Editing A Brief History of Time (section)

=== Chapter 3: The Expanding Universe ===
[[File:CMB Timeline300 no WMAP.jpg|thumb|400px|The [[expansion of the universe]] since the Big Bang]]
Hawking describes how physicists and astronomers calculated the relative distance of stars from the Earth. Sir [[William Herschel]] confirmed the positions and distances of many stars in the night sky. In 1924, [[Edwin Hubble]] discovered a method to measure the distance using the [[apparent magnitude|brightness]] of [[Cepheid variable stars]] as viewed from Earth. The [[luminosity]] and distance of these stars are related by a simple mathematical formula. Using this, he showed that ours is not the only galaxy. 

In 1929, Hubble discovered that light from most galaxies was [[red shift| shifted to the red]], and that [[Hubble's law| the degree of redshift is directly proportional to distance]]. From this, he determined that the universe is expanding. This possibility had not been seriously considered. Einstein was so sure of a static universe that he added the [[cosmological constant]] to his equations. Many astronomers also tried to avoid the implications of general relativity, with one notable exception: the Russian physicist [[Alexander Friedmann]].

In 1922, Friedmann made two very simple assumptions: the universe is identical wherever we are, ([[homogeneity and heterogeneity|homogeneity]]), and that it is identical in every direction that we look, ([[isotropy]]). It follows that the universe is non-static. Support was found when two physicists at [[Bell Labs Holmdel Complex|Bell Labs]], [[Arno Penzias]] and [[Robert Woodrow Wilson|Robert Wilson]], found [[Discovery of cosmic microwave background radiation|unexpected microwave radiation]] coming from all parts of the sky. 

At around the same time, [[Robert H. Dicke]] and [[Jim Peebles]] were also working on [[microwave radiation]]. They argued that radiation from the early universe should be detectable as the [[cosmic microwave background]]. This was what Penzias and Wilson had found. 

In 1965, [[Roger Penrose]] used general relativity to prove that a collapsing star could result in a singularity. Hawking and Penrose proved together that the universe should have arisen from a singularity. Hawking later argued this need not be the case once quantum effects are taken into account.