Inflationary Theory
Alan Guth proposed a mechanism that, almost by accident, turned out to be an elegant solution
to these questions. While investigating another problemÑrelating to magnetic monopolesÑhe realized that the early universe could have been a "pressure cooker" of negative energy. In other words, the vacuum that existed at the time of the big bang was not a stable but an unstable, high-energy vacuum. As the universe started to cool into a more stable state, the enormous negative pressure of the "false" vacuum drove a wildly accelerating
expansion that Guth called "inflation" to distinguish it from normal expansion. During this period of inflation the universe would have doubled its size every 10¦7y seconds, swelling from the size of a proton to the size of a grapefruit. All of this would have occurred before the universe was 10¦76 seconds old.
Inflation solved the smoothness problem essentially by ironing out the universe. It would have been much smaller at the very beginning of time, so that points in the sky now very distant from one another would indeed have been in contact, which could account for its observed homogeneity. Inflation also flattens the universe, because any curved surface that gets big enoughÑincluding four-dimensional space-timeÑeventually looks flat.
According to inflation, the universe is a small patch of false vacuum that grew exponentially. This flatness, in turn, requires an ð of 1, so that accounting for the missing mass becomes an even more pressing problem as inflation is increasingly accepted. Also, according to inflation, all the matter and energy in the universe condensed out of the false vacuum. The negative energy, or expulsive force, is exactly balanced by the attractive pull of gravity. In a sense, this means that the total energy is zero, which is what led Guth to conclude that the universe may be "the ultimate free lunch." After inflation the universe went back to the leisurely rate of expansion that Hubble first detected in the 1920s.
Inflation does leave behind its own set of conundrums, however. For example, did it make the universe so smooth so fast that no structures could have formed? Some theories propose that imperfections in the "freezing" process could have provided the seeds for the growth of galaxies and other structures. One of the most successful modifications to the theory calls for a slower, less abrupt type of phase transition that would allow the universe to cohere while retaining some form of structure. It has also been suggested that variations in the density of the original energy bubble caused by quantum uncertainty could produce structure, as matter and radiation condensed out of the false vacuum. At present no theory works completely or gives the right scale of clumping at the end of inflation to produce the structures of the universe.
Inflation also vastly increases the universe's size, because it implies that the observed universe is just a very small fraction of the real one, which is probably trillions of light-years across. There could also be many other bubble-universes besides our own, inflating in their own space-times. Today many different versions of inflation compete for attention.
Finally, when
cosmologists add up all the forms of energy and matter in the universe, including the still-not-discovered dark matter, they still come up short of the amount needed to exactly balance expansion and keep the universe flat. This missing energy, cosmologists have speculated, might be something far stranger than dark matter, because it would exert a repulsive force opposite from the attractive gravitational force. To explain the missing energy, some cosmologists have returned to an idea that Einstein called "the biggest blunder of my life." His equations of general relativity had implied that the gravitational force of all existing matter would cause the universe to collapse. To prevent that, he inserted a repulsive force that he called he cosmological constant. When Hubble discovered that the universe is expanding, the cosmological constant was no longer needed, and Einstein was happy to be rid of it. Today, however, some scientists are working on theories that would reincorporate some version of the cosmological constant, especially as astronomers in the late 1990s have found evidence suggesting that the expansion of the universe is actually accelerating. If so, it could imply that another period of inflation may be able to begin, although not until longer after our Sun burns out.