Theories of the Universe: From Aristotle to Hawking
Theories of the Universe
Aristotle
In astronomy, Aristotle proposed the existence of a finite spherical universe with the Earth at its center. This center would consist of four elements: earth, air, fire, and water. In his physics, each of these elements has a proper place, determined by their relative weight or specific gravity. Each element moves, naturally, in a straight line—earth down, fire up—toward its rightful place, stopping once achieved. Thus, the ground motion is always linear and always ends up stopping. The skies, however, move naturally and infinitely along a complex circular motion, so they must, in accordance with logic, be composed of a fifth element, which he called Aether. Aether is the top item, not susceptible to any change, that is not the place held by a circular motion.
Aristotle’s theory that linear motion always takes place through a means of resistance is in fact valid for all observable terrestrial motions. Aristotle also argued that heavier bodies fall with a specific matter more quickly than lighter ones when their forms are the same—a misconception accepted as the norm until the Italian physicist and astronomer Galileo conducted his experiment with weights dropped from the Leaning Tower of Pisa.
Copernicus
Cosmology before the Copernican theory postulated a geocentric universe in which the Earth was stationary at its center, surrounded by areas that revolved around it. Within these areas were (in order from inside to outside): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and finally the outer sphere in which were the so-called fixed stars. It was thought that this outer sphere wobbled slowly, thereby producing the precession of the equinoxes. (See Ecliptic.)
In ancient times, it was difficult for cosmologists and philosophers to explain the apparent retrograde motion of Mars, Jupiter, and Saturn. Sometimes, the movement of these planets in the sky seemed to stop, then start to move in the opposite direction. To explain this phenomenon, medieval cosmology posited that the planets revolved in a circle called the epicycle, and the center of each epicycle revolved around the Earth, tracing what they called a trajectory deferens. (See Ptolemaic System.)
Galileo
The last work of Galileo, Considerations and Mathematical Demonstrations on Two New Sciences Relating to Mechanics, published in Leiden in 1638, revises and refines his earlier studies on movement and the principles of mechanics in general. The book opened the path that led Newton to formulate the law of universal gravitation, which harmonized Kepler’s laws of planets with the math and physics of Galileo.
Galileo’s most important contribution to science was his discovery of precise physical measurements rather than metaphysical principles and formal logic. However, his books were more influential: The Messenger of the Stars and Dialogue, which opened new fields in astronomy. Beyond his scientific work, Galileo stands as an advocate of free inquiry, free of philosophical and theological interference. Since the publication of the complete trial of Galileo in 1870, full responsibility for the condemnation of Galileo has traditionally fallen on the Roman Catholic Church, covering up the responsibility of teachers of philosophy who persuaded the theologians of the findings of the heretical Galileo. John Paul II in 1979 opened an investigation into the ecclesiastical condemnation of the astronomer for possible revision. In October 1992, a papal commission acknowledged the error of the Vatican.
Tycho Brahe
Brahe never fully accepted the Copernican system of the universe and sought a compromise between it and the ancient system of Ptolemy. Brahe’s system assumed that the five known planets revolved around the sun, which, coupled with planets, went once around the Earth once a year. The sphere of the stars revolved around the Earth once a day.
Although Brahe’s theory on planetary motion was defective, the data obtained throughout his life played a key role in developing the correct description of planetary motion. Johannes Kepler, who was Brahe’s assistant from 1600 until his death in 1601, used Brahe’s data as a basis for formulating his three laws of motion of the planets. (See Kepler’s laws.)
Kepler
Kepler’s laws, three laws on the movements of the planets, were made by the German astronomer Johannes Kepler in the early seventeenth century. (See Solar System.)
Kepler based his laws of planetary data collected by the Danish astronomer Tycho Brahe, for whom he was an assistant. The proposals broke with a centuries-old belief that the planets moved in circular orbits. This was a feature of the Ptolemaic system, developed by the Alexandrian astronomer Ptolemy in the second century AD, and the Copernican system, proposed by the Polish astronomer Nicolaus Copernicus in the sixteenth century. According to Kepler’s first law, planets orbit the Sun in elliptical orbits in which the Sun occupies one focus of the ellipse. The second law states that the areas swept by the radius vector from the center of the planet to the Sun’s core are the same for equal periods; as a result, the closer the sun is, the faster the planet moves. The third law states that the ratio of the average distance, d, of a planet to the Sun, to the third power divided by the square of its orbital period, t, is a constant, i.e., d3/t2 is equal for all planets.
These laws have played an important role in the work of the astronomer, mathematician, and seventeenth-century English physicist Isaac Newton, and are essential to understand the orbital paths of the Moon and artificial satellites. (See also Mechanics.)
Newton
The British physicist Isaac Newton advanced a simple principle to explain Kepler’s laws of planetary motion: the force of attraction between the Sun and planets. This force, which depends on the mass of the Sun and planets and the distances between them, provides the basis for the physical explanation of Kepler’s laws. Newton’s mathematical discovery is called the law of universal gravitation.
After Newton’s time, astronomy branched into different directions. With this law of gravitation, the old problem of planetary motion was further examined as celestial mechanics. The refinement of the telescope allowed the exploration of the surfaces of planets, the discovery of many faint stars, and the measuring of stellar distances. In the nineteenth century, a new instrument, the spectroscope, provided information on the chemical composition of celestial bodies and new data on their movements. (See Spectroscopy.)
During the twentieth century, reflecting telescopes have been built increasingly larger. Studies of these instruments have revealed the structure of massive and distant clusters of stars, called galaxies and clusters of galaxies. In the second half of the twentieth century, developments in physics provided new types of astronomical instruments, some of which have been placed on satellites that are used as observatories in Earth orbit. These instruments are sensitive to a wide range of wavelengths of radiation, including gamma rays, X-rays, ultraviolet, infrared, and radio regions of the electromagnetic spectrum. Astronomers not only study planets, stars, and galaxies, but also plasma (hot ionized gases) that surround double stars, interstellar regions that are the birthplaces of new stars, dust grains in cold regions invisible to optical energy, nuclei that may contain black holes, and cosmic microwave background, which may provide information on the early stages of the history of the universe. (See Astronomy, radar astronomy, gamma, ultraviolet astronomy; Astronautics.)
Newton’s law of gravitation proposed a force of attraction between the Sun and each of the planets to explain Kepler’s laws of elliptical motion. However, this also means that smaller forces must exist between the planets and between the sun and bodies such as comets. Interplanetary gravitational forces cause the orbits of the planets to deviate from simple elliptical motion. Most of these irregularities, predicted on the basis of Newton’s theory, could be observed with the telescope. (See Solar System.)
Hawking
Inflationary theory (cosmology) is a theory developed in the early 1980s by the American physicist Alan Guth trying to explain the events of the first moments of the universe. According to the Big Bang theory, generally accepted, the universe arose from an initial explosion that caused the expansion of matter from a state of extreme condensation. (See Cosmology.) However, in the original formulation of the Big Bang theory, there were several unresolved issues. The state of the art at the time of the explosion was such that one could not apply the normal physical laws. The degree of uniformity observed in the universe was difficult to explain because, according to this theory, the universe would have expanded too quickly to develop this uniformity.
Guth based his inflationary theory on the work of physicists like Stephen Hawking, who had studied very strong gravitational fields, such as those found in the vicinity of a black hole or at the very beginning of the universe. This work shows that all matter in the universe could have been created by quantum fluctuations in ’empty space’ under such conditions. Guth’s work uses the unified field theory to show that in the first moments of the universe, phase transitions could take place and that a region of that original chaotic state…
Big Bang
The Big Bang, big bang literally, is the moment of “nothingness” from which all matter emerged, i.e., the origin of the universe. The matter so far, is a point of infinite density, which at one time “explodes,” generating the expansion of matter in all directions and creating what we know as our Universe.
Immediately after the time of the “explosion,” each particle of matter began to recede rapidly from each other, in the same way that an inflating balloon occupies more space as it expands its surface. Theoretical physicists have managed to reconstruct the chronology of events from 1/100 second after the Big Bang. The material released in all directions by the primary explosion is composed exclusively of elementary particles: electrons, positrons, mesons, baryons, neutrinos, photons, and so on, up to more than 89 particles known today.