We really don’t know what the real origin of the universe is, but people are studying on many theories to find out the truth. From now they have three theories to study.
The first and the most common theory is the BIGBANG THEORY, according to scientist this theory happened at approximately 13.7 billion years ago this is the moment before creation when space and time did not exist. It is said that our universe was created when a super-dense, super-hot mass exploded and began expanding very rapidly, eventually cooling and forming into the stars
and galaxies with which we are familiar. Rather than expanding outward into some preexisting vacuum, the event of the Big Bang was space itself expanding perhaps at speeds greater than light.
The Big Bang theory was originally developed in the late 1920s by Georges-Henri Lemaître, a Belgian Catholic priest and astronomer, an early advocate of solutions to the general relativity field equations which predicted our universe was expanding.
The next theory is the STEADY STATE THEORY it was developed as a result of theoretical calculations that showed that a static universe was impossible under general relativity and observations by Edwin Hubble that the universe was expanding. The steady state theory asserts that although the universe is expanding, it nevertheless does not change its look over time. For this to work, new matter must be formed to keep the density equal over time.
Because only very little matter needs to be formed, roughly a few hundred atoms of hydrogen in the Milky Way Galaxy each year, it is not a problem of the theory that the forming of matter is not observed directly. Despite violating conservation of mass, the steady state theory had a number of attractive features. Most notably, the theory removes the need for the universe to have a beginning.
For most cosmologists, the refutation of the steady-state theory came with the discovery of the cosmic background radiation in 1965, which was predicted by the big bang theory. Within the steady state theory this background radiation is the result of light from ancient stars which has been scattered by galactic dust. However, this explanation has been unconvincing to most cosmologists as the cosmic microwave background is very smooth, making it difficult to explain how it arose from point sources and the microwave background shows no evidence of features such as polarization which are normally associated with scattering. Furthermore, its spectrum is so close to that of an ideal black body that it could hardly be formed by the superposition of contributions from dust clumps at different temperatures as well as at different red shifts.
The next theory is the INFLATATION OF THE UNIVERSE this theory proposes a brief period of extremely rapid accelerating expansion in the very early universe, before the radiation dominated era called the hot big bang. This acceleration is believed to be driven by a quantum field (in effect, some exotic kind of matter) with a repulsive gravitational effect. This can be achieved if the pressure of the field is extremely large and negative
A specific example is a scalar field associated with a potential energy. Such a field "rolls down" the energy surface defined by the potential, and if it is slow-rolling can act like an effective cosmological constant, driving an exponential expansion with constant acceleration. During this epoch, any matter or radiation density other than that of the scalar field is negligible; one is left with an almost constant energy density of the field, often called a false vacuum because it behaves like the highly energetic vacuum of quantum field theory. Every 10-37 seconds the size of an inflating patch doubles with its energy density remaining constant, so the total mass in the region increases by a huge factor. Inflation ends through decay of the repulsive material into a mixture of matter and radiation, this decay taking place by quantum processes similar to radioactive decay of ordinary matter. The resulting hot expanding gas provides the starting point for the hot big bang era in the early universe.
This scenario provides explanations for some puzzles in cosmology: why the universe is so large, why it is so uniform, and why it is so nearly flat (scientists cannot detect the large-scale spatial curvature effects associated with general relativity). Most importantly, this scenario provides an explanation for the origin of large-scale structure in the universe: Clusters of galaxies arise from seed perturbations generated by quantum fluctuations in the very early universe, amplified vastly in size by the inflationary expansion of the universe and in amplitude by gravitational instability after the decoupling of matter and radiation. A major triumph of the theory is that the subtle variations in the cosmic background radiation it predicted have been observed from satellites and balloons.
