe expanded from its dense
state, it left a glow that corresponded to its hot, early state as a dense
point of squeezed mass and energy.
