The Big Bang Theory is the most widely accepted theory about the origin of the universe. The Big Bang theory states that the universe began in a state of extremely high density and the volume approaching just a point.
Then the universe has been expanding since some particular instant that marked the origin of the universe. Tracing the expansion of the universe back in time shows that the universe would have been compressed to infinite density approximately 10-20 x 109, years ago.
The universe began at that time as a big bang began the expansion. The big bang was the origin of space and time. It is tempting to think of the big bang as something like the explosion of an artillery shell. But it is more correctly an expansion of space itself, filled with hot radiation and matter. Scientists have been trying to visualize the creation of the universe implying properties cf energy and matter and by the experiments in nuclear reactors.
Nuclear reactors are used to study elementary subatomic particles and fundamental forces. In attempts to extrapolate the universe back to its origin or the big bang, different stages of the life of the universe are discussed. The time of the big bang is taken as time zero. At time 10"43 second after the big bang the universe was as simple as it could possibly be. At the present time many kinds of particles make up matter, and four different forces govern the behavior of matter. The electromagnetic force, gravitational force, strong force (attraction between subatomic particles in the nucleus) and weak force (which contribute to radioactive decay).
In the first fraction of second after the big bang all the particles were fundamentally similar and only one force (the grand unifying force) was functioning. Between 10*43 and 10"36 second after the big bang, the universe was extremely hot and the particles were exerting upon another force the single, strong-weak electromagnetic force. Due to high energy, quarks and electrons present at that time were interchangeable. Quarks are the most fundamental particles discovered which combine to form protons and neutrons. The temperature of the universe at that time was 100,000 million degree Centigrade (°C).
From 10"35 to 10"10 second of the big bang the universe was cooling and the strong and electromagnetic- weak forces were functional distinctly between quarks. Quarks and electrons were no longer interchangeable.String like concentrations appeared around which future galaxies were to be formed. After 10"10 to 10"6 second of the moment of creation the hot radiation continued to cool down and three forces; electromagnetic, strong and weak, became differentiated. After the end of this era quarks started coming together to form protons, neutrons and other subatomic particles.
Then starting from 10"5 second, the commonly known subatomic particles were formed. Quarks had combined to form protons and neutrons. However, the universe was too hot to allow the particles to combine in the form of atoms and atomic nuclei. The collisions of high energy particles would have knocked any nucleus trying to assemble. Electrons, protons, neutrons, neutrinos and anti-neutrons were involved in atomic activity characteristic of the particles.
By about three minutes after the big bang the temperature of the universe was dropped enough that protons and neutrons were able to collect to form atomic nuclei. The temperature was however too high for the formation of atoms or elements. The electrons going around the atomic nuclei would have been knocked to go astray, leaving the orbit. By about 500,000 years after the big bang the temperature was low enough to form complete atoms. Radiation were however so strong that they were preventing the particles to assemble in the form of galaxies and stars. After the formation of atoms, the matter started condensing to form early planetary bodies. These cosmic bodies could interact with one another through force of gravity. More matter was condensed to make nuclei for the formation of galaxies.
Big Bang and the Expanding Universe - Theory of the Creation of the Universe
Scientists have known since the 1920s that the universe is expanding. Most of the Earth's galactic neighbors are receding, and the farther away they are the higher is their velocity. Edwin P. Hubble who was a pioneer of modern astronomy analyzed the red shift of galaxies. He discovered that the speed at which a galaxy is receding (moving away from us) is proportional to its Alative distance. It means that the more distant a galaxy, the faster it is moving away. The principle was named Hubble's law.
It can be represented as:
Speed = H x Distance where H is Hubble's constant. A generally accepted value of H is 56 Km per second per mega parsec (a mega parsec is 3.26 million light years).