A brief history of time 8

 

Chapter 8

The origin and the fate of the universe

 

As the universe expands any matter or radiation in it gets cooler.

When it doubles its size, its temperature falls by half.

As the temperature is a measure of the average energy -or speed- of the particles, this cooling of the universe would have a major effect on the matter in it.

At very high temperatures, particles would be moving around so fast that they would escape any attraction toward each other due to nuclear or electromagnetic forces, but as they cooled off one would expect particles that attract each other to start to clump together.

Moreover, even the types of particles that exist in the universe would depend on the temperature. At high enough temperatures, particles have so much energy that whenever they collide many different particle/antiparticle pairs would be produced-and although these particles would annihilate on hitting antiparticles, they would be produced more rapidly than they could annihilate.

At lower temperatures, however, when colliding particles have less energy, particle/antiparticle pairs would be produced less quickly-and annihilation would become faster than production.

 

One second after the big bang, the temperature of the universe would fall to 10 thousand million (ten billion) degrees. At this time the universe would have contained mostly, photons, electrons, neutrinos and their antiparticles, together with some protons and neutrons.

As the universe continued to expand and the temperature to drop, most of the electrons would have annihilated with each other to produce more photons, leaving only a few electrons left over.

The neutrinos and antineutrinos however, would not have annihilated with each other. So they should still be around today. We might be able to detect them only indirectly: they could be a form of “dark matter”, with sufficient gravitational attraction to stop the expansion of the universe and cause it to collapse again.

One hundred seconds after the big bang, the temperature would have fallen to one thousand million (one billion) degrees, the temperature inside the hottest star. At this temperature protons and neutrons would have started to combine together to produce the nuclei of atoms of deuterium (heavy hydrogen) which contain one proton and one neutron.

The deuterium nuclei would then have combined with more protons and neutrons to make helium nuclei, which contain two protons, and two neutrons, and also small amounts of a couple of heavier elements, lithium and beryllium.

The remaining neutrons would have decayed into protons, which are the nuclei of ordinary hydrogen atoms.

 Within a few hours from the big bang , the production of helium and other elements would have stopped and it would continue to expand for about the next million years, without anything much happening.

Eventually, once the temperature had dropped to a few thousand degrees, the electrons and nuclei would not have much energy to overcome the electromagnetic forces between them, and they would have started combining to form atoms.

 The universe would have continued expanding and cooling, but in regions that were slightly denser than average, the expansion would have been slowed down by the extra gravitational attraction. This would eventually stop expansion in some areas and cause them to start to recollapse. As the collapsing region got smaller, it would spin faster and balance the attraction of gravity, and in this way disklike rotating galaxies were born. Other regions which did not happen to pick up a rotation, would become oval shaped objects called elliptical galaxies.

As time went on, hydrogen burning into helium and radiating the resulting energy as heat and light would form stars like our sun.

The outer regions of a star may sometimes get blown off in a tremendous explosion (supernova), which would outshine all the other stars in its galaxy.

Some of the heavier elements produced near the end of the life of a star would be flung back into the gas in the galaxy, and would provide some of the raw materials for the next generation of stars.

 Our own sun contains 2 percent of these heavier elements, because it is a second or third-generation star, formed some five thousand million years ago from a cloud of rotating gas containing the debris of earlier supernovas. Most of the gas in that cloud went to form the sun or got blown away, but a small amount of the heavier elements collected together to form the bodies that now orbit the sun as planets like the earth.

The earth was initially hot and without atmosphere. In the course of time it cooled and acquired an atmosphere from the emission of gases from the rocks. This early atmosphere was not one in which we could have survived. It contained no oxygen, but a lot of other gases that are poisonous to us but not to other primitive forms of life. They are thought to have developed in the oceans, possibly as a result of chance combinations of atoms into large structures, called macromolecules, which were capable of assembling other atoms in the ocean into similar structures. They would thus have reproduced themselves and multiplied.

 In some cases there would be errors in the reproduction. Mostly these errors would have been such that the new macromolecule could not reproduce itself and eventually would have been destroyed.

However, a few of the errors would have reproduced new macromolecules that were even better at reproducing themselves. They would have therefore an advantage and would have tended to replace the original molecules. In this way a process of evolution was started that led to the development of more and more complicated and self-producing organisms. These primitive organisms consumed various materials and released oxygen. This gradually changed the atmosphere to the composition that it has today, and allowed the development of higher forms of life such as fish, reptiles, mammals, and ultimately human race.

 This picture of the universe which started off very hot and cooled as it expanded is in agreement with all the observational evidence that we have today. Nevertheless, it leaves a number of important questions unanswered:

·         Why was the early universe so hot?

·         Why is universe so uniform on a large scale? Why does it look the same at all points of space and in all directions?

·         Why did the universe start out with so nearly the critical rate of expansion that separates models that recollapse from those that go on expanding forever, that even now, ten thousand million years later, it is still expanding at nearly the critical rate?

·         Despite the universe is so uniform and homogeneous on a large scale, it contains irregularities such as the stars and galaxies which are thought to have developed from small differences in the density of the early universe from one region to another. What was the origin of these density fluctuations?

 The general theory of relativity, on its own, cannot explain these features or answer these questions.

  How did God start the initial state of the universe? One possible answer could be that God chose the initial configuration of the universe for reasons that we cannot hope to understand. This would certainly have been within the power of an omnipotent being. BUT if he had started it off in such an incomprehensible way, why did he choose to let it evolve according to laws that we COULD understand?

The whole history of science has been the gradual realization that events do not happen in an arbitrary manner but there is an underlying order, which may or may not be divinely inspired. Why shouldn’t this order apply to the initial state of the universe?

There may be different initial conditions of the universe but there ought to be a principle that picks out one initial state.

 One such possibility is what is called the chaotic boundary conditions. These assume either that the universe is spatially infinite or there are infinitely many universes. In any case the initial state of the universe, according to this principle, is chosen purely randomly.

 But it is difficult to see how such chaotic initial conditions could have given rise to a universe that is so smooth and regular on a large scale as ours is today.

 Another principle is the anthropic principle.

The first version of it that is called the weak anthropic principle states that in a universe which is large or infinite in space and/or time, the conditions necessary for the development of intelligent life will be met only in certain regions that are limited in space and time.

The strong version of the principle states that there are either many different universes or many different regions of a single universe, each with its own initial configuration, and perhaps with its own set of laws of science.

 

 The laws of science as we know them at present contain many fundamental numbers. For example the size of the electric charge of the electron. If the electric charge of the electron had been slightly different, stars either would have been unable to burn hydrogen and helium, or else they would not have exploded. Of course there might be other forms of intelligent life that did not require the light of the sun or the heavier chemical elements that are made in stars, and are flung back into space when the stars explode.

   In order to predict how the universe should have started off, one needs laws that hold at the beginning of time.

 If the classical theory of relativity was correct, the singularity theorems that Roger Penrose and I (says Hawking) proved show that the beginning of time would have been a point of infinite density and infinite curvature of space-time. All the known laws of science would break down at such a point. One would suppose that there were new laws that held at singularities, but it would be very difficult even to formulate such laws and we would have no guide from observations as to what those laws might be.

 So, classical theory is no longer a good description of the universe.

 One has to use a quantum theory of gravity to discuss the very early stages of the universe.

 It is possible in the quantum theory for the ordinary laws of science to hold everywhere, including at the beginning of time: it is not necessary to postulate new laws for singularities, because there need not be any singularities in the quantum theory.

 We don’t yet have a complete and consistent theory that combines quantum mechanics and gravity. But we are fairly certain of some features that such a unified theory should have.

 The idea that place and time may form a closed surface without boundary also has profound implications for the role of God in the affairs of the universe. With the success of scientific theories in describing events, most people have come to believe that God allows the universe to evolve according to a set of laws and does not intervene in the universe to break these laws.

 So as long as the universe had a beginning, we could suppose it had a creator. But if the universe is really completely self-contained, having no boundary or edge, it would have neither beginning nor end: it would simply be. What place then for a creator?