Simple Nature - Light and Matter

football player with a serious nicotine addiction, Hubble did notset out to change our image of the beginning of the universe. Hisautobiography seldom even mentions the cosmological discovery forwhich he is now remembered. When astronomers began to study theDoppler shifts of galaxies, they expected that each galaxy’s directionand velocity of motion would be essentially random. Some would beapproaching us, and their light would therefore be Doppler-shiftedto the blue end of the spectrum, while an equal number would beexpected to have red shifts. What Hubble discovered instead wasthat except for a few very nearby ones, all the galaxies had redshifts, indicating that they were receding from us at a hefty fractionof the speed of light. Not only that, but the ones farther awaywere receding more quickly. The speeds were directly proportionalto their distance from us.Did this mean that the earth (or at least our galaxy) was thecenter of the universe? No, because Doppler shifts of light onlydepend on the relative motion of the source and the observer. Ifwe see a distant galaxy moving away from us at 10% of the speedof light, we can be assured that the astronomers who live in thatgalaxy will see ours receding from them at the same speed in theopposite direction. The whole universe can be envisioned as a risingloaf of raisin bread. As the bread expands, there is more and morespace between the raisins. The farther apart two raisins are, thegreater the speed with which they move apart.The universe’s expansion is presumably decelerating because ofgravitational attraction among the galaxies. We do not presentlyknow whether there is enough mass in the universe to cause enoughattraction to halt the expansion eventually. But perhaps more interestingthan the distant future of the universe is what its presentexpansion implies about its past. Extrapolating backward in timeusing the known laws of physics, the universe must have been denserand denser at earlier and earlier times. At some point, it must havebeen extremely dense and hot, and we can even detect the radiationfrom this early fireball, in the form of microwave radiation thatpermeates space. The phrase Big Bang was originally coined by thedoubters of the theory to make it sound ridiculous, but it stuck,and today essentially all astronomers accept the Big Bang theorybased on the very direct evidence of the red shifts and the cosmicmicrowave background radiation.Finally it should be noted what the Big Bang theory is not. It isnot an explanation of why the universe exists. Such questions belongto the realm of religion, not science. Science can find ever simplerand ever more fundamental explanations for a variety of phenomena,but ultimately science takes the universe as it is according toobservations.Furthermore, there is an unfortunate tendency, even among manyz / How do astronomers knowwhat mixture of wavelengths astar emitted originally, so thatthey can tell how much theDoppler shift was? This image(obtained by the author withequipment costing about $5, andno telescope) shows the mixtureof colors emitted by the starSirius. (If you have the book inblack and white, blue is on the leftand red on the right.) The starappears white or bluish-white tothe eye, but any light looks whiteif it contains roughly an equalmixture of the rainbow colors,i.e., of all the pure sinusoidalwaves with wavelengths lying inthe visible range. Note the black“gap teeth.” These are the fingerprintof hydrogen in the outeratmosphere of Sirius. Thesewavelengths are selectively absorbedby hydrogen. Sirius is inour own galaxy, but similar starsin other galaxies would havethe whole pattern shifted towardthe red end, indicating they aremoving away from us.Section 6.1 Free Waves 357