Evidence for the Big Bang
Astronomers think that the Universe started with the Big Bang. As with all science, this is based on evidence; so what is the evidence for the Big Bang theory?
A Spiral Galaxy
© Liverpool Telescope Image
Redshift of Galaxies
The redshift of distant galaxies means that the Universe is probably expanding. If we then go back far enough in time, everything must have been squashed together into a tiny dot - this is the Big Bang.
WMAP CMB map
Credit: WMAP Satellite
Microwave Background
Very early in its history, the whole Universe was very hot. As it expanded, this heat left behind a "glow" that fills the entire Universe. The Big Bang theory not only predicts that this glow should exist, but that it should be visible as microwaves (part of the Electromagnetic Spectrum).
This is the Cosmic Microwave Background which has been accurately measured and is very good evidence that the Big Bang theory is correct.
The Sun contains elements made by other stars since the Big Bang
Mixture of Elements
As the Universe expanded and cooled down, some of the elements that we see today were created. The Big Bang theory predicts how much of each element should have been made, and what we see in very distant galaxies and old stars is just right.
(You cannot look in new stars like the Sun as new elements are made in stars and so the amount of each one is different in new stars than the old ones made when the Universe was much younger).
Distant Galaxies in the early Universe
Looking back in Time
The main alternative theory to the Big Bang theory of the Universe is called the Steady State theory.
In this theory, the Universe does not change very much with time.
Remeber that because light takes a long time to travel across the Universe, when we look at very distant galaxies, we are also looking back in time.
From this we can see that galaxies a long time ago were quite different from those today, showing that the Universe has changed. This fits better with the Big Bang theory than the Steady State theory.
