Question

Some scientific evidence suggests that our universe is ever-expanding from a hot and dense initial state. Briefly describe at least one piece of evidence that supports the theory known as the Big Bang.

Answer 1

Doppler shift. ;-)

\[f = f_o\cdot (\frac{c\pm v_r}{c\pm v_s})\]
Where:
f = frequency observed by the receiver/recipient
f_o = initial frequency emitted
v_s = velocity of source, positive if the source is moving away from the receiver
v_r = velocity of receiver, positive if the receiver is moving towards the source

Light has a known speed. It's 299,792,458 metres per second in a vacuum. Space is mostly vacuum. Light can be bent and distorted by vacuum, but by looking as the shift in wavelengths of light (and using this simple formula) we can know whether something is coming or going.

Guess what astronomers discovered?

Answer 2

PS: You've encountered this phenomenon before in your life several times but most often with sound waves instead of light. (i.e.: police sirens as they drive by relative to you)

Answer 3

The most obvious piece of evidence is that the night sky is mostly dark. If the universe were infinite and filled with stars, then in any direction you looked, your line of sight would eventually intersect the surface of a star. Hence the night sky would not be dark, but something more reddish-orange, the typical color of a star's surface.

The pre-relativity explanation for this observation (often called Olber's Paradox) was that the universe was finite, or at least only a finite portion of it contained stars. But this is a bit awkward. If the universe is finite, what's outside it? Or, alternatively, why would only a small finite portion of an infinite universe contain stars?

However, if the universe is expanding, this solve the problem, because the relativistic doppler shift, mentioned by agentx5 above, means that stars at a great enough distance from us (and hence receding at a high enough velocity) have the light they emit doppler shifted out of the visible range, or (if they are receding at or above c) out of detection entirely. So they are invisible.

In other words, when you look at the night sky and fail to see most of the stars contained in the universe, you are directly observing the relativistic doppler shift that proves the universe is expanding.