Brief History of Time - II

Chapter 3: The Expanding Universe

The crux of the chapter (appears to me) is the Doppler Effect. It was observed by Edwin Hubble that the spectra of light from galaxies appeared shifted to the red end of the spectrum. This means that all galaxies are moving away from the earth. {The discovery that the universe is expanding was one of the great intellectual revolutions of the twentieth century. Even Einstein had predicted a static universe!}

Next are the Friedmann assumptions :-
1. The universe looks identical in whichever direction we look
2. The above would also be true if we were observing the universe from elsewhere

Three models obey Friedmann’s fundamental assumptions
1.The universe is expanding sufficiently slowly that the gravitational attraction between the galaxies causes it to eventually slow down. The universe begins contracting.
2. The universe is expanding so rapidly that the gravitational attraction can never stop it albeit would slow it down
3. The universe is expanding only just fast enough to avoid re-collapse

So which model describes our universe? We don’t know!! All we know for sure is that it is expanding. Even if it contracts it shouldn’t worry us, unless we have transgressed the Solar System, mankind will long since have died out, extinguished along with our sun !!

Hawking then explains that the big bang is the beginning of time. He says that all events prior to the big bang are irrelevant and should not form part of a scientific model of the universe. This further means that general relativity is only an incomplete theory; it cannot tell us how the universe started off; because it predicts that all physical theories including itself breakdown at the beginning of the universe.