For thousands of years people have been seeking answers to questions about the origin of the universe. For a long time many people thought that the universe had always existed in its present form, and that it would always remain the same. But ideas on the nature of the universe have altered as scientific understanding has increased. Evidence gathered over the centuries has shown that, rather than staying the same, the universe is always changing. The science of the universe is called cosmology. Cosmologists study the entire universe to find out how it began and how it has evolved.
Galaxies beyond our own
Edwin Powell Hubble was born and brought up in the USA and studied law at the University of Chicago. He first worked as a lawyer but then turned to astronomy. For the rest of his life he worked at Mount Wilson Observatory in California.
Einstein and Hubble at the controls of Mt. Wilson Telescope
In 1923 Hubble examined a galaxy (giant group of stars) called the Andromeda Spiral. At that time, most astronomers thought that our galaxy, known as the Milky Way, made up the entire universe. They realized that the spiral forms we now know to be other galaxies were only clouds of gases. But Hubble saw stars on the edge of the Andromeda Spiral and estimated that they lay well beyond the Milky Way. His work proved that the Andromeda Spiral was another galaxy and therefore that other galaxies existed apart from our own. Gradually he and others began to identify more galaxies.
The age of the universe
When examined closely, the light spectrum not only consists of the range of different shades, but also a series of lines. Astronomers noticed that when they examined light from stars, the shades and lines had moved toward the red end of the spectrum. This effect is known as "red shift". The reason for this is that when a source of light moves away from an observer, its wavelength increases. The greater the speed at which the light source is moving away, the greater the red shift. Hubble realized that in order to produce the shift effect, the stars must be moving away from us. He also noticed that the fainter the galaxies were, the greater the red shift. This implied that the more distant the galaxies were, the faster they were moving.
Diagram showing the various properties across the range of wavelengths and frequencies
By 1929, he was able to measure the degree of red shift to calculate the speed of the galaxies and their distance from the earth. He found that the speed increases in proportion to the distance. This is known as Hubble's Law. His work provided the first evidence that the universe is expanding. This is the key to the Big Bang Theory. In addition once astronomers were able to measure the speed at which galaxies are moving outward, they could calculate when the universe began. They now believe this happened between six and fifteen million years ago.
Creation from explosion
Georges Lemaitre was born in Belgium, He studied astronomy at the University of Louvain in Belgium and then trained as a priest. He moved to Cambridge University in England and was later appointed professor of astronomy at Louvain where he remained for the rest of his career.
In 1927, using Einstein's General Theory of Relativity, Lemaitre proposed that the universe was still expanding. He claimed that at one time it must have been compressed into a tiny atom of energy and matter. He went on to suggest that the atom blew apart in a huge explosion, scattering hot gases in all directions. This idea has become known as Big Bang theory, At the time its true importance was not fully appreciated, but most scientists now see it as the best explanation of the origin of the universe.
The Big Bang Theory
As new discoveries in space have been made, the Big Bang theory has been refined and modified. In 1970 Roger Penrose and Stephen Hawking proved that if Einstein's General Theory of Relativity is correct, then it is possible that there was a definite beginning to the universe. At this point, called a singularity, space and time as we know them would not have existed. Moments after the explosion, the universe would have been an incredibly hot fireball. It expanded and cooled until, millions of years later, hydrogen and then other elements formed. Eventually, gravity drew atoms together and galaxies began to develop.
Another modification of the Big Bang Theory, called the Oscillating Universe Theory, states that the universe is alternately expanding and contracting. If this is true, when the limit of expansion is reached, the growth will stop and gravity will pull everything back together once more. The galaxies will be squeezed together so tightly that another cosmic explosion will set the whole process off again.
Staying the same
Hermann Bondi grew up in Vienna, Austria. He moved to England and studied at Cambridge University. In 1954, he became professor of mathematics at King's College London.
In 1948 Bondi proposed the Steady State theory, which says that new galaxies form in the middle of the expanding universe to replace those moving outward. As a result, he claimed, the universe would always look the same. However, later findings contradict this. For example, in 1964, two astronomers called Robert Wilson and Arno Penzias picked up faint radio noise from space. This is now thought to be the echo of Big Bang.
