1. Big Bang Theory
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2. Big Bang theory
Theory states that in the 13.7 billion years before the Universe began it expanded from a tiny dense and extremely hot fireball to the enormous but cold and diffuse (spread out) universe we see today

3. Evidence for Big Bang 1. Redshift of Galaxies 2. Microwave Background
3. Mixture of Elements
4. Looking back in time

4. 1. What is Redshift?
Most galaxies appear to be moving away from us. This means that the light from them is “stretched” slightly and appears a bit redder than it would otherwise do.
This is similar to what happens to sound. Imagine that a fire engine is driving past you with its siren on. When it is moving towards you, the siren it quite high pitched. When it is moving away from you, the pitch drops and it sounds lower. With light, "lower" means more red and "higher" means more blue.
You can see what this looks like in the diagram below, where two galaxy spectra are compared.

5. 1. Redshift of Galaxies
The redshift of distant galaxies means that the Universe is probably expanding.
If we then go go back far enough in time, everything must have been squashed together into a tiny dot.
The rapid eruption from this tiny dot was the Big Bang.

6. 2. 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 by orbiting detectors, and is very good evidence that the Big Bang theory is correct.

7. 3. 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 in the early universe, and what we see in very distant galaxies and old stars is just right.
You cannot look in new stars, like the Sun, for this evidence, because they contain elements that were created in previous generations of stars.
As such, the composition of new stars will be very different from the composition of stars that existed 7 billion years ago, shortly after the Big Bang.

8. 4. Looking back in time
The main alternative 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.
Remember 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.

9. How long does light take to travel?
The light-year is a way of measuring distance - and not time as you might think from the name. It is the distance that light travels in one year.
You may not think that light takes any time at all to travel - after all, there is no pause after you turn on a light switch, but that is because it travels very fast. In fact, it moves 300,000 km every second. That is more than 7 times around the Earth in a single second!

10. How long does light take to travel?
The light-year is therefore a very long way, and is useful for measuring the huge distances between stars and galaxies. The following are a few examples of typical light-travel times:
To the Moon – 1.3 seconds
To the Sun – 8 minutes
To the nearest Star – 4 years
To centre of the Milky Way – 30,000 years
To the nearest Galaxy – 2 million years
Of course, because light takes so long to travel between stars and galaxies, when you look at very distant objects, you are actually looking back in time! Astronomers use this to look at the history of stars and even the whole Universe.