Our journey has been remarkably successful. We have followed the development of the universe from its earliest moments through to the present. Our picture is that the universe has been expanding ever since it came into existence some ten billion years ago. However, Einstein's theory of general relativity breaks down at extremely early times when the universe was very small. The critical time scale over which the theory does not apply is the Planck time, corresponding to 10−43 seconds. We cannot employ Einstein's theory to determine the nature of the universe before this time.

The question we will address in this concluding chapter is whether we can travel back beyond the Planck time. Are we able to complete the story and cross this final barrier, or does the breakdown of Einstein's theory at this point represent a fundamental limit to our understanding?

If we are going to discuss the origin of the universe, we must first extend Einstein's theory in some suitable way. How might we proceed to modify the theory? We may answer this question by appealing to quantum effects. We have seen in the preceding chapters how these effects become significant on very small scales. The reason why Einstein's theory breaks down before the Planck time is that it fails to take into account the quantum fluctuations that are inherently present in any physical process involving gravity.

These uncertainties are not important on large scales and can be ignored.