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Wow!The Universe is believed to be between 10 and 20 billion years old!
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The ancient Greeks struggled to understand the Universe. Thales, who lived in the sixth century BC, was a philosopher and traveler. He brought back to Greece, the knowledge and records of the Babylonians and Egyptians. While he believed the Earth was flat and floated on water like a log, nevertheless his beliefs were some of the first recorded ideas that lay between mythology and true science. Aristotle, who lived from 384 to 322 BC, put forth 3 experimental proofs that the Earth was round. However, he believed the Earth was the center of the Universe and the Sun, Moon, planets, and a sphere containing all the "perfect, fixed" stars revolved around it. Most Greeks believed Aristotle and his ideas. The exception (a century later) was a fellow named Aristarchus, who thought the Earth moved around the Sun. He found very few supporters for his ideas and is often overlooked in the history of heliocentric ideas. In the 100s BC, Hipparchus compiled a star catalog giving the stars a magnitude (or comparative brightness scale), and calculated the Moon's distance from the Earth.
The first astronomer to make truly scientific maps of the heavens, Claudius Ptolemaeus (better known as Ptolemy), came along 300 years later. The ancient Greeks believed that the Sun, Moon, and planets circled the Earth. To explain the fact that planets sometimes appear to travel backward in the sky, the Greeks thought each moved in a small circle (called an epicycle) that was itself orbiting Earth. Ptolemy summarized the theory in the Almagest, written in the 140 AD. This view of the Universe was believed for about 1500 years.
The Ptolemaic Theory held sway until 1543, when Polish monk Nicolaus Copernicus suggested that the Sun was at the center of things. The Church, however, taught that the Earth was central; Copernicus' theory was heresy. (Perhaps this is why he did not publish it until he died.) His publication was called On the Revolution of the Celestial Spheres. Needless to say, the publication of this theory was met with great hostility, however two events made the "Copernican revolution" inevitable: Tycho Brahe's precise observations of the sky and Galileo's use of the telescope.
One night in 1572, Danish astronomer Tycho Brahe saw a bright new star in the sky in the constellation Cassiopeia. (We now know he was observing a supernova). In 1604, a second supernova was observed. These discoveries undermined the foundation of the Ptolemaic system: that the outermost sphere contained all the stars and it was unchanging.
In 1609, Italian scientist Galileo Galilei heard about the invention of a spyglass, made one for himself, and turned it on the heavens. One of the first discoveries was four moons circling around the planet Jupiter. In essence, his telescope revealed a miniature version of Copernicus' solar system, with the moons moving around the planet in simple, circular orbits. The Catholic Church later forced Galileo to recant his ideas and live out his life under house arrest, but his discoveries had already changed the face of astronomy.
The beginning of modern science can be attributed to Galileo and to British genius Isaac Newton. Newton put mathematics to the observations. He worked out the equations to describe the relationships observed for motion, including the motions induced by gravity. When he applied his equations to Brahe's observations of the motions of the planets (and to the observations of Johannes Kepler, who took up the work after Brahe's death), they fit almost perfectly. The conclusions were indisputable: Aristarchus and Copernicus has been right all along! Newton had proven that it is the force of gravity which keeps all the planets in their elliptical orbits around the Sun. Newton's Laws are all based on the idea that nothing is naturally at rest. He reasoned that all heavenly bodies should be constantly moving, with no limits on space and time. This made the Church happy, since Newton's ideas are based on a Universe that is eternal and infinite. It was an uneasy reconciliation between the Church and science, but it was one that allowed science to move forward unimpeded.
In 1917, Albert Einstein proposed a description of the Universe based on his Theory of General Relativity. It inspired many other theorists, including Willem de Sitter in Holland and Alexandr Friedmann in Russia. In fact, much of today's cosmology is based on Friedmann's solutions to the mathematical equations included in Einstein's Theory. Friedmann built on the General Relativity equations to develop models that helped explain the evolution of the Universe.
A major breakthrough in our understanding of the Universe took place in the 1920s thanks to American astronomer Edwin Hubble. For centuries, astronomers believed that the Milky Way was the entire Universe. In 1923, Hubble was among the first to show that the fuzzy patches they saw in the sky through their telescopes were whole other galaxies, not parts of the Milky Way. Then, in 1929, by looking at the spectra of these galaxies, he concluded that they were speeding away from us - the Universe was expanding!
Georges Lematire came up with his own model when he heard that the Universe was expanding. He made the intellectual leap that the Universe began as a "primeval atom", something hot and dense that had exploded, causing space to expand. In the late 1940s, George Gamow, a Russian-American physicist, conceived of the Big Bang as we know it today. He and his colleagues proposed that the elements were created during a very brief period early in the expansion of the Universe. They also predicted an afterglow which would bathe the entire Universe and should be present still, arising from the hot, early phase.
In 1965, physicists Arno Penzias and Robert Wilson started to search for faint radio signals (actually microwaves) from the outskirts of the Milky Way Galaxy. They were bothered by a persistent interference - a constant signal from all over the sky - which corresponded to a temperature of -454 degrees Fahrenheit (-270 Celsius or 3 Kelvin). They thought the signal was from pigeon droppings in the telescope, but colleagues soon realized that they had in fact found the afterglow predicted by Gamow. It was important evidence that the Universe began with a hot Big Bang. More recently, NASA's COBE satellite measured and mapped this radiation in detail. All of the measurements were consistent with the Big Bang theory.
In 1979, particle physicist Alan Guth performed calculations which led to the idea of "cosmic inflation", a brief period of rapid expansion in the early Universe. Inflation solves many problems with the simple, original Big Bang theory. It explains why the Universe is so big and so smooth, why (at least) four different forces act in it today, and where the vast amounts of matter came from.
It is worth mentioning that the idea that the Universe had a beginning was not appealing to all scientists. In 1948, Fred Hoyle, Hermann Bondi, and Tommy Gold, came up with the Steady State Theory. In it, the Universe has no beginning and no end. Although expanding, it stays in perfect balance - like a pool kept full-to-overflowing by a trickle from a faucet. The "faucet" of the Universe is the continuous creation of matter from energy. They showed that only one atom per cubic mile of space per hour was needed. Yet, even this tiny amount proved to be the theory's undoing. Evidence mounted against the theory: the background cosmic heat radiation was discovered; the amount of helium observed in the Universe exactly fit what was predicted by the Big Bang; radio astronomers found that galaxies were more crowded together in the past - meaning the idea of an unchanging Universe was incorrect.
One remaining big question for cosmologists is the ultimate fate of the Universe. Will it expand forever, expand to a certain size and stop, or will it stop and begin to collapse? In 1998, data were published which implied that the Universe is expanding at an accelerating rate. For over ten years astronomers had been studying the expansion of the Universe by measuring the redshift and brightness of distant supernovae. By 1998, a few dozen of these supernovae had been analyzed in detail, enough to lead scientists to the startling discovery that the expansion of the Universe is not slowing, as had been expected, but accelerating. Combined with other cosmological studies, the supernova data strongly suggest that the Universe is filled with an unidentified form of energy (currently being called "dark energy" since we know nothing about it) which is causing the expansion of the Universe to accelerate. If these observations and analyses turn out to be correct, it would be expected that the Universe would continue to expand forever.
A QuestionWhat did scientists observe which was important evidence in support of the Big Bang theory? |
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