One of the greatest challenges of the modern astrophysics is an explanation about the origin (or formation) and evolution of the universe.
This issue has been a heated source of discussions during the past centuries and even before since man began to learn about the universe.
Following Einstein’s general theory of relativity in 1917 about the curved geometry of the universe and Hubble’s discovery that galaxies were actually receding from each other and our universe was expanding, the topic gained more momentum and since then a great deal of effort and attention has been put forward to offer a complete and coherent theory with common acceptance among astronomers.
Indeed, every theory about the origin and evolution of the universe, should be capable of addressing the following issues:
- What was the origin of the structure?
- How was the universe evolved from its earliest stages to its present status?
- What are the constituents of our universe and what is it made of?
- What physical laws govern the formation of the universe?
- What is dark matter and dark energy and their origin?
- How were the four existing fundamental fields of force in nature (strong interactions, electromagnetic interactions, weak interactions and gravity) disintegrated?
These are just some of the few questions that need to be answered by any cosmological theory. The most commonly agreed timeline for evolution of the universe from t = 0 when our universe can be tracked to its back until now is as follows:
0: The Big Bang. Time and space are created. At this stage, time, space, matter and initial conditions of the universe are indeterminate with its temperature around 1032 K!
10-43 seconds: This is the time of distinction of space and time from matter and separation of the gravitational field the other three fields. This time-span is called the quantum of time or “Planck time”. This is the smallest unit of time where it is possible to make a clock capable of measuring it.
10-35 seconds: Around this time, our universe underwent an inflationary stage with a spectacular acceleration known as inflation. This size increase, by a factor of about 1026, resulted in homogenizing of our universe (it is believed that an unknown field named inflation filed derived this size increase).
10-6 seconds: During this period the so-called electroweak forces disintegrated into the electromagnetic and weak forces that are currently observable for us. Also, at this stage, Higgs particle comes into the scene and sub-atomic particles gain mass.
1 second: The first composite particles were formed. During this period, nucleons (protons and neutrons) were created from a very hot mixture of quarks and gluons.
3 minutes: The first hydrogen and helium nuclei were formed out of protons and neutrons. At this short step, the universe expanded and cooled with such a high rate that heavier elements had no time to be created.
380,000 years: The temperature of the universe dropped enough for capturing of electrons by protons and neutrons and thereby, the formation of hydrogen atoms. Also, for the first time, due to the expansion of the universe and a decrease in the density of the existing matter, the light traveled freely through space without being absorbed by the matter. This reminiscent light from the past is what is now called CMB (cosmic microwave background) and is detectable by modern sensing devices.
30 million years: Around this time, the gaseous matter of the universe compacted enough under its force of gravity formed the earliest stars in our universe. According to computer simulations, the first stars were formed at this point, along with the formation of heavy elements.
200 million years: The Milky Way galaxy, which our solar system is a member of, was formed.
9 billion years: Our solar system and the Earth were formed.
10 billion years: Life started on Earth. It is believed that the start of life might have been due to the formation of the “primordial soup” of organic material under the joint effect of solar radiation and lightning on the earth.
11 billion years: Oxygen began to be produced and accumulated in the Earth’s atmosphere.
13.5 billion years: Early humans evolved in Africa. At this stage, modern humans began to appear on the Earth.
13.8 billion years: for the first time, humanity managed to have a detailed image of the evolution of the universe and a rough view about the steps it went through.
Although many facts about the structure and evolution of the universe have been discovered, there are still many mysteries that are yet to be explained. Dark matter and dark energy and their mass dominance in the observable part of the universe, possibility of the existence of multiverses, the process that led to Big Bang, are just a few.
Our universe, according to the best estimates, is about 13.8 billion years old, meaning that the part of the universe that can be observed by us is 13.8 billion light years in radius. This is called the “edge” or “horizon” of the universe. We cannot observe any point beyond this time span.
In other words, this is an edge in time and not space. It does not mean that our entire universe is about this size; indeed, according to recent estimates, the entire universe is much greater in size or even infinite.