Thanks to online shopping, buying things now is much easier than before. You do not like something—you can always return it back for a full refund. A physicist might say that one can turn the arrow of time back for non-used purchases. A cosmologist may comment that we use a similar time reversal method in our research.
Indeed, to understand the origin of the universe we may study its present evolution, and then solve the Einstein equations back in time. What we find is very similar to what happens then we play a movie back. At present, galaxies move away from each other. Playing the movie back shows them moving closer together. Going further back in time, we see that at some point the density of matter becomes infinitely large. This is the cosmological singularity.
Solving the same equations forward in time, starting from the singularity, one finds that all matter appears from the singularity in a huge explosion, called the Big Bang. When the original cosmic fire cools down, matter condenses into galaxies, and they fly away from each other.
The possibility to go back and forth in time is a powerful tool, which helps us to visualize the evolution of the universe. The resulting picture is so convincing that many of us are using it even now in our lectures. It tells us, essentially, that our universe is a gift sent to us from the cosmological online retailer 14 billion years ago. We can track its delivery from its origin to the present time, running our calculations all the way back to the Big Bang.
Of course, we know that one cannot turn the arrow of time back because of the second law of thermodynamics. But the standard lore of the Big Bang theory was that the expansion of the universe was nearly adiabatic, and therefore approximately reversible. In particular, it was assumed that the total number of elementary particles in the universe did not change much during the cosmological evolution.
Some parts of this picture, however, were problematic. One may wonder, for example, who paid the bill for sending us more than 1090 elementary particles populating our universe, and who made the universe uniform and suitable for life?
In the beginning of the '80s, it was found that one can solve these problems if soon after the Big Bang there was a short stage of an exponentially fast expansion of the universe, called inflation. This idea experienced numerous modifications, and now we do not even think that the universe was born in fire. Instead, it could be created from a tiny vacuum-like speck of matter with special properties, weighting less than a gram and containing no elementary particles at all. Normal matter emerged only later, when the universe became exponentially large and its original vacuum-like state decayed.
During the last 35 years, many predictions of this theory have been verified by cosmological observations, but nevertheless it is somewhat difficult to get used to it, in part because it does not match the conventional picture of the time-reversible universe.
Indeed, let us try to follow the cosmological evolution back in time in this scenario. At first, galaxies move towards each other and the universe becomes very dense, just as in the standard Big Bang theory. But then something really weird happens. Suddenly, at the moment corresponding to the end of inflation, played back in time, all 1090 elementary particles populating our part of the universe completely disappear!... And then—not a flash, not a sound, nothing, except the exponentially shrinking and disappearing empty universe.
Thus, in this scenario, the movie played back in time has a nonsensical ending. Making 1090 elementary particles instantly vanish is absolutely impossible!
But there is nothing wrong about it, if one moves in the proper time direction. According to inflationary theory, all elementary particles in the universe were created by conversion of vacuum energy to matter during the decay of the original vacuum-like state. The theory of this process is well developed, but this process is irreversible: once particles are created, they cannot be un-created.
This is a part of the mechanism that makes the new theory work: We do not need anybody to send us a huge container with more than 1090 elementary particles; a tiny package with one gram of matter is more than enough. On its way to us, this package started growing up, unwrapping itself, producing billions of galaxies containing hundreds of billions of stars. We cannot, even in our imagination, follow this package back to its origin and watch all matter dissolving into nothingness. Our universe is unreturnable, so our only choice is to accept this gift and use it to the best of our abilities.