When I set out to write a book about the Big Bang theory of the universe I was repeatedly shocked by one obvious fact. Astronomers can look, but they can’t touch. Physicists can dissect atoms, chemists can boil chemicals and biologists can examine cells, but the vast majority of astronomical objects are so far away that there is no hope of ever tasting, touching, smelling or hearing them - all we can do is stand and stare. Nevertheless, by using bigger and better telescopes and employing photography and other more recently devised technologies, astronomers have been able to discern the clues that prove that the universe had its origins in a hot, dense, compact state that expanded, leading to the evolution of stars and galaxies - and life itself.

The challenge of distance

One of the first problems facing astronomers was to work out the distances to celestial objects. Ordinarily, humans determine the distance to an object using stereo vision - we have two eyes, so we see an object from two positions, separated by roughly 10 centimetres. If the object is far away, our eyes are effectively both looking in the same direction, but as the object approaches our forehead, our eyes change their line of sight until we become cross-eyed. By comparing the lines of sight from each eye, our brain is able to work out the rough distance to the object. This is known as parallax.

Astronomers realised that they, too, can use stereo vision, because they can look at the same star in January and then in July, by which time the Earth would have travelled to the other side of the Sun, a distance of some 300,000,000km. However, the stars are so far away that the line of sight hardly seemed to change. It was not until 1838 that Friedrich Wilhelm Bessel discerned that the line of sight to the star 61 Cygni shifted by 16,000 of a degree over the course of six months, which indicated that it was 100,000,000,000,000km away.

This technique of stellar parallax is severely limited and could be used only to measure the distances to the closest stars. Astronomers had to estimate stellar distances by assuming that all stars are the same actual brightness and that apparent brightness decreases with distance or, to be more precise, with the square of the distance. In other words, if star A appeared to be one-ninth as bright as star B, it was probably three times farther away. But this technique is unreliable. Star A might be dimmer and much closer than star B - or brighter and 100 times further away.

The situation seemed hopeless until the arrival of Henrietta Leavitt, born in 1868 in Lancaster, Massachusetts. She was one of a team of women employed by Harvard College Observatory to analyse the thousands of astronomical photographs that were being taken at the start of the 20th century. The women were known as computers, as the original meaning of the word was a person who analysed data. Although forbidden to make her own observations, Leavitt developed an intimate knowledge of the data she studied and was able to notice something that had escaped the attention of her supervisors.

She studied a type of star known as a Cepheid variable, which changes its brightness periodically. Some Cepheids dipped in brightness and then peaked within a few days, whereas others dipped and peaked over the course of several weeks. She was able to show that the time taken for one cycle in brightness was a very strong indication of a Cepheid’s actual brightness - the longer the cycle, the brighter the Cepheid. So cycle length indicated actual brightness, and then by comparing a Cepheid’s actual brightness with its apparent brightness it became possible to deduce its distance.

Leavitt’s breakthrough was to be used by others to provide a yardstick for the universe.

Cosmic speedometer

The universe consists of galaxies, giant groupings of billions of stars.

Our own galaxy is the Milky Way, and initially Leavitt’s technique could only be applied to Cepheid variable stars in the Milky Way itself because Cepheids in other galaxies were too faint to be seen. However, in 1923 the US astronomer Edwin Hubble spotted a Cepheid in the Andromeda galaxy and used its variation in brightness to estimate that the Cepheid and its home galaxy were roughly a million light years away (10,000,000,000,000km).

Later observations refined and increased this measurement, but Hubble had certainly demonstrated the power of the Cepheid yardstick and the sheer scale of the universe.

Hubble’s measurement made him famous. One paper called him “Major Hubble, the titan astronomer” and he received numerous prestigious prizes and awards. Herbert Turner, Savilian professor of astronomy at Oxford University, said: “It will be years before Edwin realises the magnitude of what he has done. Such a thing can come only once to most men and they are fortunate.”

But Hubble was destined to make an even greater discovery. He was determined to measure the motion of the galaxies. He wanted to find out if they were just floating in space or whether they were moving in some systematic or random way. The technique he used to measure the dynamics of the cosmos is known as the cosmological Doppler shift. The Doppler shift is usually associated with sound - it is the phenomenon whereby an object emitting a certain note appears to shift in pitch if the object is approaching or receding. For example, a racing car makes a “yeeeeeee-ooooooow” sound as it approaches and then recedes, its engine noise shifting from a higher pitch to a lower pitch. The effect is quite obvious, because the speed of a racing car is significant compared with the speed of sound.

A similar phenomenon happens to an object that is emitting light. The light appears more blue if the object is approaching and more red if it is receding. These effects are known as red shift and blue shift. So the racing car would look a little bit bluer than normal as it approaches and a little bit redder as it recedes. However, the effect is imperceptible, as the speed of a car is negligible compared to the speed of light. But if galaxies were travelling at a more significant fraction of the speed of light, Hubble hoped that the effect should be detectable.

In 1929, he published his first set of galactic Doppler measurements. The results were astonishing. Apart from the very closest galaxies, all of them were receding and, furthermore, their speed was proportional to their distance. Hence, in a year from now all the galaxies will be further away.

Conversely, a year ago all the galaxies were closer to us. And a century ago they were closer still, and at some point in the past all the galaxies and all the matter in the universe must have been piled up in the same place.

This was the first observational evidence in favour of the Big Bang theory of the universe. If all the matter in the universe was once compact, then it must have exploded outwards and the universe has been expanding and evolving ever since. Prior to this breakthrough, the consensus was that the universe was eternal and unchanging. The Big Bang implied a universe with a finite history that was expanding and becoming increasingly less dense.

Theory married to observation

One cosmologist had anticipated Hubble’s measurements. Just a couple of years earlier, Georges Lemaitre had shown that the laws of physics were perfectly compatible with an expanding universe, although the expression “Big Bang” would not be coined until the 1950s. Instead, Lemaitre talked of “a day without a yesterday”. Unfortunately, the scientific establishment largely dismissed Lemaitre’s work. Even Einstein insulted Lemaitre by calling his physics “abominable”.

We might think that the Big Bang theory was taken seriously when Hubble published his observations. In fact, there was still a huge reluctance to accept it. The standard model of an eternal universe had become so firmly established that cosmologists fought to explain Hubble’s observations in the context of an eternal universe.

For example, the astrophysicist Fritz Zwicky suggested that the red shift had nothing to do with receding galaxies, but was simply a result of light losing energy as it escaped the tremendous gravity associated with supermassive galaxies. His so-called “tired light” theory was taken seriously for a while.

Alternatively, the British cosmologist Fred Hoyle accepted that the galaxies were receding, but suggested that new matter and galaxies were born in the increasing gaps between the older galaxies. In this way, the universe could expand but retain its overall density of galaxies, and furthermore the universe could be eternal.

The stage was set for a cosmic battle between the two views of the cosmos - Big Bang model versus eternal universe model.

Decisive test

In order to settle this controversy, it was necessary for one of the theories to put its neck on the line by making a definite prediction. In fact, both theories put themselves to the test over and over again, but perhaps the single most important test concerned a supposed “echo” from the Big Bang.

Big Bang supporters had calculated that the universe should have released a blast of radiation a few hundred thousand years after the moment of creation. They predicted that this radiation echo should still exist today as an omnipresent sea of microwaves. If somebody could find these microwaves, it would prove beyond all reasonable doubt that the Big Bang happened. Alternatively, if somebody could show they did not exist, it would prove that the Big Bang theory was wrong.

Some astronomers did not bother looking for the cosmic microwave echo, either because they did not believe in the Big Bang theory or because they did not consider it technically feasible to detect the echo.

However, in 1965, Arno Penzias and Robert Wilson at Bell Laboratories in New Jersey accidentally discovered the microwave echo while calibrating their radio receiver. This act of pure serendipity confirmed one of the greatest theories in the history of science. There was official recognition that the Big Bang theory had become the new establishment when Penzias and Wilson received the Nobel prize for physics in 1978.

This was yet another scientific breakthrough achieved simply by analysing light. Although microwaves are invisible to the human eye, they are technically a form of light, and as such it is true to say that, at last, seeing is believing.

The end of the story

Although the overwhelming majority of experts endorse the Big Bang theory of the universe, this does not mean that the theory is complete.

Cosmologists at the start of the 21st century are exploring difficult issues, such as the existence of dark matter, dark energy, inflation and - the biggest problem of all - what came before the Big Bang?

Although these questions seem impossible to answer, it is worth bearing in mind the tale of the French philosopher Auguste Comte. In 1842, he tried to identify areas of knowledge that would forever remain beyond the wit of scientific endeavour. For example, he thought some qualities of the stars could never be ascertained: “We see how we may determine their forms, their distances, their bulk, and their motions, but we can never know anything of their chemical or mineralogical structure.”

In fact, Comte was proved wrong within two years of his death, as scientists began to discover which types of atom exist in our closest star, the Sun. Perhaps the answer to the ultimate question might be closer than we think.

Simon Singh is the author of Fermat’s Last Theorem and The Code Book. His new book, Big Bang, is published by Fourth Estate at pound;20

Space websites

Simon Singh: www.simonsingh.net

NASA:www.nasa.govhomeindex.html

Astronomy picture of the day:

http:antwrp.gsfc.nasa.govapodastropix.html

Cosmology 101:

http:map.gsfc.nasa.govm_uni.html

Echo of the Big Bang: http:map.gsfc.nasa.govm_uniuni_101bbtest3.html

Cepheid Variables:

http:imagine.gsfc.nasa.govdocssciencemysteries_l1cepheid.html

Edwin Hubble: www.time.comtimetime100scientistprofilehubble.html

Georges Lemaitre: http:zyx.orgLEMAITRE.html

Penzias Wilson: www.pbs.orgwgbhasodatabankentriesdp65co.html

Unanswered questions in cosmology:

www.thebigview.comspacetimequestions.html

Mount Wilson observatory: www.mtwilson.edu

Doppler shift: http:en.wikipedia.orgwikiDoppler_shift