Several theories have emerged trying to explain the origin of the universe. Some of these theories include the Passing Star Theory and the Big Bang Theory which is the most popular theory of origin of universe. The big bang theory resulted from the observation that other galaxies were at great speed and in all directions moving away from our galaxy seeming to be propelled by some external explosive force.
The big bang theory explains that about thirteen billion years ago an enormous blast of high density and high temperature expanded carrying about light elements, large scale structure which is explained in the cosmic microwave background and Hubbles Law of Physics. After the initial expansion there followed a cooling effect which led to the formation of subatomic particles and later on simple atoms. Stars and galaxies were then formed by giant clouds of primordial elements which coalesced through gravity (Simon, 2005, p. 456).
In 1965, the background radiation of cosmic microwave discovery provided an evidence in favour of the big bang theory since big bang theory had predicted the existence of such a microwave background radiation across the universe before it was discovered. Edwin Hubble, an American astronomer observed that the distances to galaxies far away were tied to their redshifts. The interpretation of this was that all the distant galaxies and clusters were receding away from our vantage point with an estimated velocity proportionate to their distance. That is, the greater the distance they are from us, the higher their velocity of moving away from us in whichever direction (Wollack, 2010, p.65).
Since it is known that the distance between galaxies increases as time goes by, it implies that earlier the galaxies must have been closer together and the universe must have been probably denser and hotter during those times. The first subatomic particles were protons, neutrons and electrons. Simple atomic nuclei formed shortly after the big bang while thousands of years passed for electrically neutral atoms to be formed. These atoms included hydrogen, helium and traces of lithium. Stars and galaxies were then formed by gravitational coalescing of the giant elements. The framework for the big bang relied on the theory of general relativity brought forward by Albert Einstein and simplified assumptions of homogeneity and isotropy of space (Wollack, 2010, p. 63).
In the earliest phases of big bang, the universe was homogeneously and isotropically filled with a density of a very high energy and high temperatures and pressures which were rapidly expanding and cooling. A cosmic inflation was caused by a phase transition which led to exponential growth of the universe. After the inflation, the universe was composed of quark-gluon plasma with other elementary particles. High temperatures created random motions of particles at relativistic speed with particle-antiparticle pairs of varied kinds created and destroyed in collisions. At certain situations some reaction called baryogenesis violated the conservation of baryon number resulting to a small excess of leptons and quarks over antileptons and antiquarks which consequently resulted into the predominance of matter over antimatter in todays universe (Joseph, 2009, p. 212).
Thereafter there was a continuous decrease in density and fall in temperature in the universe thus a decrease in typical energy of each particle. From the small excess of quarks over antiquarks, arose a proportional small excess of baryons over antibaryons. The temperatures fell such that neither new proton-antiproton pairs nor new neutron-antineutron pairs could be created. This was followed by a mass annihilation which left only 1010 protons and neutrons of all the protons and neutrons initially created. However, none of the neither antiproton nor antineutron particles were left. Electrons and positrons also underwent a similar process. Protons, neutrons and electrons that remained were no longer in a relativistic motion. Instead, the density of the energy of the universe became dominated by photons (Gibson, 2005).
Later on a process known as big bang nucleosynthesis process, most protons if not all remained single as hydrogen nuclei. They were uncombined. In the cooling process of the universe, the rest mass density of energy of matter gravitationally dominated the energy density of the photon radiation. In 379000 years, atoms were formed through a combination of electrons and nuclei to continue through space. This radiation was what was referred to as cosmic microwave background radiation. Over quite a long period of time, gas clouds, stars, galaxies and other astronomical structures present today were formed through gravitational attraction of the slightly denser matter. The possible types of matter that formed the aforementioned included hot dark matter, cold dark matter, baryonic matter and warm dark matter (Simon, p. 560, 2005).
The big bang theory relied on the assumptions of universality of physical laws and the cosmological principle where the cosmological principle asserts that on the large scale, the universe is homogeneous and isotropic. The assumptions have been tested over the years to prove their legitimacy and applicability. For instance, the assumption of universality of physical laws has been tested and observations show that fine structure constant of the age of the universe can have the largest possible deviation of order 10-5. The general relativity has also been put under stringent tests on the solar system and binary stars scale (Wollack, 2010).
General relativity determines the distance separating nearby points using a metric. These points can be stars, galaxies or even coordinate chart laid down over all spacetime. Cosmological principle implies that on large scales, the metric should be homogeneous and isotropic. The big bang is where the space expands with time to widen the physical distance between two co-moving points and not an explosion in space but an expansion in space. The existence of horizons is a very important feature of the big bang spacetime. With the finite age of the universe, light also travels across the universe at a finite speed. Horizon places a limit on the observable most distant objects. With the expansion of space and quick receding of more distant objects, light emitted today may not visualize very distant objects. This gives us a definition of future horizon which limits the future events that we may be able to influence if there comes a continuous acceleration of the expansion of the universe, then there is as well a future horizon (Joseph, p. 208, 2009).
The big bang theory came as a result of observations of the structure of the universe and other theoretical considerations. In 1912, astronomist Vesto Slipher carried out a measure on the first Doppler shift of a spiral nebula and the result was that almost all such nebulae receded from the earth. He never grasped the cosmological implications of this test measure and in fact there were controversies whether these nebulae were island universes outside the earths milky way or not. A decade later, Alexander Friedmann, a Russian cosmologist and mathematician derived an equation from Albert Einsteins equation of general relativity which indicated the possibility of expansion of the universe contrary to the static universe model brought forward by Einstein at that time. Come 1924, Edwin Hubble measured the great distance to the nearest nebulae and proved that the systems were indeed some other galaxies. Georges Lemaitre, a Roman Catholic priest and a Belgian physicist in 1927 independently derived the Friedmans equations and gave a proposal showing that the inferred recession of the nebulae was due to the expansion of the universe (Gibson, p. 34, 2005).
In conclusion, in 1931, Lemaitre further suggested that the expansion of the universe, given backward time projection, implied that the further in the past the smaller the universe was. Then at some finite time all the mass of the universe became concentrated into one point known as primeval atom which brought about the existence of fabric of space and time. And that was the Big Bang theory origin of the universe.
Gibson, C. H. (2005). The First Turbulent Combustion P. 34-87
Joseph silk (2009). Horizons of Cosmology. Templeton Press. P. 208-243
Simon Singh (2005). Big Bang: The Origin of the Universe. Harper Perennial. P. 560.
Wollack, E. J. (10 Dec. 2010). Cosmology: The Study of the Universe. P. 56-67
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