Answer:
a. Big Bang Theory explains the beginning of the universe. The entire matter of the universe is believed to be concentrated in a single point. This point had infinite density. At time, t = 0, at the big bang, universe started expanding and cooling down. This theory is most widely accepted theory describing the beginning of the universe. It is not considered as hypothesis because of the evidences received by the scientists- detection of cosmic microwave background, red shift of the light received from the galaxies indicating that universe is expanding.
b. Cosmic Microwave background radiation is a faint radiation having average temperature of 3 K detected every where around from the sky. This is a residual radiation from 400,000 years after the beginning of the universe. The universe was very hot and dense at the beginning and thus, opaque. Hence, detection of the CMB has provided the evidence of Big bang theory.
c. Scientists have proposed three models of the universe:
1. closed universe- The universe has finite volume and curves around itself. The space-time has positive curvature.
2. open universe - The universe has infinite volume and has a negative curvature.
3. flat universe - The universe has zero curvature. It has infinite volume. The actual density of the universe is equal to the critical density of the universe. The expansion rate slows down over time. This is the most widely accepted model.
A. In the beginning there was a singularity with indefinite mass and density. It's a theory cause there are facts such as galaxies moving away from each other that can prove it.
b) Anywhere at any point of the Universe there is some radiation called background radiation. It led to conclude that the universe was hotter and much denser many many years ago.
c) Model of the universe can be defined by the value of the cosmological constant, so now it is considered to be flat. It probably means that it will expand forever.
