Origin of the Universe 13.7 bya

Expansion of the Universe

Our story starts 13.7 billion years ago (bya) with a very important observation: that the Universe is expanding. This is the foundation of our understanding of the past. It is the single most important fact in this story. So how do we know that it is expanding?

Have you ever noticed that the pitch of a moving vehicle or train rises when it comes towards you and drops as it goes past you? That is called the Doppler Effect.

While studying the light from stars which orbited each other, Austrian physicist Christian Doppler (1803-1853) suggested that their color should change in the same way. A star moving towards us should look bluer than a stationary one (this is called the blue shift) and one moving away from us should look a little redder (the red shift).

In 1912 American astronomer Vesto Slipher observed both the red and blue shifts, and he found that red shifts were far more common, especially in stars contained in spiral clouds or ‘nebulae'. So how to explain this strange observation?

Early astronomers, Isaac Newton for example, believed that the universe must be static, because the heavens were closest to God, whom he thought perfect and unchanging. Albert Einstein too believed that the Universe was static. He believed that "one would get into bottomless speculation if one departed from" this hypothesis. But the fields equations he produced in 1915 to describe how spacetime is deformed by the matter it contains (his theory of general relativity) predicted the Universe should be contracting under the force of gravity. This he could not accept, so he introduced a new repulsive force into the equations which would stop it contracting and make it stable. He called this the cosmological constant. It was a fix, a ruse to save his theory from disaster.

But in 1922 Alexander Friedmann found he could solve the field equations without this cosmological constant if he assumed the Universe was expanding. In 1927 Belgian priest Georges Lemaître rediscovered this solution and invented a theory which could account for such a situation.

Lemaître realized that the Universe must have been smaller in the past. Far back in the distant past it must have been very small indeed. Lemaître proposed the Universe expanded from an initial point which he called the "Primeval Atom". He also called it the "Cosmic Egg" which exploded at the moment of the creation.

The proof of this hypothesis came in 1929.

Slipher had already used the red shift to show that many stars are moving away from us, but to prove the whole Universe is expanding we need to be able to measure the distance to those stars. It was American astronomer Edwin Hubble who did this in 1929, using special stars called Cepheid variables to establish the distance to stars and proving that the furthest stars were moving away from us the fastest. Why should this be?

The simplest assumption is that one would see the same pattern no matter where one was in the Universe. This implies that the whole Universe is expanding, like a cake rising in an oven. All the stars move away from each other, like the currants in the cake.

When Einstein heard this he abandoned the cosmological constant, calling it the biggest blunder he ever made. However, as we will see, the idea has not gone away.

It seems that the rate of expansion of the Universe is now increasing, which has led to the theory of Dark Energy. One possible explanation is that this comes from the cosmological constant. Also, early in history, the Universe expanded very rapidly during the process called inflation. This too was caused by a form of cosmological constant. So Einstein's blunder has produced fruitful results!

We are led to our central theory about the history of the Universe: that it began very small and has been expanding ever since.