"A Brief History of Time" 3 by Steven Hawking

 

Chapter 3. 

The expanding Universe

·         Hawking and Penrose proved, in 1970, that the universe must have had a beginning in time. They based their theory on Einstein’s general theory of relativity. That proof showed 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, break down at the beginning of the universe.

There must have been a time in the very early universe when the universe was so small that one could no longer ignore the small-scale effects of the other great partial theory of the twentieth century, quantum mechanics.

 

      In 1924, the American astronomer Edwin Hubble demonstrated that ours was not the only galaxy.

In 1929, Hubble published his findings that the galaxies are moving away from us. The further a galaxy is, the faster it is moving away! And that meant that the universe cannot be static, as everyone previously had thought, but is in fact expanding; the distance between the different galaxies is growing all the time.

 If the universe was static, it would soon start to contract under the influence of gravity.

If it was expanding fairly slowly, the force of gravity would cause it eventually to stop expanding and then to start contracting.

 If it was expanding at more than a certain critical rate, the gravity would never be strong enough to stop it and so it would continue to expand forever.

 This behavior of the universe could have been predicted from Newton’s theory of gravity at any time since the 17^th century. Yet, the belief in a static universe was too strong to be changed.

 Even Einstein, claimed that space-time had an inbuilt tendency to expand and this could be made to balance exactly the attraction of all the matter in the universe, so that a static universe would result.

 It was a Russian physicist and mathematician, Alexander Friedmann, who showed that we should not expect the universe to be static.  

 

• We could determine the present rate of expansion by measuring the velocities at which other galaxies are moving away from us. But there are lots of things we don’t know so we cannot do it.

E.g  the distances to the galaxies are not very well known because we can only measure then indirectly.

So all we know is that the universe is expanding by between 5 percent and 10 percent every thousand million years.

Also, our uncertainty of the present average density of the universe is even greater.

 

• All of the Friedmann solutions have the feature that at some time in the past

(between ten and twenty thousand million years ago) the distance between neighboring galaxies must have been zero.

That time we call the Big Bang.

 The density of the universe and the curvature of space-time would have been infinite. This means that the theory of relativity (on which Friedmann’s solutions are based) predicts that there is a point in the universe where the theory itself breaks down.

Such a point is an example of what mathematicians call a singularity.

 In fact, all our theories of Science are formulated on the assumption that space-time is smooth and nearly flat, so they break down at the big bang singularity, where the curvature of space-time is infinite.

 This means that even if there were events before the big bang, one could not use them to determine what would happen afterward, because predictability would break down at the big bang. We should therefore cut them out and say that time had a beginning at the big bang.

The Catholic Church, seized on the big bang model and in 1951 officially pronounced it to be in accordance with the Bible!

• In 1965, the British mathematician and physicist Roger Penrose showed that a star collapsing under its own gravity is trapped in a region whose surface eventually shrinks to zero size. And since the surface of the region shrinks to zero, so too must its volume. All the matter in the star will be compressed into a region of zero volume, so the density of the matter and the curvature of space-time become infinite. In other words one has a singularity contained within a region of space-time known as a black hole.

Hawking reversed Penrose’s theorem in 1965 and showed that any Friedmann-like expanding universe must have begun with a singularity.

 Almost everyone accepted that the universe started with a big bang singularity, but ironically, Hawking changed his mind and “I am now trying to convince other physicists that there was no singularity at the beginning of the universe. It disappears when quantum effects are taken into account”.