Modern Theories

The story of how the universe began is told through the Big Bang Theory, also called the expanding universe hypothesis. The key evidence came from Edwin Hubble in 1920, who showed that the universe is expanding — galaxies are moving further and further apart as time passes.

To picture this, take a balloon and mark points on it to represent galaxies. As you inflate the balloon, the points move away from each other. The same thing happens with galaxies in space. But the analogy is only partially correct: on the balloon, the points themselves also expand, whereas in reality, scientists find no evidence that galaxies themselves are expanding. Only the space between galaxies is increasing.

The Big Bang Theory describes the development of the universe in stages:

• In the beginning, all matter of the universe existed in one place as a "tiny ball" — a singular atom — with unimaginably small volume, infinite temperature, and infinite density.
• This tiny ball exploded violently in the Big Bang, causing a huge expansion. This event is accepted to have occurred 13.7 billion years before the present. The expansion continues even today. As the universe grew, some energy was converted into matter. Expansion was especially rapid within fractions of a second after the bang, and has slowed down since. Within the first three minutes, the first atom began to form.
• Within 300,000 years of the Big Bang, the temperature dropped to 4,500 Kelvin, giving rise to atomic matter, and the universe became transparent.

An alternative idea was Hoyle's steady state concept, which held that the universe looks roughly the same at any point in time. But with growing evidence for an expanding universe, the scientific community today favours the expanding universe argument.

The Star Formation

Matter and energy were not evenly distributed in the early universe. These initial density differences created differences in gravitational forces, which caused matter to be drawn together. This laid the basis for the formation of galaxies.

A galaxy contains a large number of stars and spans vast distances measured in thousands of light-years. Individual galaxies range from 80,000 to 150,000 light-years in diameter. A galaxy begins to form when hydrogen gas accumulates as a very large cloud called a nebula. Over time, the growing nebula develops localised clumps of gas, which continue to grow into denser gaseous bodies, eventually giving rise to stars. Star formation is believed to have taken place some 5–6 billion years ago.

A light year is a measure of distance, not time. Light travels at 300,000 km per second, and the distance light covers in one year is one light year, equal to 9.461 × 10¹² km. The mean distance between the Sun and the Earth is 149,598,000 km, which in light terms is 8.311 minutes.

Formation of Planets

Planets develop in three stages: …