How did the universe begin? The Big Bang theory believes that around 13.7 billion years ago, the universe was a single point. The point exploded due to heat and density. Then the temperature and density decreased, the universe cooled. Items were formed and atoms gathered in clouds of gas and radiation. From these clouds, stars and galaxies formed. Earth's system formed billions of years after the Big Bang, about 4.5 billion years ago; this is about 9 billion years after the Big Bang occurred.
Scientific math supports the Big Bang theory. Cosmic radiation can be measured, and the levels are what would be expected if the explosion occurred. According to this theory, the universe is still expanding. Galaxies are still moving out from a center point of the universe. Scientists have not found objects older than 14 billion years.
The Cyclic model is an addition to the Big Bang theory. The Big Bang theory proposes space and time began at a single point. The Cyclic model proposes that the Big Bang was just a point in time in a cycle of events. It believes there will be other Big Bangs, each followed by a collapse where the universe contracts back to a single point. Then, another Big Bang will occur, and the cycle begins again.
Some scientists think the universe has a limit to how big it can get; they think it will snap back like a rubber band, drawn back by gravity. This theory is the Geocentric model. One cycle would take billions or trillions of years.
The Steady State theory was proposed in 1948. This theory states the density of the universe never changes. When stars and galaxies burn out, new stars and galaxies are created to replace them. The universe stays steady. This theory was largely discredited in the 1960s.
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