If the universe is currently expanding, this indicates that the Universe was originally much smaller.
Well, how small was the Universe before it started to expand? and how does something so tiny suddenly expand to become a huge ever-expanding Universe?
The honest answer is that we don’t know, but we have some pretty good ideas and a widely accepted idea is the Big Bang Theory.
According to the big bang theory, all of the matter in the Universe initially occupied a very small space, which was really hot and dense. Then around 13.8 billion years ago, this small space suddenly exploded, due to its enormous energy. After this explosion, it began to expand, which is still going on today.
As the space between galaxies is expanding, they get further apart over time.
In the 1960s, Arno Penzias and Robert Wilson discovered the cosmic microwave background radiation (CMBR).
The CMBR is often referred to as the smoking gun left over from the big bang. It is leftover radiation from the time when the universe began (the big bang), and it fills the entire universe.
Initially, the CMBR must have started on the high frequency, short wavelength end of the spectrum (gamma waves). However, as time has passed and the universe has expanded, it has decreased in frequency and increased in wavelength. So, it is now at the microwave portion of the spectrum.
The discovery of the CMBR was predicted by the Big Bang theory, so it provides strong support for the theory.