The Big Bang theory found great success explaining the general features of the universe, including the approximate age, the expansion history after the first second, the relative atomic abundances from cosmic nucleosynthesis, and of course the cosmic microwave background radiation. And it required only general relativity, a smooth initial state, and some well-understood atomic and nuclear physics. It assumed matter, both seen and unseen, was dominating and slowing the expansion via gravity. In this model the universe could expand forever, or recollapse on itself, depending on whether the average density was less than or greater than a certain value determined only by the present value of the Hubble constant.
However, during the late 20th century there remained some limitations and concerns with the standard Big Bang. Why is today’s density so relatively close to this critical value for recollapse, since it would have had to be within 1 part in 1000 trillion of the critical density at the time of the microwave background to yield that state? How did galaxies form given only the tiny density fluctuations observed in the microwave background emitted at the age of 380,000 years for the universe? And why was the microwave background so uniform anyway? In the standard Big Bang model, regions only a few degrees away from each other would not be casually connected (no communication even with light between the regions would be possible).
There are four known fundamental forces of nature. These are electromagnetism and gravity and two types of nuclear forces, known as the strong force and the weak force. Physicists believe all the forces but gravity unify at energies around 10,000 trillion times the rest mass-energy (using E = mc^2) of the proton (1 Giga-electron-Volt). At some point very early in the life of the universe, at even higher energies equal to the Planck energy of 10 million trillion times the proton mass, all of the four forces would have been unified as a single force or interaction. Gravity would separate from the others first as the universe’s expansion began and the effective temperature dropped, and next the strong force would decouple.
We also must consider the vacuum field, that represents the non-zero energy of empty space. Even empty space is filled with virtual particles, and thus energy. At very early times the energy density of the vacuum would be expected to be very high. During the very earliest period of the development of the universe, it could have decayed to a lower energy state in conjunction with the decoupling of the strong force from the unified single force, and this would also have driven an enormous expansion of space and deposited a large amount of energy into the creation of matter.
In the inflationary Big Bang model postulated by Alan Guth and others, the decay of the vacuum field would release massive amounts of energy and drive an enormous inflation (hyperinflation really) during a very short period of time. The inflation might have started one trillionth of one trillionth of one trillionth of a second after the beginning. And it might have lasted until only the time of one billionth of one trillionth of one trillionth of a second. But it would have driven the size of the entire universe to grow from an extremely microscopic scale up to the macroscopic scale. At the end of the inflation, what was originally a tiny bubble of space-time would have grown to perhaps one meter in size. And at the end of the inflationary period, the universe would have been filled with radiation and matter in the form of a quark-gluon plasma. Quarks are the constituent particles of ordinary matter such as protons and neutrons and gluons carry the strong force.
The doubling time was extremely short, so during this one billionth of one trillionth of one trillionth of a second the universe doubled around 100 times. In each of the 3 spatial dimensions it grew by roughly one million times one trillion times one trillion in size! This is much greater than even Zimbabwe’s inflation and happens in a nearly infinitesimal time! The inflationary period drove the universe to be very flat topologically, which is observed. And it implies that the little corner of the universe we can observe, and think of as our own, is only one trillionth of one trillionth of the entire universe, or less. There is good observational support for the inflationary Big Bang model from the latest observations concerning the flatness of the universe, given that the mass-energy density is so close to the critical value, and also from the weight of the evidence concerning the growth of original density fluctuations to form stars and galaxies.