The Cosmic Calendar ------------------- As with our Scale Model of the Local Universe, by simple direct proportions, we may also scale time. Let the origin of the Universe to have occurred at the stroke of midnight, 2002 December 31 / 2003 January 1.0. Let now be represented to occur at the stroke of midnight, 2003 December 31 / 2004 January 1.0---exactly one year later. With time scaled this way in direct proportions, the following events in cosmic history would occur at the following times on the Cosmic Calendar: (*) January 1, 12:00 a.m. (The Beginning, 13.7 billion years ago): The Universe begins in a hot, dense fireball called the Big Bang. It expands rapidly, which it continues to this day. There may also have been an early time of even more rapid expansion, or inflation, but this is under debate. Events in the early Big Bang (the first 0.01 second) are poorly understood, and are the topic of much current research. (*) The First Three Minutes (*real* minutes, not scaled): The whole Universe is as hot and dense as the center of the Sun. Nuclear fusion reactions, as which power the Sun, occur. They turn hydrogen nuclei into helium nuclei, again as in the Sun. This leaves the Universe about 3/4 H and 1/4 He, by mass---which we observe, today. (*) January 1, 12:12 a.m. (300,000 years later): The Universe cools enough to become transparent to light. It becomes dark, since there are no stars yet. Thanks to look-back time, we can still see the edge of this fireball, called the cosmic background radiation---quite literally, the echo of the fires of creation. (*) ?: The Dark Age ensues. No one knows when the first stars came into being. This is problem will be solved by your generation: the successor to Hubble Space Telescope, now called Next Generation Space Telescope (NGST), is scheduled to be launched in 2009, and is being designed for this problem. Indirect evidence so far suggests that the first stars probably had explosive births *and* deaths---and that the nuclear reactions that made their energy also made a great many chemical elements heavier than helium, with which, by exploding, seeded the Universe with carbon, nitrogen, oxygen, and other elements---the stuff of life. (*) Before January 28 (12 billion years ago): The most distant galaxies (islands of 100 billion stars, which are often disk or spiral-shaped) we can see are this old. How did they form? (*) Before April 23 (9.5 billion years ago): The disk of our Galaxy---note it's capitalized here, but for our Galaxy only---the Milky Way Galaxy, was formed. We know this because the oldest stars in the Galaxy are this old. Star formation, throughout the Universe, was probably more active at early times than now. The Sun formed *after* the time of maximum star formation. (*) August 23 (4.55 +/- 0.02 billion years ago): Formation of the Solar System. The Sun formed in about 100 million years. In contrast, the Appalachian mountains are 300 million years old. the Rocky Mountains are 60-80 million years old. the Grand Tetons are 9 million years old. The planets formed within 10-100 million years. Note the precision of the date, 4.55 +/- 0.02 billion years, found from meteorites and Moon rocks. These dates, like most that follow, were found by radioactive dating. Radioactive atoms split over time, at precise rates. Some are long-lived, with decay rates in the billions of years. They therefore can be used as clocks: a rock with much long-lived radioactive material and almost no short-lived radioactive material is older than one with nearly the same amount of long-lived material, and relatively plentiful short-lived radioactive material. (Work it out yourself!) (See also p. 210 of Kuhn.) (*) August 26 (100 million years later): The Sun and the planets were mostly formed. Much debris was left over. The planets therefore had much planetesimal bombardment---in other words, they were often hit by *big* meteorites. (*) September 16 (3.8 billion years ago): Planetesimal bombardment mostly ended, in the Late Heavy Bombardment. Most features on the Earth's Moon are at least this old. Earth's surface cools enough to solidify. (*) Before September 25 (3.5 billion years ago): Earliest signs of life on Earth. (!) Fossil stromatolites (*) November 6 (2 billion years ago): Blue-green algae make Earth's atmosphere rich in oxygen. Before this, it had been composed of chemically reducing gases, mainly from outgassing from Earth's interior. (*) December 11 (700 million years ago): First multicellular organisms. Before this, all life had been single-celled, and so relatively simple. No one knows why this happened. (*) December 15 (570 million years ago): The Cambrian Explosion, of diversity of species of life. The fossil record gets much richer, beginning in rocks this old. No one knows why. (*) December 20 (425 million years ago): First fish. (*) December 21 (375 million years ago): First insects; first forests on land. (*) December 22 (326 million years ago): First amphibians; emergence of vertebrates onto land. (*) December 25 (225 million years ago): First dinosaurs; first mammals (*) December 28 (80 million years ago): First flowering plants, preserved in fossils (*) December 29 (65 million years ago): Dinosaurs become extinct, thought to be because of a giant meteorite impact on Earth. (That an impact happened is clear, and it happened at just the time of the extinction. This is shown by microfossils abruptly becoming less common in a worldwide 65-million-year-old rock layer that is rich in iridium dust, common in meteorites but uncommon in Earth rocks. Whether the impact was from an asteroid or comet is under debate. How exactly this caused the extinctions, for example by throwing so much dust into the air that the whole Earth became dark and cold for a long time, is also unclear.) (*) December 31, 9:18 p.m. (4 million years ago): First hominid (human-like) ancestor, Australopithecus, in Africa. It was essentially an ape that walked upright. (*) December 31, 10:39 p.m. (2 million years ago): Hominids (Homo Erectus) could use tools and fire, but probably not articulate speech. Their fossils and tools have been found from Africa to China to Java. Although conventional wisdom decrees they originated in Africa, current evidence cannot rule out Asia. (*) December 31, 11:56 p.m. (100,000 years ago): First modern Homo Sapiens, our species. All living humans are descended from one woman, at about this time, who probably lived in South Africa, as shown by tracing samples of DNA, taken from people all over the world. In 1998, a fossil trackway (footprints) just this old was discovered in S. Africa. The footprints were of a modern human, probably a woman, since the feet and stride were small. It could have been her! (*) December 31, 11:57:30 p.m. (60,000 years ago): Homo Sapiens almost becomes extinct, particularly in Central Asia. Perhaps we were displaced by archaic hominids: it is not known. Gene differences in contemporary humans, however, indicate a constriction of the population at this time. (*) December 31, 11:59:00 p.m. (30,000 years ago): Our nearest relatives, Homo Neanderthalensis, become extinct. Why them and not us? They were certainly stronger, and had larger brains---but their tool use (technology) was not nearly as advanced as ours. (*) December 31, 11:59:36 p.m. (10,000 years B.C.): First agriculture, in Mesopotamia (now called Iraq). Soon after were the first human settlements: towns and cities. (*) December 31, 11:59:48 p.m. (3,000 years B.C.): First recorded history, in Egypt. Bronze Age (*) December 31, 11:59:53 p.m. (1,000 years B.C.): Iron Age in Europe and Asia (*) December 31, 11:59:59.6 p.m. (about 1830): Industrial Revolution, beginning in England (*) December 31, 11:59:59.9 p.m. (1957 October 4): Space Age begins, with the launch of Sputnik from the U.S.S.R. AND NOW FOR THE PUNCHLINE: We occupy only the very TINIEST sliver of cosmic history! A bit obvious, perhaps, but worth driving home. SOME OTHER IDEAS TO TAKE HOME with you: On Life: (*) There has been life on Earth for nearly the entire time the planet has been cool enough to have a solid surface. The exact origin of life, however, is poorly understood, and is a lively topic of research. (*) Most of this time, the only life on Earth was single-celled. Complex, multicellular life is a relatively recent innovation. No one knows why this innovation was made. On the Universe: (*) The Universe had a definite origin. We can measure how long ago it was. (*) We don't know the ultimate fate of the Universe. Current evidence suggests it will expand forever. On the Stars: (*) The stars are SUNS. This is a very big thought. (We don't see stars in the daytime because the Sun is so much brighter. Its light scatters in the Earth's atmosphere, drowning out the stars---much as city lights do, sadly.) (*) There are *very* many stars: 100 billion (1e11) per galaxy, with over 50 billion galaxies in the Universe. (*) They are powered by nuclear reactions in their cores, which also make all the chemical elements heavier than helium. This is quite literally, then, the power of creation. (*) YOU are literally made of star stuff---and so are connected to the Universe in a _very_ basic way. (Editorial comment: Who needs pseudoscience? The real thing is more amazing than anyone ever imagined.)