..zero

Initial singularity.

The universe is believed to have begun with a singularity, a state of infinite density which then began to expand. A very short time after the singularity began to expand, the universe underwent a period of rapid expansion called inflation. The expansion was very rapid, the universe expanding by a factor of in less than second.

When inflation ceased the universe was extremely hot and normal matter could not exist. A quark-gluon plasma formed. Quarks and Gluons make up particles such as Protons and Neutrons that make up atoms.

When the universe was about a ten thousandth of a second old the temperature had dropped enough for protons and neutrons to form out of the quark-gluon plasma.

The universe continued expanding and cooling and after about a second the Protons and Neutrons that had formed out of the quark-gluon plasma began to form into light nuclei. The process producing light nuclei lasted for only a few minutes and at the end of that time the universe contained substantial amounts of hydrogen and helium4 with varying amounts of deuterium, helium3 and lithium7.

After about 400,000 years the temperature of the universe had dropped to 3000°K. Prior to this, energy and matter had interacted in such a way that light could not travel through the universe. The universe was opaque. However once the temperature dropped below 3000°K conditions changed abruptly, electrons combined with nuclei forming atoms allowing light to travel throughout the universe, the universe had become transparent. This produced a signal called the Cosmic Microwave Background which was discovered by Penzias and Wilson in 1965. A map can be made from this signal giving us a picture of the universe at an age of 400,000 years.

 ..............Cosmic Microwave Background - Credit: NASA/WMAP Science Team

The universe then entered a period called the dark ages in which there was no light because there were no stars. The dark ages lasted for about 200 million years after which the first stars formed.

These first stars were not like present day stars, they were up to 100 times larger than the Sun and millions of times brighter. This was because there were no heavy elements in the universe at the time and the lack of heavy elements makes a star larger.

After 400 million years stars had begun to form into small galaxies, these small galaxies then merged with each other forming larger ones. By 2 billion years most of the small galaxies had merged.

The universe had continued to expand but the rate of expansion was slowing down due to the gravitational attraction of the matter in the universe. As the universe expands the galaxies tend to move away from each other, however gravity tries to pull the galaxies together. Gravity then works against the expansion tending to slow it down. If there is enough matter and therefore enough gravity, the time would come when expansion would stop and the universe would collapse in on itself in a so called big crunch. If there was not enough matter in the universe to create a big crunch then the universe would continue to expand but at a slower and slower rate.

It was discovered in 1998 that about 5 billion years after the big bang the expansion of the universe had begun to speed up again and has continued to speed up to the present day. The exact cause of the acceleration of the rate of expansion is uncertain but it appears it is caused by energy contained in empty space. This energy creates a form of antigravity forcing the universe apart.

You will recall that when Einstein realised that his equations indicated an expanding universe he added an extra term to his equations called the cosmological constant to cancel out the expansion and allow for a static universe. When it was discovered that the universe was indeed expanding he dropped the cosmological constant from his equations calling it the greatest blunder of his life. Now it appears that the cosmological constant does exist but in a somewhat different form. Einstein was right after all.

..second

Inflation ends.

Quark - gluon plasma.

..second

Protons, neutrons form.

.1 second

Hydrogen and helium nuclei form. 

. 400,000 ..years

Atoms form, universe becomes transparent, cosmic microwave background generated. 

Dark ages - no stars.

.200 million ..years

First stars form.

.400 million ..years 

First generation of small galaxies. 

2 billion years 

Large galaxies from merging of small galaxies. 

5 billion years 

Dark energy dominates. Expansion of universe accelerates. 

13.7 billion .years 

Present day. 

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