The Beginning of the Cosmos and the Flow of Time

There is a certain amount of arbitrariness in the definition of the moment from which we count time. We could have started counting it 753 years earlier, that is, from the founding of Rome. (In reality, these years are probably 749 and not 753, due to the fact that Jesus was born in 4 BC.) Or we could have counted it from the time of Muhammad’s Hijra, 622 years later. Or, we could have chosen Friday, February 18, 3102 BC, the beginning of the Kali Yuga.

These different choices, however humanly meaningful, are ultimately irrelevant: more than time itself, what matters is its flow. Its progression from past to future. Linear in the West. Circular in the East. Linear in the Gospel or the Quran. Circular in the Baghavad-Gita.

It is tantalizing to start the universal time from the beginning of cosmological expansion. This, however, would have two disadvantages. First, it would place the initial moment in a remote, extremely remote time. Far removed from human time.

Secondly, setting the zero time so far in the past means dealing with cosmological and gravitational effects. It means being careful about the clock we use, which must be in harmony with the expansion. We can think of it as a clock that, starting 380,000 years into the universal lifespan, sees the cosmic microwave background radiation as isotropic. Isotropic, that is, equal in all directions.

But what does it really mean to be “in harmony” with the expansion? Essentially, it means not being attracted by other bodies’ gravitational pull… and in turn not attracting them. Because this would give rise to “local motions” due to forces. Cosmological expansion, on the other hand, is not due to any force. It can be thought of as the Universe becoming increasingly “bigger,” and the class of observers who satisfy these conditions (and therefore see the cosmic microwave background radiation as isotropic) are called “comoving” observers.

We will discuss this problem again, since it is complicated. It seems perhaps too inhuman. We prefer to start from a “zero” reference, from a starting point that has significant human value. The conventional birth of Jesus as year “zero” is fine. The date of a human event. Exceptionally, extraordinarily human.

Autori: Marco G. Giammarchi e Roberto Radice