The Big Bang theory still remains the prevailing cosmological model explaining the beginning of the Universe. But it can’t answer many questions concerning its early development. Perhaps, any other theory can? Let’s try to find it out.
First of all, let’s have a look at the main statements of the Big Bang theory. Our universe was born after the Big Bang about 13-14 billion years ago. The Big Bang happened everywhere at once. At that time there were no stars, galaxies and even atoms, and the Universe was filled with a very dense, hot and rapidly expanding blob of matter and radiation. Increasing in size, it got cold. Approximately three minutes after the Big Bang the temperature fell enough to form atomic nuclei, and after half a million years electrons and nuclei integrated in electrically neutral atoms and the Universe became transparent to the light. Today it allows us to register the light emitted by that fiery bunch. This is what we call cosmic background radiation.
Initially the fire bunch was almost perfectly homogeneous. But in some areas the density was slightly higher than in others. These inhomogeneities were growing up, pulling with their gravity more and more substance from the environment, until they evolved into galaxies.
A lot of observational data speaks in favor of the Big Bang theory, leaving no doubt that this scenario is basically correct. First of all, we see distant galaxies running away from us at very high velocities, indicating that the universe is expanding. Also the Big Bang theory explains the prevalence of light elements such as helium and lithium in the universe. But the most important piece of evidence is the cosmic background radiation, the afterglow of the primary fireball, still allowing to observe and to explore it.
So, we seem to have a very successful theory. Still, it leaves unanswered some intriguing questions concerning the initial state of the universe after the Big Bang. Why was the universe so hot? Why did it begin to grow? Why was it so homogeneous? And finally, what was happening to it before the Big Bang?
All these questions can be answered by the theory of inflation that was proposed by Alan Guth about 30 years ago.
A central role in this theory belongs to a special form of matter called false vacuum. In this theory vacuum is not just a completely empty space but it is a physical object that has energy and pressure and can be in different energy states. We live in a very low-energy vacuum, and for a long time it has been believed that the energy of our vacuum is equal to zero. However, recent observations have shown that it has a bit different from zero energy (it is called dark energy).
Modern theories of elementary particles claim that except for our vacuum there are several other high-energy vacuums, known as “false”. They are characterized by high negative pressure, repulsive gravity and high instability. It usually decomposes very rapidly, turning into a low-energy vacuum.
Thus, Alan Guth assumed that at the beginning of the universe the space was in a state of false vacuum. In this case, its repulsive gravity would lead to a very rapid, accelerating expansion of the Universe. In this type of expansion, called inflation, there is a typical doubling time in which the size of the universe doubles. In cosmological inflation a tiny fraction smaller than atom inflates to a size greater than the observed part of the universe today.
As the false vacuum is unstable, it eventually breaks, causing a fiery bunch, and inflation ends. The collapse of false vacuum in this theory plays the role of the Big Bang. Since then, the Universe evolves in accordance with the statements of the standard Big Bang cosmology.
So, the inflation theory naturally explains the particularities of the initial state of the Universe, which previously seemed so mysterious. The high temperature is due to a high energy of the false vacuum. The expansion happens due to the repulsive gravity, which makes the false vacuum expand, and the fiery bunch continues growing by inertia. The universe is homogeneous because the false vacuum has always strictly equal energy density (except for small irregularities that are associated with quantum fluctuations in the false vacuum).