Big Bang

The currently accepted scientific theory concerning the beginning of the universe is called "The Big Bang." According to this theory, the universe began with a big explosion approximately 17 billion years ago. This was the beginning of time. Energy and matter formed simultaneously. Over the billions of years from that first flash of light, the universe expanded and cooled, and all matter as we know it - stars and planets, galaxies and galactic clusters, pulsars and black holes, evolved.  The universe is the collection of all the objects in space and space itself. The universe includes everything!

According to the theory, all the matter of the universe was concentrated in one small point. With one great explosion, it began to expand in every direction. Energy and expanding matter created the expanse of the universe.

Scientists believe that in the first few seconds following the big bang, the universe was a dense and extremely hot place. It expanded very swiftly.  Elementary particles formed from radiation. The temperature exceeded one trillion degrees, but quickly cooled down.

According to the theory, by approximately one million years after the big bang, the temperature had cooled down to about 7,000°F (4,000°C). By this time, the matter in the universe transformed into hydrogen and helium. The universe had a smooth, homogeneous structure throughout. However, tiny disturbances were beginning to form.

According to the big-bang theory, one billion years after the big bang, matter began to aggregate in clumps. Between them were great open spaces of vacuum. These clumps of matter were the first galaxies. Billions of stars formed within them.

Scientists believe that when the universe was 3 billion years old, the first galactic clusters began to form. Since then, the universe has cooled off considerably. Today its average temperature is about 2.7 degrees above absolute zero, which is about -454°F (-270°C).

The universe is so huge that we measure distances in light-years.

One light-year is the distance light travels in one year. This equals 5.91 trillion miles (9.46 trillion kilometers).

Our knowledge about the size of the universe depends on our observational skills. The better our equipment, the farther we can see into space. We still haven't seen the edge of the universe.

Until the beginning of the 20th century, most astronomers believed that the Milky Way was the entire universe. They concluded that everything they saw through the telescope was inside our galaxy. They believed that the universe was only 100,000 light-years across - the size of our own galaxy.