If a film of the history of the universe had been made, what would happen if you ran it backwards? At about five billion years ago our planet would disappear, for this is when it probably formed, from the debris of our solar system. Keep going back to the beginning and what happened then? The Big Bang: an explosion so powerful that its temperature and force are still being felt some 13.8 billion years later.
At least this is what scientists from the 1940s began to suggest with increasing confidence. The universe had begun from a point, an unimaginably hot, dense state, and then there was the big bang. Ever since this moment, it has been cooling and expanding, carrying the galaxies outwards from this original point. Ours is a dynamic and exciting universe, in which we are the tiniest of tiny specks. It is composed of the stars, planets and comets making up the visible galaxies; there is also much that’s invisible – black holes and the much more abundant ‘dark matter’ and ‘dark energy’.
So, did the Big Bang really happen, and can it explain the universe? Nobody was there of course, to begin filming. And what happened just before the Big Bang? These are questions that it is impossible to answer with any certainty, but they involve a lot of cutting-edge physics, as well as cosmology (the study of the universe). They have generated much debate over the past half-century or so. And it goes on right now.
Around 1800, the French Newtonian, Laplace, developed his nebular hypothesis. He was mainly aiming to argue that the solar system had developed from a giant gas cloud. It convinced a lot of people that the earth had an ancient history, which would help explain its characteristics, such as its central heat, fossils and other geological features. Many nineteenth-century scientists passionately disputed the age of the earth and of our galaxy, the Milky Way. In the early decades of the twentieth century, two developments radically altered the questions.
The first was Einstein’s General Theory of Relativity, with its important implications for time and space. By insisting that these two things are intimately related, as ‘space-time’, Einstein added a new dimension to the universe. Einstein’s mathematical work also implied that space was curved, so that Euclid’s geometry didn’t quite provide an adequate explanation over the vast distances of space. In Euclid’s universe, parallel lines go on for ever, and never touch. But this assumes that space is flat. In a flat, Euclidian world, the sum of the angles of a triangle is always 180 degrees. But if you are measuring a triangle on a globe, with its curved surface, this doesn’t work. And if space itself is curved, we need different forms of mathematics to deal with it.
Having accepted the essential truth of Einstein’s brilliant work, the physicists and cosmologists had some new thinking to do. While the revolution he brought about was largely a theoretical one, the second major development in cosmology was not theoretical. It was based firmly on observations, especially those of the American astronomer Edwin Hubble (1889–1953). Hubble was celebrated in 1990 when a space shuttle carried into orbit round the earth a space telescope named after him. The Hubble Space Telescope has recently revealed more than even he could have seen with the telescope at the Mount Wilson Observatory in California, where he worked. In the 1920s, Hubble saw further than any astronomer had ever done. He showed that our galaxy (the Milky Way) is not even the beginning of the end of the universe. It is one of countless thousands of other galaxies, stretching even farther than our telescopes can reach.
Cosmologists also remember Hubble for the special number, the ‘constant’, attached to his name. (You may remember Planck’s constant, which was a similar idea.) When light is moving away from us, it shifts the spectrum of its waves to the red end of the visible spectrum. This is called the ‘red shift.’ If it is moving towards us, its waves shift towards the other end of the spectrum, the ‘blue shift.’ This is an effect that astronomers can easily measure, and is caused by the same thing that makes trains sound different when they are coming towards you and going away from you. What Hubble saw is that light from very distant stars has red shifts, and the further away the star is, the larger the shift. This told him that the stars are moving away from us, and the further away they are, the faster they are moving. The universe is expanding, and it appears to be doing so at an increasing rate. Hubble measured the distance from the stars and the extent of the red shift. His measurements fell on a pretty straight line when he plotted them on a graph. From this he calculated ‘Hubble’s constant’, which he published in a very important paper in 1929. This extraordinary number gave cosmologists a method of calculating the age of the universe.
Hubble’s constant has been refined since then. New observations have found stars even farther away, and we can now make more accurate measurements of the red shift. Some of these stars are millions of light years away. A light year is about six trillion earth miles. It takes only eight minutes for a ray of sunlight to reach the earth. If the ray of light then bounced back to the sun, it could make over 32,000 return journeys in a year – another way of trying to appreciate the vast distances involved. And vast amounts of time. Some of what we see in the night sky is light that began its journey a very long time ago from stars that have since become extinct. To get a really precise value for Hubble’s constant, we need to know exactly how far away these very distant stars and galaxies are from us. But even with these difficulties, the constant’s importance is that it can tell us approximately how long they have been travelling. This gives the age of the universe – beginning with its Big Bang.
The Big Bang was popularised in the 1940s by George Gamow (1904–68). Gamow was a colourful Russian-born physicist who went to America in the early 1930s. He had a wonderfully creative mind, contributing ideas to molecular biology as well as physics and relativity theory. With a colleague, he explored, at the micro-level, how the nucleus of an atom emits electrons (beta particles). On the grand scale, he looked at how nebulae – massive clouds of hot particles and cosmic dust – are formed. His theory of the Big Bang, worked out from 1948 with others, built on knowledge of the smallest constituents of atoms, combined with a model of what might have happened when the universe began.
First, the constituents: the particles and forces. In the late 1940s this bit of physics came to be called quantum electrodynamics or QED for short. One man who helped make sense of it was the American physicist Richard Feynman (1918–88). He is famous for the diagrams he drew (sometimes on restaurant napkins) to explain his theories and his mathematics, and for playing the bongo drums. He won the Nobel Prize in 1965, primarily for his work on QED, which provided the complicated mathematics to describe the even smaller particles and forces that we examine below.
After the end of the Second World War, particle physicists continued to accelerate atoms and then particles in increasingly more powerful particle accelerators. The accelerators can break up atoms into their sub-atomic particles, which is like reversing what might have happened a few instants after the Big Bang. Immediately after the Big Bang, as cooling began, the building-blocks of matter would have begun to form. From the particles would come the atoms and from the atoms the elements, and so on up to the planets and stars.
As Einstein’s E = mc2 tells us, at ever-higher speeds – almost the speed of light – in the accelerators, the mass is mostly converted into energy. The physicists found that these very fast particles do some fascinating things. The electron emerges unchanged from the accelerator. It is part of a family of force-particles – the leptons. The proton and neutron turn out to be composed of even smaller particles called quarks. There are several kinds. Each comes with a charge. Combined into threes, they make up a neutron or a proton.
There are four basic forces in the universe. Understanding how they relate to each other has been one of the great quests of the twentieth century. Gravity is the weakest, but acts at an infinite distance. It is still not entirely understood, even though we have been officially puzzling about it since Newton’s apple. Electromagnetism is involved in many aspects of nature. It keeps the electrons in their orbits in the atom, and, as light, brings us daily news that the sun is still shining. Also in the atom are the strong and weak nuclear forces. These two bind the particles within the nucleus of the atom.
Leaving aside gravity, the other forces work by the exchange of special particles – force carriers – called bosons. These include the photon, Einstein’s quantum of light, which is the boson for electromagnetism. Yet, perhaps the most famous boson is the missing one: the Higgs Boson. Particle physicists have been looking it for since the 1960s. This boson is thought to create mass in other particles. Finding it would help explain how particles gained their mass in the immediate aftermath of the Big Bang. At the world’s biggest particle accelerator, the Large Hadron Collider (LHC) near Geneva, Switzerland, scientists think they caught a glimpse of it on their instruments in 2012. The LHC was constructed between 1998 and 2008 by the European Organisation for Nuclear Research (CERN). CERN itself was established in 1954. It was a cooperative scientific enterprise among several European countries, a result of the high cost of physics research, and the need for many scientists, technicians and computer staff to perform and interpret these experiments at the extremes of matter and energy.
The Higgs Boson would be an extremely useful (but not the final) part of the puzzle known as the Standard Model, which accounts for everything except gravity. And a confirmed Standard Model would move close to a ‘Theory of Everything’, possibly via string theory, an approach to analysing all these forces and particles. String theory is based on the assumption that these fundamental forces of nature can be considered as if they were one-dimensional vibrating strings. It uses very complicated mathematics. This work is still science in the making.
A lot of this micro-level particle physics is difficult to associate with the ordinary world we live in. But scientists are finding more and more uses for it in nuclear energy, television, computers, quantum computing and medical screening equipment. Beyond these important uses in our daily lives, there is much to be learned too as the idea of the Big Bang has been fitted into what can be seen and not seen in the far reaches of space.
In the 1920s, the Russian physicist Alexander Friedman (1888–1925) was one of those who quickly assimilated Einstein’s general theory of relativity into his own mathematical understanding of the universe. His Friedman Equations provided rules for an expanding universe. Friedman also wondered if it mattered that we looked out at the stars from earth. It’s a special place for us, but did this give us a unique place for seeing the universe? He said no, it didn’t matter. It’s just where we happen to be. Things would not look different if we were on some other planet, light years away. This is Friedman’s Cosmological Constant. It gives us another important idea: that matter is uniformly distributed throughout the universe. There are local variations, of course – the earth is much denser than the surrounding atmosphere. But smoothed out across all space, the principle appears to be true. Today, cosmologists still base much of their exploration on Friedman’s models. They also have to deal with mysterious things such as black holes and dark matter.
Two fellows of the Royal Society discussed the idea of a ‘dark star’ in the eighteenth century. Describing its modern equivalent, the ‘black hole’, was the work of a modern mathematical genius, Roger Penrose (b. 1931), and a brilliant theoretical physicist, Stephen Hawking (b. 1942). Until his retirement, Hawking had Isaac Newton’s old job as Lucasian Professor of Mathematics at the University of Cambridge. Together they explained how black holes are easy to imagine, but of course impossible to see. This is because they are caused by areas in space where dying stars have gradually shrunk. As their remaining matter becomes more densely packed, the forces of gravity become so strong that the photons of light are trapped and cannot get out.
There are also super-massive black holes. In 2008 the Milky Way’s very own super black hole – Sagittarius A* – was confirmed after a sixteen-year hunt with telescopes in Chile. Astronomers led by the German Reinhard Genzel (b. 1952) watched the patterns of the stars that orbit the black hole at the centre of the galaxy. They used measurements of infra-red light because there is so much stellar dust between the black hole and us, 27,000 light years away.
These super-massive black holes might play a part in the formation of galaxies and involve another part of space we cannot see directly: dark matter. Dark matter is thought to account for much more of the universe – 80 per cent of its matter – than the 4 percent of the visible stars and planets together with gas and space dust. Dark matter was first considered in the 1930s, to explain why large bits of the universe did not behave exactly as predicted. Scientists had realised there was a mismatch between the mass of the visible parts and their gravitational effects: something was missing. In the 1970s, the astronomer Vera Rubin (b. 1928) charted how fast stars on the edge of galaxies were moving. They were travelling faster than they should have been. Traditionally it was thought that the further they were away from the centre of the galaxy, the slower they would orbit. Dark matter would provide the extra gravity needed to speed up the stars. So indirectly evidence of dark matter was provided and it has been generally accepted. But what dark matter is remains a mystery – something else to be found or disproved in the future.
Modern cosmology has emerged from Einstein’s theories, from thousands upon thousands of observations, with computers to analyse the data, and from Gamow’s idea of the Big Bang. Like any good theory in science, the Big Bang has changed since Gamow’s time. In fact, for two decades after it was put forward in 1948, physicists hardly concerned themselves with the origins of the universe. The Big Bang had to contend with another model of the universe, called the ‘steady state’ one, most associated with the astronomer Fred Hoyle (1915–2001). Hoyle’s model enjoyed some backing in the 1950s. It suggested an infinite universe, with the continuous creation of new matter. In this mode, the universe has no beginning and no end. There were so many difficulties with the steady-state idea that it had only a brief scientific life.
Physicists now have information about short-lived particles and forces gathered in particle accelerators. They have observations in the far reaches of space. They have been able to refine what we know about the Big Bang. There is still a lot of disagreement about details, and even about some of the fundamental principles, but this is not unusual in science. The Big Bang model can make sense of much that can now be measured, including the red shifts of distant stars, background cosmic radiation and the fundamental atomic forces. It can accommodate black holes and dark matter. What the model does not do, is say why the Big Bang happened. But, then, science deals with the how, not the why. As in all branches of science, some physicists and cosmologists have religious beliefs and others do not. That is how it should be. The best science is done in an atmosphere of tolerance.
Excerpted from "A Little History of Science" by William F. Bynum, published this month by Yale University Press. Reprinted by permission of the publisher.