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In this lecture, I would like to discuss whether time itself has a beginning, and whether it will have an end. All the evidence seems to indicate, that the universe has not existed forever, but that it had a beginning, about 15 billion years ago. ... By that time, maybe the GATT trade agreement will have come into effect.
The time scale of the universe is very long compared to that for human life. It was therefore not surprising that until recently, the universe was thought to be essentially static, and unchanging in time. ... It was therefore natural to believe that the human race, and maybe the whole universe, had a beginning in the fairly recent past. However, many people were unhappy with the idea that the universe had a beginning, because it seemed to imply the existence of a supernatural being who created the universe. ...
This argument about whether or not the universe had a beginning, persisted into the 19th and 20th centuries. ... The Second Law, states that disorder always increases with time. Like the argument about human progress, it indicates that there must have been a beginning. ... The only way of avoiding this problem would be if, for some reason, the stars did not shine before a certain time.
In a universe that was essentially static, there would not have been any dynamical reason, why the stars should have suddenly turned on, at some time. Any such "lighting up time" would have to be imposed by an intervention from outside the universe. ... One can plot the separation of two galaxies, as a function of time. ... So the time of zero separation, would have been less than twenty billion years ago.
At this time, the Big Bang, all the matter in the universe, would have been on top of itself. ...
Since events before the Big Bang have no observational consequences, one may as well cut them out of the theory, and say that time began at the Big Bang. ... This kind of beginning to the universe, and of time itself, is very different to the beginnings that had been considered earlier. ... There is no dynamical reason why the motion of bodies in the solar system can not be extrapolated back in time, far beyond four thousand and four BC, the date for the creation of the universe, according to the book of Genesis. ... By contrast, the Big Bang is a beginning that is required by the dynamical laws that govern the universe. ...
Although the laws of science seemed to predict the universe had a beginning, they also seemed to predict that they could not determine how the universe would have begun. ... This could lead to the graph of the separation between two galaxies, being a curve that approached zero, but didnt actually pass through it, at any finite time in the past. ...
Another attempt to avoid a beginning to time, was the suggestion, that maybe all the galaxies didnt meet up at a single point in the past. ... It was argued, that as you plotted the position of the galaxies back in time, the sideways peculiar velocities, would have meant that the galaxies wouldnt have all met up. ... Thus there would be no necessity for the universe, and time itself, to have a beginning. ... It is therefore difficult to see how the universe could have been oscillating for an infinite time. ... The first theorems were designed to show that time came to an end, inside a black hole, formed by the collapse of a star. However, the expansion of the universe, is like the time reverse of the collapse of a star. I therefore want to show you, that observational evidence indicates the universe contains sufficient matter, that it is like the time reverse of a black hole, and so contains a singularity.
In order to discuss observations in cosmology, it is helpful to draw a diagram of events in space and time, with time going upward, and the space directions horizontal. ...
As we look out at the universe, we are looking back in time, because light had to leave distant objects a long time ago, to reach us at the present time. ... The point of the cone is at our position, at the present time. As one goes back in time on the diagram, the light cone spreads out to greater distances, and its area increases. ... It is this focussing of our past light cone, by the gravitational effect of the matter in the universe, that is the signal that the universe is within its horizon, like the time reverse of a black hole. If one can determine that there is enough matter in the universe, to focus our past light cone, one can then apply the singularity theorems, to show that time must have a beginning. ... One can then apply the theorem of Penrose and myself, to show that time must have a beginning.
Approximate Word count = 4005
Approximate Pages = 16
(250 words per page double spaced)
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