How did the Universe begin?

As was the case when we considered models of how the Universe is structured, when it comes to explaining the beginning of the Universe you again need to know about two competing ideas. You also need to know about the evidence that either supports or questions each theory.

One theory is the Steady State Theory. This states that the Universe has always existed and has always looked the same. On a small scale, stars are born and die but when viewed on a much larger scale, the Universe stays the same.

The other theory is the Big Bang Theory. This states that the Universe was formed by an event that happened in one place (a tiny point) at the very beginning of time, about 13.7 billion years ago. You can think of this model as an explosion of energy. There is no point in asking what came before the Big Bang because time only came into existence when the Big Bang happened.

Both theories are consistent with the expansion of the Universe, the evidence for which comes from the red shifts that we see for distant galaxies. We know that galaxies that look smaller (because they are further away) have bigger red shifts, so they must be moving away (receding) faster. And if we wind the expansion backwards, by reversing time, then this means everything must have started in one place at some time in the past – as suggested by the Big Bang Theory.

It isn’t quite as obvious to see how the Steady State Theory agrees with an expanding Universe unless you imagine that the Universe creates new matter as it expands. This may sound hard to believe – but so too is the idea that the Universe started just out of some strange “big bang”.

Fortunately, there is a further piece of evidence that we can use to decide which theory is the right one.

This evidence was discovered by accident when astronomers detected some background “noise” coming from space. The signal they detected has a wavelength that puts it in the microwave section of the electromagnetic spectrum, and since it is detected in every direction, it is known as Cosmic Microwave Background (CMB) radiation.

CMB radiation does not really fit with the Steady State Theory but it is actually a required feature of the Big Bang Theory because we should be able to detect an afterglow from the initial “explosion” – and it turns out that the afterglow should be a microwave signal!

So given that both theories are supported by an expanding Universe (red shift) but only the Big Bang Theory is supported by CMB radiation, we now believe that the Big Bang Theory is our best idea about how the Universe began.

Cosmology is a fascinating area for study because our theories about the Universe change as new evidence comes to light. But you only need to know the facts that the exam board will require, so don’t get distracted by all the latest ideas! (At least not until after you have sat your GCSE exam.)