We have so far glossed over the drawbacks of the standard Big Bang theory. We now address two of the drawbacks in this chapter: the flatness problem and the horizon problem. In the early 1980s, Alan Guth resolved these two problems with his inflationary theory. His basic idea is that the universe enters a false vacuum state shortly after the Big Bang, then tunnels out and expands exponentially. We choose to discuss Guth’s original model (now called classical model or old inflation) for pedagogic reasons. Guth’s model has many nice qualitative features; it does not work quantitatively. Therefore, A. Linde, A. Albrecht, P. Steinhardt, and others constructed new models as remedies. It is not clear which of the new models is correct, so we will discuss each of them briefly.


Critical Density Slow Roll False Vacuum Horizon Problem True Vacuum 
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