Purpose/1. The Universe/Introduction/What We See In The Sky
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Who has not looked at the sky on a starry, cloudless night and felt the wonder of living on a planet surrounded by so many mysterious shining points of light? Two thousand years ago, many thought that most of these were merely copies of each other, and that they were positioned on one of several “shells” which surrounded the Earth—a very uninspiring arrangement. We now know how incorrect that view was, and any reasonably good backyard telescope can provide hints about how our new perspective was obtained. It can be seen that these points of light are not all the same: many vary in brightness, some possess tinges of color, and, while most hardly ever change their position relative to one another, a few move about from one night to the next, and the whole rotates with the time and season.
Even four hundred years ago it was not generally known that the Earth and planets are satellites of the sun. Although the Greek astronomer Aristarchus (around 250 BCE) had guessed as much, most preferred to believe otherwise. The Catholic Church had adopted Aristotle’s cosmology, and it maintained that three concentric spheres or “shells” encompassed the Earth, with the sun and moon moving on the surface of the first sphere, the planets moving on the second, and all of the stars being at rest on the third.[1] This conformed to the notion that a perfect heaven awaited above (while volcanoes and hot springs were said to prove that hell and brimstone lay beneath). There is a well-known story of how Galileo,[2] after careful use of a telescope he had constructed, declared in 1616 that Copernicus was right and that the Earth and planets did revolve around the sun; this led to the Church placing Galileo under house arrest for the rest of his life.[3] However, the increasing use of telescopes gradually altered popular opinion, and by the middle of the seventeenth century most astronomers agreed that the sun-centred description was correct.
A six-inch telescope and some careful scrutiny will uncover numerous small, fuzzy patches of light in the night sky. Careful observation at higher magnifications shows that these are actually collections of stars, or galaxies. Galaxies, typically comprising thousands of millions of stars, mostly appear in one of three arrangements: elliptical, spiral[4] or irregular. Astronomers further find (through careful examination of photographs taken using giant telescopes) that galaxies themselves collect together in groups and clusters, and that these clumps form even bigger collections. The size of these massive groupings (called superclusters[5] and galaxy walls) can exceed many hundreds of millions of light-years.
Astronomers have also observed several pairs of galaxies colliding. This sounds catastrophic, but it produces nowhere near the (relative) damage caused, for example, when comets or asteroids crash into planets. This is because galaxies are vast (which is another way of saying that they are mostly empty space) and one galaxy can pass right through another with relatively few direct impacts (orbital disruptions would be much more likely to happen).[6]
A number of dark patches are also noticeable in the night sky, areas which seem to be devoid of light-emitting stars. One well-known dark region can be seen in the Great Orion Nebula forming part of Orion’s sword and lies some 1500 light-years away. (The reason such dark patches exist is explained in section four of this chapter.)
Stars, galaxies, superclusters, and other objects in the universe have been photographed, numbered, catalogued and intensively studied for decades. Their brilliance, variabilities, emissions, compositions, movements, ages, and much else, have been repeatedly measured by a wide range of very powerful instruments. Over years of study, it has been found that most objects and phenomena can be grouped into categories, and these have been given names (for example, white and brown dwarfs, red giants, neutron stars, gas giants, black holes, Cepheids, pulsars, quasars, novae, supernovae, and so on). Much has been learned in just a few decades about the nature and properties of members of each category, but a great deal more remains to be discovered. By studying the variations in intensity of emitted radiation, astronomers have found how to measure the distance between Earth and a particular kind of varying star called a Cepheid variable. By associating these stars with galaxies, it has been determined that the light from the most distant galaxies has been travelling for more than thirteen billion years! This means that, when viewing those galaxies, we are looking at something that is about 12x1022 kilometres away (calculated by multiplying the distance light travels in one year by 13 billion), and, perhaps more significantly, it means we are seeing light that was emitted from them as they existed over thirteen billion years ago.[7] To see these galaxies as they exist today, we would have to wait another thirteen billion years for their current emissions to reach Earth (or we would have to instantaneously travel 12x1022 kilometres to where they are, an impossibility). Knowing how far an object is from Earth has allowed astronomers to “look into the past”—the farther away a celestial entity is, the older the light we are seeing. Thus, the properties of distant objects can be examined and compared to closer (i.e., younger) emitting bodies. Such analyses have led to many meaningful discoveries about the origins of our universe and its evolution over time.
Some eighty years ago astronomers discovered that the spectral emissions[8] from all distant objects are displaced, i.e., that their spectral dark bands[9] have been shifted from their normal position. All galactic radiation (except that coming from a few, nearby, galaxies) shows spectral lines that have been moved toward the red end of the spectrum; this has been termed the “red shift.”[10] This phenomenon has been found to occur in every direction we look, and means that all distant objects are moving away from us. In fact, the farther away an object is, the faster it recedes.[11]
Unfortunately, for those who would have otherwise, this does not make us unique. Although it seems to place the Earth (and therefore humanity) at the centre of the universe, this is not the case. The true explanation is that everything is moving away from everything else, because the intervening space is expanding rather than the objects themselves moving. (They do move, of course, just as our planet moves around our sun. Our sun travels around the centre of our galaxy, and galaxies themselves move. But galactic recession is due to space expansion. Space expansion is perhaps most clearly visualized using the analogy of specks of dirt on the surface of a balloon. When the balloon is inflated, the distance between each speck increases, and those furthest apart separate at the greatest speed, but no speck is more centrally positioned than any other.)
Finding that billions of galaxies exist, each containing billions of stars, has been exciting. Finding that they are speeding away from each other was at first unbelievable, then astonishing, and it immediately claimed the attention of all cosmologists. This galactic recession (or expansion of the universe, which is another way of saying the same thing) was so fundamental and far-reaching an observation that few astronomers could concentrate upon any other issue—an explanation had to be found.
