Numerous volumes have been written about the science of cosmology over the centuries. Strange as it may sound, until the 1920’s, many scientists thought the Milky Way galaxy constituted the entire universe. Terms now familiar to the general public, such as the Big Bang and expansion of the universe, did not even exist a single lifetime ago. This article will briefly discuss how modern cosmology grew out of astronomy, astrology and physics then touch on some key problems cosmologists are working on today.
One definition of cosmology is the quest to figure out the origin and overall structure of the universe. The roots of that quest are unknown but undoubtedly began in prehistoric times. Humans have always been fascinated by celestial bodies, notably the sun, moon, stars, and planets. To primitive people living millennia before the advent of science, it was no doubt easier to imagine celestial objects as gods or goddesses rather than mundane balls of rock or gas.
Nonetheless, by ancient times, various civilizations had specialists whose chief task was to observe the motions of the sun, moon, planets, and stars. Gradually, peoples as far apart in space and time as the Chinese, the Egyptians, the Greeks, and the Maya developed solar and lunar calendars. In ancient Egypt, the priests observed that the Nile flooded its banks every August, around the time the Dog Star (Sirius) rose just prior to sunrise. Half way around the globe, the Maya developed two calendars – one solar and a second based on the orbital cycle of the planet Venus.
From ancient times through the Middle Ages, most people, with the notable exception of Hipparchus of Rhodes, accepted a geocentric view of the cosmos (sometimes called the Ptolemaic model), which placed Earth in a privileged place at the center of the universe. By late medieval times, astronomy and astrology began to diverge in Europe. The first astronomer to break with the geocentric view was the Polish priest Copernicus, whose work, On the Revolutions of the Celestial Orbs , was published posthumously in 1543. Most scholars agree that its heliocentric view inspired the work of an even more famous scientist, Galileo Gallilei.
Galileo had one advantage that no person before him enjoyed: the telescope, first constructed in the year 1609. Although his telescope was crude by today’s standards, it allowed Galileo to see mountains on the moon, four small satellites orbiting the planet Jupiter, and countless tiny stars that form the band of light called the Milky Way. Although the Catholic church put Galileo under house arrest for publishing his observations, there was no way to undo the invention of the telescope. By the late 17th century, astronomers throughout Europe were expanding upon the work of Galileo as well as that of the German astronomer Johannes Kepler, who formulated the basic equations describing planetary orbits.
The Enlightenment: Isaac Newton
It is difficult to exaggerate the importance of Isaac Newton (1642-1727) in the development of mathematics, physics, or cosmology. Newton’s theory of gravitation made the laws of physics applicable to the realm of astronomy. Newton’s work in the field of optics helped advance the development of more powerful telescopes. Finally, Newton’s discovery of calculus shaped the work of all the great scientists who followed him, including Albert Einstein (1879-1955).
The Early 20th century – Einstein’s Theory of Relativity
In 1905, Einstein published his special theory of relativity, whose core theme was that the speed of light was really the highest speed attainable in our universe. By 1916, Einstein published his general theory of relativity in which he reconfigured Newton’s theory of gravity, in which space and time are separate, absolute entities, into a universe with a four dimensional space-time matrix. As intriguing as these concepts were, they raised an even more amazing prediction – the universe could not exist in a static state; rather, it must be expanding or contracting. Little did Einstein know, but a few decades later, an astronomer named Edwin Hubble would prove this idea was correct.
Edwin Hubble – the Expanding Universe and the Big Bang
Today, the name Hubble brings to mind the orbiting space telescope launched in 1990. The astronomer for whom it is named, Edwin Hubble (1889-1953), first came to prominence in the astronomical community when he established that the Andromeda nebula was actually a galaxy in its own right, not merely a patchy cloud within the Milky Way galaxy. Soon after, Hubble and his contemporaries observed that many objects once thought to be nebulae were actually galaxies quite distant from our own.
Several years later, Hubble noticed an astounding pattern: almost all observable galaxies displayed a red shift in their spectra, meaning that light from these galaxies was being stretched, or shifted toward the red end of the spectrum. Much like the sound of a siren becomes stretched out as it rushes away from an observer, so too, the light from these galaxies is stretched, as it were, to lower frequencies because they are moving away from our own galaxy. On top of that, Hubble noted, the more distant the galaxy, the larger was its red shift, and the faster it was receding from the Milky Way. The conclusion was inescapable (although it took some scientists decades to accept): at some point in the distant past, all matter in the universe was compressed into a single point. Around 15 billion years ago, our universe burst into existence from this primordial fireball, now popularly known as the Big Bang.
Modern Cosmology – the Search for the Hubble Constant and the Fate of the Universe
Since the 1960’s, many cosmologists have tried to measure the expansion rate of the universe, in an attempt to learn its ultimate fate. The term for the expansion rate of the universe, called the Hubble Constant, is denoted Ho. Knowing the value of Ho and whether it has changed over time is the key to predicting whether the universe will expand forever and eventually burn out, or stop expanding and ultimately collapse into another fireball. Today, many astronomers are convinced the expansion of the universe is accelerating for unknown reasons. This has led them to concoct ideas such as dark energy – a sort of antigravity – to account for this bizarre phenomenon. Other cosmologists remain unconvinced; their theories center around the concept of a multiverse – the idea that our universe is one of an infinite number of universes that separate and coalesce over unimaginably long stretches of time. As with most profound questions, the debate will continue for the forseeable future.