The Big Bang Theory

Brief Outline

For centuries it had been generally assumed that the universe was largely unchanging. However, in 1916, Albert Einstein published his famous Theory of General Relativity, including a prediction that a static universe is impossible. In 1929 American astronomer Edwin Hubble provided compelling evidence that the universe is expanding.

Two years later Georges Lemaître, a Belgian physicist and Roman Catholic priest, proposed the Big Bang theory in an attempt to explain why the universe was expanding. The details of the theory were developed in the following decades.

According to the Big Bang theory the universe began as a singularity (point) that exploded into existence about 14 billion years ago. As the universe expanded, it cooled. After the first three minutes the temperature of the universe became low enough that small nuclei formed. After a few hundred thousand years atoms and molecules formed. These gases expanded in all directions throughout the universe until they were highly dispersed. Then the molecules of gas began to collapse in on themselves in local areas by gravitational attraction. The molecules within a space of about ten trillion kilometres diameter collapsed to form each star, a hundred billion stars collected to form each of the galaxies in the universe.

Our own solar system formed about five billion years ago from a cloud of dust and gas made up of the exploded remnants of previously existing stars.

Worldview

There has long been argument about whether the universe had a beginning or has existed eternally. Georges Lemaître, a Catholic priest, believed in a doctrine of creation. The opening statement of the bible tells us that the universe has not always existed, it was created by God.

"In the beginning God created the heavens and the earth."

Genesis 1:1

Lemaître proposed that the universe had a beginning – that it was created out of nothing and has been expanding ever since. His theory was consistent with his worldview. Arno Penzias, who discovered the most compelling evidence in favour of Big Bang Theory, later commented:

“Astronomy leads us to a unique event, a universe which was created out of nothing, one with the very delicate balance needed to provide exactly the conditions required to permit life, and one which has an underlying (one might say ‘supernatural’) plan.”

Arno Penzias, 1992. Cosmos, Bios and Theos

Evidence

Red-Shift of Light from Stars

Astronomers analysed the light from stars in the Milky Way with a spectroscope and discovered that some wavelengths of their spectrum were missing, because elements in the stars were absorbing the light. The emission spectra of stars have black lines at characteristic wavelengths.

Using the recently constructed Hooker telescope at the Mt Wilson Observatory, Edwin Hubble observed stars from beyond our galaxy for the first time. In 1929, Hubble published his finding that light from all the stars in other galaxies experience a red-shift. Their emission spectra have the same lines, but they have moved slightly towards the red end of the spectrum (see diagram below). Hubble found that the further away a star is from Earth the greater the red-shift.

Red-shifts may be interpreted as a result of the Doppler effect. (The light emitted by galaxies moving away from us through space would be stretched). However, stars are not moving way from us through space, rather, space is expanding and stretching the light waves travelling to Earth from the stars. The further away the star, the longer the journey for the light and the more the more the light has been stretched as the universe expands and the bigger the red-shift observed on Earth.

It was this evidence that Lemaître tried to explain when he first proposed the Big Bang theory. Lemaître deduced that if the universe is expanding it used to be smaller than it is now. An obvious starting state for an expanding universe would be a point (singularity).

Abundance of Light Elements in the Universe

In the 1940s astrophysicists were trying to answer an important question — what determined the relative abundance of elements in the universe? Could the observed abundances be matched by calculations that applied the laws of physics to an early extremely hot dense phase of an expanding universe? George Gamow succeeded in explaining the relative abundances of hydrogen (75%) and helium (25%) in the universe (with the total composition of all the other elements being much less than 1%). Fred Hoyle and William Fowler later showed that all the other elements were produced in stars, but the Big Bang theory currently provides the only explanation for the formation of hydrogen and helium.

Cosmic Microwave Background Radiation (CMBR)

In a paper published in 1948, George Gamow, Ralph Alpher and Robert Herman predicted that most of the energy in the early universe would be converted into matter as it cooled, however, some energy would be left over. They predicted that the radiation should lose energy because its wavelength would be stretched by the expansion of the universe. Originally, the radiation temperature would have been about 3000 Kelvin, but it should fall to a current temperature of about 5 Kelvin. 

In 1965 Arno Penzias and Robert Wilson, using the 15 metre Holmdel horn antenna at Bell Telephone Laboratories in Holmdel, New Jersey, detected this cosmic microwave background radiation (CMBR) at a temperature of 2.7 Kelvin, only slightly lower than had been predicted. This spectacular confirmation of the theory led to Penzias and Wilson being awarded the 1978 Nobel Prize in Physics.

 In 1990 John C. Mather, using NASA's Cosmic Background Explorer (COBE) satellite, measured the spectrum of the CMBR. The theoretical best fit curve for black body radiation at a temperature of 2.7 K is indistinguishable from the experimental data (see diagram below). Mather was awarded the 2006 Nobel Prize in Physics for this work.