A major piece of evidence supporting the Big Bang

theory is the observation that wavelengths of

light from stars in distant galaxies show a

(1) redshift, appearing to be shorter

(2) redshift, appearing to be longer

(3) blueshift, appearing to be shorter

(4) blueshift, appearing to be longer

Electromagnetic radiation from stars in distant galaxies shows

a "red-shift", that is, a shift that makes the wavelengths appear

to be longer than they really should be.

The real trick to this kind of observation is figuring out what the

wavelengths you observe really should be, since you only see them

in their shifted condition.

The only process we know of that can make wavelengths longer

is when the source of the waves is moving away from the observer.

So we assume that this is the cause of all the radiation from distant

galaxies having its wavelengths stretched, and this leads us to the

concept that the universe is expanding. There's no way to prove it,

so the expanding universe is "only a theory" ... just like gravity and

elliptical orbits and nuclear fusion, which also can't be proven.