The Story of Astronomy (32 page)

By the latter part of the 20th century, astronomers had thus gained a much better understanding of the mass content (both visible and invisible), the structure and extent, and the rate of expansion of the universe. It was only natural to return to the fundamental question that had been asked many times before. Will the universe last forever or will it stop expanding and collapse into itself? For many years cosmologists debated about whether the universe is “closed,” so that it will eventually fall back on itself and end in a “Big Crunch,” or whether it is “open” and will continue expanding for all time at an ever increasing rate. The outcome depends on the mass content of the universe, and thus whether or not there is enough mutual gravitational attraction eventually to pull everything back together.
A further option—favored by most astronomers because of the observed isotropy and homogeneity of the cosmic microwave background—was the “flat” universe; one which lies on the boundary between the open and closed universe.

Consequently, in the 1980s and 1990s, ambitious programs tried to measure the expansion of the universe more precisely than before, with the aim of determining its eventual fate. By attempting to extend Hubble's law to far further galaxies than had been possible previously, astronomers aimed to see if the relationship between velocity and distance still held, or whether it indicated the first stages of a gradual slowing, or deceleration, that would result from a flat or closed universe. The research was undertaken by two teams of astronomers, one led by Saul Perlmutter (b. 1959) at Berkeley and the other led by Brian Schmidt (b. 1967), then at Harvard Observatory. Both used observations of Type 1a supernovae to establish the distance to very early galaxies, and they independently discovered an astounding result. The expansion of the universe was not found to be continuing at a constant rate, nor was it beginning to slow down. Astronomers were amazed to find that the rate of expansion was increasing with time—it had been accelerating for the last six billion years! This result, announced in 1998, changed the face of modern cosmology, and has since been confirmed by many more and different observations
to extend Hubble's law using different ways of estimating distances.

In order to begin to understand why this unexpected acceleration had occurred, it was necessary to introduce a new concept which went by the (somewhat misleading) name of “dark energy.” Einstein's famous equation
E
=
mc
²
shows how matter and energy are related, and in some sense can be considered as equivalent entities. Astronomers were astounded to discover that to produce the accelerated expansion, dark energy had to account for around three-quarters of the entire mass–energy density of the universe. But a true understanding of dark energy remains elusive, although there are a number of current ideas.

For centuries it has been known that gravity is an attractive force; unlike electricity and magnetism it is not possible under normal conditions for gravity to be repulsive. However, suppose the fundamental nature of gravity changed from an attractive to a repulsive force on the very largest (and truly astronomical) distance scales. Then there might come a point when the galaxies drew a sufficiently immense distance apart that they began to be pushed away faster and further by gravity. Such ideas echo Einstein's original, but misguided, attempt to include in his equations a cosmological constant representing an
anti-gravity that prevented the universe from evolving. It is possible that a modified form of the cosmological constant could be reincorporated in our understanding of gravity to account for the dark energy.

An alternative explanation is suggested by the expectation from particle physics that completely empty regions of space can still produce a “vacuum energy.” It is thought that pairs of particles and their associated anti-particles are continually created out of the vacuum, only to almost immediately annihilate each other and disappear. As they do so, they produce a minute outward pressure. Averaged over the entire voids of the universe, there would be sufficient such “vacuum fluctuations” to produce enough pressure to push the universe further apart. Since this is a property of empty space, as the universe grows larger and the voids grow bigger, the effect of vacuum energy thus becomes increasingly dominant over that of gravity. However, a better understanding of particle physics is required before the vacuum energy theory can be shown to be a reality—current estimates suggest it would be much more powerful than observed even in our accelerating universe. Another rival for dark energy is the suggestion of a new force field which goes by the name of quintessence. Scientists are debating a rigorous mathematical description of quintessence, but it may be a force whose strength and importance changes through the
history of the universe. It could be linked to the very early inflationary period of expansion thought to have occurred immediately after the initial Big Bang; if quintessence then lay relatively dormant for a period, we could be in another active phase producing the current accelerated expansion.

Whatever the true nature of the dark energy, if it continues to exert its influence we can speculate on a bleak future for our universe. The galaxies will continue to fly further and further apart ever faster, perhaps leading to an eventual “Big Chill.” Even worse, if the effects of dark energy become increasingly important, it may begin to dominate over gravity on smaller scales, such as within galaxies—leading to a “Big Rip.”

The story of creation does not end with the birth of the stars. Indeed, for many other bodies in the universe, the Big Bang was just the start. Until some stars had been born, and eventually died, there could be no planets. And until there were planets there could be no life on Earth.

At the present time we have no proof that any form of life exists in the universe other than on our own planet. But before we start hunting for planets that may support life, we must understand how the Earth itself was formed. The first stars, formed hundreds of millions of years after the Big Bang, had no solid rocky planets orbiting around them. It was not until some of the stars had ended their lives with massive supernovae explosions that the space dust and the atoms of the heavier elements appeared in
abundance throughout the galaxies. Only then could the rocky planets be created. A star and its accompanying planetary system will have formed from within a giant molecular cloud which eventually collapses under gravity, and after a few million years will reach pressures and temperatures sufficient to ignite nuclear fusion; a young star has formed at the core of the cloud. But the young star does not comprise all the material in the cloud. During its formation it is surrounded by a “proto-planetary” disc that is also in the process of gravitational collapse subsequently to form the planetary system.

Individual planets form by a process called gravitational accretion. As the proto-planetary disc cools, particles of dust condense and form. As they pass close to each other, they are pulled together by their tiny gravitational attraction, until after a few millennia larger particles form. The larger lumps of matter—called planetesimals—are better able to attract additional particles and are thus more likely to grow than the smaller masses. The larger and more massive bodies continue to accumulate space debris and grow steadily. After more millennia of accumulation some have become a proto-planet, with a mass the size of a planet. Our solar system is an excellent example of planetary formation. Each early planetesimal had its own ring of space where it could gravitationally capture any smaller particle and grow a little larger. The exception is
at the region of the asteroid belt, where the strong gravity of the proto-planet Jupiter disturbed the gravitational accumulation of the debris, preventing the formation of a single larger planet.

After this evolutionary period, some of the stars had evolved planetary systems orbiting around them. Within these systems, planets known as gas giants—like Jupiter and Saturn in our own solar system—have been commonly identified. As yet, the smaller, rocky terrestrial planets are proving much harder to discover. Once a sizeable planet begins to form it can capture most of the matter within several million miles of its own orbit. Some of this matter itself condenses to form satellite moons around the proto-planet. We see very complex moon systems around all of the very massive gas giants but few in orbit around the inner rocky planets. The larger moons are true satellites that originated in the initial collapse; they are spherical in shape and they have orbits that are nearly circular. However, many other moons are smaller and irregularly shaped and often have very elongated orbits. They are not true satellites, but rather captured asteroids and comets that have succumbed to the gravitational field of the planet much later. Saturn has a spectacular ring system. Jupiter, Uranus and Neptune have fainter and less striking ring
systems. Seen close up, the rings are found to be the remains of moons that have been broken into small pieces by the tidal forces of the planet.

The Earth's moon (which we call “the Moon”) is very large compared with the size of our planet. It also has a much more complex and violent history than any of the other moons in the solar system. It is possible for a planet the size of the Earth to capture small objects to be held as satellites in orbit around it, such as we see in the case of the two small moons of Mars, but it is impossible for the Earth to capture a passing object as large as the Moon and to retain it in orbit. Over the years many theories have been suggested about the formation of the Moon. One suggested that the Moon had somehow broken off from the Earth, leaving a twin planetary system with both planets orbiting about their common center of gravity. The truth is much more complex. At one time there were two planetesimals competing with each other for the matter in the space between Venus and Mars. The one destined to become the Earth was the larger and more successful, but the second planetesimal still managed to attract a substantial amount of the matter.

The two planetesimals both had elliptical orbits around the Sun. They avoided each other for millions of
years, but then, billions of years before life began on the planet, a catastrophic collision took place between them. The impact was so great that the orbit of the Earth was considerably changed by the collision. The proto-Earth was greatly deformed, as the heat generated in the giant impact made the matter in the planetesimal molten and fluid, and a huge quantity of matter was thrown out into space into an orbit around the Earth. The other planetesimal disintegrated after the collision, except for its core, which had adhered to the proto-Earth during the collision.

After a time the two orbiting lumps of matter regained their spherical shapes and they evolved to become the Earth–Moon system. It happened that the Earth was very much the larger body. It is interesting to speculate what would have happened if the Moon had been larger and a twin planetary system had formed. Could this have created two life-supporting planets close to each other? What is not in doubt is that the tidal forces on the Earth would have been enormous, and a very different planet would have evolved. We know that the tidal forces of the Moon have played a major part in the evolution of life on our planet. It is also likely that the impact changed the Earth's axis of rotation to create the angle of the ecliptic—and therefore our familiar seasons.

The newborn Moon was so hot after the great collision that it remained molten for several millennia. As it cooled down, lakes of lava began to form on its surface and these eventually solidified to leave a crust. About four billion years ago the Earth and the Moon were subjected to a great barrage of debris from space, forming large craters on their surfaces. The heavily cratered lunar surface is testimony to this violent pounding. The Earth, however, although heavily scarred as well, was afforded some protection by the atmosphere and, over time, the weather has eroded the craters.

There is one major disappointment that has come about just because we now know so much about the solar system. Many different environments in the solar system have been discovered and explored, but there has been no positive sign of life anywhere other than on our own planet. There is one intriguing find in the form of the SNC meteorite discovered in Antarctica. It has been identified as a small piece of the planet Mars, thrown into space by a massive impact over a billion years ago with such velocity that it escaped the gravitational field of Mars and eventually landed on Earth. There is evidence to show that the rocky chunk had once been exposed to water and there is also evidence of fossilized primitive bacterial life,
but scientists think the exposure to water was on Earth and not Mars. The
Mars Global Surveyor
and the Mars rovers
Spirit
and
Opportunity
have mapped the surface of Mars in detail. There is little doubt that at one time the planet had a much warmer climate and flowing water.

There is still a chance of finding primitive life forms, and the search for microscopic life continues. There has been speculation that some of the moons of the outer planets, in particular Saturn's moon Titan that is known to harbor complex organic compounds, could be suitable sites for life but they need a much warmer environment.

As early as 1952 those hoping to find evidence of life elsewhere in the solar system received encouragement when the American scientists Stanley Miller (1930–2007) and Harold Urey (1893–1981) performed a classic experiment with the simplest of laboratory equipment. They showed that in a closed container, using heat and electric sparks to simulate lightning, simple chemical elements such as hydrogen and nitrogen with molecules of water and carbon dioxide can combine to form organic molecules. Later experiments along the same lines have produced a wide variety of organic compounds. It is safe to conclude that the DNA molecule, and therefore life itself, could form under primitive Earth-like conditions.