is thought and theories about the origin, fate, and large scale structure of the universe
. All human belief systems, including the major world religion
s, have some sort of cosmology, and so does contemporary science, which, through the study
and fundamental physics
, has arrived at the current Big Bang theory
, in which the universe arose from a point-like concentration of energy some 15 billion years ago, and has expanded ever since, forming billions of galaxies, stars, and planets.
Religious and mythical cosmologies range from simple metaphors such as the belief among some pacific northwest native American populations that the Earth is the back of a giant turtle, to complicated mystical theories, such as a Hindu belief that a giant universe, of which the Earth is only a small part, is created and destroyed every 60 billion years by the divine utterance of the single, all powerful syllable. Most people are familiar with the biblical cosmology of the book of Genesis, which forms the basis of cosmological thought in fundamentalist Christianity, Judaism, and Islam, in which an omnipotent God created the universe over the course of six days some 6000 years ago.
The scientific study of cosmology began, obviously, with an account of the aspects of the universe visible with the naked eye from the Earth. The classical era Greek and Roman philosophers, including Aristotle, hypothesized a static, constant universe consisting of the Earth at the center, orbited by the moon, sun, and other planets, all of which was surrounded by a sphere of fixed stars. This view was sufficiently compatible with and complimentary to the Genesis view of the universe that it became orthodox opinion. However, throughout the middle ages, astronomers in the Islamic world, and then in the West, refined their measurements to the point where the Aristotelian view of the universe was shown to be inconsistent with observational data. An attempt was made to correct the Aristotelian view with complicated sub-orbits for the planets, but a better explanation was the Sun-centered solar system of Nicholas Copernicus, proposed in the middle of the sixteenth century.
With Copernicus, the scientific theory of cosmology was now incompatible with the Genesis explanation, a situation that has persisted to this day. It also began a major, ongoing philosophical revolution, as the Earth was shown to occupy less and less of a ‘privileged’ or special place in an increasingly complicated universe. The advent of optical telescopes only furthered this, as other planets were shown to have moons, and additional planets, stars, and galaxies were discovered. Some idea of the scale of the universe was deduced in the early eighteenth century, as astronomers began to measure, through trigonometric techniques, the shockingly vast distances to even the closest stars.
By the late 19th century, astronomers knew that the universe was unimaginably vast, with billions of stars and possibly many other galaxies, and from fields such as geology and biology, science knew that the Earth had been around for at least millions, if not billions of years. Still the origin and shape of the universe was an open question. In the opening decades of the 20th century, three amazing profound revolutions in physics changed science and thought. The first two were relativity theory and quantum mechanics. With special and general relativity, ideas of the structure of space itself were changed. The universe could possibly have a ‘shape’ extending to other dimensions, even to the point of being a hypersphere and closing in on itself, and space-time needed a ‘fabric’ which could be deformed.
But the third revolution in physics was the most important for the study of cosmology. It was the discovery, in the early 1920s, by astronomers such as Edwin Hubble, that there were indeed billions of other galaxies, and that they were uniformly moving away from our own, at a speed that increased with their distance. This was ascertained by viewing the so-called ‘red shift’ in the frequency of the electromagnetic radiation emitted by the galaxies, which was correlated to their velocity relative to us, their observers. One proper conclusion was that at some point in the past, billions of years ago, everything that is now in the universe was concentrated in a small space and has been spreading out ever since, making the present universe extraordinarily vast but still finite. It was later shown by Stephen Hawking that's Einstein's Equations of General Relativity would permit such a 'singularity.'
This so-called Big Bang theory remained an increasingly accepted but unverified theory until the discovery of the Cosmic Background Radiation, by accident, in the late 1960s. This radiation is almost entirely uniform in all directions, thus it must originate from a time when the universe was isotropic and compact. Small fluctuations in the energy of the background radiation are interpreted to be the result of a non-uniform density of matter in the early universe, which would have eventually led to the formation of the galactic structures we see today.
In the 1990s, two further discoveries again changed our conception of the universe. One was from observing the rotation rates of galaxies, where it was revealed that there must many times more gravitationally interacting matter in the universe than we can account for with the ‘ordinary’ matter based on the sub-atomic particles that we know of and are made of, and that shines in stars. The nature of the missing matter, the so-called Dark Matter is unknown, but we know it must not interact electromagnetically, or else we would be able to detect its presence by seeing the light it emits, hence the name 'Dark' Matter.
The second discovery was the result of a very accurate measurement, made with the Hubble Space Telescope, of the velocity with which galaxies are receeding from us, which revealed that the expansion of the universe is actually accelerating, not decelerating as would be the case if the only thing effecting the acceleration were the gravitation of matter. This means that there must be a present source of the accelerating expansion, other than the initial oomph from the big bang, and it has been termed Dark Energy. The magnitude of the energy that seems to be causing the acceleration of the expansion is a parameter known as the cosmological constant.
Currently, the Dark Energy accounts for about 70% of the enegy density of the universe, while the dark matter is about 25%. Neutrinos are thought to account for another 2% or so, which leaves only 3% of the energy density of the universe being from everything that we see and are!
Determining the nature of the dark matter and the dark energy are two great unsolved questions in cosmology, but there are two other vexing problems in cosmology which are more philosophical in nature. One is the Fine Tuning Problem. It seems that only a very narrow range of values for universal parameters, such as the relative magnitudes of the cosmological constant and the matter density, or the relative strengths of the gravitational and electromagnetic interaction, would permit the formation of complex structures like galaxies, stars, planets, and life. For instance, if the relative magnitudes of the cosmological constant and the matter density were slightly different, the early universe would have either expanded too fast for structures to form from gravitational collapse, or would have so vigorously collapsed that it would have reversed the expansion before srtuctures could form. It seems like a very improbable coincidence, then, that the universe is tuned so well to permit us to exist and contemplate it.
Another problem is the Coincidence Problem. The energy density due to matter diminishes with the expansion of the universe, for matter becomes less dense as the space between it grows. However, the energy density due to the dark energy remains constant, as it appears to be a property of spcetime itself. It therefore seems that only during a brief window of time in the history of the universe would these densities be comparable, and as a result both detectable, and we just happen to be living during that very brief epoch. This again seems highly lucky and improbable.
The answers to these questions may come soon, or they may not. One thing we can certainly say is that the contemplation of cosmology is natural for a species where members seek to explain their very existence and purpose.