The Big Bang theory is the most widely accepted scientific explanation of how the Universe came into being and how it came to exist as it does today.

According to the Big Bang theory, the Universe began as a tiny, hot, dense piece of matter billions of years ago, then cooled and expanded until it reached its current size. The Universe continues to expand.

The Doppler Effect

The Doppler Effect, named for Austrian physicist Christian Doppler, refers to the way in which waves produced by an object increase in frequency as that object moves closer to the observer.

For example, suppose someone drives and ambulance past you. As the ambulance approaches you, the pitch of the siren will sound higher. As the ambulance moves past you, the siren's pitch will growlower.

The reason for this is that as the ambulance moves toward you, the sound waves that come between it and your ears are compressed. There are more sound waves in a smaller amount of space - the sound waves have a higher frequency. Your brain interprets this as the siren having a higher pitch.

The reverse happens when the ambulance moves away.

The Doppler effect works with light as well as with sound.

If an object, such as a star is moving towards the Earth, it will tend to give off higher frequencies of light (toward the violet end of the spectrum).

If it is moving away from the Earth, it will give off lower frequencies (toward the red end).

The Expanding Universe

In 1929, Edwin Hubble, an American astronomer, was examining  the spectra of about 20 galaxies. The most distant of these was 20 million light-years away.

Hubble found that all of these these galaxies' spectra lines were shifted to the red, and that there was a direct relation between the amount of redshift and how far away the galaxy was.

The farther away the galaxy, the greater the redshift. This meant that the galaxies were receding from the Earth. The farther away they were, the faster they were moving.

Hubble predicted that the same relationship would be found true for more distant galaxies.

This relationship between the distance of a galaxy and its speed is called Hubble's Law.

He explained this phenomenon by saying that the Universe is expanding.

Hypothesis of the Primeval Atom

In 1931, the discovery that the Universe is expanding led Georges Lemaître, a Belgian priest, physicist and astronomer, to develop a hypothesis about how the Universe began

Lemaître reasoned that since the galaxies are rushing away from each other, they must have been closer to each other yesterday than they are today.

Therefore, long ago, they were even closer.

Lemaître realized that if you went back far enough in time you would finally find space so contracted that all the galaxies would have been crowded together into one mass.

He concluded that the Universe began in that way.

According to Lemaître, the whole Universe was originally about the size of the planet Mars, a sphere about 4,000 miles in diameter. All the matter that we now find in the stars and nebulae of our galaxy and the other billions of galaxies was crowded into that space.

Lemaître called this original sphere of matter the Primeval Atom.

The primeval atom exploded, and the Universe came into existence. As the subatomic particles that had been held within the primeval atom flew out in all directions, space began to expand.

Lemaître's hypothesis was known as the hypothesis of the Primeval Atom.

Big Bang Theory

The English astronomer, Sir Fred Hoyle created the term Big Bang to refer to the instant when the initial mass that made up the Universe exploded and began to expand. The hypothesis of the Primeval Atom became known as the Big Bang theory.

The Russian-born American scientist, George Gamow developed the Big Bang theory further, explaining how atoms of various elements came into existence.

Cosmic Microwave Background Radiation

In 1948, Gamow and his colleagues, Robert Herman and Ralph Alpher, said that if the Big Bang had happened, then the Universe should be filled radiation.

Immediately after the Big Bang, the Universe consisted of a plasma - matter that it is so hot that subatomic particles cannot coalesce to form atoms - of hydrogen.

This plasma was full of radiation.

As the Universe expanded and cooled, stable atoms formed.

These atoms were unable to absorb the radiation that had previously filled the hydrogen plasma. This radiation has continued to exist, although it has grown fainter, since it must fill an increasingly larger universe.

In 1964, radio astronomers Arno Penzias and Robert Wilson of Bell Laboratories discovered a steady background noise coming from a very sensitive radio antenna. This noise did not come from anywhere in the Milky Way Galaxy.

Penzias and Wilson learned that astrophysicists Robert Dicke, David Wilkinson and Jim Peebles of Princeton University were hoping to find evidence of background microwave radiation, which would help to prove the Big Bang Theory.

Wilson and Penzias realized that the radiation that they had detected was exactly like the radiation that Dicke and his associates had predicted.

In 1978, Penzias and Wilson earned the Noble Prize in Physics for detecting the cosmic microwave background radiation.