The third chapter in A Brief History of Time is called The Expanding Universe. This chapter first begins to talk about "fixed stars" and how they really aren't fixed. It's just that they move so slowly, they appear fixed. The book continues by talking about how small Earth is in comparison with our solar system, our galaxy, and even our Local Group. The book talks about how people know so much about stars. Scientists look at a star's luminosity and color to learn about it. If a star is brighter, it is closer, darker means farther. The scientist also use color to determine which elements a star is made of. Light is a type of wave. The largest waveleghnths to the shortest in order are: radio waves, about a meter, microwaves, o.1 meters, infared, 0.00001 meters, visible light, 0.000001 to 0.0000001 meters, ultraviolet, o.ooooooo1 meters, x-rays, 0.0000000001 meters, and gamma rays, 0.000000000000001 meters. The longer the wavelength, the more "redder" it is, the more shorter, the more " bluer" it is. If a stationary object is giving a certain wavelength, you will recieve the same wavelength. If an object is moving away, the length will be longer, if it is moving to you, it will be shorter. This is called the doppler effect and it is explained in the picture above. Edward Hubble, who was currently using wavelenghths to measure the movement of the stars, discovered that almost all stars were on the red side of the spectrum. In 1929, he published a paper saying that the entire universe must be expanding.If the universe was expanding, shouldn't it eventually collapse? General Relativity Theory supported that the universe must be in motion. Einstien, who liked a static universe, introduced an " antigravity force" that keeps the universe in balance. The only person willing to take Hubble seriously was Alexander Friedmann. Friedmann made two assumptions about the universe: It looks approximatly identical in whichever direction you look and that this would be true wherever you were. In 1965, two American physisist, Arno Penzias and Robert Wilson, were working with a sensitive microwave detector. The detector detected an almost equal amount of microwave energy outside the atmosphere. They had accidently proved the first of Friedmann's assumptions! As for the second assumption, there has been nothing to prove or disprove it. Friedmann, seeing that the galaxies are moving apart theorized that they must have been at the same place, long ago. This led to the first Friedmann model of the universe, saying that the world began with the Big Bang and will begin to expand, and eventually contract into the Big Crunch. They're actually two other models, one stating the universe will expand forever and the other saying the universe expansion force will eventually become smaller and smaller but never quite reach zero. No one knows which model represents our universe.
A lot of people didn't like the big bang theory but it was became generally accepted. In 1965, Roger Penrose, a British mathematician said that General Relativity Theory predicted the Big Bang. He said that a star's gravity might pull all the mass into zero volume. This space-time bending phenomena is called a black hole. In the next few years Stephen Hawking used this theory, reversed it and removed technicality with complex mathematics. In 1970, Penrose and Hawking wrote a paper that proved the Big Bang. This paper used general relativity as proof, but in turn saying that general relativity is a partial theory. This therory will have to combine with the quantum theory of gravity.
