The Story of Astronomy (30 page)

The observational starting point for cosmologists is the expanding universe of galaxies receding rapidly from us. Let us imagine we reverse time and run events backward so that all the galaxies are rushing toward each other instead. It is possible in our time-reversing universe to work out how far apart they were a billion years ago. The density of matter in the universe would have been higher at that time, but the galaxies would still be very distant. However, if we keep proceeding backward in time in steps of a billion years we reach a point, long before the evolution of life on Earth, when all the galaxies were so much nearer to each other. Close to 13.7 billion
years ago, there comes a point where all the matter and energy in the universe is compressed into a tiny space. The resulting temperature is so high that most of the mass of the universe is in the form of radiation. We cannot see and measure this early universe and must rely on our understanding of the laws of physics and mathematics to determine what must have happened in the earliest phases. Even so, our understanding is severely constrained by our inability to combine relativistic and quantum physics—which is essential if we are to fathom how infinitely small yet massive concentrations of matter and energy behave.

Before tackling the Big Bang we need to understand the concept of force. Force is one of the keys to the nature of matter. As babies, we soon acquire a knowledge of the force of gravity. We discover the force of magnetism when we play with a magnet, and we discover electricity in the form of electrostatic force when we find that after pulling a plastic comb through our hair it can pick up small particles. In addition to these forces there are two others, called strong and weak nuclear force, but they act over a very limited range and can only be studied in extreme conditions. Physicists have consistently tried to find a theory that unifies all the known forces acting in the universe into a single force. Electric and magnetic forces were once thought to be distinct
from each other but, as described in an earlier chapter, they were unified by the work of James Clerk Maxwell (1831–79) in the 19th century. In the 20th century Albert Einstein (1879–1955) tried to unify electromagnetic force with gravitation, but his efforts met with little success.

Weak nuclear force was needed to explain some of the observed aspects of radioactivity. It acts on sub-atomic particles such as quarks, electrons and neutrinos. Experiments at CERN in the 1980s created collisions at such high temperatures that it was impossible to tell the difference between weak nuclear force and electromagnetic force. Weak nuclear force was shown to be a special case of electromagnetic force at certain high temperatures, and these two forces have now been successfully unified. Thus the number of forces in the universe comes down to only three: gravitational force, electroweak force and strong nuclear force. Extending the idea, physicists theorize that at even higher temperatures—such as in the initial stages of the Big Bang—all three forces would be indistinguishable. It is likely that Einstein's dream of a unified field theory was a reality in the earliest stage of creation. Physicists hope to be able to reconcile all three forces into a single force. A new particle accelerator, the Large Hadron Collider, will smash atomic particles together at energies approaching those present soon after the Big Bang. It is hoped the experiments will help us to understand how
all the forces could have been unified, and how they might have behaved in the first era of the Big Bang.

We shall now go right back to the moment of the creation and examine the presumed behavior of the universe from the first instant. We can say nothing about what happened before the Big Bang or how it was caused; such questions are probably more of a philosophical or a theological nature.

Let us turn back the clock and fix on an instant of time that we shall define as zero. We call this the Planck Era, and it lasts from time zero to 5 × 10
^–44
seconds, or one Planck unit of time. At this point, the universe would have fitted into the nucleus of an atom several billion times over. However, our limited knowledge means that there is much speculation involved in trying to envisage anything detailed about the universe at this time. We assume that it was so hot that even at the end of the era the temperature was of the order of 10
³²
kelvin, and in such hot and dense conditions, there must have existed some very exotic laws of physics. It is probable that all four forces were indistinguishable. As the universe expands out from the point of creation, it cools. At some point gravity separated or “froze out” from the other forces, heralding the second era of expansion.

This era is called the Grand Unification Epoch (GUE), since at temperatures higher than 10
²⁷
kelvin, it was sufficiently hot for all the forces other than gravity to remain unified. During this period most of the universe's energy was in the form of radiation, but under the laws of quantum mechanics pairs of elementary particles were frequently created by the process known as pair production. Particles of both matter and antimatter were created and there was a great conflict as they annihilated each other whenever they came into contact. The battle was won in favor of matter. If the forces of antimatter had won then a very different universe would have been created. The GUE lasted until about 10
^–35
seconds, at which point the universe cooled to the point where the strong nuclear force “froze out.” Although the GUE lasted for a very brief period of time indeed, it was still a hundred million times longer than the Planck Era that preceded it.

Next, the universe is thought to have entered a phase called the Inflationary Epoch in which it underwent an exponential expansion, growing to about 1050 times its previous size in under 10
^−33
seconds. Originally mooted by Alan Guth (b.1947) in 1981, the concept of a period of inflationary evolution accounts for some otherwise puzzling properties of the CMB, although we have no
direct evidence for this phase. First, the spectrum of the CMB is completely uniform in temperature, despite the enormous physical separation between different sides of the present-day universe. During the Planck Era, the universe was like a small, hot and homogenous “soup,” where every bit was in contact with every other bit, and reached a uniform temperature. Subsequent inflation then instantaneously dispersed all the matter outward in all directions to give uniformity.

The tiny temperature fluctuations of the CMB tell us that any deviations in density in the initial universe were miniscule; yet it is these slight overdensities that provide the starting points for the condensations of matter under gravity. Inflation provides a mechanism whereby such tiny fluctuations are stretched and magnified to the physical length scales of the galaxies they will later form. Finally, the angular scale of the CMB fluctuations suggests that the present-day universe has a “flat” or Euclidean geometry (where “flat” here does not have its normal linguistic sense, but is used to imply the kind of familiar geometry where parallel lines only meet at infinity, and the internal angles of a triangle still add to 180°). There would not normally be any reason to suppose that this would be the eventual geometry, unless an inflationary period stretched and flattened the universe, regardless of its initial shape.

After this period of exponential growth, the universe was dominated by quarks and antiquarks. Quarks can be seen as the building blocks of all the elementary particles. The collisions and reactions between atomic particles, and the properties of the particles, can be explained more simply in terms of quark theory.

The first part of this era, until electromagnetic and weak forces separated out when the universe was 10
^–12
seconds old, is often called the Electroweak Era. This left only electrostatic and magnetic forces united, and we know they remained united because they are the same force when met at what we would consider to be “room temperatures.” The universe was still very hot and energetic, and even as new particles were created they were destroyed and changed into high-energy photons on meeting an anti-particle. Hence radiation was still dominant over matter. The quarks were at first unable to combine with each other to form the heavier atomic particles, as the temperatures were too high. As the age of the universe approached the end of the first microsecond it cooled sufficiently for quarks to do their work. The universe entered the Hadron and Lepton Era, when atomic particles such as protons, neutrons and other baryons were created.

At last it was time for the more familiar laws of physics to rule the universe, but there were still many
interchanges between matter and energy. These quantities are related by Einstein's law
E
=
mc
²
. The universe came of age by the time a whole second had passed since the moment of creation. The temperature had fallen to a mere million or so degrees and the radius of the universe had expanded to about 187,000 miles (300,000 km)—roughly three-quarters the distance of the Moon from the Earth.

The expansion of the universe continued. There were plenty of protons and electrons around to form atoms of hydrogen, but there were even more high-energy protons that frequently collided with the newly formed atoms and caused them to split apart. It was not until temperatures fell below a million degrees that the hydrogen atoms became more stable, and then a few atoms of helium were formed.

As the universe expanded the density fell rapidly until matter was able to dominate. Particles were stable and they existed for a longer period of time, in particular the atoms of hydrogen and helium. Up to this point, the particles and photons had been tied together, continually interacting with each other. But as the particles recombined to form stable atoms, the photons became free, and many streamed toward us. The moment when the photons
were “last scattered” by the matter marks the first observable feature in our nascent universe—the CMB. We will never be able to observe the earlier universe with our telescopes; we can only conjecture what happened before the CMB from what we know of the laws of physics and mathematics.

About one billion years after the Big Bang, protogalaxies and protostars began to form. The most massive stars lived only a few million years, evolving into supernova explosions, and scattering the heavy elements they had fused in their core across interstellar space. Subsequent cycles of star formation and evolution continued to enrich their surroundings, and stars formed later in the lifetime of galaxies resembled our Sun. Some of these stars formed planetary systems, and possibly other Earth-like planets were created. At least one of these planets evolved in a fascinating way. It developed life.

Dark matter and dark energy together make up some 96 percent of all the mass and energy in the universe. But their nature remains a mystery. Dark matter was proposed decades ago to explain why galaxies hold together, while dark energy is a more recent prediction which explains why the universe is not just expanding, but is doing so at an ever-increasing pace.

Ever since the dawn of astronomy observers have wanted to know the distance to the stars and the scale of the universe. The methods by which these distances have been measured since the early days of astronomy provide a little history of its own.

The distance to objects in our solar system can be measured by the method of parallax, whereby two images of
the same object can be viewed from two places a known distance apart. The Moon, for example, can be seen at the same time against a different star background from two widely spaced points on the surface of the Earth. The difference in its position against the distant stars can be used to calculate its distance from Earth.

Measuring a parallax for the planets is more difficult, but the method still provides a way of finding their distance from the Earth. Measuring a parallax for the stars was a far more difficult problem. It took centuries to solve, but with modern detection techniques the distance of stars about 100 light years away can also be measured by the method of parallax; the baseline used for the stars is a diameter of the Earth's orbit.

In the 20th century, methods to calculate longer distances became available. The first was developed by Henry Norris Russell (1877–1957), who with Ejnar Hertzsprung (1873–1967) discovered the relationship between the magnitudes and spectra of stars. Confusingly called the “spectroscopic parallax” method, it has little to do with trigonometry, relying instead on the observed properties of stars. The H-R diagram provides the correlation between a star's spectral type and its absolute magnitude, by which we mean its magnitude if it was located at the “standard” distance of 10 parsecs from the Earth. Using the distance–magnitude relationship, the star's distance can be
inferred from its observed brightness. While less precise than trigonometric parallax, this technique enabled astronomers to estimate distances as far away as 10 kpc.

To measure distances beyond 1 kpc and as far away as 30 Mpc, the best technique is the one developed by Arthur Eddington (1882–1984) and Henrietta Leavitt (1868–1921) using the Cepheid variables. The Cepheids are variable stars glowing brighter and dimmer over a period of several weeks or days. They have a well-known period–luminosity relationship; thus if we know the period of variability of a Cepheid then we know its absolute luminosity. By comparing this with the luminosity observed from Earth we can calculate its distance. Cepheids separate into two types, commonly known as population I and population II, according to their luminosity. Both are useful distance indicators, but the brighter population I Cepheids cover a much wider range of distance and we can observe them in our neighboring galaxies, giving us indicators to estimate the distance to the nearer galaxies.