The beginning of the Universe

It is now generally accepted by most astronomers that the Universe as we know it began with an unimaginably huge expansion which occured some 14 000 000 000 years (1.4x10¹⁰ years). We call this the Big Bang. The age of the universe is therefore 1.4x10¹⁰ years.

Time and space both originated at the same time with the Big Bang. Before that there was no space and no time – the Big Bang ‘created’ space and time. We cannot ask what happened before the Big Bang because before that moment nothing existed – no space and no time!

It is interesting to compare time with distance and think of a scale where a distance of one metre represents a thousand million years. The age of the Universe is then represented by a distance of around 14m, a million years by only 1 mm and one human lifespan by 0.1 μm!

The fascination of Cosmology is the ability to 'look back' to the beginning of the Universe and predict what may have happened then.

Moments after the big bang

Time after the Big Bang	Nature of the Universe	Temperature
10-43s	Grand unification epoch
10-34 - 10-10 s	Electroweak epoch Quark epoch
10-10s	Particle soup dominates	1015 K
1 s	Neutrons and protons formed	1010 K
3 mins	Helium nuclei formed	109
300 000 years	Microwave background fills the universe	6000 K
500 000 years	Temperature falls further. Infra red.	750 K
1 million years	Atoms form. Stars and galaxies exist The universe becomes transparent
1 billion (109) years	The first stars. Heavy elements form.	18 K (-255 oC)
14 billion years	The present day	2.7 K (-270.3 oC)

The Big Bang - the beginning of the Universe

As thousands of years passed the Universe cooled from the initial enormous temperatures of the Big Bang (in the region of 10¹⁵ K).

At some time in the past, roughly 500 000 years after the Big Bang the Universe became dark. The radiation emitted had passed over the barrier between visible and the infra red. From then there was no light until the primeval matter had condensed into atoms and these had slowly grouped together under gravitational attraction to make the embryo of a star. Eventually the temperature in the centre of these stars had become high enough for nuclear fusion to take place - the first star was born and blazed out into the darkness of space - there was light!

The currant filled loaf of bread 3D model of the expanding universe

Imagine the Universe to be like a loaf of bread filled with currants. As the bread is cooked it expands - every currant moving away from every other currant. As far as each currant is concerned it is at the centre of expansion because it "sees" currants moving away from it in all directions. This is just like the expansion of the Universe - the universe being represented by the loaf of bread and the currants representing the galaxies

Another good analogy is to thread three or four large polystyrene balls (to represent galaxies) onto a length of elastic, one end of which is fixed to a hook on the wall or a secure retort stand. If the free end of the elastic is pulled all the balls separate from each other.

The point about all these analogies is that they make their own space as they expand. Similarly before the universe began to expand there was no space! Actually there was no time either - both space and time were "created" at the start of the universe.

The Echo of the Big Bang

As time passed since the Big Bang the Universe cooled. The temperature in deep space dropped and dropped. The background radiation moved into the infrared and the cooling continued.

The radiation from interstellar space at present represents a temperature of 2.7K (- 270.3 ^oC) and this radiation is known as the Echo of the Big Bang. If you look at a "detuned" TV signal some of it (admittedly less than 1%) is the Cosmic Microwave Background Radiation (CMB) – the Echo of the Big Bang. The CMB was thought to be uniform but discontinuities have been discovered within it.